ampeg svt-3 pro bass problem

[Jazz P Bass]

I have a SVT3 Pro on the bench at the moment.
There is NO pop at turn off.
In fact there is a slight buzz as the power supply drains & the amp is still outputing a signal. 2 seconds elapse until the output relay falls out.
What is strange is the 18 second ON delay.
Is this a normal "let the tubes warm up" delay time?

[Jazz P Bass]

Is there any chance to get rid of it?

Does the amp "POP" when no signal is applied?
Did you try going in the "Power Amp In" jack?
That would isolate the "POP" to the preamp or the output section/ power supply.

[spy]

What us strange is the 18 second ON delay.
Is this a normal "let the tubes warm up" delay time?

My SVT3 PRO has the 18sec ON delay, too.

My amp pops without any signal at the input. Not even a cable is plugged in. It pops exactly when the output relay falls out.

EDIT: I tried with a cable plugged in at the Power Amp In jack and no pop!
Also, I tried Preamp Out with another amp and the pop is still there. Weird?! I thought it's caused by the output relay.

[Jazz P Bass]

Corrupt relay contacts?
If the board is out, temporarily jumpering the contacts may help find the problem.
Did you try replacing the 12AU7 tube? V2.
The one on my bench had a bad one.
It goofed up the 12AX7 before it. V1.
It did not kill it, but the waveform was screwy.

[spy]

Yes I replaced both V1 and V2 and pop is still there.

I don't think it's the relay because I took the output from Preamp out and the pop was audible at the other amp. Check my edits at the previous post.

[Jazz P Bass]

Yeah, must have been typing at the same time!
Jumper the FX send & return?
Monitor TP14 on the preamp schematic.IC2.
If the turn off spike is there, work your way back.

[spy]

yup! I did this too!

Don't know if this makes any sense but when I take off one of the preamp tubes (from the other board) so the input stage isn't working, I don't have a pop. Tried to replace these tubes but with no success.

[Jazz P Bass]

Monitor TP14 on the preamp schematic.IC2.
If the turn off spike is there, work your way back.

[spy]

Spike is everywhere!

[Jazz P Bass]

Everywhere?
Certainly not at the input jack.
If working backwards from IC 2 is not helping, work forward.

Is it on any of the power supply rails?
That may lead to a clue that something is dumping voltage.

[J M Fahey]

Check that D13, the 1N914 diode in parallel with the relay coil isn't open.
*If* it is, teplace it with a more robust 1N4002 to 4007; I see the 914 stretched to its limits there.

[Jazz P Bass]

Pretty sound call on the diode.
That may be what is "dumping" voltage.

[spy]

Thanks for that J M Fahey!

I must unsolder the diode to measure it! Anyway, I'll do it. With the coil in parallel it measures around 400mV both ways.

[spy]

Problem may solved!
Thanks to JM Fahey that pointed out the diode, I took off the PCB of the PSU and output stage. I checked the diode and it's ok but look what I found!


It's the pin of one of the big electrolytics you can see over here,


And, I was thinking to take off this PCB to check for cold joints from the beginning! I would have saved a lot of time!

Anyway, I'll refresh all the joints and hope that no pop from now on. Also, it's a good idea to replace that diode just in case. I have some spare 1N4007s.

[spy]

I have a SVT3 Pro on the bench at the moment.
There is NO pop at turn off.
In fact there is a slight buzz as the power supply drains & the amp is still outputing a signal. 2 seconds elapse until the output relay falls out.
What is strange is the 18 second ON delay.
Is this a normal "let the tubes warm up" delay time?

Mine (NOW) doesn't have too!
Thanks for pointing this out from the beginning so I started to search about it!
And thanks to be suportive all the time.

By the way, does anyone know how to set the bias? I have done all these it'll be great to set the bias of this amp.
I read somewhere that I must set it to read 25mV across each .47 output resistor. Is this right? I don't want to blow the output stage after all this.

Found it, SVT-3 Pro Bias Setting