Re: Adjusting a 7A13, and a few other questions
On Mon, 23 Jul 2012 20:13:41 -0400, Brad Thompson <brad.thompson@...> wrote: 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.
73--
Brad AA1IP It is part of the +50V input filter. The identical type and value C553 next to it is part of the -50V input filter. In both cases, a 50 volt capacitor was used at 50 volts but maybe wet slug tantalums do not need to be voltage derated? I have seen Tektronix not derate capacitor voltage before.
|
--- In TekScopes@..., ken scharf <wa2mze@...> wrote:
I've been offered a Tek 549 scope that will probably need some repairs.
Anybody know anything about this model.
There is a very recent thread comparing the 549 to the 564. Go back just a few days to read it. The 549 introduced a new type of phosphor for bistable storage that has much faster writing rate than the 564. But it is extremely easy to burn. Tek warned about this in both the manual and even the catalog page. You might want to fire it up and check for a permement image in the storage target. Not many 549's were made so CRT replacements are hard to find. Tek made a bazillion 564's, and even NOS CRTs in the orginal Tek boxes are still around. You might think the 549 has the same vertical amp as the 545B, however it is different. I am not sure, but I suspect this is because the 549 does not use PDA as the storage target circuits need to run near ground potential. This probably puts the CM voltage of the deflection plates at a quite different voltage than the 545B. (I never examined the circuit, so this is speculation on my part) - Steve
|
Re: Adjusting a 7A13, and a few other questions
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.
|
Re: Adjusting a 7A13, and a few other questions
--- 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
|
Re: Tek IC clone possible?
--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
Not necessarily.?? If you have a block diagram and a description of the device that you want to replace, post it.?? From there, maybe we as a group could suggest what might be able to be used to substitute the internals.
In addition to its logic diagram, you need to look at the circuit it is used in. Because Tek made ICs are single sourced, Tek engineers could rely on parameters that are typically "unspecified" as being consistent. Things such as input and input impedances, pull up resistor values, Ft and Miller C of the transistors, etc. would not change as they may with different vendors' version of the same logic circuit. Hence, in Tek designs you often see analog signals driving inputs or outputs driving analog signals which depend on these characteristics. Simple substitution of a functionally logic equivalent circuit may not operate if these conditions are not duplicated, possibly using external components. The first Tek made ICs of the part numbers you mentioned were fabricated on a 200 ohm/square process ¨C Tek's first. This is not a very fast process (transistor Ft and switching speed) by today's standards. In some cases you may need to slow down output transitions to substitute FPGA equivalents. - Steve
|
Re: 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. 73-- Brad AA1IP
|
I've been offered a Tek 549 scope that will probably need some repairs. Anybody know anything about this model.
|
Adjusting a 7A13, and a few other questions
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.
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:
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.
Thanks,
Keith Ostertag
|
Re: Tek IC clone possible?
Not necessarily.? If you have a block diagram and a description of the device that you want to replace, post it.? From there, maybe we as a group could suggest what might be able to be used to substitute the internals.
|
--- On Mon, 7/23/12, Fabio Eboli wrote:
From: Fabio Eboli
Subject: Re: [TekScopes] Tek IC clone possible?
To: TekScopes@...
Date: Monday, July 23, 2012, 3:43 PM
?
I think there is a problem with my english :)
I was thinking to a small pcb with some smd components
to fit in place of the original ic.
The uC I usually use for simple tasks are AVR micros.
but I think I didnt understand your question.
Fabio.
KeepIt SimpleStupid <keepitsimplestupid@...> ha scritto:
> Why not post what you have?? Is this stuff FPGA material?? It might
> take a major investment in tools.
>
> --- On Mon, 7/23/12, Fabio Eboli <fabioeb@...> wrote:
>
> From: Fabio Eboli <fabioeb@...>
> Subject: [TekScopes] Tek IC clone possible?
> To: TekScopes@...
