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SVT III Pro schematics

#21
That’s why we need to see what is happening when things are not working correctly and compare to when they are working.
 
#22
I just remember I measured these some time ago:
TP6 (should be 2VACp-p, 310VDC).
-ok output: 0.67VAC (100Hz), 335VDC.
-no output: 0.67VAC (100Hz), 335VDC.

TP7 (should be 20mVACp-p, 280VDC).
-ok signal: 136VDC, 77mVAC.
-no signal: 138.6VDC. 0VAC when completely dead, when signal was restored; 55.7mVAC. The drop in AC is probably due to warm up. 10 min later it was: 45mVAC.

What does TP7 AC drop say?
 
#23
It says that the DC voltage is low there. NOW, we need to identify WHY.

If you remove the two tubes on that power amp PCB, the voltage after the variable stage should rise to approx. what it's supposed to be. If it doesn't, it indicates a problem with the variable plate voltage circuit.

If it does, it doesn't mean that it's not the plate voltage control circuit, BUT it could also be a problem with either tube or the resistors or C3 surrounding the tubes. With the tubes out of the circuit (and the power disconnected), wait a few minutes, verify that the power supply is fully discharged, you should be able to measure most if not all in circuit.

If they all check out, and replacing the tubes (swapping with known good tubes) doesn't change things, it points to the plate voltage control circuit. If that's the case, I would expect that C5 is leaky, pulling down the base control voltage on the pass element. That said, if any of those resistors are way off spec, that too could cause the same symptoms.

C5 needs to be 47uF/450V because the initial conditions can be higher than the steady state conditions.
 
#24
agedhorse said:
If you remove the two tubes on that power amp PCB, the voltage after the variable stage should rise to approx. what it's supposed to be. If it doesn't, it indicates a problem with the variable plate voltage circuit.
Click to expand...
This would be TP7?

agedhorse said:
If it does, it doesn't mean that it's not the plate voltage control circuit, BUT it could also be a problem with either tube or the resistors or C3 surrounding the tubes. With the tubes out of the circuit (and the power disconnected), wait a few minutes, verify that the power supply is fully discharged, you should be able to measure most if not all in circuit.
Click to expand...
What do you mean about measure in circuit, but with power disconnected? What am I then measuring?
 
#25
Yes, TP-7

Measure the values of the resistors using a DMM. The power needs to be disconnected because any leakage from a cap across the power switch (many amps have this) can corrupt the measurements.

- R1 and R2 don't matter, they aren't involved.
- R8 and R81 need to have one leg lifted.
- be sure not to damage the PCB, I understand that they are no longer available as a service part (and were expensive when available)
 
#27
R3 - R9, R81. Only need to lift one leg on R9 and R81.

This might be a good time to learn how this circuit is supposed to work, and why those resistors can be measured in-circuit w/ tubes pulled.
 
#28
Finally I received the new 47uF/450V caps, and I replaced all 3 (C5, C14, C15). I know C14/C15 wasn't necessary, but it took 3 minutes and $4 extra once the PCB was off anyway. The old caps had ESR of 0.6 to 0.9ohms. New ones had 0.4.

Now TP7 is a solid 307.7VDC @ 10 tube gain (before it was 138VDC). It's a bit high as the schematics say 280VDC. Anything to worry about? Could something else be ready for replacement as well?
Actually, TP6 is 335VDC (should be) 310VDC and TP8 324VDC (should be 300VDC).

The TP7 AC in the schematics lists 20mV, but I get approx. 1mV (jumps between 0.4 and 1.3mVAC). Is this a maximum value, so anything less is good? When I turn the tube gain pot, it jumps up to 50-60mV then immediately drops to about 1mV again.

Before I replaced the caps, I measured all resistors you said.
All is dead on except R5 (150k 1%) which reads 153.1k. Would this cause any problems?
 
#29
The ESR wasn't the problem, if you were to measure the capacitance you would find that it's going to be way off.

153.1k is plenty close.

I don't know about TP7, I believe there's a standard signal level and settings to achieve that measurement but I haven't looked at it.
 
#30
I figured they wanted a closer value 148.5-151.5k since they chose a 1% resistor.

I measured the caps directly after desoldering. But I re-measured them just now and ESR was a bit higher now when cold. However, capacitance is dead on (and way inside their 10% spec).
C5 was 46.50uF (ESR 1.41) C14 was 46.6uF (ESR 1.01) and C15 was 47.4uF (ESR 1.43).

Actually, almost all cap replacement I've done (that solved a problem at least), has had dead on capacitance, but often (but not always) the ESR was a lot higher. In this SVT case, it was as much as 4 times higher.
 
