Ping Enzo

[clyde1]

Hi Enzo,
Awhile back you had posted info on your site (Ampix?) concerning modding the Ampeg SVT's protection circuit. I'm unable to locate it and have an SVT that needs this surgery. Could you help me with this? Thanks in advance.
Clyde(1)

[Enzo]

www.ampix.org is not my site, it is a part of this site. It is a palce for all AMpagers to post their photos.

I'll try to figure out what you want and see if I can find it and post it. Can you be more specific?

[clyde1]

Thanks for the reply Enzo. The protection circuit is overly sensitive on the new reissue SVT, shutting down the amp. You had posted an Ampeg factory fix addressing this issue. I read it about a year ago, knowing that an amp near me was doing this. Now the owner wants it fixed but I can't find the bulletin anymore. I guess the other alternative would be to remove it altogether like the original but this is rather unnerving to me considering the cost to replace components on this amp should something untoward happen.
Clyde

[Enzo]

Maybe you mean this?

[clyde1]

Thanks Enzo, I don't quite get the last character of the diode. Is it another "0" as in 1N3070?-looks like a backward "D", not sure what greek letter that is.

[Fragger]

http://music-electronics-forum.com/a...hp/t-6615.html

Enzo
03-29-2008, 07:26 PM
Look at the schematic. Those diodes are across R8, which is the input to the power amp. All they do is clamp the input signal. Same as clipping diodes in some guitar amp. The 1N3070 has a forward voltage of 1v. And a reverse rating of 200v. Current isn't an issue, but it is rated 500ma.

For all the world, it is a beefier 1N4148 (forward 1v, reverse 100v, forward cirrent 200ma) Both have a 1 second surge rating of 1A. Plenty for signal. And you are not likely to encounter in excess of 100v on a signal jack. Unless somethign with a hot chassis is connected there. SO a 1N4148 would likely work fine. However, Mouser sells real 1N3070 for 5 cents each...

[Enzo]

Never think up reasons not to be careful.

They are 1N3070 No "D" in it. Make sure the diodes are not already worked into your board. They included then in later revisions, so don;t look for kludged on diodes, they would be right in the copper tracework. Having two sets wouldn;t hurt anything, but why bother?

[clyde1]

Enzo, would you have the March, 1995 bulletin mentioned in the one you posted? I know this amp has had nothing done to it. Thanks again, invaluable
to all, you are.

[Enzo]

If I had it I'd have posted it. If I find it, I still will post it.

Your board may have never been modified, but that doesn't mean it wasn't made after the service bulletin, because at that point they reworked the board and those diodes are now a part of it.

[clyde1]

Finally got the amp, model# SVTAV, ser.# BJDQ00008, model year '03 or '04.
The bulletin states all models following the ser.#'s quoted would be updated, from 1997 on. I don't find anything in this amp to reflect that, no diodes, no cap, R1 on the audio output PCB (preamp board?) is 22K, not the 68K suggested in the bulletin. Thanks for the thread search Fragger, it had slipped my mind as I didn't get the amp until just now, almost a year later.

[Enzo]

The newer models incorporate the mods, they are not modded boards. Instead of tacked on, the diodes will just be in the row of small parts with everything else. PArt numbers may change with the versions. I have no idea what you have if different. ALl R1 does is set the DIFFERENCE between teh high and low gain inputs.

[clyde1]

Yeah the diodes are there, so I assume the cap is too (farther under the chassis). I wonder why this beast keeps shutting down then.

[Enzo]

That is the reason I never start a repair by looking for some sort of "known issue" list. People tend to think their circuit failure is some sort of systematic thing far too often.

When the amp shuts down, you have to determine just exactly what is shutting down. Then through classic troubleshooting, you look back through the circuit to see what controls THAT thing, then what controls the controller, and so on.

[clyde1]

Thanks for your patience Enzo. Apparently the amp has been working quite well the last few months, really intermittent intermittentness, but it's unreliable enough nobody trusts it to gig with, shame. I'll suggest they try a power conditioner for it as I don't really have a decent way to torture it in order to make it fault. If it was mine, out would go the protection circuit.
Thanks again.