Maxtrac PA board damage-Reverse Polarity diode?

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Xnke
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Maxtrac PA board damage-Reverse Polarity diode?

Post by Xnke »

I have been going through my stack of these things to sort the "Parts" radios from the "projects" radios, and found a nice D51MJA9JA5AK. Clean, no dings, all the buttons there, but rattles...so before even powering it up, I popped the PA cover and found CR2170's body seperated from the leads. It appears that it ran hot enough the solder melted and the body popped free from the two leads either side of it. Looking at the service manual, it appears to be a reverse polarity protection diode...which would have worked great until it melted down and went open circuit.

The PA board is an HLB3049A, so that means 36-42Mhz, I think? Not useful to me but I'm hoping the logic board and front panel are still good-they are the 16 pin and 32 channels, and I was hoping to re-purpose them on one of my 2-channel 42-50mhz PA/RF board assemblies.

Anyone know what that diode is outside of the motorola service number? I can't seem to locate one, and more importantly, if I do locate one what are the chances that replacing it will bring the radio back to life?
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Bill_G
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by Bill_G »

On the logic, near the 16 pin plug, there is a solder down fuse that may have blown preventing the radio from turning on, or the radio may have been set for ignition lead required by cutting the fuse out.
Xnke
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by Xnke »

I never did power the radio up-Checking the PA transistors showed they had an emitter-base short, and 20K from base to collector.
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Bill_G
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by Bill_G »

It's not unusual to see a ohmmeter B-E short in a class C RFPA because there may be a coil to ground. It's part of the tank circuit. Unless the transistors are obviously cooked and discolored, clip out the burnt protection diode, and apply power to the radio with a small value fuse (5A) in the mains.
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jackhackett
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by jackhackett »

I've seen those diodes fall apart like that many times. I think it's more from vibration than anything else, because they usually still checked okay with a diode tester.
They're not just a regular diode, they're a TVS (Transient Voltage Suppressor) diode, they protect against both reverse polarity and voltage spikes. Basically a big zener diode. The Motorola number was MR2525L.
Xnke
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by Xnke »

That's exactly the case-the diode body checks out fine. No blown fuses, no burned traces or blown caps that would be consistant with high voltage transient or reverse polarity, just that diode broken.

Is there a modern/available replacement that should be put back in, or is it a motorola service parts number only?

This is a 36-42Mhz unit anyway-not useful to me-but it may be useful to someone else so I want it to work before I pull the logic board and front face. Someone else may need the RF board and PA someday.
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train_radio_guy
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Re: Maxtrac PA board damage-Reverse Polarity diode?

Post by train_radio_guy »

Here's the spec sheet on the MR2525L - Overvoltage Transient Supressor:
- http://pdf.datasheetcatalog.com/datashe ... 9377_1.pdf

As for a directly equivalent part, NTE, Mouser, & Digi-Key are turning-up nothing. Perhaps a parts radio might be a perspective donor, otherwise Motorola Parts might be your only choice.
"Trust me, I know what I'm doing!" - Sledge Hammer
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