Oscilloscope Repair

[pierreretief]

Ok, I think I got it, I think its P3 and when I measured DC, halfway with the postion knob the "white wire" pin measured 55V and "black wire" pin measured 118V... Both measuring 0V AC

However, when I measure AC, it is 0V until the trace becomes wonky... then the "black wire" pin measures 33V AC and the "white wire" pin measures 0V

So I think we have found the bad side, however, is the black wire y- or y+ ???

I feel we are getting somewhere...

 

[unclejed613]

actually it's more properly called a ramp voltage in a scope with triggering. a sawtooth voltage is a free-running ramp voltage with a very short retrace time. the ramp is one cycle of a sawtooth that then waits for a trigger pulse to start again. the voltage ramps at a very contant rate, and quickly retraces at the end of the sweep, then waits for a trigger pulse.

follow the wires to the board, which half of the amp feeds the white wire, and which half feeds the black one? if the wires go to a connector, follow the traces under the board back to the amp.

 

[shimniok]

follow the wires to the board, which half of the amp feeds the white wire, and which half feeds the black one? if the wires go to a connector, follow the traces under the board back to the amp.

I would expect as you trace back you will run into either:

R82 / C28 / Q14 / Q16 --> Y-

or

R79 / C23 / Q13? / Q15 (on the sch. the trans isn't labelled) --> Y+

I can double-check on my scope if I get a minute (ie, between baby's feedings)

We're getting there... and thanks for the awesome info, unclejed613 !!!!

Michael

 

[pierreretief]

Right, the black wire runs to the top section, Y+ and the white wire is Y-.

I have also confirmed that the one not labeled, is Q13, and it is 2SC3423. I downloaded its datasheet and it says that Vce is max 1V but i just measured 100V between C and E, so maye thats our problem?

Realised I dont have enough 1uF caps to replace them, so I am trying to see if I can narrow it down while I wait for the parts.

 

[shimniok]

Right, the black wire runs to the top section, Y+ and the white wire is Y-.

Cool!

I have also confirmed that the one not labeled, is Q13, and it is 2SC3423. I downloaded its datasheet and it says that Vce is max 1V but i just measured 100V between C and E, so maye thats our problem?

My read of the datasheet is that in the Maximum Ratings table Vceo (as they call it) is 150V. In the Electrical Characterisitcs table, they list Vcesat (ie, the Vce when the transistor is saturated) at 1.0V. Of course, it may still be a problem.

What's Vce for Q13's twin-- Q14? (Does it depend on position?)

What about Vce for Q15 and Q16 ? (Do they depend on position?)

It might be worth doing some diode tests on these four. Info on diode testing BJTs: **broken link removed** -- and pic from this site in case it helps:

**broken link removed**

Here's the datasheet for Q15/16 (2SC1360) for the pinout:
2SA1360 Datasheet pdf - TRANSISTOR SILICON PNP EPITAXIAL TYPE (PCT PROCESS) AUDIO FREQUENCY POWER AMPLIFIER APPLICATIONS - TOSHIBA

 

[unclejed613]

Also, fwiw, I did some reading in

Amazon.com: Troubleshooting & Repairing Consumer Electronics Without a Schematic: Homer Davidson, Homer Davidson: Books

about transistor testing.

In addition to the diode test, you can:

1. Test Vbe - should be 0.6V for NPN and 0.3V for PNP ; incorrect reading means bad transistor
2. Diode test across C and E -- very low reading means leaky transistor
3. Test Vc, Ve, and Vb relative to common ground -- in an NPN, Vc is highest, Vb next, and Ve lowest. In an NPN Ve is highest, then Vb, and Vc is lowest. Low NPN collector (or low PNP emitter?) voltage could mean leaky transistor; Higher than normal NPN collector (or PNP emitter?) voltage could mean open transistor; close voltages between C, E, B could mean leaky transistor

Michael

Vbe for silicon (PNP or NPN) should be 0.6V or so. the 0.3V would be a germanium transistor, and you won't see one of these in any modern equipment.

#3 above is confusing.... and assumes a common emitter amp. parts of this circuit are current sources and cascode amps, so #3 above doesn't apply at all.

 

[shimniok]

Ok, to avoid confusion I'll delete that post

 

[unclejed613]

after a lot of years, the best method i've found of found is to unsolder 2 leads of the transistor and let them float and check the transistor, sometimes i only unsolder the collector lead depending on the circuit. the collector lead on all of these transistors is the center one. just remember to resolder them before applying power......

 

[pierreretief]

Wow, that looks like a great book.. Did you buy it?

