Re: Tek R7704 with curve tracer
Yeah. If I didn't already have a 7704A and all the same plug-ins (well, that FET adapter looks custom) I would jump on it. Steve H. On Sun, Nov 28, 2021 at 8:25 AM Michael W. Lynch via groups.io <mlynch003= [email protected]> wrote: WOW! What a package! I would love to have it. But alas, that is not
going to happen! Someone is gonna get a deal.
--
Michael Lynch
Dardanelle, AR
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Hi Mark,
If my calculations are correct, R594 is dissipating about 186mW. The specified 1/4W does seem to be closer to the edge than necessary but a 1/2W at the time may have been larger than there was room for. I plan to go with a 1/2W for it (unless I use the 6.8k I have on hand which, if I recall, may be 1W but still small enough to fit.
Given that R594 did what it did and that I see some minor discoloration of some of the larger resistors, I do need to check those as well. I think that resistor was probably a carbon film as I don't think a metal film would've done that(?).
Thanks,
Barry - N4BUQ
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----- Original Message ----- From: "Mark Vincent" <orangeglowaudio@...>
To: "tekscopes" <[email protected]>
Sent: Sunday, November 28, 2021 9:41:21 AM
Subject: Re: [TekScopes] 7B50 - No Trace Barry,
R594 being high is because it was too small in wattage. It should be 1W,
.
R408, R534, R582, R598 and R599 should also be 1W. The 16,500 ohm resistor can
be made with two 33,000 ohm 1/2W resistors in parallel. The 8250 ohm resistor
can be a CCF60 type. R416 is 1/8W. A 1/2W would be better because it is
dissipating almost as much as the resistors is rated in wattage. I always see
if resistors are too small in wattage. Having seen many underrated resistors be
bad is why I always look and replace those that are too small. On the 50V
supplies, it is common to see resistors too small in wattage causing the board
to darken and causing the resistor to be bad/open. There are ones on the 15V
supplies that also should be raised in wattage, e.g. about 15V across around
1000 ohms. These types should be 1W. I have found that the increase in wattage
makes the item better in operation because the resistance drift is lower
causing lower noise, drift, etc..
Mark
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Re: 492 spectrum analyzer sweep linearity
I don't think you can say that the fault is present at TP4061. -5V as measured here should correspond to the left graticule.
So far it seems that it's OK up to the output of U2060 (when I measure the voltage at pin 6 with the trace at the right graticule it is close to the inverse of the voltage at the left most graticule).
This is the older version of the board which have a couple of matched pair transistors.
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Re: 492 spectrum analyzer sweep linearity
Interleaved: On 11/28/2021 5:59 AM, Jeff Dutky wrote: Harvey,
Okay, I see the (I think) differential amplifier that drives the horizontal deflection plates (starting with Q4025/Q4038 and ending with Q1024/Q1031/Q1043/Q1049), but it sounds like the fault is already present at TP1061, which is the output of the op amp U6061, and is buffered from the horizontal deflection amplifier by two other op amps (U7073 and U2060), so I doubt that a fault in the deflection amplifier board could be causing the low voltage at TP1061 that Michael is seeing. Using another scope, if you grab the sweep at the U6061 output, run it to the horizontal input, and put timing pulses on the vertical inputs, that ought to tell you about the linearity.
Maybe R7051 and R7081, if they were both below spec, could be dragging down the output of U6061, but U7073 would have to be in really bad shape to be letting anything further down the line have any effect at TP1061.
The voltages at U6061 are OK?? supply wise? If U6061 is easy to replace, I'd try that and take a look at the parts around it, if it's producing the sweep.? It's possible that bad supply voltages could cause amp nonlinearity in the circuit.
Still, I'm fully prepared to be wrong. I'm out of my depth, and I'm not entirely sure I understand what Michael has measured in the circuit.
Also, I don't see much trimming in the deflection amplifiers. There is a gain trim pot across U2060 (R2069), and two trim caps on the inputs to the differential amps driving the deflection plates (C4057 and C5021), but I don't really understand what the trim caps are doing.
Likely peaking the frequency response of the amplifier.? Checking the linearity per stage could be useful, or maybe just seeing what the manual says about the appropriate adjustments. Harvey -- Jeff Dutky
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Still looking for a *PAPER* manual for a Tektronix 2225 Oscilloscope in UK or EU
Please excuse the X-Posting.