> Date: Monday, July 23, 2012, 2:10 PM
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> ?
>
>
>
>
>
>
>
>
>
>
>
>
>
> How hard would be to build clones of logic ic's
>
> for the 7000 scopes?
>
>
>
> I'm thinking to "simple" things like 155-0013
>
> that should be pure logic, or 155-0049 that is
>
> the logic driving the timebases.
>
>
>
> I have at least one 155-0013 broken in the
>
> 7704A and probably one 155-0049 non working
>
> in the 7B92A. The first is documented, at
>
> least for logic functions, in the 7704A service
>
> manual. The second should be documented
>
> in the "Tek-made ic reference" that is
>
> available somewhere from a manual seller.
>
>
>
>
>
>
>
> ----------------------------------------------------------
>
> This message was sent using IMP, the Internet Messaging Program.
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
----------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.
|
Re: Stereo Power Amp Problem
Please take a look at the power supply
fuses and be certain you have good fuses in all four positions. ?It is
abnormal for this amp to have excessive offset on the outputs.? You state
that you have .1V offset on one channel, that¡¯s wrong and is where you
should begin to look.? Can you put a low level 300mV sine wave on the amp¡¯s
inputs and look at the waveform on the output?? If you don¡¯t have a
complete sine wave then tell us what you see and we can advise further.? Many
of the transistors in this amp are out of print so start with the fuses and
proceed with caution.
?
Kevin
?
Kevin Collier
Director of Engineering
Warner Bros Studio
Facilities, PPS
voice-818-954-1725
mobile-818-652-6030
?
?
?
|
Re: 7633 readout problem, advice or hints needed
Do the adjustment procedure for R2214 which can drift over time. The procedure is at the end of page 5-53 in the service manual and can be done independently of other adjustments except maybe the power supply voltages. It basically comes down to installing a bunch of plug-ins and adjusting R2214 to the middle of the range where the character scan is correct. R2183 is adjusted in the same way. If either cannot be adjusted such that all characters are correct, then something else is wrong. My 7854 has a similar problem with my 7T11A where it converts ps to ns sometimes but I have not bothered to adjust it. On Mon, 23 Jul 2012 20:32:39 -0000, "max_3289" <max_3289@...> wrote: @David:
if i use a 7A26 only CH1 is affected (upper middle text), CH2 is ok (lower middle text). I prefer using the 7A22 to get the "?" too.
R2214: i did not *adjust* the pots, i did simply remember the position (easy, because of characters on the device), deudjusted for some degrees and then back to the original positions, just to move the contacts a bit.
7493: levels seem to be ok, if you send me your mailadress, i can send back some screenshots of a DSO (obviously its not possible to attach files here?)
@Albert:
not yet, i was not able to play around to get the right connections in a useful way (next step...)
Max
--- In TekScopes@..., David <davidwhess@...> wrote:
Do both channels of the right vertical bay still show up? If one is
missing, then that may be easier to track down and repairing it could
fix the bad character problem.
This pattern of errors indicates a problem between U2159, the right
vertical bay, and U2244 causing a decoding off-by-one error. Leakage
is a definite possibility.
How did you adjust R2214? The same off-by-one error is showing up in
at least two different time slots and two different columns so I would
suspect something in common between them which leaves R2214, the time
slot connections to the center bay, and pin 8 of U2250.
Check the levels of the outputs (pin 8, 9, and 11) from U2250 with an
oscilloscope. Pin 8 only goes high when the right vertical bay is
selected. Maybe it is not switching as completely as it should. If
you have a 7493 available, you could just swap it. A 74LS93 or
74HCT93 should work as a substitute.
|
Re: 7633 readout problem, advice or hints needed
@David:
if i use a 7A26 only CH1 is affected (upper middle text), CH2 is ok (lower middle text). I prefer using the 7A22 to get the "?" too.
R2214: i did not *adjust* the pots, i did simply remember the position (easy, because of characters on the device), deudjusted for some degrees and then back to the original positions, just to move the contacts a bit.
