Carvin 1981 XT-112 (X-Amp) Excessive DC Ripple?

[LessPaul]

Short version:
I'm chasing an intermittent volume change on drive channel, which is switched in via a 5 volt relay. The relay seems to engage properly and both NO and NC contact resistance is negligible. However, the relay activation applied voltage is only 3.7v with over a 1.5VAC ripple. The voltage is derived via a half-wave rectifier off the 6VAC heater circuit into a 470mfd cap and is used to drive two LEDs and the 5VDC relay. The ripple seems excessive to me, but I have little experience with such setups. A few electrolytics are on the way, but I wanted to know if this actually needs rectifying. <-- Pun not intended, but fitting.

Long version: Curses upon Carvin. There have been several entirely different circuits sold under the catalog number "XT-112" with the vast majority not matching this amp. This was the first year (or year and a half) of X-Amp production, easily identified by the single input jack. Rumor has it that this design was too close to the Mesa Boogie Mark series and that Carvin was sued, thus the switch to an entirely different design. Dunno if that is so. The best schematics I have match about 95%. In addition, there are numerous jumpers and other obvious (hopefully) factory mods, and I'm really not wanting to circuit trace this darn thing. I'm sure this is the rule of the road for amp repairs, so I'll stop whining now.

[g1]

I would just hardwire across the relay terminals that connect in 'drive' and see if the problem is solved.
If not, you can carry on looking for the fault elsewhere.

[LessPaul]

I would just hardwire across the relay terminals that connect in 'drive' and see if the problem is solved.
If not, you can carry on looking for the fault elsewhere.

Good idea. Thank you.

[Jon Snell]

A schematic would help but 1.5volts AC ripple seems hight to me.
I would replace the 470uF capacitor.

[Delta362]

See if the attached schematics help in your case.

Several years ago I fixed a Legacy 1 amp head that had a channel switching issue after about 45 minutes of playing. In that instance, it turned out to be a failing 7805 regulator.

[LessPaul]

See if the attached schematics help in your case.

Thank you for posting these. These are the same as what I've been trying to work from, and are the closest to what I have. I've started chasing out the wiring and drawing a schematic, and there are major differences.
The motherboard of my amp is marked "Carvin X100-b". This is definitely not the amp later marketed as a 100B, but instead a derivative of the design you posted.

The one input, four volume control (Rhythm Vol / Master and Lead Drive / Master) and relay switched amps don't appear to have been made for much over a year, and sadly, I've been unable to find a schematic that matches this 'b" version board. To make confusion even greater, there are places where it appears components have been removed, and also a few cut traces. Whether these are factory or modifications by an owner are hard to tell.

Clearly, I don't need to dive this deep for the above-mentioned "drive" issue. In fact, I believe that simply exercising the relay has resolved that issue. But it is my own amp, and I'd really like to make sure all is as it should be.

[Delta362]

Hope the caps help out with your ripple. That circuit doesn't have much to it.

For curiosity sake, did you compare it to the slightly newer 4 input version schematic to see if the mods were leaning towards those changes?

I've attached a few other variations of the X amp schematics. While X-Series Amp Schematic.pdf is a slightly newer schematic, it does indicate that it covered some variation of the XT112. Perhaps yours was headed in this direction.

[LessPaul]

For curiosity sake, did you compare it to the slightly newer 4 input version schematic to see if the mods were leaning towards those changes?
.

Yes, I have compared them, and unfortunately, other than the layout of the control knobs on the front panel, they have very little in common. I'm starting to get a pretty good idea what I've got here. There are differences in this amp from the reference schematic, but the biggest issue is several questionable user mods. My goal is to undo these and try to get back to as factory as possible.

For example, the entire volume boost circuit, which would bypass the 47k resistor tying the mids tone pot to ground, was deleted and the pot hard-wired to ground. The boost switch and the deleted transistor were re-purposed to switch in part of the drive circuit chain. And there is clearly more. All in all, it isn't really worth the effort I'm investing, but I've taken it on as a challenge. I'm a man on a mission!

I do very much appreciate your efforts. Thank you.