Ampeg SVT 3 PRO fan speed question

[Randall]

SVT-3 PRO comes in for distorted low volume, but seems OK when cranked. Sounds like dirty switching jacks to me, so I give it a good cleaning, and man were they ever fithy. Thinking that took care of it, I moved on to the second issue, no DI output. I had tuner out, but no DI. What I did notice however, is that when I plugged into the tuner out jack or the 1/4" DI out, the cooloing fan went into high speed, and slowed right down when removed. I started tracing back from the line out board, and noticed one of the connectors was installed one pin to the right of where it was meant to be.

So, looking at sheet two of the schematic where J5 on the main board mates with J8 on the line out board, it was connected as follows:

J5 pin 1 (16v) to N/C
J5 pin 2 (-16v) to J8 pin 1 N/C as drawn
J5 pin 3 (pre EQ send) to J8 pin 2 N/C as drawn
J5 pin 4 (ground) to J8 pin 3 (pre EQ side of switch)
J5 pin 5 (pre EQ tuner out feed) to J8 pin 4 (ground)
J5 pin 6 (post EQ feed) to J8 pin 5 (tuner out jack)
J6 pin 6 (post EQ side of switch) to N/C

So I see where the tuner out jack was passing the post EQ feed intended for the switch, and the other signals were either grounded or dead ended. But what I can't explain is why the fan was going on full when those above mentioned jacks were engaged. I don't see a connection?

ampeg_svt-3-pro.pdf

page 10

[glebert]

So I don't even get why the +/- 16V even goes to that header. Those voltages are used in the fan speed control but don't see the cause either. Is the ground on the break out board actually ground or does it have to go through pin 4 on J5?

[Randall]

I'm a little unsure about where ground is on this..

[Enzo]

The +/-16 is used all over the amp, not just the fan control.

I have no idea why they ran them out to that jack board. Perhaps in other designs they used a couple op amps instead of a transformer to drive the low-Z XLR out. Or wanted it to be available in case they did that in the future.

Signal ground is on pin 4.

Earth ground and power supply common meet on the power supply page.

I am unsure where signal ground joins earth and power common.

[J M Fahey]

Yes, the dual Op Amp idea is most reasonable (and practical and effective) BUT you know what the Marketing Dept said: "hey, the older the Technology the more it sells!!!" so let´s add a "Real Transformer" [tm]
"Will it be too complicated to finish the head in green Leather and polished Brass hardware?"
What about real gaslights as pilot lights, instead of cold clinical buzzy SS Leds?.