7493: levels seem to be ok, if you send me your mailadress, i can send back some screenshots of a DSO (obviously its not possible to attach files here?)
@Albert:
not yet, i was not able to play around to get the right connections in a useful way (next step...)
Max
toggle quoted message
Show quoted text
--- In TekScopes@..., David <davidwhess@...> wrote:
Do both channels of the right vertical bay still show up? If one is
missing, then that may be easier to track down and repairing it could
fix the bad character problem.
This pattern of errors indicates a problem between U2159, the right
vertical bay, and U2244 causing a decoding off-by-one error. Leakage
is a definite possibility.
How did you adjust R2214? The same off-by-one error is showing up in
at least two different time slots and two different columns so I would
suspect something in common between them which leaves R2214, the time
slot connections to the center bay, and pin 8 of U2250.
Check the levels of the outputs (pin 8, 9, and 11) from U2250 with an
oscilloscope. Pin 8 only goes high when the right vertical bay is
selected. Maybe it is not switching as completely as it should. If
you have a 7493 available, you could just swap it. A 74LS93 or
74HCT93 should work as a substitute.
On Mon, 23 Jul 2012 12:02:37 -0000, "max_3289" <max_3289@...>
wrote:
Now i have reseated *all* reseatable parts except cables on the board (ICs, Qs) and deadjusted/readjusted all pots. Also i interchanged U2180 and U2190.
All contacs were clean.
I installed a 7A22 (to get the "?" character also) and switched from position to position and got out the following:
0S (instead of 10V)
4S (5V)
1S (2V)
0S (1V)
50S (500mV)
10S (200mV)
00S then 00V then 100V then 100mV (100mV)
-> the "m" appeared slowly in weird way, here i started
from beginning:
1V (10V)
5V (5V)
2V (2V)
1V (1V)
500V (500mV) "m" not here again
200V (200mV)
100V (100mV)
-> gave a 3rd beginning:
10V
5V
...
everything OK down to 10?V, "m" correct now for some hours.
Lets see later with another cold start.
Max.
--- In TekScopes@..., "max_3289" <max_3289@> wrote:
Hi Albert, Jerry,
first i measured:
TP2119: -14,94 VDC with 3,75 mVAC ripple
TP2117: 4,96 VDC with 3,45 mVAC ripple
TP2115: 15,05 VDC with 9,94 mVAC ripple
-> ok.
Then, while thinking how to change the connections for Albert's proposal, i saw the characters all correct!
Obviously i touched some device while measuring the voltages (i have moved out and in all socketed transistors long before without success...), perhaps a bad soldering somewhere.
I will observe for the next days, by good luck the problem was gone and no IC affected.
Thanks a lot for your help!
Max.
BTW: is it possible to get email notification only for dedicated topics?
--- In TekScopes@..., jerry massengale <j_massengale@> wrote:
Hi,
There are 3 power test points on the readout board. Make sure all 3 voltages(+/-15V and 5V) are ripple free and stable with the 5V reading 4.8V or better.
Jerry Massengale
-----Original Message-----
From: max_3289 <max_3289@>
To: TekScopes <TekScopes@...>
Sent: Sun, Jul 22, 2012 12:14 pm
Subject: [TekScopes] Re: 7633 readout problem, advice or hints needed
Hello Albert,
backplaneboard A1, J2 contact A37 to readout board A13, U2190 pin 12 -> low ohmic
backplaneboard A1, J2 contact A38 to readout board A13, U2190 pin 11 -> low ohmic
backplaneboard A1, J2 contact B37 to readout board A13, U2180 pin 12 -> low ohmic
backplaneboard A1, J2 contact B38 to readout board A13, U2180 pin 11 -> low ohmic
aAll the diodes on the protection circuit board of A13 are ok.
Connections are ok, connectors are all clean.
After cooling down over night, there were seen some false characters again for some time.
