Re: Adjusting a 7A13, and a few other questions
If you're going to buy new pots, why not try cleaning the old ones first? Nothing to lose if it doesn't work.
Don Black.
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On 24-Jul-12 11:16 AM, keithostertag wrote: Thanks Steve. I haven't yet seen the sections that address adjusting the lower ranges. Just glancing through the manual I feel I am not compentent or equipped to do much on this 7A13. If I'm not confident about cleaning the pots it may be best to just buy them (about $20 for both at QService). And thanks for informing me not to move any of the input components.
Keith
--- In TekScopes@..., "Steve"<ditter2@...> wrote:
--- In TekScopes@..., "keithostertag"<keitho@> wrote:
I am just now getting around to looking at my 7A13, bought on eBay awhile back. I've never used a differential amp. Serial # B236701.
I'm following the basic operating instructions, page 2-4 of the manual. Steps 9 and 10 are:
9) Vary the VOLTS/DIV switch from 10mV to 50mV while observing the CRT trace.
10) Adjust STEP ATTEN BAL so that the trace does not move while varying the VOLTS/DIV switch.
OK. Does this mean _only_ the range between 10mV and 50mV? Yes, There are other Balance tweeks for other ranges.
The reason I ask is that while that range is easy to adjust in that manner, if I vary the VOLTS/DIV switch through it's entire range I get quite a change when going from 10mV to the lower settings of 5,2, and 1 mV. I just want to know if that's normal/expected or not.
Observation: The VAR BAL pot works, but must be turned almost all the way CW in order to adjust the trace for no movement when varying the VARIABLE control.
Second question: I feel the Comparison Voltage (Vc) pots R690A (fine) and R690B (coarse) both should be cleaned. Can someone give me a few tips on how to clean these, whether this type pot can/should be dismantled or not? Here's a photo:
Don't dismantal the pots. Check the archive for substitutes for "no-noise" - the cleaner Tek used to use which is no longer made.
I thought I'd show you a cap I found which has visible leaking (C505):
Would you assume this is due to age, or does it suggest a closer look at that part of the circuit is warranted?
Finally, I may want to pay someone to do a full calibration on this for me, if it is not too dear. Recommendations? Feel free to contact me off-list. Will probably cost a small fortune. I have not cal'ed a 7A13 in years, but I remember the procedure is nearly 100 steps and takes several hours for an experienced technician who is not familiar with this specific instrument.
If you dissassembled anything or physically moved any of the input components or wiring, the CMRR balance tweaks will need to be readjusted to maintain the great CMRR performance this amp is capable of. If no component replacements or dissassembly was done, these tweaks tend to be very stable and not require any adjustment during a routine calibration.
- Steve
Thanks,
Keith Ostertag
------------------------------------
Yahoo! Groups Links
|
Re: Adjusting a 7A13, and a few other questions
I haven't used it yet but I've just bought a Deoxit kit based on their reputation. It contains Deoxit for normal contacts, Deoxit gold for gold contacts, Deoxit shield for harsh environments and Deoxit fader for conductive tracks (pots). Perhaps someone has used the Deoxit fader cleaner and can coment on its usefulness for Tektronix pots.
Don Black.
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On 24-Jul-12 10:24 AM, Steve wrote: --- In TekScopes@..., "keithostertag"<keitho@...> wrote:
I am just now getting around to looking at my 7A13, bought on eBay awhile back. I've never used a differential amp. Serial # B236701.
I'm following the basic operating instructions, page 2-4 of the manual. Steps 9 and 10 are:
9) Vary the VOLTS/DIV switch from 10mV to 50mV while observing the CRT trace.
10) Adjust STEP ATTEN BAL so that the trace does not move while varying the VOLTS/DIV switch.
OK. Does this mean _only_ the range between 10mV and 50mV? Yes, There are other Balance tweeks for other ranges.
The reason I ask is that while that range is easy to adjust in that manner, if I vary the VOLTS/DIV switch through it's entire range I get quite a change when going from 10mV to the lower settings of 5,2, and 1 mV. I just want to know if that's normal/expected or not.
Observation: The VAR BAL pot works, but must be turned almost all the way CW in order to adjust the trace for no movement when varying the VARIABLE control.
Second question: I feel the Comparison Voltage (Vc) pots R690A (fine) and R690B (coarse) both should be cleaned. Can someone give me a few tips on how to clean these, whether this type pot can/should be dismantled or not? Here's a photo:
Don't dismantal the pots. Check the archive for substitutes for "no-noise" - the cleaner Tek used to use which is no longer made.
I thought I'd show you a cap I found which has visible leaking (C505):
Would you assume this is due to age, or does it suggest a closer look at that part of the circuit is warranted?
Finally, I may want to pay someone to do a full calibration on this for me, if it is not too dear. Recommendations? Feel free to contact me off-list. Will probably cost a small fortune. I have not cal'ed a 7A13 in years, but I remember the procedure is nearly 100 steps and takes several hours for an experienced technician who is not familiar with this specific instrument.
If you dissassembled anything or physically moved any of the input components or wiring, the CMRR balance tweaks will need to be readjusted to maintain the great CMRR performance this amp is capable of. If no component replacements or dissassembly was done, these tweaks tend to be very stable and not require any adjustment during a routine calibration.
