General Safety Question: 70's Solid State Amp

rlh5599

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I picked up an Epiphone E-20-B 20 watt Solid State amp this weekend. (don't judge, it was really cheap). I works great, no hiss, pops or hums, but also no 3 prong power cable, and I'm fairly certain it has never been serviced in any fashion.
My question is: Do we perform the same types of capacitor service and power cable replacement as the tube amps?? I have yet to open it, and it runs fine and actually sounds pretty good, albeit a little under powered...
Thanks!
 

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Jon Snell

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It would be prudent to look at a service manual/schematic (if there is one left) and fit a 3 pole mains lead for your own safety, removing any death caps that are likely fitted.
The capacitors suffer the same as valve amplifiers.
 

rlh5599

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It does have a schematic under the amp in the top part of the cabinet. figured I'd check the schematic when I get ambitious enough to take it apart. and then plan on checking values etc just like servicing anything else that age..
Thanks!
 

Blrfl

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I found a photo of the schematic for the E-85 which is in the same series and probably built much the same way:

Screenshot from 2022-01-18 07-34-35.png


The internal voltages aren't very high (30 and what looks like 15), but three things stand out on the primary side of the transformer:

There's a death cap between what should be the hot side of the plug and the chassis. That needs to go.

The fuse is downstream of the power switch and should be upstream to minimize anything not protected if the fuse blows.

The power cord is probably not polarized. At least re-fit it with a polarized plug so the hot side (small blade) runs to the fuse. That will prevent any part of the amp from being connected to the hot side of the supply if the fuse blows. Better would be to install a grounded cord and connect the ground to the chassis.

If you have the chassis out to take care of those things, I'd at least disconnect the filter caps from the circuit and check to see if they're still in spec. Replacing them is worth doing if you plan to keep the amp awhile.
 

Gunny

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How's the hum? It would certainly be worth it to replace the power supply filter caps in addition to what was suggested by Birfl.
Does the tremolo circuit sound good? If it's not working or too slow/fast, replace the 3 caps in that area.
I'd replace all of the electrolytic caps. After that, you've pretty much done the best you can. If possible, connect to another, good, speaker to be sure the amp is sounding good. The original speaker is likely not the best after all this time?
 

rlh5599

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very little hum, even with a Squier Bronco Bass single coil pick up, pretty much no hum with humbuckings. No trem, it's a straight "20 Watt Bass Amp" with a single heavy duty 15, according to the wiki..
I'll plan on the 3 prong and re-wiring the on-off-fused area , etc... plus servicing the filter caps, etc..

Thanks for the responses..
 

Ronsonic

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very little hum, even with a Squier Bronco Bass single coil pick up, pretty much no hum with humbuckings. No trem, it's a straight "20 Watt Bass Amp" with a single heavy duty 15, according to the wiki..
I'll plan on the 3 prong and re-wiring the on-off-fused area , etc... plus servicing the filter caps, etc..

Thanks for the responses..
If'n it ain't broke, don't fix it. I sounds like you've got the right idea. Do the three-prong and while you're in there check for leaky caps and do a general cleaning.

Have fun and ignore any amp-snob tendencies.
 




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