MAESTRO PHASER in SG SYSTEMS CMI 2x12

[t7mackie]

I got a good deal on a SG Systems CMI 2x12 amp recently from Craigslist. It's a pretty funky amp. The overdrive tone is crap but it has wonderful clean tone, it's built like a road case and it is ungodly loud - and I own a Hot Rod Deluxe! Loud & clean. It's a perfect pedal amp.



Anywho, it's in relatively good shape, it just needs some TLC. I looked under the hood tonight and I appear to be the first human being inside in 45 years. No leaking caps and nothing appears too out of order.

Except... the built in Maestro Phaser doesn't work. The only sign of any life is when I flip the, "low" switch there's some static.

I think I'll start by replacing the cap's, transistors & IC's - in that order.

Any guidance is greatly appreciated. There are schematics for the amp on the gooogle but I'm not sure the available schematics for the Maestro are a perfect fit for what's inside the amp.

Here's a pic of the Maestro PCB:



I looked underneath the PCB and other than a couple spots of what appears to be flux at a few of the thru-hole points nothing jumps out as suspect.

Thanks!

-t

[iainpunk]

i think that is the accurate schematic, but without a look on the other side of the board, we can't be sure.
have you tried setting the trimmers?

although they might not look like they are bad, electrolytics can be bad without visual badness.
might want to start replacing those...

(welcome to the forum, i'm not sure if i have seen you around before, so if this is a repeat, ignore it)

cheers, Iain

[Mark Hammer]

There was one of those for sale...cheap...not far from me.  You got the lucky one, with only a phaser in need of attention.  The one near me was missing functioning power tubes.  I looked those up and when I realized replacing them was going to more than double the purchase price of an amp I didn't really need in the first place, I passed.

Like any FET-based phaser, you're going to have to fiddle with the trimpots to get it going.  Happily, Small Bear has 2N4303 equivalents available, if it comes to that.

[PRR]

"doesn't work"?? No sound, or just no phasing?

> start by replacing the cap's, transistors & IC's - in that order.

Signal tracer first!!

Yes, the bias could be whack (but you say nobody has been in there?) and caps may be past their best-by dates. But wanton part-swopping is usually a poor plan.

[idiot savant]

These are really cool amps!

I've worked on several of them. There was a backline company in town that had a ton of them, so there's a bunch of them floating around here.

They originally used a pair of 8417 tubes for 100 watts! Though it's common to see them modded for 7027, 6550 and other output tubes since the 8417 tubes are super rare.

They had a unique footswitch connector and pedal setup that did speed switching for the phaser, and another for fuzz on/off.

schematic: https://www.prowessamplifiers.com/schematics/manuals/SG100.html

I would look at the power supply first. All of the ones I've seen needed new caps. The diodes, and zeners for the 12V rails are worth cheking as well.

If I remember right, I had to replace the rocker switches on one of them, and the big rotary switch on another. Though that was probably just due to the heavy stage usage of the ones I've seen.