Punch 60X2

[seankane]

Working on a Punch 60x2 PC-1089-B

Customer sent it to me and said it had distortion in one channel. Silly me I didnt test it just started to check and repair the obvious.

I replaced output fets in both channels, source resistors in both channels, A56 and A06 in both channels, gate resistors in both channels.

Powered up the amp idles perfect, set the bias and both channels bias fine.

Plays clean at low volume, but the higher the volume the more distortion. Both channels are like this.

I drove a 100Hz sine wave into the amp via head unit. One channel only shows the upper half of the wave form. The other channel only shows the lower half of the wave form.

I checked pins 1 and 7 of the 3 op-amps and could only get the upper half of the wave form.

I dont have clean audio on the op-amps pins 1 and 7.

I replaced C102 and C103 and also C202 and C203. This made no difference.

Is there something I am missing?

[Perry Babin]

Do you have negative regulated voltage on all of the op-amps?

[seankane]

Yes -12.95 and +13.95

[Perry Babin]

If you have the schematic diagram, send it to me.

babin_perry@yahoo.com

[seankane]

The only one I have in the X transana series is the 100X2 schematic.

Think it would be similar?

[Perry Babin]

OK. I have that one. It's a 1090 vs the 1089 board so it should be nearly identical.

Do you have the clipped signal on both output pins of U101?

[seankane]

Its a wierd signal I would say yes. It barely has a signal but looks like a bunch of small squares. Both pins 1 and 7 on U101.

1V 2MS

[Perry Babin]

If there is an R25 and R26 near the regulators, find the terminals of those two resistors that are directly connected (0 ohms). Do you also read 0 ohms between that common connection and the RCA shields (no RCAs connected)?

Do you have a clean signal (the same as you have on the center conductor of the RCA jacks) on the input pins (3 and 5) of U101?

[seankane]

Ok this is wierd I fired up the amp today and it played fine but the longer I played it OR the louder I turned the volume the more the sound degraded.

I decided to check the input of the head unit after reading your last post. The signal coming from the head unit also only showed the top half of the waveform. I have never had a problem with it but maybe something went wrong with it.

I switched head units and the amp seems to play fine. I do have a few questions.

Can the head unit sitting directly on top of my power supply cause something to fail inside of it?

Second question could a faulty treble pot cause distortion in a channel (s)? I was adjusting it up and down and the further I turned the pot down (decreasing treble) it seems to sound even a little better.

[Perry Babin]

Having the head unit on top of the power supply isn't a problem unless it's blocking vent holes.

Look at the output with the treble at a higher level. Many of these amps had problems with oscillation at higher treble settings. Some were so bad that they could cause the amp to overheat and fail. A defective treble pot could cause problems.