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Thread: Ampeg G-212 Hiss in Channel 2

  1. #1
    Member RWood's Avatar
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    Ampeg G-212 Hiss in Channel 2

    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!


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  2. #2
    Lifetime Member Enzo's Avatar
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    First suspect: noisy op amp - presumably the 4739.

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  3. #3
    Member RWood's Avatar
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    Okay, I was just looking that one up. They do seems to be available.

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    Lifetime Member Enzo's Avatar
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    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.

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  5. #5
    Member RWood's Avatar
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    Great idea ...... Thanks Enzo!

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    Don't forget the joker g1's Avatar
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    Quote Originally Posted by RWood View Post
    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.

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  7. #7
    Member RWood's Avatar
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    US seller on the bay, name is tube_buyer.
    Appears to have a lot of surplus parts.

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  8. #8
    Member RWood's Avatar
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    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?

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  9. #9
    Lifetime Member Enzo's Avatar
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    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.

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  10. #10
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    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.

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  11. #11
    Lifetime Member Enzo's Avatar
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    So once it quits, hit the reverb ICs with some freeze spray and see if they wake up.

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    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.


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  13. #13
    Don't forget the joker g1's Avatar
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    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.

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    Old Timer J M Fahey's Avatar
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    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.

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    Juan Manuel Fahey

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    Quote Originally Posted by g1 View Post
    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?

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  16. #16
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    It looks like IC 103 handles the reverb return. It is the one marked 709 CG and on the parts list it is N5709A. So I guess they have the "709" in common.
    There is also a .22 cap, C121, leaving the op amp which I suppose could be leaky but it's a film cap so my wild guess is that the IC is bad.

    If that's the consensus here I'll look for a replacement. Would the RC4739DB work there? I've seen NTE909D listed as a possible sub. Other ideas?

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  17. #17
    Don't forget the joker g1's Avatar
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    Nothing funny with the footswitch jack or the pot?
    I'd go with the NTE part (make sure you get the D suffix), they show a listing at Parts Express: https://www.parts-express.com/nte-90...inear--nte909d

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  18. #18
    Lifetime Member Enzo's Avatar
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    It is just a simple op amp. It shouldn't be hard to cobble in a TL072 or even TL071 in its place.

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    OK But how does one cobble when the pins ain't in the same places?

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    Lifetime Member Enzo's Avatar
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    There is always the dead bug method. TAck glue the IC to the board upside down, legs sticking up...like a dead bug. Then short wires down to the appropriate holes in the circuit board.

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    Old Timer J M Fahey's Avatar
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    4739 was a revolutionary Op Amp ... way back then, the first ever *designed specifically* for audio and not instrumentation or missile guidance analog computers.

    It was used by everybody and his brother , including Kustom ans a host others, plus it was *the* Op Amp in Craig Andertonīs book.

    Problem is that it stopped being made and it had its own unique pinout, incompatible with others.

    Now we have many vintage nostalgia lovers and if I donīt remember wrong, at least one site is offering "converter" mini PCBs, where you mount a little modern SOIC dual Op Amp and proper pins get routed to proper DIP sized external pins.

    Serch for it, I will do the same.

    EDIT: hereīs one
    RC4739 uA739 MC1303 Drop In DIP Board Ultra Low Noise OP AMP NE5532 Upgrade
    not just a replacement, but an upgrade.
    personally Iīs trust this fresh made adapter more than any "presumed NOS" offering, specially if coming from Asia.
    Only problem is that *I think* the mini 5534 Op Amp comes unsoldered, I woukd only buy it if fully assembled, tested and ready to solder in the old PCB. So ask him.

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    Juan Manuel Fahey

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    Thanks for the ideas and the context. I got a lot more hits searching for RC4739 without the suffix.

    I did order a pair of them from a seller in CA. We'll see (hear) what they do.

    I also ordered a pair of NTE909D from a place in Seattle, Vetco, but just got an email saying they are out of stock. So it looks like I'll be dead-bugging the one for the reverb socket. Looks like the pinout on that one is different than the RC4739.

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  23. #23
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    Wrapping this up - the NTE part fixed the reverb. I got it from NTE Parts Direct, a very good experience from chatting online on Monday to confirm they had it, to having it in my hands on Thursday. I also got some 14-DIP sockets from them and a few other things.

    The RC4739DB came in today from CA and it does work. It was Made in Korea. It does not sound as good as the one working original part.

    The tone controls on this thing are impressive. The midrange, in particular, provides quite a range of sounds. The reverb - - meh, it's a little tank - - but the Tremolo can go really, really slow, which I like.

    Thanks, as always for the responses and the assistance. I've learned a lot about solid state amps from you guys that I would have never learned. Much appreciated!


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  24. #24
    Don't forget the joker g1's Avatar
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    Is that a Firebrand SG? Nice. Maybe not, looks like the headstock is black.

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    Member RWood's Avatar
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    Yeah it's a "faded". Everything I want in an SG (and a low price).

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  26. #26
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    One other thing I meant to ask. When I bought the filter caps I also got a set of four output transistors. I got MJ15003G from Mouser.
    I never installed them, though, as the output was fine after I put in the caps and set the bias. Is there any reason I should install the new devices anyway?

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  27. #27
    Don't forget the joker g1's Avatar
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    Nope.

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    cool...thanks!

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