Any ideas?
Max.
--- In TekScopes@..., "Albert" <aodiversen@> wrote:
Hi Max,
This makes contacts A37 and B37 suspect.
Remember this is a first check. The fault could also be in a readout board channel selector IC for instance.
Albert
Hi Albert,
thanks a lot; just tested the following:
I have 2 pcs. 7A22, 1 pc 7A26, 1 pc 7B53A, 1 pc 7B85.
The 7A26 in left position shows both channels ok, in centre position only CH2 (bottom middle position readout) ok, CH1 (top middle position) no readout.
7B53A and 7B85 timebases show no difference (except additional 7B85 readout bottom right).
It seems to be the problem of the backplane-connector you mentioned.
Did you read the other readout problems thread, concerning a R7903? not yet; have not known it yet.
Max.
|
Re: 7633 readout problem, advice or hints needed
Hi James,
Shouldn't your question be posted in a new topic? I don't see the relation with the 7633 problem.
Albert
toggle quoted message
Show quoted text
--- In TekScopes@..., "James Peebles" <jpeebles@...> wrote:
Does anybody know if a factory bank2 diagnostic failure on a TDS7104 is real or what?
|
Re: Tek IC clone possible?
I think there is a problem with my english :) I was thinking to a small pcb with some smd components to fit in place of the original ic. The uC I usually use for simple tasks are AVR micros. but I think I didnt understand your question. Fabio. KeepIt SimpleStupid <keepitsimplestupid@...> ha scritto: Why not post what you have?? Is this stuff FPGA material?? It might take a major investment in tools.
--- On Mon, 7/23/12, Fabio Eboli <fabioeb@...> wrote:
From: Fabio Eboli <fabioeb@...>
Subject: [TekScopes] Tek IC clone possible?
To: TekScopes@...
Date: Monday, July 23, 2012, 2:10 PM
?
<snip>
How hard would be to build clones of logic ic's
for the 7000 scopes?
I'm thinking to "simple" things like 155-0013
that should be pure logic, or 155-0049 that is
the logic driving the timebases.
I have at least one 155-0013 broken in the
7704A and probably one 155-0049 non working
in the 7B92A. The first is documented, at
least for logic functions, in the 7704A service
manual. The second should be documented
in the "Tek-made ic reference" that is
available somewhere from a manual seller.
<snip>
----------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.
---------------------------------------------------------------- This message was sent using IMP, the Internet Messaging Program.
|
Re: Stereo Power Amp Problem
A few thoughts I have about this problem:
What is the volume setting? Does the noise level change when changing the volume level?
Is this in a noisy RF environment? Do you have flourescent lights on? I do telecom work, and whenever I run across a noisy phone with the speakerphone buzzing, I pull out my fox/hound tracer. With a quick push of a button, it will amplify any noisy environment in the AF range for the most part. I will wave it around the room monitoring the audio. Whenever I shutoff the flourescent lights, it will immediately quiet down. Another great way to use it is to trace the problem in the amplifier. It will especially work well because it is AF, not RF. It is very sensitive too. I have placed a signal from the fox signal generator on the pre-in and quickly traced it thru the amplifier by just placing the tip near the amplifying components. Just don't place it on the 120VAC directly, I did that once and blew the first stage amplifier out. Just a small transistor and easy fix. I also use it to see if slow processors are active. They make noise too. It is a very smart investment for troubleshooting. You can use it to judge the amp stages by the loudness in/out.
I would sub out the power supply too if it can be easily done. But I would say most of all ensure of the bonding. ground out the chassis to earth ground.
I have been reading up on my NEETS Basic Electronics manual and have just been studying power supplies. A very good book and free too. It might be dated, but so is your P/S. Just google NEETS, and you can get the whole series too.
Place the amplifier in a completely different environment. Did you plug in some other device in the house that might be backfeeding on the wall outlet?
And of course, isolate everything.
