A hello and an SVT assistance request

[kent]

Hello gang. This is the first post I have made to this forum in a few years. It's been so long that my original email address that I registered with is gone so I had to reregister. In any case this forum was a great help when I was starting out building amps back before the kids chewed up the time.

Here's my issue. I have a friend's newer SVT Classic with no output that he wants me to look at. The power amp section works fine as I can plug into the effects loop in with a test signal and hear it working. It has a nasty hum from the pre section. The tubes have all been popped into a different amp and tested out fine. When the amp is switched off it starts to work OK as the caps drain. I don't see any signs of filter caps blowing. The closest schematic I can find is the old SVT stuff on schematic heaven and the like so even if I check voltages on the tubes I don't know where they should be.

If you folks would have any suggestions on where to go from here I would be thankful.

[PRNDL]

I'd begin by testing the power supply electrolytics. Maybe the last ones are going.

Then measure (and post) all the tube voltages. There may be minor differences in bias, but the voltages should be similar. Let us know what the tube is used for (preamp, phase inverter and type, reverb, etc.)

Also check the connections on the effects loop. They often get dirty or the solder pulls off the board, which causes all kinds of problems.

[kent]

Thanks for the reply. Here's what I've got.
Tube 1:
pin 1 - 212V
pin 2 - 1.49V
pin 3 - 3.41V
pin 6 - 211V
pin 7 - 0V
pin 8 - 1.63V

Tube 2:
pin 1 - 330V
pin 2 - 67V
pin 3 - 107V
pin 6 - 219V
pin 7 - 2.2V
pin 8 - 3.98V

Both of these are preamp tubes (since I've verified the power amp section is working fine) and the heaters are working OK. The closest schem I've found on the web with voltages is here:
http://www.kbapps.com/audio/schemati...svtpreamp.html
Obviously SVT's have changed a bit since then but if you take the second channel and monkey with the where the cathode follower is I can see where the voltages I got came from.

Now the question. As far as the filter caps for the preamp section are concerned what is the best way to check for bad/leaky caps? I've often wondered about this.

The effects loop is working OK.

Thanks.

[Enzo]

Oh, contact Loud Technologies and ask for the SVT Classic schematic set. Here is the preamp.

If the filter caps were going, I'd expect the power amp to hum as well. If the higher B+ nodes are smooth, hum won;t reappear on the lower nodes.

If you want to check for filter cap function, set your meter to AC volts and measure the B+. That will ignore the DC and show just the ripple.

Does the master volume turn the hum up and down? That is an important question.

The heaters run on a DC supply. Is that supply clean?

The preamp feeds its signal to the effects loop circuit before it enters the power amp proper. The loop is on the power amp board. There is an IC driving the loop. It is not in the return circuit, but if that IC is screwed uo it COULD insert hum. Check the +/-15VDC rails the ICs run on.

[kent]

After a week of family stuff I'm back at the problem amp. Here's what I've got so far:

+-15V supply to the IC's are OK.
Filter cap C1 has .002V ripple on my meter (in other words not much).
Heater supply has .037V ripple (also not much).
Hum does not go up or down with the master volume control.

As stated earlier the power amp section works fine when I put a test tone into the power amp input section of the effects loop. Although the hum does not go away when I do this. Also there is no signal from the preamp out when I run a test tone through the input. So still no output.

Any thoughts?

[Enzo]

So the signal leaves the preamp where that 0.4v point is on the preamp sheet.

Then it goes to the power amp board, through the loop jacks, and back to the preamp. From there it is routed back to the power amp input. SOund right?


You get no signal from the preamp out. If none gets there, then it won;t get into the power amp either. You didn;t mention what was happening at that IC that drives the loop send. Any DC on pins other than supply? That is IC2A. Is there DC on the preamp out, the effects send or the effects returns jacks?

Plug into the effects return, does THAT pass signal to the power amp? Does signal come out the effects send? There are cables connecting those two boards - they merit examination.

And then basic signal tracing. APply signal to the input. Now is there signal at that 0.4v point at the end of the preamp? Is ther signal at the master control? Is ther signal on the EQ controls? At the gain control?

How about the DC voltages around that V2 cathode follower at the peramp output?

[kent]

Bingo!

DC on the preamp out. Checked the opamp itself and it's there on those pins (but not on the pins for the IC2B section). Pulled the preceding tube and found no DC before the blocking cap that separates the two or on the power amp in after putting a plug in the jack to disconnect it from the preamp section - which isolates the problem to IC2. Found a spare TLO72 and popped it in. Voila - she works! The hum was mostly a badly biased power amp. Thanks greatly for your help!

kent