SX-980 Offset Issue

tsd71

RIP Tom Petty
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Finished restoring a real nice non working 980, recapped all boards, replaced transistors throughout, also replaced diodes..all according to previous parts lists. Now the problem, the unit will not come out of protection, the right amp will not adjust to 0.00VDC offset. The left channel is fine, the right channel fluctuates between 2.0VDC and 6.5VDC. Checked and rechecked all caps and transistor placements all looks good. This is all on 100 watt DBT, I haven't juiced it up full house power yet. Here are some readings from PS and Amp card:

Power Supply: AWR-156 spec in brackets

pin 20: +36.0 (33v)
pin 22: +51.50 (50v)
pin 23: +24.50 (23v)
pin 25: -24.85 (23v)
pin 27: -51.85 (50v)
pin 28: -13.25 (13v)
pin 16: +13.25 (13v)
pin 18: +12.85 (7.v)

Right Amp Card AWH-073

1. -51.85
2. +16.75
4. +24.50
8. +54.75
12. +43.65
22. -43.65

Right and left amp cards are measuring the same except pin 10 which is the DC offset measurement. Idle current is easily dialed in to spec also.
 
Finished restoring a real nice non working 980, recapped all boards, replaced transistors throughout, also replaced diodes..all according to previous parts lists. Now the problem, the unit will not come out of protection, the right amp will not adjust to 0.00VDC offset. The left channel is fine, the right channel fluctuates between 2.0VDC and 6.5VDC. Checked and rechecked all caps and transistor placements all looks good. This is all on 100 watt DBT, I haven't juiced it up full house power yet. Here are some readings from PS and Amp card:

That's why it's not coming out of protection. Was this what you experiencing before doing the work?
 
It would have be helpful to note if the offset pot affects the range of fluctuation (end to end).. i.e at zero ohms what do you get and at full ohms what do you get, and whether or not there is any change.

Pull q2 and see where the offset goes.

Try subbing q1 with 2 ksa992's (pnp) and see if the offset tames down.
 
The pot just barely affects the range of fluctuation. Today I'm reading about 2.5VDC at zero and full. I pulled Q2 and got the same results, then installed 2 gain matched KSA992's and got the same results. I'm going to swap pots with the other working board. Switched pots same results but a little lower, adjusting the pot does nothing it fluctuates on its own, anywhere from 0.650VDC to 2.5VDC. it's weird it goes down then up, down then up.
I used KSC2073 and KSA940, instead of the recommended MJE's(on semis) but I've done that before with no problems.
 
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what other transistors are still original? 6 way test each or just test replace AND check all associated resistors.

keep q1 (the precious 2sa979) out until everything is resolved.

likely it is a damaged (but not yet completely bad) transistor.

I would focus on q3, q4, q5 because idle current does adjust and if q6, q7 or the outputs were doing it, the idle current would be all over.

This IS a direct DC feedback amp, so there is an off chance that C14 if leaky or R21 if open could have strange effects. TRY swapping C14 and R21 from the other channel IF you want to try this as a last resort.


--- ehhhh ------------

never did a parts list for the amp, this covers all transistors for completeness, NOT because I think you are going to need them.
this is a quick, off the cuff substitution analysis, by someone who isn't at the top of his game right now.

looks like q3 q4 (2sc1885) ksc1845
q5 (2sa912 200mhz) has some power dissipation (0.3W) so 512-KSA1013YBU(50mhz, probably FINE) or
512-KSA1220AYS(175mhz)
drivers:
q6 (2sd381, 120v, 1.5a 20w 25-250hfe 40mhz)
q7 (2sb536, 120v, 1.5a 20w 25-250hfe 40mhz)
are heavily neutralized by 330 pF b to c feedback caps
you can use:
512-KSA940TU pnp to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.62
512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.66
for DC testing - but try to retain the originals
because I am tossed up right now over:
863-MJE15033G pnp to-220 bce 250v 8a 50w 30mhz 70hfe $1.30 watch the Vbe turnon!!
863-MJE15032G npn to-220 bce 250v 8a 50w 30mhz 70hfe $1.30
because the idle current adjustment range on the pot might be adversely affected (can't go below zero ohms to get current down!!)
or
757-2SA1930(Q,M) pnp to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver
757-2SA1837(F,M) pnp to-220 bce 230v 1a 20w 70mhz 100-320hfe 70mhz nsa driver
757-2SC5171(Q,M) npn to-220 bce 180v 2a 20w 200mhz 50-320hfe 200mhz nsa driver
757-2SC4793(F,M) npn to-220 bce 230v 1a 20w 100mhz 100-320hfe 100mhz nsa driver
but their insane bandwidth may not be tamed by a mere 330pf \resulting in oscillation problems?
 
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Mark, every transistor has been changed out on this board, I used:
I think I got most of these from a SX-1080 rebuild I did.

Q1 dual KSA992's
Q2 KSC1845
Q3 KSC3503
Q4 KSC3503
Q5 KSA1381
Q6 KSC2073
Q7 KSA940
Q8 KSC1845

I also just changed out most of the 1/4 watt resistors and now have burning on R22 on a 100 watt DBT, it's been a bad day.....F&*K
 
180 ohms. It's holding back a lot of current.

Disconnect the base of Q4 and do a quick reading at the center out (speaker) of the amp. The same as reading for DC neutral. Does it go to zero or remain at a plus voltage?

Did you check D1 and 2 for voltage at their anodes? A big voltage difference is a issue. Or did you give up for the day?
 
Tomorrow is another day, it was good saturday and a bad sunday, have a 3 day work week ahead so I'm gonna get this done this week for sure.
 
Had some time so I installed KSC2383 at Q3& Q4, KSA1013 at Q5, and changed out both trimmers. Now the unit powers up and after adjusting the new Bournes pot the relay clicked in nicely. Still having a DC issue on this channel though it fluctuates up and down from -0.450VDC to +0.450VDC from red lead pin 10 and black lead bare metal chassis. . The relay stays engaged the entire time of fluctuation. Also measuring from the speaker jacks I'm getting next to 0.00 VDC from both channels. This is still on 100 watt DBT with the outputs disconnected.
 
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Mark had you pull Q2 and check to see if the DC stabilizes, well at least I think that's what he wanted.
Any chance you feel up to it? It's only a half volt either way.
I'm not sure what, or if I would know this result: One each probe on the bases of Q4 and Q3 and check the DC volts in relation to the two. If it's fluctuating up and down, it could be the Differential pair. I know you sub a pair of 992's. I'm more curious than knowledgeable. :D I know! Experiment on your own stuff! :D
 
I'm thinking about putting the original differential pair and see if she calms down. When Mark had me pull the Q2 the offset was over 6VDC and it did not make a difference. I will do that now with the much lower DC I'm getting now. At least I have relay.
 
Pulling Q2 makes the DC go up and over 1.5VDC, the pot does not adjust with the KSC1845 pulled.
 
Reinstalled Q2(KSC1845, and reinstalled the original Q1and the DC lowered a bit. Pulled the jumper wire, made it worse, 0.650VDC. Not sure how to tackle this, I remember awhile back where Echowars installed a 330uf cap somewhere on the amp board to stabilize the DC on a SX-980. Can't find it though.
 
It won't adjust as it's missing the 1845. But it's a clue that the center goes to positive and doesn't go to zero. Did you ever read the voltage on diodes D1 and D2's anodes?
 
That could be it but a 1.5v positive on the center is a little much. I'll start looking too!
 
It could be a poor connection some where on the board. Emitters, a resistor.
 
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