- Steve
Thanks,
Keith Ostertag
------------------------------------
Yahoo! Groups Links
|
Well it looks like I got my 2336YA up and working. I have let it run for 2 hours on 3 different occasions , showing the calibrator and it's doing fine. I couldn't really determine which part was causing the falt ... 155-0218-00 or 155-219-00 ... and it turned out to be the 155-219-00. So if any one needs a used but good 155-218-00, iv got one for sale.
Anyone have an idea what parts ( bipolar transistors?) this part use?
Mike KC7NOA
|
Just be very gentle with the var/cal switch/pot set screw. The plastic will
be brittle and if you over tighten the set screw, it will snap the plastic. Go
ahead, ask me how I know.
?
Tom
?
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----- Original Message -----
Sent: Tuesday, July 24, 2012 12:28
AM
Subject: Re: [TekScopes] Tektronix
549
?
ken scharf <wa2mze@...> wrote:
>
(I'm still trying to figure out how to get
> inside my 454 to fix the
ch #1 vertical cal control which is jammed with
> the v/d
knob.)
That one's probably easier than it looks. Yell if you want
to
attack it.
-ls-
|
ken scharf <wa2mze@...> wrote: (I'm still trying to figure out how to get
inside my 454 to fix the ch #1 vertical cal control which is jammed with
the v/d knob.) That one's probably easier than it looks. Yell if you want to attack it. -ls-
|
I'm currently winding for 545B's and 547's, and I have a winding
machine in development that will make it possible to handle most
of the rest of the transformers... soon.
-Chuck Harris
Tom Brown wrote:
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I don't know who is winding HV xfmrs for these scopes these days. A
prince of a fellow in Florida did it for years (I did a lot of testing
for him while he was developing his design) but I believe he sold the
gear and doesn't do it anymore.
They're cool scopes though and everybody oughta have one....or two... :-)
Tom
AE5I
|
Re: Stereo Power Amp Problem
| I have enough experience with this sort of issue so I can sort the wheat from the chaff, AND THERE'S A WHOLE LOTTA CHAFF BLOWIN' AROUND HERE! ?First, this has nothing to do with rectifier recovery times. ?Similar diodes are used everywhere, and only in recent years have the audio-fools gotten the hots for fast-recovery diodes for 60 Hz power. ?(Tek never went for this nonsense.) ?It probably is not electrolytic caps, either. ?Except in some SMPS and TV applications where they are highly stressed, they last 50 years or more (except in many Tek scopes ;-) ). ?Original post said amp worked, but this bears scrutiny. ?I always test them by running them into 4 or 8 ohm dummy loads, look for clean sine waves and symmetrical clipping, and measure THD; this could be AFU and you might still get an OK sounding
watt into a speaker. ?In which case, you have the usual blown amp repair situation. ?You may also have an open filter cap or bad regulator; this is not typical and would produce high AC ripple with the output. ?From what I've read so far, none of this has been?exonerated. ?So I'll just have to shoot in the dark like everyone else. ?This looks like a grounding problem to me, which almost never shows up with simple ohmmeter testing. ?First off, don't use a 3rd pin power cord ground, IMHSHO, it's just asking for hum troubles. ?(Although it sounds like you have more of a "buzz" problem, i.e. high-harmonics/spikes.) ?The 120 Hz on the output seems to me to be the current spikes charging the main filter caps somehow leaking into the signal path. ?When you touch the chassis and have an audible effect is a dead gave away. ?The first thing I'd do is tighten the mounting screws on the PCB's, filter caps (if
any), input jacks, and anywhere else. ?Then I'd carefully inspect the grounding strap between the main filter caps; this is usually short and thick, with the power transformer CT connected to it. ?It may be made of thick copper soldered together, and the solder could be cracked or simply a cold joint, esp if it was built from a kit. ?If it still isn't fixed, then there's a couple more tricks. ?Take a clip lead, and try connecting various ground points together; the most important are the input grounds and the above mentioned point. ?You can also take a Tektronix scope with the chassis floating, connect the probe ground to the amp input ground, and go probing for any power line signal at other ground points. ?You probably want to use a 1X probe and max gain. ?You might also be dealing with outside interference. ?Are you next to an industrial welder or early warning radar? ?To check for this, take a portable AM
radio, tune it between stations, see if you pick up any buzz and if you do, move the radio around to zero in on it. ?BTW, this is a quick and dirty way to track down nearly any source of radiated RFI. ?If you can get through all this, then the amp will be fixed. ?Or give up, and ship it to me!
Art |
|
Re: Adjusting a 7A13, and a few other questions
The better capacitors are needed for switching power supplies so there was not a lot of consumer demand for them back when the 7A13 was produced. Aluminum electrolytics are a lot better than they used to be so except in high reliability and wide temperature range applications, wet tantalums are not really needed except in very high performance AC coupling applications like oscilloscope front ends and some low noise circuits. Even now power supply designers often overlook the ripple current rating for capacitors leading to short capacitor life. On Mon, 23 Jul 2012 20:46:03 -0500, "Rob" <rgwood@...> wrote: Thanks David, I saw the post but missed the logic on which was good and why.
(as well as 2 of the links via "speed reading" I guess...sigh...)....
Anyway, much appreciated..
I do not remember the ESR and ESL on caps being something to concerns
oneself with back when I worked on consumer electronics 20 years ago.