In telecom, whenever I troubleshoot a noisy circuit/wire pair, It is almost always an unbalanced cable pair. Whether it is due to a wet cable changing the capacitance between conductors, or one of the conductors has a lower resistance to ground thru a damaged cable. Make sure your pre-amp signal cable is OK. Also ground/jumper out the preamp in center pin of the RCA connector. If it is picking up noisy environment, this should quiet it down.
Make sure you did not break the solder trace at the preamp in RCA. If it did on the ground, it will howl!
Just looking at the specs of this amp, it is very impressive!
Nick
toggle quoted message
Show quoted text
--- In TekScopes@..., "Bill McDonald" <BillM256@...> wrote:
A correction and an addition in red below.
From: Bill McDonald [mailto:BillM256@...]
Sent: Monday, July 23, 2012 11:23 AM
To: 'TekScopes@...'
Subject: Re: Stereo Power Amp Problem
All,
Many thanks for your thoughtful comments, suggestions and questions; my
responses, in no particular order, are below.
1. Hum is equally on both channels.
2. Happens when amp is connected to preamp or disconnected.
3. Happens when light dimmer is off or amp is on a different circuit.
4. Happened while installing new speakers. During the process there
was an accidental power up that may have popped something. Installation also
involved moving line-level inputs around, and this is where we may have
caused the problem. I'm increasingly thinking input stages of each channel.
5. Speaker output: Approximate sine wave at 120 60 Hz, ~.1 v p-p. Very
short duration spikes at each zero crossover, several volts in amplitude.
One channel has a dc offset of ~ -.1 volt, the other channel is ~ -.05
volts.
6. Grounds confirmed between power supply, amp boards and speaker
outs.
7. I see no physical indication of component problems on the amp
boards or relay board.
8. After reading your comments, and doing further research of my own,
I start to suspect the saw-tooth riding the +/- 90 rails may be normal and
the failure is downstream, maybe decoupling in the amp. However, this would
require a common failure on both channels. But considering #4, that's a
possibility.
9. I've attached the manual, with schematic on pg. 16. If attachment
doesn't work, the manual is available at
.
10. Also worth mentioning, the amp has run perfectly for more than a
decade, so it's not a defect in design or a new part, e.g., Brad's point
about rectifier reverse recovery time.
Thanks again!
Bill
|
Re: 7633 readout problem, advice or hints needed
Does anybody know if a factory bank2 diagnostic failure on a TDS7104 is real or what?
toggle quoted message
Show quoted text
-----Original Message----- From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Albert Sent: Monday, July 23, 2012 12:00 PM To: TekScopes@... Subject: [TekScopes] Re: 7633 readout problem, advice or hints needed Hi Max, IDENTIFY is coded at the highest column current (>= 1mA), so maybe less influenced by leakage. You interchanged U2180 and U2190, but the faults below still show a one-character *column* shift. So the fault is not in these ICs. What about my previous suggestion to temporarily use plug P66 (main interface board) for J2 readout? It would narrow your fault search. Albert "IDENTIFY" is always correct now, even when "m" is missing or "0S" displayed.
character generator?
Max. Now i have reseated *all* reseatable parts except cables on the board (ICs, Qs) and deadjusted/readjusted all pots. Also i interchanged U2180 and U2190.
All contacs were clean.
I installed a 7A22 (to get the "?" character also) and switched from position to position and got out the following:
0S (instead of 10V)
4S (5V)
1S (2V)
0S (1V)
50S (500mV)
10S (200mV)
00S then 00V then 100V then 100mV (100mV)
-> the "m" appeared slowly in weird way, here i started
from beginning:
|
Re: 7633 readout problem, advice or hints needed
Hi Max,
IDENTIFY is coded at the highest column current (>= 1mA), so maybe less influenced by leakage.
You interchanged U2180 and U2190, but the faults below still show a one-character *column* shift. So the fault is not in these ICs.
