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Old 11th January 2009, 10:14 PM   #1
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Default ampeg svt 2 pro preamp problem

I have a little problem with an ampeg svt 2 pro.

It was boiling and suddenly got quiet... (itīs a friends amp) He told me that the peak indicator started flashing, it was losing power and then it was quiet... No overheating indication.

I found out that it seemed to be an condenser (C2) that might be the bad component, i have replaced it now. Itīs better but not good. (the boiling sound dissapears totally when i push the mute switch, so the error has to be in the preamp)

Can it be something with the mute circuit? Or the tuner out? Or just oxide in the pots and switches?? Or perhaps C1??

The boiling sound is very low now, but itīs there and i donīt like it....


Best regards


Erik
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Old 12th January 2009, 12:11 AM   #2
llwhtt is offline llwhtt  United States
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The most common problem I've seen with these amps is broken solder joints/bad connections on the footswitch jack on the rear panel. It is seldom used so the contacts get dirty and there is no output for no apparent reason. The other problem with that jack especially if the amp is moved around a lot is broken solder joints underneath the circuit board. The only bad part of the repair is getting the main circuit board out, it is the only way to deal with the problem. Since it is a major pain to remove the board it would be best to replace the jack curing either problem at one time. Even if this is not the problem most repairs are going to require removing the board, check this jack while the board is out.

Craig
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Old 12th January 2009, 06:49 PM   #3
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Thanx for the answer, but i donīt think that is the error since the noise is following the gain potentiometer, and dissapear totally when i push the mute button.

I replaced the C2, before the replacement i had a voltage of about 10 mV on the input, now it is about 1 mV, so something must have been strange with it..

Or, a silly question, the voltage values in the circuit diagram is for gain structure, right? With 11 mV on the input....

I was thinking about bad solderings too, perhaps i should check them a little bit more.

The amp seems to be modified at the drive button and a few more wires, better ground and so. Is that normal modifications from service bulletins or so??

Anyway, the boiling is very low so maybe i leave it there, and suggest a better service with replacing potentiometers and buttons to remove bad contact...

//Erik
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Old 12th January 2009, 07:48 PM   #4
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It is well known problem.

Tube sockets soldered to PCBs have short rugged legs, so all Ampeg SVTs I fixed developed cold solder joints there.
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Old 13th January 2009, 08:15 PM   #5
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On this amp there is no footswitch, found a lot of schematics for the amp, and there is two versions, one with, and one without footswitch. But this version still have the mute circuit so i suppose someone have removed the footswitch connector...


Can the J176 switch cause any errors? And, do i rellay need it? I think i could remove it to avoid one error source. The amp will mute anyway if i unplug the bass.... Am i right??
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Old 13th January 2009, 10:31 PM   #6
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A silly question:

exactly what does milpoly mean? Some of the capacitors are marked milpoly.....

could it be the 10 uF/400 V capacitor that makes the noise? Have resoldered the tube socket and some other components around... Or could it be the capacitor 0,1 uF? (C3) It really has to be there somewhere....
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Old 14th January 2009, 02:29 AM   #7
llwhtt is offline llwhtt  United States
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Q3(J176) is the mute JFET, if you pull it out it would defeat the mute function(the Mute LED will still work tho), and if it is the problem the amp should work. Have you verified if is the preamp or the power amp? If you plug a source into the PWR AMP IN jack do get a signal? Or plug the PRE AMP OUT into another amp, does that produce a signal? If the PWR AMP IN jack is dirty or has broken solder joints you'll get no sound. Anyway I have the schematics here so ask away. What troubleshooting equipment do you have? The amp will work with or without C2, it's cathode bypass and is part of the attenuator circuit, high gain with, low gain without. If it is shorted V1B would be biased a little weird but should still produce sound, probably distorted though.

Craig
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Old 14th January 2009, 09:48 AM   #8
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It definitely in the preamp. If i turn the input gain up or down, the level of the sound follows it.When i push the mute button, itīs gone. When i remove the plug itīs gone (shortened input) but when i insert a 1/4-plug halfway (so just the tip contact is shortened) itīs there. When the input connector is grounded on the ring (plug totally out), noise is gone..

When i push the gain button, the level of the noise follows.... Have tried bypassing the mute switch (in out position, by soldering it on the board), no difference.

Unfortanely the amp is modified so i have tuner out, neither any footswitch, if i had i would try to listening on the tuner output and see it it follows the gain switch....

I found all schematics i think, thanx anyway.

I donīt know if i were lucky, but before i replaced C2 there was more noise...

I donīt have any more equipment than a (good) multimeter and a simple tone generator (oscilloscope in the basement, but itīs old and big and my wife will go crazy if i take that up too....)
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Old 14th January 2009, 11:35 AM   #9
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I mean of course i bypassed the gain switch..... By soldering it in out position.
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Old 1st March 2010, 07:55 PM   #10
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Problems with same amp again... I replaced the capacitors in the preamp and the boiling was gone. But, it seems to be problematic again. I have tried tapping the tubes, and one of the AX7A is ringing a little bit more than the others, can this cause the amp suddenly get quiet for a while?

Should i use any special brand of tubes to replace them? I think the owner of the amp never has changed the tubes since he bought it, so perhaps it is time....

How much microphonic effect is normal in the tubes? Hard to answer of course, but please try...
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