D&A PHANTOM DUAL POWER AMP "BLOWING A/C FUSE "

[TIGERGRIZ]

Hi guys, /gals, I appreciate any help in advance, I am good with a soldering iron and with google and I would take this amp to a certified repair guy , but the one in my area is so busy he does not reply, so all I am asking is where to start to check the circuit for trouble, history, the amp was running fine, a little low on power so I bought all nos tested 100% 6LQ6 Tubes, installed them and sizzle pop blew main power A/C fuse, so I removed all tubes, looked for damage bulging caps, burnt diodes ETC. could not find visual damage, so in goes new 30 amp A/C fuse all tubes still removed, and when powered on pop goes the fuse, so now I have checked the actual power switch to see if it went to ground when flipped on and no, nor do the wires down stream as far as I could understand how to trace, the amp does have several shady areas I could see, like wires going through a hole under the transformers with no protection, or restraint but it is so tight under transformers I cant see if any wires are making contact through frayed insulation, it appears not, I have a couple good multi meters, but I am not a tech so any next steps from a person of experience would be greatly appreciated,
I take a lot of pictures as my eyes are not what they used to be, helps me magnify things, hoping it helps, oops also the ending 3343 jpeg pic of 2 watt what. diodes for the rectifiers circuit is my work, one blew up, causing the charred red wires behind them, the amp worked fine after the diode replacement, man did I just have an epiphany the leads to top of tubes might be touching ground, back out to check NOW! no that would be to easy still blows fuse
Ken

 

[Dmans]

Check all main power supply rectifiers and filter capacitors. Check all (if any) low voltage (12v) rectifiers as well.

From your pictures, the filter capacitors look to be original. Bad Juu-Juu! They have to go!!

73
David

 

[loosecannon]

best advice i can give you is some advice NomadRadio gave to me at some point.

get a transformer that has a 120 volt primary and a 12 volt secondary.

power the radio with the 12 volt side so that all voltages in the amp are 1/10 of what they would be.

now you can test the transformers and HV sections without risking your life.

if you measure 35 volts out of the HV transformer then you know it would be putting out 350 volts in normal operation.
LC

 

[TIGERGRIZ]

best advice i can give you is some advice NomadRadio gave to me at some point.

get a transformer that has a 120 volt primary and a 12 volt secondary.

power the radio with the 12 volt side so that all voltages in the amp are 1/10 of what they would be.

now you can test the transformers and HV sections without risking your life.

if you measure 35 volts out of the HV transformer then you know it would be putting out 350 volts in normal operation.
LC

ok, I like that, use low voltage, so is it as simple as applying 12-volt a/c to the power cord?
thank you for the help.

 

[nomadradio]

Before you can check a transformer, whether at normal or reduced voltage you need to unhook the shorted circuit that's overloading the transformer.

The filter capacitors are usually only good for 20 or 30 years, tops. An amplifier that had low mileage might get closer to 40 years from those caps before they fail and short.

But this amplifier is older than that.

When filter capacitors short, the overload surge can also cause the rectifier diodes to fail as a short circuit. Since they are the first thing the transformer's HV winding feeds into, this tends to trip the fuse quickly.

Using your meter's ohms scale to check for shorts in the filter caps and rectifier diodes would be a good start.

73