Author Topic: Tektronix 7603 Oscilloscope - Repair Log  (Read 1280 times)

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Offline sean0118

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Tektronix 7603 Oscilloscope - Repair Log
« on: July 24, 2021, 06:16:56 am »
Following on from the success of the Tek 7623A repair I thought I'd have a look at this 7603. This one came with a 7D01 Logic Analyzer plugin installed.

This thing is super dirty and crusty in some places inside, so I think one of the first things I'll do is clean it up a bit. I checked the voltage rails and they were out by maybe 1% but trimming the -50V brought them back within range.

Although, it seems this scope does have some major issues. The trace is doing some very strange things with and without plugins installed. Turning the Intensity knob has no effect, but turning up the Readout Intensity knob causes a strange pattern I don't like the look of, if you know what causes it let me know.  ???





With no plugins:



With plugins from 7623A installed:



With the Readout Intensity turned up:









Board locations:



Power rail test point Locations:



A5 Vertical Amplifier:



A6 Horizontal Amplifier:





A8 Z-Axis Amplifier



A3 Readout Board

« Last Edit: July 24, 2021, 06:28:43 am by sean0118 »
 
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Offline sean0118

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #1 on: July 24, 2021, 09:25:54 am »
Okay, I made a little bit of progress. The vertical amplifier outputs are way off from what I would expect. It's not a square wave, and it's way too slow (period should be ~1ms). The period doesn't change when I change the time base div either.

But, turns out there's massive 100Hz ripple on the -15V! it's pretty much dropping out to 0V then going to -20V. The multimeter was measuring about 14.8V which I thought was good enough, just shows I should have connected a scope to the power rails earlier.

I suspect the main bulk storage capacitor for the -15V may have gone bad, which would explain why it's 100Hz (rectified from 50Hz). But I guess there could be a faulty component pulling down the -15V too.

Vert Amp Outputs:





-15V rail:
 
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Offline wn1fju

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #2 on: July 24, 2021, 11:42:17 am »
I've been burned many times by voltmeter readings when checking power supplies, since the voltmeter will just give you an average value.  I always look at the power rails with a scope even if the voltmeter readings seem perfect.  In fact, even if a piece appears to be working correctly, it's always a good idea to consult the oscilloscope.
 
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Offline bd139

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #3 on: July 24, 2021, 12:55:51 pm »
This will be a fun one. Whole power supply probably needs recapping. Then there’s the game of hunt the tantalum capacitor to play when it still doesn’t work. This journey is well worth it though. Good luck.  :-+

There’s a good thread here regarding the power supply and using adapter boards for the capacitor https://www.eevblog.com/forum/testgear/tektronix-76037613-repairs/
« Last Edit: July 24, 2021, 12:57:23 pm by bd139 »
 
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Offline David Hess

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #4 on: July 24, 2021, 04:17:48 pm »
My first suggestion was going to be massive ripple from the power supply because of worn out input capacitors; I have seen it before.
 
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Offline sean0118

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #5 on: July 25, 2021, 04:37:06 am »
This will be a fun one. Whole power supply probably needs recapping. Then there’s the game of hunt the tantalum capacitor to play when it still doesn’t work. This journey is well worth it though. Good luck.  :-+

There’s a good thread here regarding the power supply and using adapter boards for the capacitor https://www.eevblog.com/forum/testgear/tektronix-76037613-repairs/

Yeah that's what it really needs. I'm actually planning to auction these off for charity here on the eevblog though (they are from work so can't sell them). I don't want to spend much on them because I won't be able to recover the cost, but hopefully whoever wins the auction gives it the full re-cap it deserves. Thanks for linking the other thread, it's good to have it here as a reference. ;)

My first suggestion was going to be massive ripple from the power supply because of worn out input capacitors; I have seen it before.

Yeah it makes sense. I'd like to know what causes that pattern when the Readout Intensity is turned up. At first I was worried it was a problem with the CRT itself, but it almost looks like transistor Vds / Ids curves rotated 90 degrees, might just be a coincidence though.



I replaced C811 which directly follows the rectifier on the -18V line. I only had a 10,000uF which is really only a temporary fix as C811 was 14,000uF. I'm not sure if its ripple current rating is adequate for the long term either. I also added a wire to join the two -18V planes on the PCB that were connected by the multiple pins of C811. Unfortunately I forgot to take a photo of this while the module was out.

The ripple has improved a lot, now only ~1V on the -15V rail, probably due to the replacement being too small. The CRT intensity still doesn't adjust and the trace is still off centre etc. But the Readout Intensity adjustment doesn't produce that strange pattern anymore.

But, the -15V is now at -20V, so I assume the excessive ripple killed the regulator. Hopefully the -20V hasn't damaged anything.



A10 Rectifier:



A12 Signals out:

« Last Edit: July 25, 2021, 04:47:37 am by sean0118 »
 

Offline sean0118

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #6 on: July 25, 2021, 10:06:09 am »
Okay, I made some more progress.  ;)

The main pass transistor (Q958) for regulating the -15V tested okay. But the smaller one that drives it (Q956) did not, not a complete short, but didn't pass the 'diode test'. Vbe and Vbc were both about 0.47, but were 0.8V in reverse instead of open.

