Ampeg SVT-350H problem

[stevenrb718]

Yes, nothing at TP9 or 10. I get 65v at both other legs of Q4, and nothing at Q6. At R20 and R25, and R21 and R24, I have 65v and -65v respectively. I checked the 1N3070s at D27 and D27 earlier, and they're ok. The ceramic resistors are all giving me 1ohm rather than the rated 0.47. Also, rather than the called for 25mV +- 5, I'm getting a solid 0.958V on each one. Adjusting the trim does change the resistance of the pot itself properly, but it doesn't change the voltage across the ceramic resistors a single bit.

[Enzo]

When you measure a 100k resistor, the ohm or so your probes add doesn't matter. WHen you try to measure 0.47 ohms, it does. For low resistance measures, always short the probes together and see what reading you get. You will then subtract that amount from the resistor reading. SO if your probes have half an ohm resistance, and the resistor also has half an ohm, that adds up to a 1 ohm reading.

It is like weighing your cat. You weigh yourself, then you pick up the cat and weigh yourself again. The difference in the readings is the cat.

Those resistors are either going to be open or they will be OK.

OK OK, 99% of the time.

[stevenrb718]

Oh yeah, I forgot about that. The resistance of the probes are about half an ohm, so the ceramic capacitors are reading fine. The bias adjust pot doesn't seem to be adjusting the voltage properly, as I'm getting a solid 0.958V regardless of pot position.

[Enzo]

What do you mean you are getting 0.958v ON each resistor? You mean to ground? If you are getting a whole volt ACROSS each half ohm resistor, then they are sitting there conducting TWO WHOLE AMPERES. If you are just measuring to ground that just means you have a DC offset on the output. Is that the case? The bias doesn't adjust that. The bias note says 25mv ACROSS each resistor. It says more or less, average them.

[stevenrb718]

I'm measuring 3.1mV regardless of pot position. I assume this is not saturating the transistors properly? Would that point to my MPSA06s at Q5 and Q6 as actually being faulty?

[Enzo]

C12 shorted or real leaky?

[stevenrb718]

No, I pulled C12 previously and did it again for verification and it's tested perfect.

[nevetslab]

I'm measuring 3.1mV regardless of pot position. I assume this is not saturating the transistors properly? Would that point to my MPSA06s at Q5 and Q6 as actually being faulty?

THAT IS AFFIRMATIVE!! I'm not sure why you don't measure any voltage at TP9 & TP10, but having close to 1V across any of your MosFET Source Resistors means they are turned on hard. You have troubles in your bias circuit, which involves Q5 & Q6. If that circuit was working, you'd be able to turn the MosFET's completely off, so there's NO Voltage across their Source resistors. I would reccomend you lift one end of each of the Gate resistors AND one end of the Source resistors on the six MosFET's, taking them completely out of circuit while you get your Bias circuit operational. THEN, when you can adjust the voltage between the N-Ch and P-Ch gate busses, which is TP9 and TP10, I'd adjust the bias pot so you have minimal voltage at both test points. Now it will be safe to restore the Gate and Source resistors to put the MosFET's back into circuit.

THEN, while monitoring Source resistors, carefully adjust the bias to where you start seeing millivolts across all six of your MosFET's Source resistors. We have no idea how well matched they are at this point, so be wary of any that are twice the 25mV nominal voltage needed across each one. If you get to where you finally have near-equal Source Voltage thru those 0.47 ohm resistors, you've achieved proper bias. AND, you should see proper voltages at TP9 & TP10.

[stevenrb718]

Well I took a nice, long break away from the amp as it was bothering me. I replaced both Q5 and Q6 with new transistors. The old ones both measured lower hFE than the new ones, but nothing changed. I'm still not getting voltages at TP9 and TP10. What am I missing?

[nevetslab]

Well I took a nice, long break away from the amp as it was bothering me. I replaced both Q5 and Q6 with new transistors. The old ones both measured lower hFE than the new ones, but nothing changed. I'm still not getting voltages at TP9 and TP10. What am I missing?

Maybe a bad (open) bias pot? Or any of the resistors in the bias adj string R27 thru R30, including the bias pot. If that string is open, I believe you'd have NO current flow thru Q4 & Q13, hence NO voltage at TP9 & TP10. Man, this amp is one mean bugger to keep taxing your/our ability to find where the fault is.

As I had suggested in my last post, lifting up the gate and source resistors on the MosFET output stages is recommended, so you're JUST working with this Driver Stage, and not suddenly have the output stage turn on hard enough to kill any of the MosFET's. You still have a fault in this bias circuit that needs to be found. It won't work until you solve that.

[stevenrb718]

Maybe a bad (open) bias pot? Or any of the resistors in the bias adj string R27 thru R30, including the bias pot. If that string is open, I believe you'd have NO current flow thru Q4 & Q13, hence NO voltage at TP9 & TP10. Man, this amp is one mean bugger to keep taxing your/our ability to find where the fault is.

As I had suggested in my last post, lifting up the gate and source resistors on the MosFET output stages is recommended, so you're JUST working with this Driver Stage, and not suddenly have the output stage turn on hard enough to kill any of the MosFET's. You still have a fault in this bias circuit that needs to be found. It won't work until you solve that.

Yeah, this amp has been a real PITA. I really thought it would be smooth sailing after fixing the preamp section, only to find out more problems. I checked the bias pot and all 4 resistors across the bias adjust section, and all are A-OK. Looking at the schematic, I don't see anything that may be shorting to ground either. I pulled out the thermal camera to see if anything was clearly shorting either, and didn't find anything. I did see that the couples of resistors at R54, R67/68, and R69/70 are extremely hot, but I'm assuming that's normal being the flame proof blocks. I think I already have, but I guess the only thing I can do now is test every component that connects into the TP9/TP10 line to see if anything is leaking or awkwardly shorted to ground. I guess maybe a joint could've melted into a ground trace contact?... really grasping at straws now.

[nevetslab]

R67/68 & R69/70 are 10W resistors, dropping the supply voltages that run the power amp down to +/- 16V Zener regulated, and are continuous duty, so they will ALWAYS be running hot, but no doubt within their rating in circuit.

I assume your test leads are measuring ok. When you short them together on your low ohms scale, what do you read? Typically around 0.1 ohm or less for proper probes.

At this point, I'd measure from component lead to component lead in the Voltage Gain stage and bias circuit. Essentially the series current strings from V+ thru all components to V- for those stages. This is now looking for any open traces between components, since thus far, nothing is found to be defective, but....you're NOT getting voltage readings at TP9 & TP10. If there IS a current path it should measure a voltage drop, or there's open circuit(s) someplace yet to be found.