What about my previous suggestion to temporarily use plug P66 (main interface board) for J2 readout? It would narrow your fault search.
Albert
toggle quoted message
Show quoted text
"IDENTIFY" is always correct now, even when "m" is missing or "0S" displayed.
character generator?
Max. Now i have reseated *all* reseatable parts except cables on the board (ICs, Qs) and deadjusted/readjusted all pots. Also i interchanged U2180 and U2190.
All contacs were clean.
I installed a 7A22 (to get the "?" character also) and switched from position to position and got out the following:
0S (instead of 10V)
4S (5V)
1S (2V)
0S (1V)
50S (500mV)
10S (200mV)
00S then 00V then 100V then 100mV (100mV)
-> the "m" appeared slowly in weird way, here i started
from beginning:
|
Re: Stereo Power Amp Problem
I also suspect a grounding issue.
Such questions will gladly be answered at Electronics_101 group, where they are on topic.
ST
toggle quoted message
Show quoted text
On Mon, Jul 23, 2012 at 8:44 PM, Denis <xyzzx_adv@...> wrote:
Bill,
? The information you provided about the amlifier power rails sounds normal.? I don't think you have an amplifier problem but a grounding issue.?
|
Re: Stereo Power Amp Problem
Bill,
? The information you provided about the amlifier power rails sounds normal.? I don't think you have an amplifier problem but a grounding issue.? Your comment about the chassis being 'touch sensitive' when the scope was connected to the amplifier circuit ground and the sensitivity to power plug polarity sounds like a floating circuit issue with the amplifier and scope chassis/cases acting as an antenna.?
?
? As I read the schematic the amplifier circuit is floating (not connected to an external ground).? Power line noises (SCR light dimmers produce high frequency transients locked into the AC line frequency.? High frequencies couple through stray capacitances of the circuit wiring and?power transformer in the?floating amplifier circuit.? Normally, the amplifier chassis is grounded to power?green wire ground (the 'U' shaped power plug pin)?but with unbalanced audio lines you don't want multiple ground connections (typically the lowest signal level point is grounded and everything else uses that ground)? so the preamp is normally the ground connection.
?
? You state that this has worked for a long time, SO something has changed, please provide more details:
?
* What have you done since the time it worked properly (you mention new speakers)
?
* Confirm that the scope is floating (the power plug ground pin is not connected to power green wire ground terminal)
?
* Is the chassis 'touch sensitive' without the scope connected
?
* As another commenter questioned, what happens if you short each input connector to ground (solder a jumper wire between the RCA plug tip and shell)
?
* Describe more details of the inadvertant power up of the amp, how long was it on, what was connected, any shorted leads
?
* Where does your audio system connect to ground (power line green wire ground)
?
? You mention moving the line input leads as well as trying new speakers which does raise input connection issues, some things to try:
* Create a shorting input plugs (RCA plug with jumpered input) for each input?- what is the hum output level
?
*?Measure the resistance from the RCA jack shell and the chassis while trying to move the jack?- should be zero ohms
?
* Install a temporary grounding path from chassis to?power 'U' pin with a 1000 ohm resistor (measure the AC voltage across the resistor).? Ideally there should not be any voltage present.? Report the voltage, hum output level, and if the shorting input plugs have any effect (also the effect of having the scope ground clip connected the amplifier)
?
Denis
?
From: billm256 To: TekScopes@... Sent: Sunday, July 22, 2012 4:55 PM Subject: [TekScopes] Stereo Power Amp Problem
?
Hello,
?
Please pardon me for submitting this non-Tektronix question. However, I know there's a lot of general electronic expertise in this group, and I really need help (and I am using my Tek 2245 to troubleshoot this problem).
?
Issue is with my Hafler DH-500 stereo power amp, which has picked up a raspy hum on the output. I'm hoping someone with more power supply experience than I have will be able to help.
?
General description of the power supply circuit.
1.?????? Classic linear power supply.