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#31
I just checked the bias voltage. I clipped onto the 47R 5W resistors in the cooling channel. Since PCB is now mounted back, I can't see the traces, but I figure all left resistors belongs to one set of MOSFETs (either the P or N-channel), the right ones for the opposite. So I didn't bother measuring all (since they have a lot of that rubbery white gunk on their legs). No signal, no load.
I measured the right side, 3rd resistor from the top (top being the rear panel): -5.1mVDC.
I measured the left side, 2nd one from the top: -2.5mVDC.
(multimeter leads: positive on right leg, negative on left)

Seems awfully low (should be approx. 25mVDC?). Although my father said it played really really clear (and LOUD).
Should I go ahead and adjust it (slooowly, as I understand the trimmer pot is extremely sensitive)?
 
#32
ESR measurements are made at very high frequencies, so for electrolytic capacitors they can be misleading.

I don’t know why you are measuring the correct cap value, in circuit I would expect the cap to either be almost open or very leaky.

47R doesn’t sound right 0R47 might be what you mean? A 47 ohm resistor may be used in the driver circuit (I haven’t looked) but 0.47 ohms is more likely a source resistor so be sure you are measuring what you think you are measuring.
 
#33
Yes, I don't know why, but replacing C5 fixed the random dropout problem, and plate voltage doubled and is up at around 300V again.

Yes, my bad, 0R47. They are the source resistors. Do you think I should adjust the bias voltage closer to 25mVDC? Or should I measure around more for other faulty/out of spec components before adjusting trimmer pot AP1?
 
#35
okwestern said:
Yes, I don't know why, but replacing C5 fixed the random dropout problem, and plate voltage doubled and is up at around 300V again.

Yes, my bad, 0R47. They are the source resistors. Do you think I should adjust the bias voltage closer to 25mVDC? Or should I measure around more for other faulty/out of spec components before adjusting trimmer pot AP1?
Click to expand...

if you don’t hear any distortion, nor see any crossover distortion than leave it alone.

The cap was bad, possibly intermittent, this is not uncommon.

byacey said:
I've seen caps do funny things after unsoldering.
I'm just guessing, but perhaps the heat on the leads causes the internal foil oxide to reform, or eliminate some leakage temporarily.
Click to expand...

Agreed, this is not uncommon.
 
#36
agedhorse said:
if you don’t hear any distortion, nor see any crossover distortion than leave it alone.
Click to expand...
I'm not sure sure how the bias adjustment affects the amp. Since this is the source of the MOSFET, would a lower voltage over the resistor mean the MOSFET outputs less power?
It's not my amp, but my fathers, I don't know how it should sound (and how much I can safely crank it up). And I don't have the cabinet to test, just a BA115HP combo (which at the time has the amp removed, so I guess I could hook it up to this). But it's way too small for the SVT?
 
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#38
When I had my SVT-3 pro in the shop (blue/orange logo from the mid 90's), the MOSFETs were terribly balanced. Measure the drop across all the resistors. In my amp prior to service, the values ranged from 3mv to 25mv. The tech installed a new set of matched mosfets and replaced the resistors as well. New bias across all averaged to 25mv, with the lowest being 23.8 and the highest 26.9. Cleaner when pushed than before. The tech indicated this is pretty common. He usually keeps a set or two of mosfets for the 3 pro on hand as he has done this a lot.
 
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#39
gstaats said:
When I had my SVT-3 pro in the shop (blue/orange logo from the mid 90's), the MOSFETs were terribly balanced. Measure the drop across all the resistors. In my amp prior to service, the values ranged from 3mv to 25mv. The tech installed a new set of matched mosfets and replaced the resistors as well. New bias across all averaged to 25mv, with the lowest being 23.8 and the highest 26.9. Cleaner when pushed than before. The tech indicated this is pretty common. He usually keeps a set or two of mosfets for the 3 pro on hand as he has done this a lot.
Click to expand...
Mosfets have a positive temperature coefficient, so if one gets hotter than the others, it conducts less and the other parallel devices pick up the slack. It's nice to be able to match them up reasonably close at idle, but when they start passing appreciable current, they may not be so well matched as what they appear idling. At high power levels, this is really where you want them tracking fairly close to each other.
 
#40
The MOSFET balance is rarely that bad in practice, and no it not common, because the source resistors provide local negative feedback which along with the positive tempco, help them balance.

I have a lot of experience with both bipolar and MOSFET designs (including many of my own), and if they behaved that way, they wouldn’t work so well.
 

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