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?
Midland 76-900 internal PSU fault
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- Dead Key
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- First Name: Richard
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Midland 76-900 internal PSU fault
Last edited by mm3stb on Sunday 7th Mar 2010, 3:16, edited 1 time in total.
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Re: Midland 76-900 internal PSU fault
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.
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.
Re: Midland 76-900 internal PSU fault
If any one of D1 to D4 had gone short curcuit it would have blown the mains fuse.
Other than that, as Scott says.
Other than that, as Scott says.
How far is it.
Twice it's length from halfway.
Twice it's length from halfway.
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- Dead Key
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Re: Midland 76-900 internal PSU fault
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?
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?
Re: Midland 76-900 internal PSU fault
Richard,
Have you checked the components that Scott told you about and what did you find ?.
Have you checked the components that Scott told you about and what did you find ?.
How far is it.
Twice it's length from halfway.
Twice it's length from halfway.
-
- Dead Key
- Posts: 21
- Joined: Saturday 1st Aug 2009, 9:29
- First Name: Richard
- Location: Aberdeen, Scotland
Re: Midland 76-900 internal PSU fault
Sorry havent had a chance to get the soldering iron out yet. Will keep you updated
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- Dead Key
- Posts: 21
- Joined: Saturday 1st Aug 2009, 9:29
- First Name: Richard
- Location: Aberdeen, Scotland
Re: Midland 76-900 internal PSU fault
Just to update you That's the radio repaired.It was ZD1 was short circuit. Thank you so much for all your help!