Tubetech MP1A Phantom problem

[madreza]

Hi Guys .... I have a phantom problem on a MP1A
in fact it reads 28-30V
tried resoldering the PSU, no result ....
any idea ?

 

[SSLtech]

Reads 28-30 volts...WHAT exactly?

DC?

AC?

Assuming you mean DC, I might hazard a guess that you're reading an average and that you have a "clipped ripple" that has lots of dips below 48V.

Switch it to AC, and I bet that you have a high reading also...

If that's the case, you need to replace  a lot of electrolytics.

 

[madreza]

Yes it was DC , and when reading AC I have 72V

 

[gyraf]

I think the early version MP1A uses the 48V for both the tube heaters (in series) and the P48. Which means that there's relatively high load on the 48V - and this is probably the most common fail point of the early MP1A.

Check rectifier, pass transistor - and replace reservoir electrolytic.

Jakob E.

 

[madreza]

Reservoir  electrolytic for the 48V is the 2200uF 63V , right ?

 

[mjrippe]

I don't have the schematic, but that sounds correct.  Caps for anything else will likely be much lower or much higher voltage (25v or 450v).

 

[madreza]

thanks guys for helping
I changed the 2200uf and didn't change anything
I'm gonna Check the rectifier ....

 

[usekgb]

You should definitely NOT have 72V AC on the phantom power line!  Time to examine the power supply.

 

[madreza]

Thanks guys

So, I changed the 2200uF , the rectifier and the T4 on the schematics ( bc337 )
Still have 35V DC. ( didn't have time to check anything else :/ )