However, I know that my chosen addiction is a big step above the stuff I
worked on in those days. The education I receive via this forum is
invaluable and I appreciate you taking the time. If/ when I am finished with
the mainframes and plug-ins I don't ultimately want to keep....
I want to be able to tell people what I replaced and why, and hopefully give
them something that will last another decade. This education from time to
time will help me in that goal.
Rob
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of David
Sent: Monday, July 23, 2012 8:17 PM
To: TekScopes@...
Subject: Re: [TekScopes] Adjusting a 7A13, and a few other questions
I mentioned a couple of replacements for this capacitor in my earlier post.
The capacitor in this case is used for bulk power supply decoupling so low
ESR and ESL is important while leakage is irrelevent. The 109D 10uF 50V has
an 8 Ohm ESR and 160 mA ripple current specification at 120Hz.
An organic polymer electrolytic of the same value should be a superior
replacement if its super low ESR does not cause circuit oscillation which
would not be a problem here because of the relatively high wire resistance
from the mainframe power supply.
A very good low impedance aluminum electrolytic would need to be at least
twice as large achieve the same ESR and ripple current rating but would be
half the price of the organic polymer electrolytic capacitor.
On Mon, 23 Jul 2012 19:50:42 -0500, "Rob" <rgwood@...>
wrote:
Forgive me jumping into a post but the question is related and I know
Keith so he won't mind.... (much)....
When choosing a sub for the particular cap referenced here...
Is a standard electrolytic (by standard electrolytic I mean a 'good'
105C,
etc.) an ok sub or must one go with a wet tantalum?
Thanks in advance for answering
Rob aka Keith * smile *
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On
Behalf Of Brad Thompson
Sent: Monday, July 23, 2012 7:14 PM
To: TekScopes@...
Subject: Re: [TekScopes] Adjusting a 7A13, and a few other questions
On 7/23/2012 6:05 PM, keithostertag wrote:
I am just now getting around to looking at my 7A13, bought on eBay
awhile back. I've never used a differential amp. Serial # B236701. <snip>
I thought I'd show you a cap I found which has visible leaking (C505):
Hello, Keith and the group--
That's a so-called "wet slug" tantalum capacitor. Its clear outer
jacket is yellowed, likely due to overheating.
IIRC, the electrolyte in the wet-slug capacitors is sulfuric acid, so
when you remove the failed capacitor, I'd recommend swabbing the
corroded area with a weak baking-soda solution to neutralize any
remaining acid, followed by swabbing with distilled water to remove any chemical residues.
Inspect for corrosion damage to all PC-board traces in the area and
repair as necessary.
Wet-slug caps don't like to operate with a reverse bias and are likely
to fail prematurely, so check the circuitry for upstream component
failures that might have back-biased the capacitor.
------------------------------------
Yahoo! Groups Links
|
Hi Ken-
It's a cool instrument. It's the only storage mainframe for letter and 1 series plug-in units. When running a 1L5 at slow, high resolution sweeps, it can be indispensable!
Basically, when resurrecting one, the main thing to know is that it has probably either (1) had a replacement high voltage transformer installed or else (2) it needs one. I've worked on a lot of 549s and they all needed new high voltage transformers. And, once it was installed, they all worked like a champ.
The symptom is this: many of them will run for a while....sometimes a few minutes, sometimes longer.....and then the trace will bloom and fade away as the HV drops.
What's happening is that the HV transformer becomes lossy and heats up. As it heats up, it gets more lossy and the screen voltage on the oscillator continues to rise in order to compensate. Finally, the whole thing runs out of room to compensate and the HV falls. You can check for the problem by watching the screen voltage of the oscillator as the unit runs. After a good HV xfmr has been installed, the screen voltage drops right back to where it's supposed to be and stays there. From memory, I think it's something like 23 volts when operating properly.... That's from a long time ago, though, and I could be wrong about the voltage.
I don't know who is winding HV xfmrs for these scopes these days. A prince of a fellow in Florida did it for years (I did a lot of testing for him while he was developing his design) but I believe he sold the gear and doesn't do it anymore.
They're cool scopes though and everybody oughta have one....or two... :-)
Tom
AE5I
|
Re: Rigol 1102E vs. 2430A vs. 2440?
I considered the same question a couple years ago and went with a 2440
because the Rigol lacks peak detection. They have envelope detection
which they call peak detection but that is not the same.
I could make good use of variable persistence in a DSO which the 2440
lacks but I can use my 7834 for that.
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On Tue, 24 Jul 2012 02:42:38 -0000, "ducoar" <ducoar@...> wrote: The first step in getting help is to admit you have a problem. I have multiple scope disorder. There I said it. As I write this, there are at least ten Tek scopes around me fighting for space. (I don't count the Tenma 72-330, which has been banished to the garage, but I digress) Receiving the TekScopes daily digest probably just compounds the problem ;-) but I am having fun.
About 3 or 4 years ago I renewed my interest in electronics, which grew into the desire to add "a scope" to the bench. After some success in (no small thanks to the experts here) repairing a 2236 and a 2445, my analog 'collection' grew to include another 2445, a 2467b, and 3 2465s(2 of these still need repair). I tell myself, "you know, U800s are rare so it's not a bad thing I have so many scopes around." :-) My actual plan is to end keep the 2467B, and add 2445B & 2465B. So for analog I have a plan.
