Epiphone SC210 - internal damage

[Jodah]

Hi SS Guitar! I'm trying to learn a little about amplifier repair and figured this would be a good place to start...seen lots of helpful information here in the past.

I recently bought a yard-sale special Epiphone SC210 2x10 combo amp in the hopes of using it as a project amp to learn some basic repair techniques. The amp will power on, and all LEDs on the front (channel switching, chorus, etc) will turn on. The speakers have a quiet hum that's amplified slightly by turning the onboard chorus effect on...it doesn't sound like a bad amp, just like one without any sound coming out of it However, it refuses to play anything from either input ("High" & "Low"). There's no popping when I insert cable into the jacks, and the headphone jack doesn't allow me to listen in..there's simply no noise.

I popped open the chassis for a look-see and found that two of the small three-post electronic components had come loose from the board. They're right near each other, located in brackets marked "FET1" & "FET2". The leg posts are still plenty intact, they're just not attached to the board (it looks like the result of a poor solder joint; the amp is made in Korea, after all). I tried to resolder them into place and didn't have any more luck in getting guitar sound to come through the speakers, though they still hum just like any dormant amp. I also checked the jacks, on the off chance they were damaged. They weren't...they work fine.

Can anyone offer me some advice on where to go next? There's no obvious damage other than the FET1 & FET2 components being loose....do I just need to try a cleaner solder joint on them in particular or is there another component with the same symptoms (powers on, no playing sound, plenty of dormant sound)? Remember, I'm really trying to learn how to do this and how to know without getting a second degree in an engineering field, so if you've got ANY general tips or tricks too, I'm happy to hear them.

Thanks in advance!

Lex

[Enzo]

Can we come up with a schematic?  it doesn;t seem to have an FX loop, so that won't be the trouble.  Inspect your controls closely - are any cracked across their wafer?  And check the input jack solder.  DO the volume and EQ controls and channel switching affect the SOUND of the background noise at all?  If so, that tells us the noise signal is passing through the amp, not coming from the end of the processs.


I would point out to you that soldering is not a very difficult skill to master, in particular if that is what you do all day.  Korean people are just as capable of it as someone on this side of the ocean.  Even in this country, production solderers are not hired for their electronic knowledge.  it is a repetitive task, like sewing jackets together, or putting bags of potato chips into shipping boxes.

Furthermore, when things are on circuit boards, they are not usually hand soldered anyway.  Boards are usually soldered all at once on a wave soldering machine.  And to be honest, the wave machine will run about the same whether the guy running it is named Wong or Bubba.

[J M Fahey]

Hi. Is this the beast?  http://16streets.transverserecords.com/index.php?topic=3730.0
If so, I have a couple of doubts.
To begin with, it seems to have an effects loop, an usual suspect.
What I don´t get is the logic of the Tweed/Vintage (?) look with a *very* SS stereo chorus amp.
Oh well, maybe they were overstocked with Tweed and just had to use it.
More important than that, I wonder about the "three post thingies", can you supply a couple pictures of the board and them in particular?
What´s written on them?
I don´t even ask for a schematic finding it an impossible task.

[Jodah]

Hey all,

Thanks so much for the responses! I've done a little work on the amp so far, trying to research as much as I can. I'm REALLY starting from scratch here lol

Enzo, thanks for all the info. I'm not very familiar with any sort of mass production technique, all I have to go on is reputation, and when I open an amp and find components hanging freely in the wind, I get frustrated and blame the first thing I can see. Certainly, no offense meant.

J.M., yes, that is the amp (I actually have a silverface version, but it's identical in specs). It does have an FX loop. I found out that the "three post thingies" are in fact transistors (FET = "field effect transistor"). I was able to remove the old ones...they had "K 9E K30A Y" on them. I replaced the two transistors with new ones from Radio Shack (non-switching models). However, the amp still won't play. I've figured out that the problem is somewhere in or around the inputs. When the amp is plugged into power and turned on, I can actually touch caps inside it and get a local radio station playing through the speakers, and I can flick the reverb tank and get a nice "sproingy" sound. The volume and EQ controls don't affect the sound, though the input jacks don't provide even a popping or clicking...there's simply nothing. When the amp is powered up, I can run my hands along the bottom side of the PCB and get all sorts of fun feedback and popping noises, but no input sounds from the jacks. All the solder joints are in perfect condition, including those at the input jacks.

I figure there's something in the initial section of the amp, near the jacks, causing the problem. I suppose it could be the FX loop...I have tried to plug another amp into the effects "send" to see if the inputs are processing that far, but I can't get anything out. With the way the little "radio" trick (touch a cap, get an AM signal) works, though, I can say that the speakers are in working order and that it's definitely a problem getting guitar sound into the amp, not amplified signal out of it.

I haven't been able to find a schematic. I'll check all the wafers for cracks, that's a start. Anyone have any other suggestions?

[teemuk]

Not really.

I'd start from thorough visual inspecting that solder joints are ok – especially in components under lot of wear (the heaviest ones, those that are touched and those that heat up the most).

If no luck, I'd start sketching the schematic (unless Epiphone/Gibson can provide one), bit by bit, while doing systematic troubleshooting on components.

Since it sounds like a preamp failure you might try plugging a signal generator (e.g. computer), feeding sine wave, to the input  and then follow the signal path (with a scope or multimeter) from the input stage up to the point where the signal disappears. That should find the stage that has the problem. Tracking the signal path that way might speed out the troubleshooting process.

I'm also quite sure that everything "upstream" from the reverb injection stage is ok.