Author Topic: Tek 2235 no sweep  (Read 554 times)

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Offline andy911john

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Tek 2235 no sweep
« on: March 13, 2022, 02:26:28 am »
Hello all,
I purchased a Philips CD player a little while ago.   OK, actually about 38 years ago, the first thing I purchased when I started working for a living.  So, after 38 years it started making some bad audio noises and then stopped making sound altogether.  I had a look at it checked a few things, PSU etc. nothing obvious.   I purchased a (parts) Tek 2235 scope on eBay hoping I could persuade it into life.  No luck, fuse blown, many missing parts, so, having been bitten by a bad eBay decision....obviously, I bought another one...also didn't work, but with a little persuasion, I managed to get this one working.  I did replace all the dodgy caps in the first scope and got all the power supply rails to a good place but no sweep.
Now, the normal approach here would be to use the good scope to diagnose the CD player.....right?   But, now that I have researched the 2235 so much, I am actually keen to make the non working one work.   
Symptoms - I get beam from both inputs but no sweep. Horizontal shift has no effect. Beam find does move the trace a little bit toward center (normal position is just left of center).  ch1 and 2 attenuation seems to work - i.e trace moves up and down but no left to right.
So, I have been looking at the circuit diagram and I assume that if beam find is working that the last 6 transistors in the path are probably OK.  Before beam find, there is a preamp that takes the sweep input and routes to the output stage.
I have managed to find transistor equivalents for the final stage but I am worried that I may need to replace the preamp.   I can't find a source for this part (U760 on the timing board..from memory)

So, apart from "stop wasting time trying to fix that that junk and get on with fixing the CD player!"  any sage advice would be appreciated.   
I was hoping that this would be a common problem....but most of the issues I have read about seem to be pretty unique.
Best regards
A
 

Offline bob91343

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Re: Tek 2235 no sweep
« Reply #1 on: March 13, 2022, 04:34:49 am »
 I presume you have the service manual.  On the schematic, look at the horizontal deflection amplifier and see that the voltages are proper.  At some point there is supposed to be a signal fed to it.  Is it getting a signal?  Can you put an external signal in?  Does it work in XY mode?

The problem is either the amplifier isn't working or it isn't getting an input.  If the latter, the problem could be in the sweep circuit.
 

Offline m3vuv

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Re: Tek 2235 no sweep
« Reply #2 on: March 14, 2022, 07:21:54 am »
had the same issue with a tek 453 scope,started trying to measure cap esr values(disk ceramic,using j diddys 5 transistor esr meter,couldnt get a measurement but on most i had a small meter deflection),i found one on the sweep board that gave zero meter deflection,replaced it and it fixed the sweep,maybe worth a look.
 

Offline andy911john

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Re: Tek 2235 no sweep
« Reply #3 on: March 20, 2022, 03:56:31 am »
M3vuv,
Many thanks for the reply.
I did measure the waveforms as detailed in the manual - most looked OK but I get no output from Pre-amp.   But before I conclude that as the issue, I will check caps...
I don't have a cap tester (yet), any idea what the offending cap was doing, or should have been doing? 

I'm going to re-read the manual and re-do the waveform checks.   I'll also change out caps.

A

 

Offline CMTan

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Re: Tek 2235 no sweep
« Reply #4 on: March 23, 2022, 07:19:18 am »
What instrument do you have with you for investigation?
 


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