The digital side, is still pretty new to me. My first digital scope was a 2430A, which only required a good cleaning and re-seating of some connections to bring it back to life. I really liked the ability to save a waveform with the 2430A. However the Rigol's ability to save directly to USB vs. the challenge of GPIB for the 2430A won out, and a Rigol 1102E was added to the collection. I also have a 2440 that still needs some tweaking (intensity control circuit).
Comparing digital scope specs isn't clear cut for me (yet), so my question is (and I hope it's not a silly one) do I gain anything by keeping the 2430A or the 2440 along with the Rigol?
On the surface the 2430A (150MHz) and the 2440 (300MHz) should stomp the Rigol's 100Mhz, but I'm told Rigol's sample rate & memory is the real measure. However being the Tektronix junkie that I am (You'll get my 2467B when you pry it out of cold dead hands) tells me not to dismiss the 2430A/2440 yet... or at least without asking here first!
|
Rigol 1102E vs. 2430A vs. 2440?
The first step in getting help is to admit you have a problem. I have multiple scope disorder. There I said it. As I write this, there are at least ten Tek scopes around me fighting for space. (I don't count the Tenma 72-330, which has been banished to the garage, but I digress) Receiving the TekScopes daily digest probably just compounds the problem ;-) but I am having fun.
About 3 or 4 years ago I renewed my interest in electronics, which grew into the desire to add "a scope" to the bench. After some success in (no small thanks to the experts here) repairing a 2236 and a 2445, my analog 'collection' grew to include another 2445, a 2467b, and 3 2465s(2 of these still need repair). I tell myself, "you know, U800s are rare so it's not a bad thing I have so many scopes around." :-) My actual plan is to end keep the 2467B, and add 2445B & 2465B. So for analog I have a plan.
The digital side, is still pretty new to me. My first digital scope was a 2430A, which only required a good cleaning and re-seating of some connections to bring it back to life. I really liked the ability to save a waveform with the 2430A. However the Rigol's ability to save directly to USB vs. the challenge of GPIB for the 2430A won out, and a Rigol 1102E was added to the collection. I also have a 2440 that still needs some tweaking (intensity control circuit).
Comparing digital scope specs isn't clear cut for me (yet), so my question is (and I hope it's not a silly one) do I gain anything by keeping the 2430A or the 2440 along with the Rigol?
On the surface the 2430A (150MHz) and the 2440 (300MHz) should stomp the Rigol's 100Mhz, but I'm told Rigol's sample rate & memory is the real measure. However being the Tektronix junkie that I am (You'll get my 2467B when you pry it out of cold dead hands) tells me not to dismiss the 2430A/2440 yet... or at least without asking here first!
Thanks for your advice.
|
Thanks Keith:
My next door neighbor has the scope at his business. He hasn't used it for years and it is gathering dust there. I'll have to help haul it out of there as it's too heavy for him to lift (so he claims). He doesn't think it is fully functional, but I thought it would make a nice restoration project as the Tek 500 series are usually easier to work on than the newer 400 series (I'm still trying to figure out how to get inside my 454 to fix the ch #1 vertical cal control which is jammed with the v/d knob.)
I hope the CRT on the 549 is still OK since they seem to be hard to find. I'd rather try to restore it then let some audiofool rape it for the tubes.
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On 07/23/2012 09:49 PM, Keith Knox wrote: Hi Ken,
I have eight Tek. 549 Scopes. I had six and just got two more in a lot
of scopes I picked up.
I also have a spare good used CRT + a new in the box CRT.
I just finished up three Tek. 555 scopes and will be calling the 549
scopes to the bench as my next project.
If you get the 549 scope and need some help, I will be glad to help.
You will also find many others on this group that are willing to help.
Keith
--- On *Mon, 7/23/12, ken scharf /<wa2mze@...>/* wrote:
From: ken scharf <wa2mze@...>
Subject: [TekScopes] Tektronix 549
To: TekScopes@...
Date: Monday, July 23, 2012, 8:00 PM
I've been offered a Tek 549 scope that will probably need some repairs.
Anybody know anything about this model.
|
Re: Help! Tek 2235 Signal on Ch1 appears on Ch2
Hi John,
The power supply voltages look good. -8.59, 5.2V, 8.62V, 30.07V, 102V
I checked the voltages around U130 and U180 and they appear to be as stated on the schematic.
I did verify that the phantom Ch1 signal on Ch2 does not invert when I flip the switch.
Unfortunately I don't have easy access to a scope to trace each channel.
- -Mark
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--- In TekScopes@..., "johncharlesgord" <johngord@...> wrote:
Mark,
Yes, I think those are Tek proprietary chips.
Do you have another scope available? If so, you could follow stage by stage to see where in the CH2 chain the CH1 signal first appears.
(I assume you have done the usual check of power supply voltages. A wrong bias voltage might allow some path through the diodes or transistors used for channel switching.)
--John Gord
--- In TekScopes@..., "Mark" <mjurras@> wrote:
Thanks John,
I do have the service manual and I was looking at U130 & U180 pin 14. It switches between +0.70 and -0.65 when switching between CH1 and CH2 alt and chop. The panel switches appear to be noise free and the scope has low hours on it. It used to work correctly for me.
