fixing unicord stage 25 amp

[Mizno]

i have a unicord stage 25 solid state amp that i dug out of storage recently when i plug it in and turn it on the power light does go on but no sound is produced. i have eliminated the jacks, fuse, and the speaker as a problem... what would be the most common cause for a problem like this? any help, suggestions are very appreciated

[J M Fahey]

Hi Mizno. To begin with, google around a lot and try to get the schematic, otherwise it will be very difficult to help you.
Just for starters, check the speaker by desoldering one of its wires and touching the speaker´s terminals with a 9V battery; it should "click".
If it does, reattach the wire and power the amp, you should hear some kind of click or pop in the speaker.
Good luck.
J M Fahey

[teemuk]

...Also, those Stage amps were just an alternative brand name of Unicord products so you might get lucky finding a schematic to a similar circuit by checking out the schematics of Univox and Westbury amplifiers. All those were made by Unicord. They established that Stage brand because Univox was regarded as a low-quality and cheap brand and by introducing a new brand they could offer a product that was seemingly more prestigeous.

As far as I know, you won't find any schematics by searching "Stage" (I tried once) but you can find a few Stage amp schematics if you search for Westbury or Univox schematics. Despite the different brand and cosmetic appearance those amps were often 100% the same under the hood.

Some reverse engineering is also a very valid option. Those are very simple circuits and sketching out a few areas won't take longer than half an hour. Some of those amps have very transparent circuit boards too, in which case tracing out the circuit should be a very easy task because you can see where the traces run from components and you don't have to turn around the board constantly to follow through the circuit. Sketching out the schematic by yourself is also the only way to get a 100% accurate circuit diagram (assuming you make no mistakes in drawing, of course). There is simply no reason why you would need to find a factory schematic to fix these things. We are not talking about some miniature SMD-part thing with thousands of components and hyper thin board traces.

Is it one of those circuits with a FET preamp and a FET input stage in the power amp? If I remember right, all those amps were almost identical. If you can find one schematic you can guess 90% of rest of the circuit's architechture and after that verifying your guesses is a quick thing to do.

You likely should try to focus on the area you troubleshoot and systematically eliminate all the basic blocks, power supply, preamp, power amp etc. Maybe you can try feeding some signal to the volume potentiometer and see if it comes through from the power amp (which would suggest a preamp problem) - or something along those lines. And more importantly, since the amp seems to have a fault, limit its current until the fault is located and fixed. Power light may come on while you are cooking some components. Have you opened the amp up?

[Mizno]

ok i got it to work... for a few days it shorted out recently tho and the fuse before the transformer keeps blowing when the amp is turned on would this indicate a bad transformer or yet another problem? the first was a bad track between the two input jacks and the rest of the amp.