GK 1001RB 2 - Problem passing self check

[nevetslab]

I haven't pulled up the document on the LM3886T Tweeter Amp (Power Amp IC), where Mute2 is being sourced from it's pin 8. Did you test the Tweeter Amp output's DC level? I often find that IC having failed. As I recall, Q6's B-E junction serves as the constant current source for base current of Q2 diff amp up at the input stage, all passing thru Q4 & Q5. Mute 1 control line is pulled high when Q20 turns on, controlled by the 555 timer circuit U2. I've never stopped to study this amps' control system, so if there's an issue with the Tweeter Amp, and that Mute 2 line isn't going high, does that prevent the front end of the amp from turning on? Pin 8 on the LM3886T is normally pulled negative....down near the neg supply level. So, I think when the Mute1 control line is pulled up, Q1 is turned on, and pulls Pin 8 of the LM3886 low, with resistance R5 to limit how far down that goes. Good question.

LM3886T.pdf

[nickbme07]

Thanks for some clues guys!! I will start looking around. Mute2 is definitely not going high

R5 -1.8v

LM3886 pin8 -1.7v, also verified on C7 leg -1.7v

[nickbme07]

LM3886 for thoroughness. Pin1,5 40v. Pin3 0v. Pin4 -40v. Pin8 -1.7v. Pins 9, 10 both 0v, as well as the tweeter header J3

[nickbme07]

Still nothing on bias test points, not even a blip at plug up. Q21 seems to test ok in circuit. It's collector is getting 100v and base and emitter at .22v. R49 is 0v. I've replaced D15 and R50. Watching Q21 during power down, the emitter drops faster than the base voltage.

I can't seem to track where the fault is being shown to Q15.

[nickbme07]

Interesting.. I followed the .5v through the D10 and D11 and found that R37 lifted is reading 0.750k ohms. Edit. Well it's making a liar out of me, however it's now showing open. Can a 680ohm suffice, or a 1m? Those I have on hand, thanks!

Nevermind false alarm (I'm losing my mind) I had my meter off of auto.. the R37 is fine.

[g1]

I think 680 ohm would be ok for test purposes. It should be replaced with correct value though.

[nickbme07]

When attempting to trace the fault, I find Q19 base is being fed 1.2v from R34, which starts as 84v at the Q15 collector, E is 97v, B at 101.5v

[nickbme07]

I think we are the right track to getting the mute1 up, but I can't seem to figure out how Q15 gets triggered.

[nickbme07]

Is it possible it's the 555 timer? Can I test this in circuit?

[nickbme07]

Still nothing on bias test points, not even a blip at plug up. Q21 seems to test ok in circuit. It's collector is getting 100v and base and emitter at .22v. R49 is 0v. I've replaced D15 and R50. Watching Q21 during power down, the emitter drops faster than the base voltage.

I can't seem to track where the fault is being shown to Q15.

Q21 does have 100v on the collector but the base and emitter are showing 0.02v basically.

[nickbme07]

Looking through BIAS supply circuit.. D5 and D9 show .05v. The supply coming from Q14 is 0v with E and B having 101v and Q11 having the opposite, also 0v on collector. Q13 E, C and B are all .06v

[g1]

When attempting to trace the fault, I find Q19 base is being fed 1.2v from R34, which starts as 84v at the Q15 collector, E is 97v, B at 101.5v

Do you have Q15 e and b readings reversed? It is a PNP so base would be more negative than emitter to turn on.

Also, I don't think you have reported emitter voltages for Q24 and Q27?

[nickbme07]

Well, I went to take measurements today and it decided to turn blue... As i am sitting there like umm ok, the fuse blows. I smell hot electronics, no smoke. I pull heatsink and now Q22 Q23 Q24 all test SHORTED .. Q7 E to C is shorted as well. Everything else seems to pass in circuit.

[nevetslab]

Well, I went to take measurements today and it decided to turn blue... As i am sitting there like umm ok, the fuse blows. I smell hot electronics, no smoke. I pull heatsink and now Q22 Q23 Q24 all test SHORTED .. Q7 E to C is shorted as well. Everything else seems to pass in circuit.

OH, I HATE the sound of that!

I've never tried this trick on the GK 1001RB or higher rated power circuits of this style, but....in checking the rest of the circuitry, I've sometimes disconnected the drive circuits to the power amp stage, as it's usually just current gain, with all of the signal amplification done ahead of it. So, you often can disconnect the back end (have to be through in this) and then jumper across the bias stage (or lift the parts so that's not in circuit, and tie the collectors of the voltage gain stage together. C19 already does that, from an AC standpoint. What you then have is a high voltage low current signal amplifier....a discrete op amp, if you will.

Here, as you're dealing with a stacked output stage, you 'd have to disconnect the upper tiers drive connections by lifting up D1 and D12. You may have to connect feedback to the two joined collectors of Q14 & Q11, lift the leg of R10 and tack it to the connection between Q14 & Q11's collectors. That forms a voltage gain amplifier, lacking the current/power gain of the back end. Get that circuit so it's stable DC wise.

Of course easier said than done, as the parts density here on this amp makes me wanna shake my arms up and scream when it ain't behaving. I've used that trick during product development on power amp circuits before. Though I've never designed any stacked-rail output stages. I did do the layout for one towards the end of my stay with BGW Systems. I forget the model, but did keep completed production PCB's of it for samples of my work tucked away.

Good luck on this amp!

I seem to recall I'd been getting the replacement semis' direct from GK, as they own the GK gear in our rental inventory. When I've had output stage failure, they always include the driver xstrs 2SA1837's and 2SC4793's.

Did you loose any of the emitter resistors?

[nickbme07]

Thank you for your suggestions there, Steven. Unfortunately this amp may have been a terrible choice for my first solid state (normally only dealing with tube).. I have now decided to throw in the towel on this one and accept defeat. I learned a lot from the troubleshooting and will be better prepared for the next hunt. I appreciate all the responses and support. Maybe the tranny can be used for another amp project.