Punch 45 PC-045C pre mosfet power supply help

[mikep28]

I have a old punch 45 circa 1986-87. It has a power supply problem that I can't seem find the issue with. When I got the amplifier in it has one power supply transistor blown D44VH10 and a D40D8. I replaced both D44V and both D40D8. Also replaced both 4148 switching diodes. The thermal swith was also open. It was replaced. Now get ~14v at the D44's but amp does not show any rail voltage. I get nothing at anything on that side of the diagram. So therefore no LED. I Have another identical amp sitting side by side. All the a56 and a06 read the same. Any idea where to look.

 

[Perry Babin]

Did you buy the D40s from a reliable source?

 

[mikep28]

Got them from unicorn electronics I think. Static bagged and such. Buy a lot of 8 bit CPU IC from him. And a lot of static 5101 rams. All have been 100%. So I hope so

 

[Perry Babin]

Try using MPSA06s in their place.

The D40D8 has been virtually impossible to find for a decade or more.

 

[mikep28]

I'll try but they read same as the other amp.

 

[Perry Babin]

They may be OK but checking with a multimeter (DC test) does not tell you if the transistor will work as it should at high frequencies.

 

[mikep28]

I don't work on enough unregulated power supplies to understand the circuit properly. What generates the frequency for the power transistors?

 

[Perry Babin]

It's a self oscillating power supply. They can function in a number of ways but this one uses windings on the transformer to generate the drive signal. When switched on, a pulse is generated. This pulse starts the cycle and it will oscillate until something forces it to stop.

Self oscillating supplies can be regulated or unregulated.

 

[mikep28]

Gotcha. I understand.

Tried 2 MPSA06 in place of them. Same results. However probing around the emitter and base I hear high pitch whine of the transformer and the LED light illuminates. Then it builds rail voltage. About a second then off it goes.

 

[Perry Babin]

If you power up normally (no probing), does the large 120 ohm resistor get hot? If not, check the solder connections for it.

Have you replaced the small electrolytic capacitors in the power supply area (particularly any with a pink tint to the previous gray cover)?

 

[mikep28]

R10 stays cool. I assume it's R10 120 ohm 2 watt. I've not replaced any of the small caps. R10 did get hot before I replaced the shorted D0D8.

 

[mikep28]

No R10 does not get hot. I did get warm when one of the D0D8 's was shorted. I haven't replaced any capacitiors.

 

[Perry Babin]

If it's not getting hot, it could have bad solder connections or something in series with it could be open.

Also confirm that you read 0 ohms across the thermal switch.

 

[mikep28]

Only thing in series directly with it (R10) is B+ and the collector at Q5 (a06). The thermal switch reads 0

 

[mikep28]

If I hold my Fluke 12 meter set to diode check. Black lead on chassis ground, and red lead on the base of either D0D4 (now A06) or Q5 (a06) it will cycle power on for a second or so, shut off and repeat about every 3 or 4 seconds.

 

[mikep28]

If I hold my Fluke 12 meter set to diode check. Black lead on chassis ground, and red lead on the base of either D0D4 (now A06) or Q5 (a06) it will cycle power on for a second or so, shut off and repeat about every 3 or 4 seconds.

Let me add to this : it does it also to the smalls leads on the left side of the transformer and other components in that area..

 

[Perry Babin]

Short Q5 from collector to emitter.

 

[mikep28]

Short Q5 from collector to emitter.

Yeah I already tried that before I saw your reply. It turns on. LED lights up. It produces rail voltage. I don't get any voltage on the op amps at pin 4 or 8.

 

[Perry Babin]

Did you check all of the op-amps? The one closest to the power supply is switched after a delay. The others are powered up with the supply.

 

[mikep28]

Top 2 closest to the LED show voltage -17.16 and +17.16. The one closest to the supply shows nothing (this is with transistor jumped) nothing does anything without it jumped.