Oscilloscope Repair

Q11:

This is the strange one. I have replaced it with a BC183. And have noted the different terminal labeling, and double checked it, so it was insterted correctly. Note the sudden jump in DC-Vce. I made a not that it makes that sudden jump just before its in the fully up position. I would say 9 tenths of the way...

And note the little bit of AC on Vce...

DC - Vbe - Wonky - 0.65
DC - Vbe - Middle - 0.62
DC - Vbe - Up - 2.69

DC - Vce - Wonky - 4.11
DC - Vce - Middle - 4.73
DC - Vce - Up - 1.12 ***

AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Wonky - 0
AC - Vce - Middle - 0
AC - Vce - Up - 0.091

Ok, that works great -- keep it up with the other transistors...

We'll figure this sucker out for sure.

Michael

I think something else is causing Q11 to misbehave.

I suspect there is another transistor that will misbehave when trace is wonky...

I need to look at the ckt to have a better clue -- but we have family over and I need to go be social now So, will try to look at it later tonite along with your other findings.

Michael

Q13:

Ok, this one gave some funny readings, so I made some more. usually there havent been much of a difference between the wonky signal and completely down, since completely down is also wonky, sometimes. dont really understand why it shifts... When you set it to a wonky picture, after about 2 min its a straight line. almost like it "warms" up or something...

Anyway, here follows Q13's results. You will see that I indicated a few more tests with the DC Vce, like (3/4) which refers to a straight line, approx 3/4 way up the screen. And note AC-Vce for the wonky part, quite a jump just below and above... which tells me thats quite close to the prob...


Q13:

DC - Vbe - Wonky - 0.624
DC - Vbe - Middle - 0.029
DC - Vbe - Up - 0.012

DC - Vce - Down - 0.01
DC - Vce - Wonky - 39.2
DC - Vce - (3/4) - 101.8
DC - Vce - Middle - 93.6
DC - Vce - Up - 96.8

AC - Vbe - Wonky - 0.012
AC - Vbe - Middle - 0.185
AC - Vbe - Up - 0.237

AC - Vce - Down - 0.18
AC - Vce - Wonky - 22.1
AC - Vce - Middle - 0.25
AC - Vce - Up - 0.29

Interesting...

Keep 'em coming.

It is possible there is some oscillation that stabilizes over time (ie, it is a damped oscillation) until you upset the balance of the circuit again. At least, that is my guess.

Will wait until we get all the readings ...

Michael

Cool, thanks again, Im gonna continue now till I got all of them...

Have decided to add the complete bottom reading also, just in case...

Q14:

DC - Vbe - Down - 0.57
DC - Vbe - Wonky - 0.57
DC - Vbe - Middle - 0.56
DC - Vbe - Up - 0.67

DC - Vce - Down - 86.9
DC - Vce - Wonky - 86.7
DC - Vce - Middle - 39.6
DC - Vce - Up - 0

AC - Vbe - Down - 0
AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Down - 0.07
AC - Vce - Wonky - 0.01
AC - Vce - Middle - 0.11
AC - Vce - Up - 0

Q15. Note that Both DC and AC volts start appearing at Vce as soon as it starts to become wonky. Just as it becomes a straight line, it drops to zero...

Q15:

DC - Vbe - Down - 0.536
DC - Vbe - Wonky - 0.592
DC - Vbe - Middle - 0.628
DC - Vbe - Up - 0.639

DC - Vce - Down - 99.6
DC - Vce - Wonky - 79.02
DC - Vce - Middle - 0
DC - Vce - Up - 0

AC - Vbe - Down - 0
AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Down - 0.07
AC - Vce - Wonky - 23.8
AC - Vce - Middle - 0
AC - Vce - Up - 0

Q16:

DC - Vbe - Down - 0.674
DC - Vbe - Wonky - 0.669
DC - Vbe - Middle - 0.606
DC - Vbe - Up - 0.6

DC - Vce - Down - 12.7
DC - Vce - Wonky - 13
DC - Vce - Middle - 58.9
DC - Vce - Up - 105.3

AC - Vbe - Down - 0
AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Down - 0.1
AC - Vce - Wonky - 01
AC - Vce - Middle - 0
AC - Vce - Up - 0

Q17:

DC - Vbe - Down - 0.73
DC - Vbe - Wonky - 0.731
DC - Vbe - Middle - 0.739
DC - Vbe - Up - 0.752

DC - Vce - Down - 7.8
DC - Vce - Wonky - 7.8
DC - Vce - Middle - 5.4
DC - Vce - Up - 2.5

AC - Vbe - Down - 0
AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Down - 0
AC - Vce - Wonky - 0
AC - Vce - Middle - 0
AC - Vce - Up - 0

Q18:

DC - Vbe - Down - 0.723
DC - Vbe - Wonky - 0.723
DC - Vbe - Middle - 0.724
DC - Vbe - Up - 0.723

DC - Vce - Down - 7.8
DC - Vce - Wonky - 7.7
DC - Vce - Middle - 7.7
DC - Vce - Up - 7.8

AC - Vbe - Down - 0
AC - Vbe - Wonky - 0
AC - Vbe - Middle - 0
AC - Vbe - Up - 0

AC - Vce - Down - 0
AC - Vce - Wonky - 0
AC - Vce - Middle - 0
AC - Vce - Up - 0

Ok, sjoe, now Im tired. I think lets chew on that, and see if u can maybe spot something that you think might be the cause of all this. Il get the cap voltages tomorrow. Thanks again

Ok, so I think Q9 and Q10 appear to be fine. I say that because (a) there's no AC voltages on either and (b) their measurements 'mirror' each other -- e.g., Q10's measurements for "down" are similar for Q9's measurements for "up", and vice versa.

