Peavey VMP-2 advice

[Johnny1234]

Hi all it''s been a while.
I am working on my Peavey VMP-2 pre amp.

The problem is it has a flat response at 1kh but falls down 6db at 300hz on channel one.
EQ and filters are off.
Oh no!
Relays all fire and switch from one pole to the other.
Swapped all 4 tubes from one channel to the other ....Same problem
Changed all 22uf 25 volt (circled in diagram) ....Same problem
The jumper on both channels is bypassed leaving C164 open circuit.
All plate to ground, and grid to cathode voltage are within reason from one channel to the other.
Starting to get baffled....
Diagram is attached
Any ideas greatly appreciated.

 

[JohnRoberts]

As I recall that VMP had a simple Baxandal EQ, have you confirmed it is flat, when it says flat?

JR

 

[Johnny1234]

Hi John,
EQ and filter switches are in off position. Also the controls are set flat.
Do you think the .75uf caps could become a problem?
Many Thanks

 

[JohnRoberts]

It's a stereo unit so compare the two channels to see where it goes off the rails.

JR

 

[flyingeyepro]

pink noise and and RTA can quickly locate where the signal deviates. be very careful of HV of course...

 

[CJ]

Dang where did you get the schemo? I had to trace that by hand.

 

[Johnny1234]

I will post it to the schematics forum this afternoon.

 

[JohnRoberts]

I will post it to the schematics forum this afternoon.

I didn't see the drawn by or approved signatures in the title block. The design should be approved by JCS (Jack Sondermeyer RIP). I think the junior engineer working on the project was Sean something.

I am not a tube guy but I made some contributions to the feature set (I was involved in AMR marketing among several things)...

Having the gain knob go up to 11 was our homage to Spinal Tap. I had them limit the EQ range to only +/- 6dB to make it harder to get a bad tone. There were a couple subtle features inside, like no transformer on the hi Z output for linearity. I also asked Jack to design in the capability to reduce the open loop gain (with a PCB header plug-in jumper) to make the NF less effective at reducing distortion and giving it more of a potential "tube sound".

I was worried that it might sound too clean for some tweaky Tube mic preamp customers. We shipped them set for low distortion and I waited for customer complaints. Nobody complained and I told Jack he could just hardwire it for clean from the factory.

JR

 

[Johnny1234]

I will post it to the schematics forum this afternoon

I will post it to the schematics forum this afternoon.

The schematic is already on groupdiy.
So no need to pollute the forum.

 

[Chandra]

Looks like the polarity of the capacitor is wrong (reversed) on the schematic. Did you wire per schematic?
Regards.

 

[Johnny1234]

Looks like the polarity of the capacitor is wrong (reversed) on the schematic. Did you wire per schematic?
Regards.

Are you referring to C171 ?

 

[Johnny1234]

Well today I un soldered the 2 5 pin ribbon connectors P102 and P114. Inserted signal into the EQ buffer amps of good and bad channel V2 V2 and V6 V7 and they test identical. Did the same thing with the filter buffer, output amplifier through output transformer and also both test the same. Must be problem in the EQ, Filter board. Next going to change the mylar caps as all solder joints look impeccable.

 

[Johnny1234]

Some sound advice if repairing this unit. First I definitely suspected the filter board. So against my own tuition I replaced the mylar caps in the circuit, still no good. Next rigged up unit using all channel 1 electronics and channel 2 EQ and filter board, I was so frustrated and just needed to know. Test showed 100% the filter board at fault. I continuity tested the channel 1 EQ filter board and found broken wire link between the 2 .047 .47 and R204. Long story short, always check all these links and just not from link to link, but component to component, as this is the weak point of this unit. Lesson learnt the hard way, LOL

 

[Johnny1234]

I hope I save some one all the BS I went through.

 

[Johnny1234]

Final question on this unit. Today I am installing the CM-27101 output transformers by Cine Mag. Where should the Faraday shield be connected. Any advice greatly appreciated.

 

[Johnny1234]

Would it lead to any problems if I put 1uf capacitors in place of the .78uf capacitors.
Many Thanks

 

[JohnRoberts]

Would it lead to any problems if I put 1uf capacitors in place of the .78uf capacitors.
Many Thanks

It depends on where in the circuit those 0.78uF capacitors are. If they are in an EQ circuit a different value will result in a different frequency.

Final question on this unit. Today I am installing the CM-27101 output transformers by Cine Mag. Where should the Faraday shield be connected. Any advice greatly appreciated.

Faraday (electrostatic) shields generally connect to a low impedance ground.

JR

 

[Johnny1234]

It depends on where in the circuit those 0.78uF capacitors are. If they are in an EQ circuit a different value will result in a different frequency.

Faraday (electrostatic) shields generally connect to a low impedance ground.

JR

Hi John, You have always been a great help. I am specifically looking at these two capacitors.

 

[JohnRoberts]

I am not a tube guy, so maybe somebody can give you a better answer.

My speculation is that 0.78uF is not a common value so was probably used for some specific reason.

JR