Midland 76-900 internal PSU fault

[mm3stb]

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 Defpom]

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.

[zodiac]

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

[mm3stb]

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?

[zodiac]

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

[mm3stb]

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

[mm3stb]

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