Polytone 103

[mokomon]

I have a 1975 Polytone 103 guitar amp with one 15" speaker . It has some transistors that need replacing , couple of resistors out of range and some electrolytic caps in need of replacement. Anyone have anything to share from previous repairs that I might need to check on this amp that are maybe unusual ? I am just checking the wiring in it compared to the schematic I have , as someone has changed the wiring to the main electrolytics.

[52 Bill]

That one would be pre-chip power amp, so it will be a very simple standard design circuit. Most of the driver transistors will be obsolete, but can be replaced with modern types.

Watch for loose hardware as the cabinets will vibrate everything loose. I just had one that had a heatsink hanging by the wires and the screws floating in the bottom of the cab. Have the filter caps been replaced with smaller modern ones?

[mokomon]

Yes this is the model with discrete transistors in the power amp. I am in the process of changing all the electrolytics and one of the power transistors . I might even just change all the transistors , seeing as one of the output transistors is shot.I have substitutes . Someone has changed the big main filter caps already , I have yet to test them . I'm still trying to sort through the rewiring job they did on them, Crimp connectors....ugh .One of the 50uf caps on the power amp board had been exploded due to being installed wrong polarity , looks like.

[mokomon]

This is the output amp

[52 Bill]

The exploded cap does not look original to me. I think that the right side driver transistor is supposed to have the heatsink with the diode thermally coupled to the case.

Be sure to check all of the emitter resistors for the outputs and the drivers.

[mokomon]

Yes that cap is not original , and I removed the heat sink from the transistor so I could read the number on the transistor. I will be checking all the transistors , I found one already that is supposed to be 4.7ohm and it had drifted to 8ohm.

[mokomon]

The main caps in this amp ( 4000uf) are connected with leads that have the ends crimped on . This is not factory done , is it ?

[52 Bill]

The main caps in this amp ( 4000uf) are connected with leads that have the ends crimped on . This is not factory done , is it ?

Yes, that is normal on a lot of these amps. But you should check them to be certain that they are tight.

[Tom Phillips]

Is that exploded cap installed backwards? (I.e. reverse polarity)

[mokomon]

yes that exploded cap had been installed backwards during some previous attempt to repair this amp . One of the transistors on the preamp board had been replaced also and the base lead had solder on it but was not actually attached to the circuit board. I'm finding all sorts of small issues. Hopefully next week I can find more time to work on this amp.

[mokomon]

Anyone have a owners manual for a Polytone 103 ? I like to have as much literature on old amps as I can find.

[Tom Phillips]

Anyone have a owners manual for a Polytone 103 ? I like to have as much literature on old amps as I can find.

In case you have not seen it yet, there are Polytone schematics & links posted in the thread at http://music-electronics-forum.com/t34687/. No owner's manual but I did see mention of the model 103 power amp schematic.

[mokomon]

Yes I have all the schematics I need . Just hoping someone might have a manual.

[52 Bill]

Yes I have all the schematics I need . Just hoping someone might have a manual.

I don't know if I have ever seen a real owner's manual for any of the Polytone amps. We were dealers for those things back in the '70s and '80s and I don't remember them having any specific printed stuff other than the occasional schematic and sales literature.