Epiphone Valve Junior Mystery

[marc61]

Nice little amp but, all of a sudden it's not working. Tubes are lit, speaker is working however no sound. Opening it up reveals only one flaw. This red wire from behind the speaker input which is not connected but does not seem to have a soldered base it was disconnected from .

Not sure if the pictures will give any insight but, suggestions are appreciated.

[johnallg]

Marc, even zooming I can't see for sure, but I believe where the black wire is soldered inside that white outline on the board there will be another hole and the red would go there, I would think. Possibly they are marked - black and + red?

Does your camera have a closeup setting so you can take a close pic of this area?

[jps]

It is pretty hard to really see what is going on. Can you unscrew the jack from the chassis and show the other side of the circuitboard the jack is mounted to?

[beatlefreak]

From a website concerning mods to the Valve Junior:

Output impedance switch

The unused red wire coming from the output transformer in the Valve Junior is actually for 8 ohm output. Drill a hole to the right of the speaker jack, install a SPDT toggle, connect the tip of the output jack to the switch wiper, and the red and brown wires to the two other lugs on the switch. This allows you to match the output impedance of your amplifier with that of an 8 or 4 ohm speaker cabinet by flipping the switch.

[jps]

Unfortunately, that does not answer why his amp doesn't work now.

[schoolside]

Did you check the fuses on the transformer lines?

[beatlefreak]

Unfortunately, that does not answer why his amp doesn't work now.

No, but it does explain why the red wire isn't attached.

[jps]

I'm sure Marc appreciates that.

[beatlefreak]

Jeff, as he was thinking the loose wire is a flaw, I'm sure he does appreciate knowing that it's not.

Here is a schematic I found of the Valve Junior online. It shows some mods that were done, but does show the basic circuit path. This can help you, Mark, troubleshoot your amp. It's a fairly simple amp, with a two stage preamp and one power amp tube. Use an ohmmeter (with the power off) to trace circuit paths and check the resistor values. Also ohm the primary and secondaries of both transformers. This will be a good place to start, and if nothing else, will eliminate components that are not the problem.

[marc61]

definitely helps to know the red wire is not an issue. I found the same schematic. will probably check over the next few days. shame as I hope I can fix it myself, because it's not worth taking to a shop to repair.

[schoolside]

Hi Marc, either you didn't hear me or you are ignoring me? If you look at the wires around the transformers, you will see a a few? inline 1amp fuses. I would check those first. Second possibility is the output power tube, being lit does not mean it is working.
FYI: the caps in the chassis are charged with about 350 volts sooooooo, I will suggest you put the chassis back in the amp and check the stuff I mentiond first because I think you are not ready to deal with the inside!

[beatlefreak]

Dwight's right - Check any fuses you see first. An ohmmeter should show a dead short across them. Probably a good idea to pick up a spare set of tubes, as there's only three. You can swap them out, one at a time, and at least eliminate them as the problem. As for the caps, they can be discharged - see here.

Basic rule of troubleshooting - When the source of the problem is not apparent, eliminate things that are not the problem to narrow it down.

[soundmasterg]

The usual way to troubleshoot this would be to inject a signal into the input of the amp, with an oscilliscope monitoring the output of the amp, and trace the signal through the amp stage by stage. In this amp there aren't many stages and no phase inverter so it would normally be quite simple to find the problem. If you don't have a scope or a signal generator, or a way to inject signal into the amp, then it makes it a bit more difficult. There are tools you can make with a cap, some wire and a speaker to have a sort of signal tracer, but you still need a way to put signal into the amp. I've got one of these amps too nad plan to mod it at some point, but bone stock it sounds ok and it works well, or at least mine does. Have you tried different tubes yet? That is the first thing to try.

Greg

[marc61]

Still working on this as my son expressed interest in getting it working again. I checked the fuses. They seemed ok (which is to say not blown)

Someone told me there's a trick with a 9v battery to see if the speaker is working. Anyone know this?

*edit - got it - according to the test, speaker is in fact working

[jps]

Disconnect the speaker from the amp and put the leads on the battery,you should hear a somewhat loud "click" and see the cone move in or out depending on which way you put the leads on the battery (positive to positive should make the speaker move forward/outward from the cabinet).

Hmm, I see Detective Seligman figured it out.