Punch 200.4 PS problem?

[RJCRAFTS]

Hi All, I have a RF punch 200.4 that every 2nd or third time turn on it will draw excessive current and I can hear th PS whistle. When it starts properly all channels check fine for output and distortion. I was leaning towards replacement of the TL494 but thought I would ask the forum for thoughts. Anyone else seen somthing like this?

Thanks

[Perry Babin]

You may have an intermittently shorted transformer. Power it up via a 10 amp fuse or a current limiting resistor and twist and push/pull on the transformer to see if the short occurs intermittently.

[RJCRAFTS]

Thanks for the replys Perry,

I have been busy and just getting back to this one. I tried what you sugguested but dosn't seem to be the problem area. I have ordered and will replace the PS FETS. Also thinking replace the TL494. Your thoughts?


Thanks
RJ crafts

[Perry Babin]

It sounds like it's an intermittent connection.

Can you push or tap on the board to make it draw excessive current?

Had you done any work on the amp before this problem first appeared?

After pushing/twisting the transformer, does it still draw excessive current?

Do you have a current limiter so that you can measure the voltage at several points when the amp is trying to draw excessive current?

[RJCRAFTS]

Hi Perry,

Pushing, Tapping hard, Moving transformer does not have any effect on when amp decides to draw excess current and whistle.

No parts replaced by me yet but I don't know the history of this amp. Purchased used. I can see some solder marks but dosen't look like the parts removed from heatsinks.

I have a regulated 30 amp PS with CC control. I can limit the curent draw with the PS.

When opeating normal current draw is about 0.9 amps at idle.
When malfunctioning i have seen current draw at 15 amps.

Seems like every other turn on it goes into excessive current draw. Works fine when working outputs

Any suggestions on checking bias setting?

Thanks for the help.
RJcrafts

[Perry Babin]

With the current limited to ~10 amps, measure the voltage across the source resistors (large resistors near the audio output transistors). You shouldn't read more than a few millivolts across them. Compare the voltage when drawing excess and operating normally. If one channel has significantly more voltage across the resistors when the amp is drawing excessive current, you know which channel is causing the problem. If the voltage is the same in both channel at all times, the problem is likely in the power supply.

Don't adjust the bias at this point.