2.?????? Step-up xformer.
3.?????? Bridge rectifier.
4.?????? On the output of the bridge, 2, 20,000 mfd, 100 V electrolytic filter caps between ground and each?of the +/- 90 VDC rails.
5.?????? There's a .01 mfd, 1000 V disc cap across the xformer connections to the bridge. I mention this because it's one of very few things the rails share.
?
General symptoms:
1.?????? There is a ~.35 V peak to peak, 120 Hz distorted saw-tooth riding the plus and minus 90 VDC rails. That its frequency is 120 Hz suggests to me it originates after rectification, i.e., the rectifier's output is 120 Hz.
2.?????? The saw-tooth is essentially identical on each rail except they are 180 degrees out of phase.
3.?????? The saw-tooth varies ~.05 V in amplitude yet triggers the scope reliably and presents the same sweep repeatedly. I.e., displaying 6 cycles of saw-tooth on the scope, amplitude variations repeat on the same cycles sweep after sweep. (Seems very strange to me.)
4.?????? Amplitude of the output hum does not vary with the input signal.
5.?????? Amplifier continues to function other than the hum.
?
A couple of symptoms lead me to suspect the power supply (as opposed to a downstream component putting noise on a rail).
1.?????? Reversing the power plug changes the amplitude of the output?hum.
2.?????? Changing the setting of an SCR light dimmer on the same house circuit changes harmonics on the hum. I.e., AC distortion from the dimmer is getting thru the power supply.
3.?????? Both rails equally affected.
?
One final observation that seems odd to me: When I hooked ground on the scope to circuit ground on the amp, the chassis of the amp became very sensitive to being touched by me in that touching it produced significant 60 Hz sine wave hum on the output. The scope is isolated from ground at the outlet as is the amplifier.
?
In conclusion, I expected to find one of the electrolytics deteriorated, and to see AC riding one rail only; that it's showing up equally on both rails is mystifying to me. That it's a saw-tooth vs. a sine wave is also indicative of something. That it's 180 degrees out of phase and 120 Hz seem to pin its origin to the rectification process. Again, the small disc cap across the bridge's input may be of interest, but its function is not apparent to me. (I'm putting a replacement cap on order and should have it installed before end of the week.)
?
Thank you for wading thru this lengthy description. Any help will be greatly appreciated!
?
Best regards,
?
Bill McDonald
?
|
Re: Tek IC clone possible?
Since Tektronix used a bipolar IC process, a lot of the logic inputs and outputs are currents instead of voltages which would require external level shifting and interfacing. The power supply and compliance voltages are higher than any programmable logic will support and the logic is largely asynchronous. There are some specialized analog programmable chips that might work but they are rather esoteric. Once you add needed external interface circuits, any space saved would be gone. I think it would be better to design a surface mount hybrid as has been done with some HP customs ICs from that era but even there, space constraints would be a problem. I do not see the economics making sense with so many busted or worn out instruments still available for parts. On Mon, 23 Jul 2012 11:21:44 -0700 (PDT), KeepIt SimpleStupid <keepitsimplestupid@...> wrote: Why not post what you have?? Is this stuff FPGA material?? It might take a major investment in tools.
--- On Mon, 7/23/12, Fabio Eboli <fabioeb@...> wrote:
From: Fabio Eboli <fabioeb@...>
Subject: [TekScopes] Tek IC clone possible?
To: TekScopes@...
Date: Monday, July 23, 2012, 2:10 PM
How hard would be to build clones of logic ic's
for the 7000 scopes?
I'm thinking to "simple" things like 155-0013
that should be pure logic, or 155-0049 that is
the logic driving the timebases.
I have at least one 155-0013 broken in the
7704A and probably one 155-0049 non working
in the 7B92A. The first is documented, at
least for logic functions, in the 7704A service
manual. The second should be documented
in the "Tek-made ic reference" that is
available somewhere from a manual seller.
|
David Hess
