Ampeg SJ12-T Trem on the fritz

[Prairie Dawg]

Any of you folks ever work on one of these with an inop trem? It made scratchy noises for a few days and then quit altogether last night.

Thanks

[Enzo]

Got voltages at the trem tube? Is it oscillating?

[Prairie Dawg]

That's where I'm going next.

[Prairie Dawg]

There's voltage but nothing in the circuit that could be dodgy except a JFET in there that switches the vibrato on and off so I ordered a handful of them from Mouser. The low voltage DC to run the solid state stuff is OK. I'll give the JFET a whirl next. As it is a relatively easy amp to work on and the board is damage tolerant for us ham fisted types, I just went ahead and replaced all the resistors around the trem circuit. It seems to have cured the occasional crackling.

I do not think I am the only person who had this problem either.

[Enzo]

I kinda meant are the voltages more or less correct. DO you see about 80 some volts when the trem is off? Is the4re about 2.7v or so on the cathode? The schematic shows gate voltage on th JFET, hows that match yours?


No, I am sure other peaople have had the trem fail in just about any amp that has a trem.

Is the tube oscillating? Print says about 45VAC of trem oscillation on that tube plate, anything there?

every trem has two parts, the low frequency oscillator or LFO, and an interface to the signal path to allow that LFO to modulate the signal. The first thing to do is determine if the trem failure is due to the LFO or the interface. I mean you could have something as simple as the intensity pot went open at the top end. And an LFO failure could esily be caused by an open speed pot. And "open" pots include broken solder on their legs.


The JFET could be involved, sure, but find out, don;t just guess. Pull the tube. Now with POWER ON, and TREM enabled, go to the empty socket, set your meter to resistance and measure resistance between pins 7 and 8. That is where the source and drain of the JFET go. Now if the thing measures some low resistance like 200 ohms or something, AND there is -15v on the JFET gate, then the JFET is stuck on. But if you measure a high resistance, then the JFET is turned off as it should be.

Note the schematic calls out the gate voltages for that JFET over by the footswitch jack stuff. -15vDC ON, and 0v DC OFF. You get that? If those control voltages are not happening, the JFET cannot turn itself off to let trem happen. Does your footswitch make those happen? If the control circuits are not working, then neither will be the trem they control.

[Prairie Dawg]

I made a lucky guess when I ordered some J112 JFETs. I got this amp for personal use a while ago and the trem worked. There was the occasional crackling and popping which made me think that maybe some of the plate load resistors were causing it. As they're easy to change I went ahead and did that as well as freshening up the grid and screen resistors. A little while later the trem started making scratchy noises and then failed completely. The foot switch had no effect on it at all, installed or not.

The J112 cured the problem, but thanks to your advice I now know a lot more about how the system works in this amp.

[Prairie Dawg]

As a point of information the problem reappeared again, and I was determined to get to the bottom of it. I removed the back cover, dialed in some vibrato to the point where it was making scratchy noises and started chopsticking, poking and prodding. After a while it seemed that the end connected to the circuit board with a molex connector was involved. I could shut the vibrato off and on by tugging gently on the cable. At the same time, if I connected it up to the footswitch board and footswitch I could see that I was interrupting the power to the LEDs. So this morning having a little time to work on it I pulled out the main circuit board and and resoldered the molex connector attachments, and went over the board and gave a touch to any joints that looked dodgy, including a few on the ribbon connectors.

The problem seems to be definitively solved at this point.