Tektronix 2235 problem

[lee_retzkin]

I understand both of you. I suppose I could bring it to work, but that might get a little sticky. I know someone close by that has a scope and if I get too frustrated i'll get together with him. I'm off tommorow, will try to post a pic then

 

[JimB]

that's why I said to use PRESENCE and ABSENCE of the DC input signal

Ahhh... got it!
Switch the DC signal on/off while probing with a meter.

JimB

 

[ronsimpson]

I would get a second scope. Connect the probes like in the second picture. With the second scope trace the calibration signal from the input through the attenuator and through the amplifier. I know you don't have a second scope but this is what a scope is made to do.

 

[KeepItSimpleStupid]

For more extensive help, join the Tekscopes Yahoo group. Once your an approved member, check the archives first.

Recapping and NVRAM are known issues as well as U800 and power suppples.

These are not likely to be your firt order problem.

The tant caps leak and destroy the copper vias. The NVRAM is beyound its retention time. It stores the calibrations and there is arisk of data loss on removal. Its not socketed and requires a Low Profile socket.

some have created herioc steps to save the NVRAM.

There may be two styles: a dallas now maxxim chip and a backup battery.

Both require more than casual effort.

 

[rumpfy]

re your post 9 and my post 14, is there any progress?
I have a file called "TEK2235foldouts_sm.pdf. I got it off the net. Do my references and your diagrams agree with the locations I talk about. I cant see that you've had the covers off yet. To provide help I think we need to agree that we have the same circuit diagrams and nomenclature.
The fact that you say the vertical position controls work suggests the thing is 99% OK.

 

[KeepItSimpleStupid]

For those wanting manuals: http://ko4bb.com/manuals/index.php?dir=Tektronix/Tektronix_-_2235

 

[lee_retzkin]

yes, all of the circuit sections you've referenced I've found in my schematics, so I feel pretty confident we're on the same pages. I plan on doing some scope stuff tomorrow as it's my weekend. I'm a tech at the usps and have weird days off. And yes, the thing is out of its case. I'll get some pics of the inside as well as the front settings. I don't think the 22xx series uses a U800 as has been mentioned in another post. I see those commonly in the 24xx series. I could be wrong.

L

 

[rumpfy]

I think thats right. 'X800' components seem to be related to Z axis modulation.
The input attenuators need to be checked. At the 50 mV per division setting, the input side of Q13 is switched straight through. Given an input of say 5 divisions = 0.25 volt, you should be able to track a DC input from the input connector through to the Q18 base without trouble, using a sensitive voltmeter. At the emitter of Q18, the 2,5,10,20 switching is done in the output attenuator R19. Same is for ch2.

 

[lee_retzkin]

So far I've only done channel 1. My camera is charging, pictures soon.

Input: .28 VDC

50mV/div. DC coupled

Quiesence: Q18 E=0V B= .74V C= 6.7V

With input: E=.27V B=1.01V C=6.47V

I even went a bit further and looked at the output of U30 with no input. it was 5.02V at pins 9 and 13. This carried over to the main board connector P9103 Pins 2 and 3; same voltage. So at least on Ch.1 it looks like the attenuator circuitry is good, yes?

L

 

[rumpfy]

So far so good. Keep going towards the switch. The output after R19 is attenuated in that 'special' R19 so be careful there.

 

[lee_retzkin]

The Q102 and 103 collector voltages are way off. They should be -4.5v , but they're like -1.3v. The voltages at the emitters and bases are what they should be. All the resistors appear to be good. Could I have found the problem? Hard to believe.

L

 

[DerStrom8]

The Q102 and 103 collector voltages are way off. They should be -4.5v , but they're like -1.3v. The voltages at the emitters and bases are what they should be. All the resistors appear to be good. Could I have found the problem? Hard to believe.

L

Not yet. You need to figure out WHY the voltages are way off. I recommend checking bus voltages, and if they are off then you should look at the voltage regulators.

You're part way there though

 

[lee_retzkin]

As I've said early on, the 1st thing I checked were the low voltages at their source, then in the signal path areas I'm checking. the q102 and 103 have their rail voltages

 

[rumpfy]

And, is what is happening in Channel 1 the same as channel 2? Q102/3 you say are at -1.3 volt so this suggests there is too much current flowing into the junction of r104,105 and 106. Because the signal after U30 is now a differential signal, as the input to ch1 increases, the voltage at q102 collector will increase, and the voltage at Q103 collector will decrease. The current in R106 will remain constant.
Is the attenuator switches at R10 pins 5, 4, 3, 2 working OK?. The 50 mVolt signal at q18 emitter will appear at U30 pin 4 as a 5 mVolt signal. Can you detect anything at U30 pin 4 OR can you measure any voltage BETWEEN pins 13 and 9, at U30?
AND, check what is happening in Channel 2 is the same as in Ch 1.

 

[lee_retzkin]

I went back to the beginning, but this time something told me to inspect the attenuators, so I unplugged them and found a pin folded on the 1ox for channel 1. I straightened it and re-inserted it. I also noted the voltage was a little high at pin 3 of u30 and adjusted the R25 pot, which didn't change it much. I then cleaned those small gold contacts for the voltage switching. Lo and behold, Channel 1 was there showing the cal signal I was hooked up to. No channel 2. That was yesterday. This morning I popped off the ch 2 attenuators; they looked ok. Cleaned those gold contacts. Checked the voltage at pin 3 of U80 and found it quite low; like -0.1v instead of the nominal -0.6V. I exercised the R75 pot and the voltage jumped to -0.7v. Checked Ch 2 and there it was. I want to send a pic, but I get a too large message. That was a bit of a strange one; for me at least. Not entirely sure what specifically did the trick, but I'm pretty happy this morning. Also, I fixed that annoying clear plastic dial on the V/D knob that finally broke in half. I probably wasn't on 50mv when I did that tracing a few posts back. I need a new one for Channel 2. Thanks for everyone's guidance and suggestions; it led me to the problems; albeit indirectly. I've read a lot of posts about many different problems, read the back to basics that tek published and just kept at it. I wonder how long this thing's been sitting around? Probably awhile. Anyway, time to put it back in its case for now. May be i'll calibrate it later if I can get the equipment together or find someone. The Retzkin Death Grip is alive and well. Cheers!

L

 

[ronsimpson]

{Happy Face!} Good Job!

 

[JimB]

It is good to hear a success story.
Well done.

JimB

 

[lee_retzkin]

OK. Figured it out.

 

[DerStrom8]

Great work! Glad to hear you got it working!