LC III Dyna Corp PSU not playing ball even after a recap

[XKSTEVE]

I had a dead DynaCorp LC PSU and after a recapping and through clean before and after new caps, it has sort of come back part way but seems to lack enough current to power the LC III (or LC). I had been using it but had to turn off the machine due to a system bomb and then it would not power on again.

Here is where I am at currently.

Both Grounds are connecting to the chassis ok (no resistance)
measuring between both grounds independently I have
- 5.027 volts on BLUE
+11.89 volts on YELLOW
+5.285 Volts on ORANGE

But when connected to the LC I get a faint speaker click or hum, no fan, no start and now the voltages drop dramatically to the following:
-3.02 Volts on thy -5 volt on the BLUE
2.854 Volts on the +12 volt YELLOW
1.642 Volts on the +5 ORANGE

I have checked my joints for continuity and capacitor orientation plus had a good look at all the other joints and found no issues.

Obviously there is some other component playing away here, but has anyone got any ideas where to look further? PSU are not my strong suit so please treat me like a novice learning about them from scratch.

Cheers Steve

 

[Nixontheknight]

This might only apply to the Macintosh Classic analog boards, but there might be a potentiometer to fix these voltages, but I'd also recommend checking the resistors and seeing if their resistance matches their markings

 

[jdmcs]

Double-check that you didn’t accidentally desolder anything… ask me how I know.

I’ve heard the opto-isolator is a common failure point in the LC power supplies. I’ve never seen a DynaCorp one in-person, so I don’t know which component that is.

 

[jdmcs]

Just saw another thread, with a picture…

The opto-isolator is probably U1, U2, or U3. I can’t read the markings on the chips, so I’m identifying candidates by the number of pins on the chip only.

Also, another candidate area is the LM393 near the output wires. If that comparator isn’t working correctly, the power supply won’t properly regulate voltage.

 

[XKSTEVE]

Just saw another thread, with a picture…

The opto-isolator is probably U1, U2, or U3. I can’t read the markings on the chips, so I’m identifying candidates by the number of pins on the chip only.

Also, another candidate area is the LM393 near the output wires. If that comparator isn’t working correctly, the power supply won’t properly regulate voltage.

Hi,

Thanks for that information.

Been busy with other stuff of late and a long winded trip to the Isle of Man in the middle of it all so PSU has been left alone for a while.

I have since then now done the following:
replaced,U1,2 and 3 UC3842AN Current Mode PWM Controller DIP8 and 2 xPC 817
replaced the comparator chip LM393N
replaced the small transistors MOS2907 and TL431CLP
replaced the large diodes and regulator on the heat sink MTP3960 and MBR1045
Replaced the -5v reg on the board L7095

All with very little change in fact it is getting worse (+4 volts, - 4 volts and now +8volts

i think I am going to draw a line under it now and just bin it as it is beyond my skill set to repair and the costs so far are more than a replacement PSU due to min order qty requiremenus

Thanks again to all tho use who put forward suggestions.

 

[joshc]

Did you replace the small capacitor on the high voltage side as well?

 

[XKSTEVE]

Did you replace the small capacitor on the high voltage side as well?

Hi, yes I replaced that capacitor as well. And repaired a small bit of trace damage leading to it.