Something is still weird with Q11 -- BUT only for the trace "up" so I don't think it is directly related to our problem. It looks like Q13 is acting weird (not conducting, as evidenced by nearly 0 Vbe) only with the trace in the middle to up positions.

I think it's really interesting that there's 22VAC on Vce (and nearly 0VAC on Vbe) -- only with the trace wonky (and < 1VAC in all other positions) This is similar to what we saw last time. To me this suggests the AC isn't coming in thru the base (bias voltage) which in the case of Q13 is in common with Q14 (and Q15 and Q16) -- it's the reference voltage.

I still think we have yet to find the root of the problem -- I fully expect to see a transistor going weird at the middle / wonky positions. I think it will be either Q19, Q7 or Q5. But I could be wrong.

I don't think anything really changed since changing Q11, so we can continue testing other stuff upstream and downstream as well as testing the caps.

Will continue thinking about this...

Michael

Ok, can you please tell me what kind of measurements are you expecting, like what are the things to look out for? So that I know when I measure it, that I might have found the problem.

Then, do you want me to measure Q17 and Q18 again?

And lastly, doesnt AC on the transistor mean that the problem must be "before" the transistor, since the transistor itself cant be responsible for the AC, or am I missing something?

I didn't see the post on Q17/18 (etc) -- sorry. Need to get caught up.

So, what I am looking for ... the transistor conducts when Vbe ≥ ≈0.6V -- if it is less, then it is turned off (no conduction between C and E). I *think* that if the transistor is on but current cannot flow between C and E then Vbe might be quite different (like with Q11) The more that the transistor is 'turned on', the lower the C-E voltage drop. So whether a transistor is conducting or not, and how much, may help us figure out what is wrong with the circuit. Is one side of the vertical amp shutting off when it shouldn't (when trace is wonky)

As for the AC... honestly, I'm not 100% sure what I am looking for but I think I will know it when I see it. At this point, I am guessing that the AC is due to an oscillation somewhere in the circuit. So if nothing else we can at least rule out some transistors that don't show any AC. Maybe we can look at Vb with respect to ground on 'suspect' transistors and see if there is AC on the base of any of them that is getting amplified through the circuit onto the Y output.

Anyway, I will look at the data and see if anything jumps out at me...

We're only seeing significant AC on Vce for Q13/15 so far (Don't know about Q19 yet) and only when the trace is wonky. I also see AC on Vbe for Q13.

There is some other stuff that is going weird when the trace is mid/up position (like Q11, Q13, and Q17). But I don't (yet?) see this as related.

Very curious to see test results for Q5/6 and Q19/20.

Michael

btw.... i'm still following this thread......

there was one DC Vbe reading you had that showed 2.6 volts or something like that (don't remember which transistor, as the thread listing while i am editing doesn't go back that far). was that possibly a Vbe of opposite polarity? because a forward biased Vbe can't be that high unless the B-E junction is open.

EDIT: ok, it was Q11. if that's an inverse Vbe (i.e. reverse biased) then something is messed up around Q11 causing the reverse bias.

if you have any doubts about it, the readings on Q12 should be a mirror image of those on Q11 (all changes should be equal but opposite... this whole amplifier operates like an electronic "seesaw").... as a matter of fact, you can use the same principle throughout the whole amp, the transistors on top should be mirror images of the ones on the bottom.

unclejed613 said:

EDIT: ok, it was Q11. if that's an inverse Vbe (i.e. reverse biased) then something is messed up around Q11 causing the reverse bias.

Click to expand...

Q11 is seeing a high Vbe (inverse or not) only when the trace up, but not when it is low and wonky. So, I can't figure how the Q11 Vbe weirdness would relate to the trace going wonky when the trace position is down. Hmm.

But, we *are* seeing ~22VAC on Vce for Q13/15 only when the trace is wonky -- this AC of course is what's causing the weird trace.

I'd really like to figure out where that AC is coming from. It's got to be coming from somewhere in the circuit that is not common to both sides.

I still think it'll be really helpful to get AC/DC, Vce/Vbe measurements for the remaining transistors.

I also think later we might need to get AC measurements with respect to ground for the transistors -- to trace down where the AC is present in the circuit and where it isn't.

I figure if we at least know where the AC is -- and where it isn't -- we'll have some idea of what components are suspect.

Michael

one of the reasons i'd like to see measurements from the top chain as well as the bottom chain, is that this is a long-chain differential amplifier. it's the difference in voltage between the Y+ and Y- plates that deflects the beam. we've seemed to concentrate on the Y+ side of the amp, but the Y- side could be just as much at fault here... a difference in current in one pair of current sources could be what we're looking for. i agree that something is definitely odd about the behavior of Q11, but with a Vbe of over 2V? something must be biasing Q11 way beyond cutoff for such a high Vbe to exist without "letting the smoke out"..... i tend to think that the odd Vbe reading is actually a reverse bias 2.xx volts, not forward. the B-E junction would have to be open circuit for that to exist as forward bias. the only other possibility is that he's not measuring directly at the transistor leads, which opens the possibility of a hairline crack in the PC board or something else like a flaky resistor.

It seems like we're only seeing big AC on one deflection plate, between Q13/15.

Will go try to learn about long chain differential amp...

When Q11 Vbe is high, Q13 Vbe=0.012. None of the other trannies have a Vbe that low.

Michael