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Old 10th January 2013, 05:52 PM   #31
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Hi.

So the lightbulb tester is a keeper.

But what bothers me more is that I see -38 DCV on the output (DVM connected to signal(red) and ground(black)) without speaker connected.
And if I turn the Bias pot, it will go lower.


The Bias pot works as it should. If resistance is max, LightBulb goes darker, if I turn the pot smaller it will go brighter.

So no issues on how the mosfets react.

But DCV is leaking into output. And its coming from the IRFP9240 side.

Last edited by served; 10th January 2013 at 06:14 PM.
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Old 10th January 2013, 06:57 PM   #32
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Some resistors that were measured and were good.
This transistor Q4 is the only unknown link. I will take more readings.

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Old 10th January 2013, 07:30 PM   #33
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I also found this
ampage > Music Electronics > Guitar Amps > Repair & Maintenance : 1 : Ampeg SVT 3 Pro output has DC

Its a webcache, but there is a very similar problem with the same amp.
All the parts he already checked didn't lead to victory. I will see what he has done.
I am having problems to find the correct forum, my browser says no connection for this link. So I can't see if it continues or not.

So there he looked over all the Zener diodes. C6 C7; Removed AutoBias circuit.Removed Q3 and Q4 function. But maybe it would not give a result if Q4 would be broken. I will get a new Q4 tomorrow to see what happens.

This voltage is not coming from Feedback. Because between R13 and TL074 the voltage is -15VDC

I am starting to think that thouse isolators between MOSFETS and heater are conducting. I will change them tomorrow.

Last edited by served; 10th January 2013 at 07:57 PM.
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Old 11th January 2013, 12:54 AM   #34
JMFahey is offline JMFahey  Argentina
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Please don't shotgun it, introducing more unknowns.
Back to the bulb limiter for now, no speaker, no audio in.
Measure and post:
1) Output rail voltage.
2) + and -V rails.
3) bias voltage, measured from Q2 emitter to top of R10.
Give me 2 values: with trimmer AP1 on max. (10K) and when set so bulb limiter starts becoming shinier (meaning output MOS are starting to turn on).
Thanks.
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Old 11th January 2013, 02:23 PM   #35
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Hi.

So here is what i found.

Measured with DVM. Black to ground. Red to meas.point

There is -42,3VDC on the neg side of D7.
-41,3 on the pos side of D7
-39,7 on the pos side of D5
-37,5 on the neg side of D5


I will measure the points you asked in an hour. This was what I got yesterday evening.

Last edited by served; 11th January 2013 at 02:26 PM.
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Old 11th January 2013, 04:06 PM   #36
JMFahey is offline JMFahey  Argentina
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"Output rail" is the speaker output and whatever is connected to it.
"+/-V rails" are the nominally +/-65V ones and anything that's connected to them.
"Bias voltage" was defined above in "3".
For now, lift one end of R12 and one of R18, to put the servo amp (1/4 TL072) out of the picture.
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Old 11th January 2013, 04:30 PM   #37
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So I don't know if this is what you asked for. (this doesnt replay do your final post but previous)

Bias pot set to 0 resistance: low bias
1)-38,3V
2)-95,6V
3)3,4V

Bias pot set to as high as it allowed (amp tuns off if it is turned pass this point, the bulb will glow a bit more.

1)-33,6
2)90,3
3)3,8v

Last edited by served; 11th January 2013 at 04:38 PM.
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Old 11th January 2013, 06:39 PM   #38
JMFahey is offline JMFahey  Argentina
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1) Output rail voltage. 1)-38,3V to -33,6 Fine, about what you posted earlier.
2) + and -V rails. 2)-95,6V to 90,3 Don't understand. +/-V rail schematic values are +65V and - 65V , I expected somewhat less because of the bulb limiter. You provide a single value, too high, the first time with a negative value, then a positive one. I'm baffled.
3) bias voltage, measured from Q2 emitter to top of R10. 3)3,4V to 3,8v Well, that's reasonable.

Now, I have a problem with:
Quote:
Bias pot set to 0 resistance: low bias
because it's the exact opposite .
Please check it.
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Old 11th January 2013, 08:32 PM   #39
llwhtt is online now llwhtt  United States
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Have you checked Q3, Q4, D2, and D3? These are the only places you can get more than 65VDC. Since the output has DC offset in the negative direction I'm thinking you have lost your + rail.

Craig
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Old 12th January 2013, 10:42 AM   #40
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Hi.

Yes you are correct. My mistake.
I did the measuring like this:

Bias pot set to max resistance: low bias
1)-38,3V
2)-95,6V (it was measured from -65V to +65V)
3)3,4V

Bias pot set as low as it allowed (amp tuns off if it is turned pass this point, the bulb started to glow a bit more.

1)-33,6
2)90,3 (it was measured from +65 to -65)
3)3,8v

Step2
R18 and R12 lifted.
Changed Q4, Q2; Q5
And measured again (it can be that bias pot is not set exactly like the previous measurement. I will try to get as close as I can.

Max Resistance
1)-23.1
2) -39,2 +39,6
3)3,4

Bias.
1)-14.0
2)-32,0 +32,0
3)3,5
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