1272 cutting out?

[hitchhiker]

I have a 1272 racked up with neve xformers that has been reliable for years . But recently it stops passing audio after being on for about 10
minutes. When I turn off the power it works again until the power supply
caps drain off. Wait a while and turn on and works again for a minute or two and on and on...

The power is steady 24v.

Is this a transistor gone bad or cap?

 

[hitchhiker]

hi Butta , there's no diodes in the circuit.

No charred resistors , I'll check them for shorts and report back.

 

[hitchhiker]

checked all parts and no shorts .

Does anybody have a 1272 schem with typical transistor voltages ?

 

[SonsOfThunder]

Some techs would use a can of "Circuit Freeze" or whatever you might call it, I use a can of contact cleaner (non-conductive spray) to troubleshoot stuff like this to see if it is just temperature.

HTH!

 

[owel]

Sounds like heat issues... is any component getting hot? Check around the PSU... then check all the transistors of the 1272.

 

[hitchhiker]

all parts stay room temp. All voltages seem to be about what I remember
when I bread boarded one a couple years ago. I didn't think to write down voltages at the time.

I guess I'll try replacing the tants now...

 

[DrFrankencopter]

Akkk I wrote a post but previewed instead of posting.....

Sounds like a coupling cap has failed short, or is at least leaky to me. Check the tantalums around pin U, pin L, and pin P. These are good bets to have gone leaky. In fact, if you're in there, you might as well replace all the electrolytics (assuming they're > 10 years old).

I figure that DC is leaking into the audio path, and forcing a stage onto the voltage rail. Becasue your output transformer will not pass DC, you will hear this as the amp going silent (cutting out). Distortion will likely rise as at gets closer to cutting out (due asymmetrical clipping). As you cut the power, the DC falls, and the eventually the stage comes out of saturation, and you can hear sound again briefly.

That's my best guess....

Cheers

Kris

 

[Tekay]

The 2N3055 should get a little warm! check the 12ohm resistor that feeds the output section of the BA283!

 

[hitchhiker]

I replaced all the coupling tants (U,L,P&S) and she's been working for an hour, so one must have been bad.

thanks guys :sam: :guinness: :thumb:

Lance