Constant Hum from 1970's univox

[multaflor]

Hey Everybody

Ok I just got this old Univox solid state amp from the early 70's.  Its kinda in good shape, with a spring reverb and tremolo and 2 ten inch speakers.

BUT there is a CONSTANT hum when I turn on the amp, with or without a guitar plugged in.   Its quite loud.  I have attached a schematic of the U150RX which is close enough to my U150R model.

So far I have replaced the power supply capacitor (the 2200 uf and 35V one, {the schematic says 2000uf but that wasnt what the actual part said})  and cleaned all the contacts.

Does anybody have any suggestions to what it could be?  Should the amp become parts? THANKS

[Enzo]

The first question is this, is there ONLY the very loud hum, or is the guitar sound still heard along with it?

And futher, looking at he speaker cone, when the amp is turned on, does the cone move one direction and then stay there?  Is ther DC voltage across the speaker?

If the guitar sound is heard along with the hum, then your filtration is not working.  A bad cap could be responsible, but a good cap can;t work either if the traces to and from it are not intact.

If the amp output is blown, you get DC voltage across the speaker - disconnect the speaker and measure for voltage on the speaker wires.  You would not hear the guitar along with the hum.

[multaflor]

You can hear the guitar along with the hum.  I'll check the speaker cone and voltage tonight and get back to you. Thanks

[Joe]

The schematic shows a 1000uF output capacitor, so there shouldn't be any voltage across the speaker, unless that capacitor is bad (which could very well be the problem.) Replace that and see what happens.

[Enzo]

Oops, didn't read the schematic first.

OK, it is a single supply amp, that means there ought to be about 18v on the + end of that cap.  If there is more like 30 or close to zero, then the output stage has serious issues.

With the outputs separartly driven it is possible to have a blown one and still get a little signal I suppose.  But if you can hear guitar, I start to think the problem is more likely powr supply, specifially that 2000uf ccap.

[multaflor]

I just replaced the 2000uf capacitor near the power supply.  I am going to try to replace the 1000uf capacitor near the speakers next.  Thanks

[phatt]

Hi
Don't forget to check the 2 diodes on the PSU,, if one goes fut it will still work but with much greater hum.  ,,, At least I think that's what happens. 
Phil.