The amplitude of Ch1's signal on Ch2 is about 66% of the original signal.
Also Ch1 affects Ch2 when in X-Y mode which is the mode I need most at the moment.
It appears U130 and U180 are used to mux the 2 channels is this correct? The are marked TEK. Are these proprietary chips?
- -Mark
--- In TekScopes@..., "johncharlesgord" <johngord@> wrote:
Mark,
If you don't already have the service manual, get it from KO4BB.com:
<<>>
I would assume the problem has something to do with channel switching, which is shown on the "Vertical Preamap & Output Ampl" diagram. The "foldouts lg" pdf seems to have the best scan of that circuit.
Try exercising the various vertical mode switches to make sure it isn't just dirty contacts. See if the various combinations of "CH1 BOTH CH2" and "ADD ALT CHOP" make sense in some positons and not in others--that my give you some clues.
--John Gord
--- In TekScopes@..., "Mark" <mjurras@> wrote:
My Tek 2235 has a problem, The input signal on Ch1 also is displayed by Ch2. This occurs whenever CH2 is displayed (Both or CH2 only) . It occurs even if Ch2 input is set to DC, GND or AC. When I adjust the vertical position of CH1, Ch2 moves. If I put the input on Ch2, Ch1 is not affected.
Can anyone help me troubleshoot this?
|
Hi Ken,
?
I have eight Tek. 549 Scopes. I had six and just?got two more in a lot of scopes I picked up.
I also have a spare good used CRT + a new in the box CRT.
?
I just finished up three Tek. 555 scopes and will be calling the 549 scopes to the bench as my next project.
?
If you get the 549 scope and need some help,? I will be glad to help.? You will also find many others on this group that are willing to help.
?
Keith
|
--- On Mon, 7/23/12, ken scharf wrote:
From: ken scharf <wa2mze@...>
Subject: [TekScopes] Tektronix 549
To: TekScopes@...
Date: Monday, July 23, 2012, 8:00 PM
?
I've been offered a Tek 549 scope that will probably need some repairs.
Anybody know anything about this model.
|
Re: Adjusting a 7A13, and a few other questions
Thanks David, I saw the post but missed the logic on which was good and why.
(as well as 2 of the links via "speed reading" I guess...sigh...)....
Anyway, much appreciated..
I do not remember the ESR and ESL on caps being something to concerns
oneself with back when I worked on consumer electronics 20 years ago.
However, I know that my chosen addiction is a big step above the stuff I
worked on in those days. The education I receive via this forum is
invaluable and I appreciate you taking the time. If/ when I am finished with
the mainframes and plug-ins I don't ultimately want to keep....
I want to be able to tell people what I replaced and why, and hopefully give
them something that will last another decade. This education from time to
time will help me in that goal.
Rob
toggle quoted message
Show quoted text
-----Original Message----- From: TekScopes@... [mailto:TekScopes@...] On Behalf Of David Sent: Monday, July 23, 2012 8:17 PM To: TekScopes@... Subject: Re: [TekScopes] Adjusting a 7A13, and a few other questions I mentioned a couple of replacements for this capacitor in my earlier post. The capacitor in this case is used for bulk power supply decoupling so low ESR and ESL is important while leakage is irrelevent. The 109D 10uF 50V has an 8 Ohm ESR and 160 mA ripple current specification at 120Hz. An organic polymer electrolytic of the same value should be a superior replacement if its super low ESR does not cause circuit oscillation which would not be a problem here because of the relatively high wire resistance from the mainframe power supply. A very good low impedance aluminum electrolytic would need to be at least twice as large achieve the same ESR and ripple current rating but would be half the price of the organic polymer electrolytic capacitor. On Mon, 23 Jul 2012 19:50:42 -0500, "Rob" <rgwood@...> wrote: Forgive me jumping into a post but the question is related and I know
Keith so he won't mind.... (much)....
When choosing a sub for the particular cap referenced here...
Is a standard electrolytic (by standard electrolytic I mean a 'good'
105C,
etc.) an ok sub or must one go with a wet tantalum?
Thanks in advance for answering
Rob aka Keith * smile *
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On
Behalf Of Brad Thompson
Sent: Monday, July 23, 2012 7:14 PM
To: TekScopes@...
Subject: Re: [TekScopes] Adjusting a 7A13, and a few other questions
On 7/23/2012 6:05 PM, keithostertag wrote:
I am just now getting around to looking at my 7A13, bought on eBay
awhile back. I've never used a differential amp. Serial # B236701. <snip>
I thought I'd show you a cap I found which has visible leaking (C505):
Hello, Keith and the group--
That's a so-called "wet slug" tantalum capacitor. Its clear outer
jacket is yellowed, likely due to overheating.
IIRC, the electrolyte in the wet-slug capacitors is sulfuric acid, so
when you remove the failed capacitor, I'd recommend swabbing the
corroded area with a weak baking-soda solution to neutralize any
remaining acid, followed by swabbing with distilled water to remove any chemical residues.
Inspect for corrosion damage to all PC-board traces in the area and
repair as necessary.
Wet-slug caps don't like to operate with a reverse bias and are likely
to fail prematurely, so check the circuitry for upstream component
failures that might have back-biased the capacitor.
