M49 power supply problem

[James Nash]

I tried to replace the 220V power transformer in my M49 power supply
with a 110V power transformer, but as luck would have it, I seem to have
gotten a bum 110V power transformer... the new transformer seemed OK
when I tested it, but it shorted out and blew the power supply fuse the
moment I hooked everything up.

So, I put back in the old 220V transformer and rechecked the voltages
coming out of the power supply (using a 220V step-up transformer at the
wall)... the plate and filament voltages were both OK, so I hooked up
the mic again. No output... strange. I unplugged the mic and rechecked
the voltages coming from the power supply, only to find a clear problem:
the filament voltage is still fine--4Vdc, but the plate voltage is way
too low--should be 120Vdc, but instead it's only 1.5Vdc.

The fuse-blowing current surge caused by the shorted transformer must
have damaged some other component in the power supply, I guess ?

Anyone have a suggestion what would likely cause the B+ supply voltage
to drop from 120Vdc to 1.5Vdc ? Would a damaged electrolytic cap cause
this kind of symptom ? Without tracing through the entire circuit, does
anyone have a suggestion where I should start tracking down the problem ?

Thanks so much!

James

[Scott Dorsey]

James Nash > wrote:
>I tried to replace the 220V power transformer in my M49 power supply
>with a 110V power transformer, but as luck would have it, I seem to have
>gotten a bum 110V power transformer... the new transformer seemed OK
>when I tested it, but it shorted out and blew the power supply fuse the
>moment I hooked everything up.

If you disconnect all the wires from the secondary and only have the
primary hooked up, does it blow the fuse?

What is the DC resistance of the primary?

Did you put the proper fuse in? With a 110V supply, the thing is going
to pull twice as much current as with a 220V supply, just because it wants
to see the same power. So you will need a different fuse.

>So, I put back in the old 220V transformer and rechecked the voltages
>coming out of the power supply (using a 220V step-up transformer at the
>wall)... the plate and filament voltages were both OK, so I hooked up
>the mic again. No output... strange. I unplugged the mic and rechecked
>the voltages coming from the power supply, only to find a clear problem:
>the filament voltage is still fine--4Vdc, but the plate voltage is way
>too low--should be 120Vdc, but instead it's only 1.5Vdc.
>
>The fuse-blowing current surge caused by the shorted transformer must
>have damaged some other component in the power supply, I guess ?

No, my bet is that something that is pulling down the supply made the fuse
blow when you put the 110V transformer in there.

>Anyone have a suggestion what would likely cause the B+ supply voltage
>to drop from 120Vdc to 1.5Vdc ? Would a damaged electrolytic cap cause
>this kind of symptom ? Without tracing through the entire circuit, does
>anyone have a suggestion where I should start tracking down the problem ?

Bad cap might do it. Bad rectifier too. Or maybe you damaged a wire
on the transformer pulling it out and putting it back in.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[James Nash]

In article >,
(Scott Dorsey) wrote:

> James Nash > wrote:

> >Anyone have a suggestion what would likely cause the B+ supply voltage
> >to drop from 120Vdc to 1.5Vdc ? Would a damaged electrolytic cap cause
> >this kind of symptom ? Without tracing through the entire circuit, does
> >anyone have a suggestion where I should start tracking down the problem ?
>
> Bad cap might do it. Bad rectifier too.


Thanks for the advice, Scott! Turned out to be a rectifier problem...
one of the diodes was open. I replaced it and everything's working
properly now. Thanks for pointing me in the right direction!

James