Ampeg G-212 Hiss in Channel 2

[RWood]

Hi folks,

I've got a G-212 solid state amp on the bench and have a problem with a volume-dependent hiss on Ch 2.

Here is the schematic

Channel 1 works fine. Ch 2 uses the same pcb as Ch1 but it goes to the Tremolo and Reverb before going to the power amp. The hiss is volume and tone dependent, going away completely when the input is grounded out or volume is all the way down.



Here's what I've done so far:
New filter caps
Set bias on power amp
DeOxit all the pots and switches
Touched up all the solder joints on Ch2 pcb
Replaced 2 caps on Ch2 that were the wrong value - previous repair; had to repair the traces(hiss was present before I changed the caps)
Sprayed freeze-spray on each component on Ch2 board, No difference

Here are some pics:

Ch2 pcb:



Ch1 pcb:



Tremelo and reverb board:



Here's the whole thing. Thanks in advance!

[Enzo]

First suspect: noisy op amp - presumably the 4739.

[RWood]

Okay, I was just looking that one up. They do seems to be available.

[Enzo]

Before ordering something, try swapping the one with the other channel's IC. If the problem moves with the IC, it is bad. If the problem stays, it wasn't the IC.

Even if 4739 are not available, we can cobble a modern quad op amp in its place.

I looked, I no longer have any in stock.

[RWood]

Great idea ...... Thanks Enzo!

[g1]

Okay, I was just looking that one up. They do seems to be available.

From US or China? I would not trust any coming from China to be the real thing.

[RWood]

US seller on the bay, name is tube_buyer.
Appears to have a lot of surplus parts.

[RWood]

Right on the money, Enzo..... swapping the 4739s as you said, now channel 1 has hiss while ch 2 is fine! I will roll the dice and get some from the guy in CA.

Now a new issue just popped up. While I was playing through it the reverb started dropping out and then after 10 seconds or so it stopped altogether. I don't know much about reverb circuits. Where would I start to troubleshoot this?

[Enzo]

Reverb is just another amp circuit. it has two parts. One is the drive, a small amp that puts energy into the reverb springs - not unlike a small speaker driver. That is an NE540 IC. IS there signal on the drive cable?

The other half is the reverb return. Pull the cable from the OUTPUT jack on the reverb pan, and touch the tip of it. Get hum? If so the return is working. Of course the revrb control needs to be up for that. The return IC is another 4739.

Determine which half is the problem, then deal.

[RWood]

Thanks; It'll be a day or two before I can test that.

I did notice something last time I fired it up. The reverb worked at first, then developed some noise (white noise with occasional pop). Reverb still working, and then the noise rapidly increases before it then goes silent and reverb stops altogether. The whole process took about three minutes.

Turning the amp off for about a minute then back on replicates the scenario but it only takes another minute before the noise ramps up and the reverb shuts down.

[Enzo]

So once it quits, hit the reverb ICs with some freeze spray and see if they wake up.

[RWood]

No change when sprayed.

The two DIP14 ICs on the reverb board are mounted in sockets, and neither appears to be original.
The one on the left which I believe is IC101 is marked 709CG. Here it is:



The one in the middle is an 8-pin, soldered to the board, and I think this is IC102, the trem oscillator. LM566CN (may be original)



The one on the far right is in a socket, marked CA3046. Not sure if this is IC103 or IC104, as I am coming up one IC short on the board compared to the schematic. Need to get in there with a meter and suss things out.

[g1]

IC's used to be round metal cans.
On the right hand side of the schematic, in the notes, it tells which IC is for each designation. The 3046 is IC105.

[J M Fahey]

Those ICs must be original, first because they are old as the pyramids and currently unavailble and second for the `71 date code on one of them, others should be about same vintage.

[RWood]

IC's used to be round metal cans. .

And so they were! Thanks for the hint - IC 104 is indeed round, so all ICs on this board are accounted for.

As for the diagnostics on the reverb - after it cuts out, the "drive" is still alive. I took the RCA plug from the tank input and sent it to my bench amp and the signal was there.

There was nothing on the return plug, though; it was silent / no hum etc.

What to check next?