Overhauled AU-101, right channel failed after 20 min

[Sci525]

Hi everyone,

I overhauled my 1971 AU-101 yesterday. New electrolytic caps, small transistors, and power supply diodes (transistor substitutions below). Sounded good for 20 minutes before the right channel went out. Turned it off and back on and the right channel was on for 5 min before going out again and for good. Left channel is fine and both fuses are good. Its clean and while the factory soldering looks rough, it ran fine for many years.

Transistor substitutions
CDC-8000 -> KSC2690 AYS
CDC-9000 -> KSA1013 YTA
2SA640 -> ZTX795 A

This amp does not have a variable bias resistor. I took some quick voltage measurements across the speaker terminals and both hovered in the 3-10 mV range, however I did not remeasure after warm up. In circuit measurements of transistors after failure show that the 2SC1060 output transistors all read the same across channels, however the KSC2690, KSA1013 and ZTX795 read differently on the right and left channel. In circuit measurements of both channel's transistors varied from my understanding of out of circuit measurements so I'm not certain how to interpret the values.

My un-educated guess is that poor bias caused the right channel to overheat and I'll need to replace these transistors as well as replace the fixed bias resistor (reading 85 ohms) with a 1/4W 200 ohm trimmer.

Can I get any expert advice? I only have one KSC2690 left so I'd really like to get this right on the next go.

Thanks!
Joe

 

[Robisme]

Verify correct pin out installation of the ZTX.

Also, I use KSC2690A and KSA1220A for the CDC transistors.

Looks like it does have bias adjustment.VR 801

Rob

 

[Sci525]

Thanks Rob.

The later AU-101s did have a bias adjustment. Those boards are F1302-2. My driver board is F1302-1 with fixed resistors.

The ZTX looks good (I think, I have made this mistake before). Image attached.

It does seem like most use the KSA1220A over the KSA1013, but it looked compatible and its running fine in the left channel.

Joe

IMG_20190121_123319

• Sci525
• Jan 21, 2019

 

[Robisme]

The pin out of the ZTX is EBC. If the lines on the PC board mean anything, it's installed differently than the other transistors. Was the original ECB? If so, it looks like schematic shows the center hole is the collector. How is the other channel ZTX orientated?

Interesting on the bias circuit change.

Rob

 

[Sci525]

Yep, it appears your right. The ZTX is actually replacing a flying saucer XA-495. I checked the old saucers and they are EBC. I swapped both of my ZTXs. The volume on the left (working channel) is greatly improved and I have a low volume degraded signal on the right channel.

This is making me think the issue might be on the tone board. When I rotate the balance pot to the right, both channels turn off and I can't get a smooth response from the right channel on the MM. The confusing part is that it worked perfectly for 20 minutes and then just shut off. I usually think of a slow and scratchy failure for potentiometers. Looks like I'll need to replace it and report back.

Thanks again.

 

[Sci525]

Ah, it was a dirty switch, the source/play back.

Its sounding great. Much better now that the ZTX are in the right position. With them in the wrong position it still sounded better than the cheap computer speakers its replacing.

Cheers

 

[Robisme]

Glad to hear you got it sorted out.

Rob