I have various PDF manuals but I find it much easier working from a paper manual and I have a couple of these units to repair.
Must be in the UK or EU (US shipping is a killer).
Regards,
Dave
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Re: Tek R7704 with curve tracer
On Sun, Nov 28, 2021 at 04:53 AM, Wallace Gasiewicz wrote:
This is not mine
It's not mine either. But did anyone read the whole thread in the blog? IMO... it appears, the seller is doing more "phishing" than a Chinese factory trawler. I'm not saying it's a bad deal... and more importantly... in the spirit of caveat emptor... I'm not saying it is. (In my experience, both assertions are possible.) I wish people wouldn't post these links on Tekscopes... unless they 'knew' they're not going to be trouble. -- Roy Thistle
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Barry,
R594 being high is because it was too small in wattage. It should be 1W, . R408, R534, R582, R598 and R599 should also be 1W. The 16,500 ohm resistor can be made with two 33,000 ohm 1/2W resistors in parallel. The 8250 ohm resistor can be a CCF60 type. R416 is 1/8W. A 1/2W would be better because it is dissipating almost as much as the resistors is rated in wattage. I always see if resistors are too small in wattage. Having seen many underrated resistors be bad is why I always look and replace those that are too small. On the 50V supplies, it is common to see resistors too small in wattage causing the board to darken and causing the resistor to be bad/open. There are ones on the 15V supplies that also should be raised in wattage, e.g. about 15V across around 1000 ohms. These types should be 1W. I have found that the increase in wattage makes the item better in operation because the resistance drift is lower causing lower noise, drift, etc..
Mark
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Re: Tek R7704 with curve tracer
WOW! What a package! I would love to have it. But alas, that is not going to happen! Someone is gonna get a deal.
--
Michael Lynch
Dardanelle, AR
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To keep too much noise off the list, Ozan was gracious enough to work with me off-list to find the problem with my 7B50. It turned out that R594 (the collector supply resistor for Q592) had gone very high (spec'd at 6.81k 1% and was over 18k). I put a 10k resistor in parallel with R594 and the trace started working. I'll get R594 properly replaced but, for now, it's quite wonderful to finally see a trace with that timebase.
I must say that I was quite surprised to find that resistor had changed value so drastically. I can't tell from looking at it and the parts list doesn't specify, but it resembles either a carbon film or metal film and I wouldn't have suspected there'd be a problem there but anything's possible.
Ozan has been so helpful to me - very patiently asking questions about the state of various voltages in various circuits, suggesting more places to look for problems, and finally, zeroing in on the Sweep Start Amplifier as at least one of the possible trouble point, and I want to thank him publicly for his help.
Thanks to everyone else who offered suggestions for this as well. I'm not the best at analyzing some of these types of circuits and it is a great help to know there are others here who are willing to provide help.
Thanks,
Barry - N4BUQ
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----- Original Message ----- From: "n4buq" <n4buq@...>
To: "tekscopes" <[email protected]>
Sent: Thursday, November 18, 2021 5:09:24 PM
Subject: Re: [TekScopes] 7B50 - No Trace Hi Ozan,
After repairing the connection to S260, I gathered another set of readings. The
setup was the same as before (e.g. NORM, EXT, TIMEBASE, LEVEL midway on the
negative slope):
Base of Q224 : 0.80V
Base of Q234 : 0.89V
Base of Q244 : 9.00V
Base of Q254 : 8.30V
Voltage at TP262 : 12.44
Voltage across R262 : 0.63V
Voltage across R267 : 0.02V
Value of R267 : 3.0 ohms
The voltage at the base of Q224 stays steady when rotating the SLOPE pot
The voltage at the base of Q234 varies with the position of the SLOPE pot
(higher or lower depending on direction)
I don't know if any of that changed enough to be significant but I thought I'd
at least do that again after seeing the broken connection at S260.
Thanks again,
Barry - N4BUQ
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Halden,
This is a common failure on these scopes. Take the two that you suspect as "good" and install both in the "A" channel, see if this gets you an "A" trigger. There is a serial number break on the manual. Make sure you get the manual that applies to your s/n. You can probably find the applicable manual at TEK Wiki or BAMA.
These parts were originally supplied as a "Matched Set" TEK P/N 151-1042-00. They can be "matched" using a curve tracer and probably other methods as well. Commercial replacements also include SF50D31 or U2885J, there can be other possibilities as well..
Good luck!.