------------------------------------ Yahoo! Groups Links
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Re: Adjusting a 7A13, and a few other questions
Those wet slug tantalums are considered high reliability, wide temperature range, low noise, low leakage, and low impedance capacitors. They have higher ripple current ratings and lower impedance than aluminum electrolytic capacitors of the same value. In this case their low leakage and low noise are irrelevant. At the time that the 7A13 was produced, good low impedance aluminum electrolytics and organic polymer electrolytics (OSCON style like you see in a lot of current PC motherboards) were not available. Solid tantalums were not much less expensive than wet tantalums and are less reliable and do not perform as well. An organic polymer capacitor should be better in all aspects except for rated lifetime and temperature range. They actually have such a low ESR that they can cause phase margin problems in power supply control loops but that is not a problem in this case. A very good low impedance aluminum electrolytic of 2 to 4 times the value would also work as a good substitute. On Tue, 24 Jul 2012 01:21:09 -0000, "keithostertag" <keitho@...> wrote: So David- what are the practical differences between these types of caps that you mention? Echoing Rob's question here- especially in this case where this C505 is simply a supply cap, does it need whatever is special about a wet electrolytic, or a tantalum? Wouldn't any decent electrolytic do just as well, in this case? I think that's what you are saying, but I am still curioius about the properties of the other types.
Keith
--- In TekScopes@..., David <davidwhess@...> wrote:
I love my 7A13 vertical amplifiers. I have two of the new style that
I rebuilt into working condition and two of the old style that I may
someday combine into one working one.
On Mon, 23 Jul 2012 22:05:25 -0000, "keithostertag"
<keitho@...> wrote:
I am just now getting around to looking at my 7A13, bought on eBay awhile back. I've never used a differential amp. Serial # B236701.
I'm following the basic operating instructions, page 2-4 of the manual. Steps 9 and 10 are:
9) Vary the VOLTS/DIV switch from 10mV to 50mV while observing the CRT trace.
10) Adjust STEP ATTEN BAL so that the trace does not move while varying the VOLTS/DIV switch.
OK. Does this mean _only_ the range between 10mV and 50mV? The reason I ask is that while that range is easy to adjust in that manner, if I vary the VOLTS/DIV switch through it's entire range I get quite a change when going from 10mV to the lower settings of 5,2, and 1 mV. I just want to know if that's normal/expected or not. It means only from 10mV to 50mV. The Step Attenuator Balance actually
applies from 10mV to 500mV but only those lower three range positions
need to be adjusted because they are the most sensitive.
When you switch from 10mV to a lower range, then use the x10 Balance
which is below the Step Attenuator Balance. Internally there is a
separate x10 stage used when you switch from 10mV to 5mV and below or
if pull the center control out between 10mV and 500mV.
Observation: The VAR BAL pot works, but must be turned almost all the way CW in order to adjust the trace for no movement when varying the VARIABLE control. Assuming that nothing is broken here which seems likely, there is an
internal calibration adjustment (page 5-10 of the service manual)
which is used to move the Variable Balance adjustment into the center
of its range. The first 7A13 I rebuilt and recalibrated had the same
problem.
Note there is a whole set of internal balance adjustments (step
attenuator, x10, and var) which are used to bring their respective
front panel adjustments into the middle of their ranges.
Be sure to do all of the above adjustments (and calibrations) AFTER
the 7A13 has warmed up which is at least 15 to 30 minutes. That you
have to make the internal calibration adjustments while the 7A13 is
not enclosed in the oscilloscope means they will drift a little do to
the temperature change when it is inside of the mainframe but it is
not a big problem.
Second question: I feel the Comparison Voltage (Vc) pots R690A (fine) and R690B (coarse) both should be cleaned. Can someone give me a few tips on how to clean these, whether this type pot can/should be dismantled or not? Here's a photo:
The 10 turn pot can certainly be cleaned (I have done it before) but
if the problem is an intermittent wiper, that probably will not fix
it.
For the single turn pot it looks like you could drop contact cleaner
into the rear seam of the control shaft. If that does not work then
the usual procedure is to carefully drill a small hole or two into the
rear enclosure so the contact cleaner can be squirted inside. I have
a little oiler I filled with MG Chemicals Super Contact Cleaner for
just those sort of jobs.
I thought I'd show you a cap I found which has visible leaking (C505):
Would you assume this is due to age, or does it suggest a closer look at that part of the circuit is warranted? That one sure leaked. The board area under the bad capacitor should
be scrubbed clean with a toothbrush and water and maybe a solvent
before putting the replacement in. That is the first wet slug
tantalum I have seen that failed by leaking. I believe they use
sulphuric acid as an electrolyte so I would start with a scrub using
water and baking soda. Luckily that is not a sensitive area of the
board.
C505 is part of the +50V input filter and nothing around it is
critical as far as leakage. You can leave it out until a proper
replacement is found but clean the board even if you do not replace it
soon. It is a 109D series wet slug tantalum which new costs upwards
of $25 now:
A solid tantalum replacement would be about $15.
This one should make a good modern substitute for it and C553 right
next to it:
A good low impedance 100V 47uF aluminum electrolytic with radial leads
would be $1 at most.
Finally, I may want to pay someone to do a full calibration on this for me, if it is not too dear. Recommendations? Feel free to contact me off-list.
