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Radio Make and Model: Midland 76-900
PCB Number: PTBM134A0X
PLL Number: LC7137
Test gear available: DMM, AMM, Dummy load, Frequency Counter.
Symptoms: No output from PSU

What is working: Works fine on external power

What has been tried: nothing yet

Did it suddenly happen ?: not sure
Anything else that may be of help?:

This is a friends radio. He purchased it from a car boot sale as a dead radio which was true when using 240AC but when connected to 12v DC works fine.
After some probing about i found that the main board is getting +12 but not -12v. Looking at the schematic i think the most likely culprit is the Inverter Diodes on the voltage regulator PCB. I haven't had a chance to remove them for testing yet but is there anything else that would give these symptoms.
Another thing is I am struggling to find anyone that stocks 1n4503 diodes but I have some 1N4504 diodes and the only difference i can find apart from appearance is the 503 diodes are 300v and the 504 diodes are 400v so would I be correct in assuming that i could use the 504's?

The usual culprit that dies in that power supply is the main current regulator, Q201, sometimes I have found the Zener diode dead (ZD1, ZD2), or Q1.

To answer your question, you could use a different diode there OK, as long as they can handle the current they will be OK, as the voltage is low anyway, many radios use a diode bridge instead of individual diodes, either way works fine.

If any one of D1 to D4 had gone short curcuit it would have blown the mains fuse.
Other than that, as Scott says.

Thanks for the reply.
If one of the diodes had gone open circuit would it give the symptoms i have?
If Q1, Q201, ZD1 or ZD2 fail would i still get +13v to the main board and no 0v?

Richard,
Have you checked the components that Scott told you about and what did you find ?.

Sorry havent had a chance to get the soldering iron out yet. Will keep you updated

Just to update you That's the radio repaired.It was ZD1 was short circuit. Thank you so much for all your help!