--
Michael Lynch
Dardanelle, AR
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Tek R7704 with curve tracer
This is not mine, but I thought someone in the seller's vicinity might want it. Sounds like a good deal to me.
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Re: 492 spectrum analyzer sweep linearity
Harvey,
Okay, I see the (I think) differential amplifier that drives the horizontal deflection plates (starting with Q4025/Q4038 and ending with Q1024/Q1031/Q1043/Q1049), but it sounds like the fault is already present at TP1061, which is the output of the op amp U6061, and is buffered from the horizontal deflection amplifier by two other op amps (U7073 and U2060), so I doubt that a fault in the deflection amplifier board could be causing the low voltage at TP1061 that Michael is seeing.
Maybe R7051 and R7081, if they were both below spec, could be dragging down the output of U6061, but U7073 would have to be in really bad shape to be letting anything further down the line have any effect at TP1061.
Still, I'm fully prepared to be wrong. I'm out of my depth, and I'm not entirely sure I understand what Michael has measured in the circuit.
Also, I don't see much trimming in the deflection amplifiers. There is a gain trim pot across U2060 (R2069), and two trim caps on the inputs to the differential amps driving the deflection plates (C4057 and C5021), but I don't really understand what the trim caps are doing.
-- Jeff Dutky
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Hi all,
I just acquired a 465B at a ham radio swap meet. The fellow said it worked and when we plugged it in we got a line on the screen. I didn't think to run a scrap of wire from the cal bar to an input. When I got it home, I found out it doesn't trigger. It does have appropriate levels of vertical deflection, though.
I found another thread here where Fabio helped John troubleshoot the same problem on his 465. I followed the same guidance and had the same results as John had. Thanks you guys for doing this on the forum!
Unfortunately, my copy of the 465B service manual does not include the full schematic diagram of this board. But by examining the 465's service manual and my 465B's circuit board for similarities, I found that the JFETs at the same place on my 465B as the bad one on the John's 465 are different from each other. So one of them is probably bad. They're Q7324. The marking is 1042/8041.
The same type JFETs on the 465B's B channel (Q7124) are also different from each other. I now have 2 that show around 200 mV (DMM, diode test mode) between source and drain in both directions and 2 that show open, both directions. All four show a normal junction between terminals 2 and 3. I'm just learning about checking JFETs, and tentatively concluded that the ones that show current flow between D and S are the good ones. I'm tempted to swap them around and put the good ones on the A channel, but there's probably a reason 2 of these devices failed. I don't want to destroy either of the only good ones I have.
Does anyone here know what conditions tend to destroy these devices? If it's another failed device in the circuit, I should probably try to fix that, too. All power supply voltages are spot on, and the trigger level potentiometer wiper swings between + and - about 8.5V.
Or do they just die of old age? They're about 40 years old I think.
Another idea is to install a couple JFETs that are entirely unlike these. I have several J310s on hand. Characteristics are quite different, but not by an order of magnitude. Would they work enough for me to get some triggering on strong signals just so I know that I've found the culprit?
Thanks!
Halden
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Re: 492 spectrum analyzer sweep linearity
There's several ways an amplifier can go non-linear.
One takes a perfect amplifier and then rolls off the? frequency response, so not enough HF.? This could produce a sweep that is fine at low sweep speeds, and starts to get odd at higher sweep speeds.? If you can't do anything with the sweep speed, then that's a non-issue for now.
The other is more subtle.
What Tektronix did on some of the scopes (and likely they used it a lot) is a circuit that looks like this:
Somewhere you have a sweep generator, producing a sawtooth. Check that for linearity with another scope and look at the timing pulses.? If the sweep is linear, then....
Tektronix used an emitter coupled differential amplifier.? lt's an amplifier that has an A and B input, and produces at the output an out of phase and an in phase copy of the input.? Tek then ran this through an amplifier that had cross coupling to give you compensation and gain control.? The output of that amplifier drove each deflection plate, the + and the -.? They typically drove one input, and signal wise, grounded the other.? This allowed one single polarity sweep signal to be split into a + going and - going sawtooth.
One side of the screen is mostly driven by the + part of the amplifier, the other by the - part.? If one side of the amplifier chain is non-linear, the the waveform on that side will be non-linear.
One good reason for an amplifier to be non-linear is that it is being run into saturation.? Since the amount of non-linearity is small, it'll be subtle.