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Re: Adjusting a 7A13, and a few other questions
On 7/23/2012 8:50 PM, Rob wrote: Forgive me jumping into a post but the question is related and I know Keith
so he won't mind.... (much)....
When choosing a sub for the particular cap referenced here...
Is a standard electrolytic (by standard electrolytic I mean a 'good' 105C,
etc.) an ok sub or must one go with a wet tantalum?
Thanks in advance for answering
Rob aka Keith * smile *
Hello, Rob/Keith and the group-- If the tantalum capacitor were used in a timing circuit, I'd go with an exact replacement, but since it's used as a power-line filter, a modern electrolytic capacitor should work fine. My choice would be to use a 15 uF +/-20%, 63 volt, 85C, axial-leaded aluminum electrolytic capacitor such as Mouser's p/n 594-2222-021-38159 (Vishay p/n MAL202138159E3). That capacitor is 10 mm long, which might provide a drop-in fit for the 10 uF 50V tantalum cap. The 20 percent tolerance spans a 12 uF to 18 uF range, which takes care of the 10 uF requirement. The 63-volt rating provides a slight voltage margin. While a cap rated for 105C would offer a generous thermal margin, I'd use the 85C part for size and availability reasons. I might even have some 10 uF 50V NOS wet-tantalum caps in my parts inventory... if I can find the right storage carton. 73-- Brad AA1IP
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Re: Adjusting a 7A13, and a few other questions
So David- what are the practical differences between these types of caps that you mention? Echoing Rob's question here- especially in this case where this C505 is simply a supply cap, does it need whatever is special about a wet electrolytic, or a tantalum? Wouldn't any decent electrolytic do just as well, in this case? I think that's what you are saying, but I am still curioius about the properties of the other types.
Keith
toggle quoted message
Show quoted text
--- In TekScopes@..., David <davidwhess@...> wrote:
I love my 7A13 vertical amplifiers. I have two of the new style that
I rebuilt into working condition and two of the old style that I may
someday combine into one working one.
On Mon, 23 Jul 2012 22:05:25 -0000, "keithostertag"
<keitho@...> wrote:
I am just now getting around to looking at my 7A13, bought on eBay awhile back. I've never used a differential amp. Serial # B236701.
I'm following the basic operating instructions, page 2-4 of the manual. Steps 9 and 10 are:
9) Vary the VOLTS/DIV switch from 10mV to 50mV while observing the CRT trace.
10) Adjust STEP ATTEN BAL so that the trace does not move while varying the VOLTS/DIV switch.
OK. Does this mean _only_ the range between 10mV and 50mV? The reason I ask is that while that range is easy to adjust in that manner, if I vary the VOLTS/DIV switch through it's entire range I get quite a change when going from 10mV to the lower settings of 5,2, and 1 mV. I just want to know if that's normal/expected or not. It means only from 10mV to 50mV. The Step Attenuator Balance actually
applies from 10mV to 500mV but only those lower three range positions
need to be adjusted because they are the most sensitive.
When you switch from 10mV to a lower range, then use the x10 Balance
which is below the Step Attenuator Balance. Internally there is a
separate x10 stage used when you switch from 10mV to 5mV and below or
if pull the center control out between 10mV and 500mV.
Observation: The VAR BAL pot works, but must be turned almost all the way CW in order to adjust the trace for no movement when varying the VARIABLE control. Assuming that nothing is broken here which seems likely, there is an
internal calibration adjustment (page 5-10 of the service manual)
which is used to move the Variable Balance adjustment into the center
of its range. The first 7A13 I rebuilt and recalibrated had the same
problem.
Note there is a whole set of internal balance adjustments (step
attenuator, x10, and var) which are used to bring their respective
front panel adjustments into the middle of their ranges.
Be sure to do all of the above adjustments (and calibrations) AFTER
the 7A13 has warmed up which is at least 15 to 30 minutes. That you
have to make the internal calibration adjustments while the 7A13 is
not enclosed in the oscilloscope means they will drift a little do to
the temperature change when it is inside of the mainframe but it is
not a big problem.
Second question: I feel the Comparison Voltage (Vc) pots R690A (fine) and R690B (coarse) both should be cleaned. Can someone give me a few tips on how to clean these, whether this type pot can/should be dismantled or not? Here's a photo:
The 10 turn pot can certainly be cleaned (I have done it before) but
if the problem is an intermittent wiper, that probably will not fix
it.
For the single turn pot it looks like you could drop contact cleaner
into the rear seam of the control shaft. If that does not work then
the usual procedure is to carefully drill a small hole or two into the
rear enclosure so the contact cleaner can be squirted inside. I have
a little oiler I filled with MG Chemicals Super Contact Cleaner for
just those sort of jobs.
I thought I'd show you a cap I found which has visible leaking (C505):
Would you assume this is due to age, or does it suggest a closer look at that part of the circuit is warranted? That one sure leaked. The board area under the bad capacitor should
be scrubbed clean with a toothbrush and water and maybe a solvent
before putting the replacement in. That is the first wet slug
tantalum I have seen that failed by leaking. I believe they use
sulphuric acid as an electrolyte so I would start with a scrub using
water and baking soda. Luckily that is not a sensitive area of the
board.