 

[pierreretief]

Ok, removed all 4 transistors and tested them, they all checked out ok

 

[pierreretief]

Though they measure differnent voltages.... between C and E, and Vbe is 0.7V for one, but I guess thats close enough

 

[pierreretief]

mmm ok. For Q14 the voltage is about 80V and doesnt change much with the position knob. For Q13 though, its about 100V with a straight line and when the signal goes wonky its about 50V and when it goes even lower, I get zero volts...

 

[shimniok]

I just finished measuring the Vce's of Q13-16 on my P3502. Here's what I got.


Vce
Trans (Bottom, Middle, Top)
Q13 ( +5.7, +48.8, +91.1)
Q15 (-91.4, -52.0, -6.3)

Q14 (+89.6, +42.8, +5.9)
Q16 ( -7.2, -53.5, -91.3)


The negative voltages are for the PNP transistors of course. The Vce of all four transistors vary smoothly from top to middle to bottom as I sweep the position knob smoothly.

Sounds like your Q13 is behaving similarly to my Q13 ?

But sounds like your Q14 is not showing voltage variance like my Q14 does -- since that is the Y- side which we *thought* was normal, you might want to doublecheck that to be sure.

I think it'd be worth checking Vce on Q15 and Q16 to compare to mine.

Do you see AC voltages across Q13 and Q15 both? Or just one of them?

As always I will defer to unclejed613's sage advice

Michael

 

[pierreretief]

Ok, I went back and did the same tests. This time double checking the whole time.. so this is deffinately correct... Its just the polarities that will differ, since I maybe didnt have the same probes on the pins, but that shouldnt make a difference right? Since it will be the same voltage, just with a different sign.. Right, here goes...


Vce
Trans (Bottom, Middle, Top)
Q13 ( 0.0, 93.1, 96.8)
Q15 ( 99.5, 0.0, 0.0)

Q14 ( 87.1, 45.2, 0.0)
Q16 ( 12.5, 66.3, 105.0)

As far as AC is concerned... Everything measures 0V except when I make the trace wonky and then only Q13 and Q15 both measure 26V (AC) on Vce

Can you please tell me what all this means??? Why I am getting AC there, and what these voltages mean? And what these 4 transistors do?

Any idea where the fault is?

 

[unclejed613]

ok, i had to go get a separate copy of the schematic so i could refer back to it. i should print it, but for now i'll just switch screens back and forth, so if i go down rabbit trails now and then be patient..... first of all what would really help is a chart showing Vbe of all of the transistors 1) trace all the way up, 2)trace doing it's "dance of death", and 3) trace all the way down. mind you that this is the voltages measured B-E on all of the transistors, not ground referenced.

what would help next is the voltages ground referenced of all of the diode voltage sources under the same 3 conditions. the diode voltage sources are the anode of D14 (the zener over at the right going from the +120V rail to the top of C33, i need the voltage at C33), and the voltage at C26 and the voltage at C25. since these are reference voltages, any instability in these will cause lots of strange things. let me know if there's any AC on these references when the trace goes crazy.

next i need you to tell me any AC voltages showing up across C16, C17, C18, C20, C22 when the trace flakes out.

 

[pierreretief]

Ok, so the measurements done so far did not help at all?

 

[shimniok]

Hang in there -- I think they are helping! Don't be discouraged!

It is just that when you are diagnosing a circuit made of a hundred discrete components, I am guessing one needs to take quite a few measurements to seek out the bad component(s)

Michael

PS: I believe the idea of Q13/15 is to sort of set a precise voltage drop sent to the plates. I don't really have a clear idea of how the constant current or amplifier circuits work -- I am kind of fumbling around. I would just be testing every single thing imaginable until something stood out. Anyway what I notice is that in your case, Q13/15 are either all the way on or all the way off. That has *something* to do with their base current, which is related to Vbe, which is probably why we want to measure that next.

 

[pierreretief]

Ok, quick update, I go some brand new caps and I replaced C17 C18 C20 and C25. Still exactly the same.. At least now we know its not thos caps

 

[pierreretief]

Unclejad, sorry, what do you mean with

"i need the voltage at C33, and the voltage at C26 and the voltage at C25" ?

Do you mean the voltage across them or ground referenced? And if its ground referenced, do I take the measurement at the positive terminal?

 

[pierreretief]

Ok, as for the other tests you requested, here are the results:


UP "WONKY" DOWN

Q13 0V* 0.5V 0.6V

Q14 0.6V 0.5V 0.5V

Q15 0.6V 0.5V 0.5V

Q16 0.5V 0.5V 0.5V


* That voltage looked like the odd one out, so I checked it again, this time moving the position knob little by little, and at 2 and a half divisions above the halfway line, it measures 0.4V and at 3 div it measures 0V, so this is correct. Hope that helps...