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Gorilla TC-110 guitar amp works fine EXCEPT the reverb control introduces a loud

Started by psafloyd, July 20, 2018, 08:08:21 AM

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psafloyd

I picked up this amp for not very much as it wasn't working properly. From the description, it sounded like dirty pots so I took the plunge and bought it.

After giving it a thorough clean, the amp turns out to be not a bad amp – the 12" speaker undoubtedly helps – and all the issues were resolved. Except one.

When you turn up the reverb knob from off, a hum is gradually introduced. It becomes very loud at maximum, though I would never use reverb at that level.

The chances are I would never use the on board reverb, either, instead use EH Holy Grail Plus. However, I'm not planning on keeping this amp and would like it working fully to maximise the return as I sell these kinds of items to raise money for charity.

I cannot find a schematic – and I have looked – but I have attached some pics of the chassis and amp and here is a link to a piece of video demonstrating the problem.

https://youtu.be/kYj17XQkhdI

One thing I can tell you is the cable on the reverb is securely plugged into the tank and also the PCB. Whether they're the right terminals, I can't tell you.

Though I have done a lot of reading and YouTube viewing since my last post, I am an electronics novice and haven't a clue, so any tips gratefully received.

phatt

Sounds like the pickup from tank section is disconnected.
The cable may look intact but it's likely lost ground connection which is why it hums so bad. Similar situation when you unplug your guitar lead causing no signal *Open circuit* causing hum, really obvious with the gain high.

Just by gently wiggling the cable at the tank plug you may get signal.
If you get intermittent reverb then it's most likely the plug.
They are mostly small RCA type plugs and often break inside the plug. the plug looks like the common sealed type and not fixable so you may have to replace that plug.

The other possible stuff up is the input and output cables have been reversed this can also cause bad hum and little or no reverb sound.
Easy to check,, just swap them over.
Phil.


psafloyd

Quote from: phatt on July 20, 2018, 09:12:45 AM
Sounds like the pickup from tank section is disconnected.
The cable may look intact but it's likely lost ground connection which is why it hums so bad. Similar situation when you unplug your guitar lead causing no signal *Open circuit* causing hum, really obvious with the gain high.

Just by gently wiggling the cable at the tank plug you may get signal.
If you get intermittent reverb then it's most likely the plug.
They are mostly small RCA type plugs and often break inside the plug. the plug looks like the common sealed type and not fixable so you may have to replace that plug.

The other possible stuff up is the input and output cables have been reversed this can also cause bad hum and little or no reverb sound.
Easy to check,, just swap them over.
Phil.

Thanks, Phil. Yes, they are the typical RCA type plugs on the reverb. I'll check those out as you suggest.

I had thought to switch the input/output cables, but the lengths suggested they'd been installed that way. I won't assume and double check.

Might it be at the other end? The revert cable terminate in 2 x 2 spade fittings that attach to the PCB. I suppose someone could have switched them by accident.


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psafloyd

Quote from: Enzo on July 20, 2018, 10:21:58 AM
Or the coil on the reverb is open.
OK, not sure what that means, but I'll look it up. Thanks again.


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Enzo

On each end of the reverb springs is a coil.  That is called a transducer.  The two wires to the jacks are coming from that coil at each end.  If the coil wires break, you can detect that with your ohm meter.

SO with the cables removed from the reverb unit, measure resistance across each jack.    I don't know what resistance you will find, it could be 2 ohms or 200 ohms.  But what we don't want to see is an open indication.  That would tell us a wire is broken inside the reverb.   It will either be good or open, it won't be "wrong".

psafloyd

Quote from: Enzo on July 21, 2018, 08:50:40 PM
On each end of the reverb springs is a coil.  That is called a transducer.  The two wires to the jacks are coming from that coil at each end.  If the coil wires break, you can detect that with your ohm meter.

SO with the cables removed from the reverb unit, measure resistance across each jack.    I don't know what resistance you will find, it could be 2 ohms or 200 ohms.  But what we don't want to see is an open indication.  That would tell us a wire is broken inside the reverb.   It will either be good or open, it won't be "wrong".

Thank you, Enzo.


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psafloyd

Enzo and Phil,

Many thanks for your advice. I took the tank out and checked the cables attached. They did.

With the reverb up, I bounced the springs and sound indicated the out was OK, so I checked for broken connections or bad solder joints. All looked fine.

I ran my multimeter over the in and out and got 50 and 200 ohms. So I disconnected the cables from the tank and gave the connections a really good clean, including running some 0000 wire wool around the outside portion of the connectors on the tank and what do you know? This was where the problem lay.

Once I had a better connection, not only did the reverb work, the hum was almost totally removed.

Thanks for the helps, gents. It gave me some focus and I now have a far better understanding of these units, now.


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phatt

Yes those RCA plugs get corroded over time and cause problems.
Glad to hear it worked out well. :dbtu:
Phil


dlbraly


psafloyd

De nada. I'm sure I won't be the only one faced with such problems.