If you can turn off the sweep, turn down the intensity, then go through and see if the collector voltages on the + part and the - part of the amplifier match.? The horizontal positioning should be about centered.? If one stage or side is off then the trace will not center at the control's mid position.
You could put timing pulses in on a scope, then use the sweep waveform from each collector to drive the H input of another scope, and perhaps see where the nonlinearity begins.
there are typically centering and gain adjustments which ought to be at reasonable settings.
Harvey
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On 11/27/2021 10:03 PM, Jeff Dutky wrote: That sounds much more likely than my suggestion, but analyzing the amplifier is outside my skill / expertise. Would there need to be trimming circuits to adjust the linearity of the horizontal amplifier, or to adjust for nonlinearity in the CRT?
¡ª Jeff Dutky
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Re: 492 spectrum analyzer sweep linearity
Have you tried replacing U6061? TP1061 is the output of that op amp, and there's no component in the feedback path that could have drifted, which implicates the op amp itself. An LM318 would be a cheap and easy fix, if that's the failed component.
There's also a voltage divider (R6058/R6052) and bypass cap (C5058) on the non-inverting input of U6061. I wonder what the effect would be if any of those resistors or the capacitor were off spec or failed.
-- Jeff Dutky
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Re: 492 spectrum analyzer sweep linearity
That sounds much more likely than my suggestion, but analyzing the amplifier is outside my skill / expertise. Would there need to be trimming circuits to adjust the linearity of the horizontal amplifier, or to adjust for nonlinearity in the CRT?
¡ª Jeff Dutky
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vintageTEK museum mouse pads
The museum is now selling mouse pads with the round CRT and vintageTEK logo on our eBay store. All proceeds support the non-profit museum. Enjoy.
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Re: OTish: ROM/RAM bank switching in the 2467 et al.
On Fri, Nov 26, 2021 at 9:13 PM Mark Litwack <mlitwack@...> wrote: I was going to write a Tek-specific disassembler that understood the later
model banking natively, but hadn't gotten around to it. Maybe IDA can do
it?
I'm using Ghidra, which almost certainly will allow automating the task of resolving function references through bank switching. The problem is that I'm a total Ghidra n00b :). It looks like the 6800 disassembler I ginned up ( ) is correctly lifting the instructions to P-code, and from there it should be possible to script up something that creates the right references. It's a bit of a complication that a call through a bank switch is typically done through a thunking routine that sits across the two banks, something like: SERVICE_FUNCTION_IN_BANK1_THUNK: Bank0: JSR SWITCH_BANK1 Bank1: JSR ACTUAL_SERVICE_FUNCTION Bank1: JMP SWITCH_BANK0 where the successive instructions above are on back-to-back addresses, but in alternate ROM banks. A call from bank0 to bank1 is then as simple as: Bank0: JSR SERVICE_FUNCTION_IN_BANK1_THUNK I think it would be ideal if the notion and particulars of the bank switching could be plugged into Ghidras existing analysis mplementation, as that way it would seamlessly trace control flow across the bank switching. After thinking on it a little bit, I think it'd be reasonably straightforward to abuse the processor description by adding a banking register (BR) to the processor, and change all PC addressing to reference BR+PC. This would make it possible to describe the ROM layout in an extended, flat address space. Another way to crack this would be to write an emulator. Mame has a 6800 CPU implementation, and the 2400 memory map and registers are sufficiently documented that emulating them should be feasible. This would at least expose all instructions and control flow for features that are exercised in the emulator run.
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Re: 492 spectrum analyzer sweep linearity
..that should read +5V at the right most graticule.
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Re: 492 spectrum analyzer sweep linearity
On Sat, Nov 27, 2021 at 01:09 AM, Jeff Dutky wrote:
R5105
I'm pretty convinced that it is an issue with the deflection amplifier. According to the calibration setup, in manual sweep, the voltage at the test point TP1061 is -5V when the cursor is at the left most graticule, 0 in the middle and right most graticule. I can adjust the center and right to be correct but this means that when I move the dot to the right most graticule it measures -4.7V. I can get the 'horizontal out signal' (which is tapped before the deflection amplifier) and apply it to to the X drive of an oscilloscope in XY mode and it is linear in time. (externally triggering the 492 with the waveform generator). This implies that the sweep generator is linear and X deflection amplifier (or the tube!) is non linear. I'll try to plot the input voltage against the right and left HV drives to see if this shows me what side of the driver is faulty.
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vk2bea
Michael W. Lynch