Oscilloscope Repair

[pierreretief]

Oh, and I get 0V AC accross C16 C17 C18 C20 C22

 

[pierreretief]

And I cant seem to find D14...

 

[unclejed613]

ok, if we can get the voltages across the caps i listed (DC as well as AC) it might help, as well as the Vbe of Q9, 10,11,12 with the 3 positions of the control. it's starting to look like the problem is actually in earlier stages of the amp. since Q13 was biased off, it is beginning to look like the problem might be with Q9 or Q11. it seems that something isn't sourcing enough current here, so either a voltage reference is acting up or one of the transistors is breaking down. each half of the amp also has it's own feedback loop, and there might be a problem here as well (R78,79 and R65,66,67 form the voltage divider for the feedback on the Y+ side, and on the Y- side it's R81,82 and R69,70,71)

if you find the anode of D11 or you find C33, the voltage measurement there will still give us one of the voltage references.

 

[pierreretief]

Sorry, I see now the voltages I posted earlier seemed to not be alligned the way I typed them, maybe you should make a copy because its kind of difficult do read it. Thanks for the advice, I will do that later today and post the results

Thanks again

 

[unclejed613]

that's ok, i was able to figure out what went where. this amp is supposed to be a class A amplifier, which means all of the transistors should be biased on at all times, and Q13 is not. this puts Q13 in a very nonlinear portion of it's "curve". Q11 feeds Q13, so this is where we are looking next. somewhere you're losing b-e drive current for Q13, which is why it biases at 0V. compare it against Q14. Q13 should be mirroring Q14 (i.e. when Q13's bias is high, Q14's is low) and it's not.

 

[shimniok]

Watching along eagerly!

 

[pierreretief]

Oh, dont know if this helps, but it looks like somewthing might be "warming up" since the "wonkyness" gets smaller over time and finally becomes a straight line, and if I move it down again, the same happens, and if I move it down again, it doesnt happen, so it like "warms up" untill a certain point, dont know if this makes sense or helps at all

 

[pierreretief]

and it makes a loud hiss sound...

 

[pierreretief]

Ok, the DC values for the caps are

C16 = 7 V
C17 = 10.1 V
C18 = 11.92 V
C20 = 21.3 V
C22 = 11.8 V

 

[pierreretief]

Sorry, that was for the wonky part...

 

[pierreretief]

Ok, for the UP and DOWN positions are exactly the same voltages...

 

[pierreretief]

Right, and for the transistors, I get the following:

Q9:

WONKY = 0.660 V
UP = 0.675 V
DOWN = 0.659 V

Q10:

WONKY = 0.674 V
UP = 0.659 V
DOWN = 0.676 V

Q11:

WONKY = 0.702 V
UP = 2.68 V
DOWN = 0.709 V

Q12:

WONKY = 0.694 V
UP = 0.689 V
DOWN = 0.700 V

 

[pierreretief]

And the voltage at D11 is 119.6 V

 

[shimniok]

Hopefully unclejed will pipe in... at first glance, wtf is up with Q11 (2.68V in the up position?!) Wished I could be of more help.

 

[shimniok]

This circuit diagram is really hurting my head. There *has* to be a mistake on the diagram showing +12V tied to Q13's Emitter / R74. That cannot be. Moreover, there isn't a 12V source tied to Q14's emitter / R75. Ok, that makes more sense now...

So, I don't see how Q11 could ever have a Vbe of 2.68V ?? (Up position) Or does that indicate a situation where Q11 can't conducting, because Q13 is biased "off" ? There's a Vbe potential but no voltage drop due to Ib?

EDIT: Isn't the bias for Q13/15 fixed??? I don't see where it is sourced other than the 12V source and hte 120V source. Is Q19/Q17 behaving properly?? Could Q17 be failing to bias Q19 properly? Or could Q19 just be jacked up? Seems like the fundamental problem is Q13/15 are either conducting fully or not at all... ? I donno...

Michael

 

[pierreretief]

I think Unclejad must come and share his valuable knowledge!

 

[shimniok]

To explain more of what I was thinking in my last post... and I don' t know if any of this is right, wrong, close, etc.

If I'm reading the diagram right it seems like Q13/15 and Q14/16 are biased (ie, voltage/current at at the same constant voltage), so it seems to me the only way Q13/15's Vce could be fully "on/off" (0V or ~100V), is if something upstream (Q19) or downstream (Q11) is preventing them from conducting current.

It seems Q19/20 are going to conduct a certain amount all the time (e.g., Q20 is biased via R101/102 & C34) and additional base current is drawn by Q17/18 to turn on Q19/20 harder. This happens when---according to the manual high frequency---feedback from R7?/R79/C23 or R62/R61/C28 causes them to conduct to varying degrees. Presumably these R/C networks are high pass filters (being dumb about that topic I'll have to go check )

Meanwhile Q11's (Q12's) base gets a voltage signal that varies as more or less current is drawn by Q9 (Q10) from the 120V source at VR6 thru that balancing pot as well as either R72 or R73. In turn, Q9/10 is activated by collector current from Q5/6.

Does any of that make sense? Or am I totally off base?

Assuming it isn't off base, if it were me, and if I was really bored and didn't want to wait to hear back from unclejed to set me straight, I'd get up/down/wonky measurements for Q9/10's Vce and Vbe. I'd also diode test Q11 if I hadn't yet.

Michael

 

[unclejed613]

i think you found the culprit..... Q11 is going open when you turn the control. Q11 feeds Q13. try replacing Q11 and see what happens. you might still have problems but i think that will cure the "wonky".

sorry for the delay, but i've been migrating to a new PC. lost about 5 gigs of data thanks to microsoft's heavy-handed disk tools. was trying to reinstall win2k after moving my drives to a new machine. deleted a partition by mistake, and since microsoft's partition tool deletes the FATs at the same time, can't bring the data back. almost thought i had lost my linux as well, because win install reordered the partition numbering too..... darn... lost the schematics for the impedance analyzer i'm designing. gotta rebuild it from memory.

 

[shimniok]

Sux re: data loss

Q11 appears to be a 2SC458D -- found a datasheet here: 2SC458DTZ datasheet pdf datenblatt - Renesas Technology Corp - Silicon NPN Epitaxial ::: ALLDATASHEET ::: --- As for sourcing one of these... that might be a little tricky. I've seen sources for 2SC458 (but not D specifically) the far east on the web; 2SC458 search, 2SC458 datasheet, 2SC458 buy, 2SC458 sell - IC2IC.COM : marketplace for brokers and distributors of electronic components. I assume those would be ok (?) Can't find on mouser or digikey, though.

Let's see...another option is NTE's cross reference search. They show an NTE85 as a replacement. **broken link removed** -- datasheet here: https://www.electro-tech-online.com/custompdfs/2009/06/nte85.pdf

Mouser carries them: NTE85

Fairchild has a cross-ref search too (Cross Reference Search). They list BC183 and a number of others as a replacement for Hitachi 2SC458D -- Product Folder - BC183 - NPN General Purpose Amplifer -- mouser carries these too (BC183)

Hope this helps!

 

[pierreretief]

That helps alot... Thank you very much for that, I need to go away, today, for 3 weeks, so unfortunately it has to wait, but I will try and order the part so long, since it might take that long to arrive in any case...

Thanks for all the help, will keep you posted