Q956 is a 2N5859, but I replaced it with a 2N1893 because it's all I had in that package. It's not a good replacement, 0.5A vs 2A max collector current. I won't be running it long until I get a proper replacement.

Replacing Q956 was enough to bring it to life. However, it seems the oscillator on the reference board is dead, even though it's set to 1kHz only 4V DC is coming out. The photo taken below is using the 1kHz ref from my Rigol. The horizontal scale seems a bit off, might be a calibration issue, or maybe I still have some voltage rail ripple.


« Last Edit: July 25, 2021, 10:19:01 am by sean0118 »
 
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Offline bd139

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #7 on: July 25, 2021, 10:33:22 am »
That’s looking pretty hopeful  :-+
 
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Offline harrison@cs.unc.edu

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #8 on: August 11, 2021, 02:26:38 pm »
I had similar problems with one of my 7603's. No high voltage led me to the rectifier board. No +5 or +130. I tested all the filter caps and
found that 5 out of 7 were leaky or just open. I replaced all of them with newer versions that are 105c tolerant. I tried to buy low ESR, but
the proper values were back ordered. After replacing all the caps, the +130 was still missing. I had checked for the raw 130 from the xformer
and it was correct. Still no +130.  The bridge rectifier was testing good while in the circuit, but still no +130. The problem was that one of the
xformer taps was intermittent. i had to remove one of the replaced caps to get to that connection. I resoldered and had all the voltages
present at the regulator board.
I didn't have a trace because no plug ins were installed. I put in the 7B53A and powered up again. The magic smoke left the scope!!
Two tantalums on the vertical and one on the 7B53 gave up life. I replaced them all and a few others. Calibration did not suffer and it has
been a good scope for me. I have 3 more on the way to repair and hopefully sell later.
nChapelHeel
 
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Offline sean0118

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #9 on: August 22, 2021, 05:33:39 am »
Okay I narrowed down the horizontal issue a bit more, I think the problem is actually in the 7B50 horizontal module.

One thing that threw me off a bit is the 4V ref is 1kHz +/- 20%, so it's not good as a reference for the horizontal. So I used the 1kHz ref from my Rigol instead, which should be the most accurate source I have.

I went through the calibration procedure for the horizontal gain of the 7603 (as best I could without a calibration module). First I set the 4V ref to DC and calibrated that to my multimeter, I set it back to 1kHz and swapped the vertical and horizontal modules. This gives vertical traces spaced horizontally by the 4V ref, which allowed me to calibrate the horizontal gain independent of the horizontal module.

Except when I swap the vertical and horizontal modules back to their correct locations the horizontal scale is still incorrect. I put the modules into my Tek 7623A  and it's off by the same amount. I should be getting 10 divisions at 0.1ms/DIV but I'm only getting 8.4 divisions:


Calibrating the horizontal gain (4V 0.5V/DIV)


Calibrating the horizontal gain (4V 2V/DIV)


Tek 7603 1kHz 0.1ms/DIV


Tek 7623A 1kHz 0.1ms/DIV

 
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Offline David Hess

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #10 on: August 23, 2021, 03:23:49 am »
I went through the calibration procedure for the horizontal gain of the 7603 (as best I could without a calibration module). First I set the 4V ref to DC and calibrated that to my multimeter, I set it back to 1kHz and swapped the vertical and horizontal modules. This gives vertical traces spaced horizontally by the 4V ref, which allowed me to calibrate the horizontal gain independent of the horizontal module.

Standardization is only required if you expect to swap modules between oscilloscopes which was never really intended.

Before I got a standardizer plug-in, I modified my 7A16A amplifier with a couple of test points at the interface so that I can connect a multimeter to measure the DC and AC signal voltage at the interface, which as I recall is 25 millivolts per division but someone should check me on that.  Many multimeters have plenty of resolution and accuracy at that level to calibrate the mainframe better than required.  AC calibration can be done with a low frequency square wave and either AC RMS or AC averaging with a correction factor of 11%.
 
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Offline sean0118

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #11 on: October 26, 2021, 03:36:39 am »
Okay it died again!  ::)

I left it on for a bit then came back to find the trace was gone. Measuring the power rails I found they were all much lower than usual, which pointed to an issue with the -50V rail as it's the reference for the others.

When turning the scope on I found the -50V rail increased very slowly and never reached -50V. I suspected the 5.6uF electrolytic capacitor C889 in the reference circuit had gone leaky, however I lifted one leg and found it to be okay.

I then found R890 (5.49k) had gone open. It biases the zener diode reference, so without it the zener voltage was much lower than normal and C889 charged very slowly.

I replaced it with a CPF15K4900FEB14 (5.49k, 1%, 25ppm, 1W), it's much smaller than the original resistor, however the service manual specs it as only needing 0.5W. I also re-soldered some of the regulator pins as they looked possibly cracked and some had way too much solder. One or two of the pads were already lifted slightly so it may have been touched up before.

With R890 replaced the scope works again and all the power rails are in spec.   :D








 

Offline youngwug

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Re: Tektronix 7603 Oscilloscope - Repair Log
« Reply #12 on: October 28, 2021, 09:32:05 am »
What is the measured value after removing the R890(5.49k)?
 


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