C505 is part of the +50V input filter and nothing around it is
critical as far as leakage. You can leave it out until a proper
replacement is found but clean the board even if you do not replace it
soon. It is a 109D series wet slug tantalum which new costs upwards
of $25 now:
A solid tantalum replacement would be about $15.
This one should make a good modern substitute for it and C553 right
next to it:
A good low impedance 100V 47uF aluminum electrolytic with radial leads
would be $1 at most.
Finally, I may want to pay someone to do a full calibration on this for me, if it is not too dear. Recommendations? Feel free to contact me off-list.
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Re: Adjusting a 7A13, and a few other questions
I mentioned a couple of replacements for this capacitor in my earlier post. The capacitor in this case is used for bulk power supply decoupling so low ESR and ESL is important while leakage is irrelevent. The 109D 10uF 50V has an 8 Ohm ESR and 160 mA ripple current specification at 120Hz. An organic polymer electrolytic of the same value should be a superior replacement if its super low ESR does not cause circuit oscillation which would not be a problem here because of the relatively high wire resistance from the mainframe power supply. A very good low impedance aluminum electrolytic would need to be at least twice as large achieve the same ESR and ripple current rating but would be half the price of the organic polymer electrolytic capacitor. On Mon, 23 Jul 2012 19:50:42 -0500, "Rob" <rgwood@...> wrote: Forgive me jumping into a post but the question is related and I know Keith
so he won't mind.... (much)....
When choosing a sub for the particular cap referenced here...
Is a standard electrolytic (by standard electrolytic I mean a 'good' 105C,
etc.) an ok sub or must one go with a wet tantalum?
Thanks in advance for answering
Rob aka Keith * smile *
-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of Brad Thompson
Sent: Monday, July 23, 2012 7:14 PM
To: TekScopes@...
Subject: Re: [TekScopes] Adjusting a 7A13, and a few other questions
On 7/23/2012 6:05 PM, keithostertag wrote:
I am just now getting around to looking at my 7A13, bought on eBay
awhile back. I've never used a differential amp. Serial # B236701. <snip>
I thought I'd show you a cap I found which has visible leaking (C505):
Hello, Keith and the group--
That's a so-called "wet slug" tantalum capacitor. Its clear outer jacket is
yellowed, likely due to overheating.
IIRC, the electrolyte in the wet-slug capacitors is sulfuric acid, so when
you remove the failed capacitor, I'd recommend swabbing the corroded area
with a weak baking-soda solution to neutralize any remaining acid, followed
by swabbing with distilled water to remove any chemical residues.
Inspect for corrosion damage to all PC-board traces in the area and repair
as necessary.
Wet-slug caps don't like to operate with a reverse bias and are likely to
fail prematurely, so check the circuitry for upstream component failures
that might have back-biased the capacitor.
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Re: Adjusting a 7A13, and a few other questions
Thanks Steve. I haven't yet seen the sections that address adjusting the lower ranges. Just glancing through the manual I feel I am not compentent or equipped to do much on this 7A13. If I'm not confident about cleaning the pots it may be best to just buy them (about $20 for both at QService). And thanks for informing me not to move any of the input components.
Keith
toggle quoted message
Show quoted text
--- In TekScopes@..., "Steve" <ditter2@...> wrote:
--- In TekScopes@..., "keithostertag" <keitho@> wrote:
I am just now getting around to looking at my 7A13, bought on eBay awhile back. I've never used a differential amp. Serial # B236701.
I'm following the basic operating instructions, page 2-4 of the manual. Steps 9 and 10 are:
9) Vary the VOLTS/DIV switch from 10mV to 50mV while observing the CRT trace.
10) Adjust STEP ATTEN BAL so that the trace does not move while varying the VOLTS/DIV switch.
OK. Does this mean _only_ the range between 10mV and 50mV?
Yes, There are other Balance tweeks for other ranges.
The reason I ask is that while that range is easy to adjust in that manner, if I vary the VOLTS/DIV switch through it's entire range I get quite a change when going from 10mV to the lower settings of 5,2, and 1 mV. I just want to know if that's normal/expected or not.
Observation: The VAR BAL pot works, but must be turned almost all the way CW in order to adjust the trace for no movement when varying the VARIABLE control.
Second question: I feel the Comparison Voltage (Vc) pots R690A (fine) and R690B (coarse) both should be cleaned. Can someone give me a few tips on how to clean these, whether this type pot can/should be dismantled or not? Here's a photo:
Don't dismantal the pots. Check the archive for substitutes for "no-noise" - the cleaner Tek used to use which is no longer made.
I thought I'd show you a cap I found which has visible leaking (C505):
Would you assume this is due to age, or does it suggest a closer look at that part of the circuit is warranted?
Finally, I may want to pay someone to do a full calibration on this for me, if it is not too dear. Recommendations? Feel free to contact me off-list.
Will probably cost a small fortune. I have not cal'ed a 7A13 in years, but I remember the procedure is nearly 100 steps and takes several hours for an experienced technician who is not familiar with this specific instrument.
If you dissassembled anything or physically moved any of the input components or wiring, the CMRR balance tweaks will need to be readjusted to maintain the great CMRR performance this amp is capable of. If no component replacements or dissassembly was done, these tweaks tend to be very stable and not require any adjustment during a routine calibration.
- Steve
Thanks,
Keith Ostertag
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David Hess