SVT 4 PRO Compression circuit no working

[cerrem]

Hi...
I have my personal SVT 4 PRO .....
Compressor circuit just stopped working without any incident... ie, it wasn't dropped....
Looked over the schematic and see the compressor circuit in the front end .....
Have a show this Sunday and have limited time to work on it...
I know there a few posters that routinely service these... I think one person is located in Burbank...
I am aware that these PCBAs are notorious for loose solder joints and poor connections on the ribbon headers as well as poor contacts in the jacks...
Asking if this is a commonly seen issue and solution, since I am trying to get this done soon..
Just trying to save some time with tips....
Many Thanks....

[J M Fahey]

Glad to suggst somthing if possible but please search for and post schematidc here.
Doubly so if you are in a hurry.
And please exactly describe the symptoms, why do you think it isnīt working or what did it do before which it does not do now.

[Mick Bailey]

Nevetslab is the guy in Burbank who posts a lot of detailed information on these amps.

The compressor is located on the main preamp board and isn't reliant on interconnections for operation. The solder joint problems are mainly related to the power amp PCB; rectifier, fan regulator, output transistors and a handful of other joints but this doesn't rule out any other area.

This amp has a simple compressor. First check would be that the pot is secure, clean and is working OK. If loose either the solder joints or the pot legs can crack. My suspicion though is the vactrol (marked OPO on the schematic) may not be working (the LED can fail) So my second check would be to test that.

SVT4 PRE.pdf

[cerrem]

OK...Thanks for the tips...Makes sense..
Looking at the schematic...this compressor looks to be basically a shunt regulator... not many components in there..
Thanks for posting the schematic...
Much appreciated...

[J M Fahey]

Ok, but:
* what was the original complaint?
* why do you think you have a problem?

or if this is hard to understand: you say it stopped working ... why do you (or amp owner) think that?
Please describe symptoms.

[cerrem]

I use the compressor quite often... If I don't use the compressor when I slap and pop I would blow the speakers at the volumes I play at...
So turning the compressor knob has no effect on the bass dynamics....amp sounds like it normally does without the compression effect..
I know there is a problem, because I have been playing this amp for 4 years 2 to 4 times a week.....
Circuit is fairly simple ... I'll just test the usual basic stuff before tearing it down...
Thanks...

[Mick Bailey]

A tip on checking the vactrol that I use (you may already know this);

You need two DMMs - one set to diode check and the other to resistance. With the amp powered off connect the resistance meter to the resistive side of the opto and it should read 1M or greater. Now connect the other DMM to the LED side and watch the resistance - it should drop to less than 5K Ohm - the diode test will light the LED. Make sure the diode is actually conducting - you will see the voltage drop reading if it is. I have a set of hook probes just for this job.

If it reads good, then when you're hitting a note with the compressor turned up you should see the DC on the LED side rise.

Edit; the dark reading will be closer to the 50k gain pot value, as the resistive side is in parallel to the pot.