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Volting A.M. Only Mobiles (Uniden/Cobra)

LeapFrog

LeapFrog

Wielding Hanlon's Razor
Feb 15, 2016
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Anchorage, Alaska
Well gentlemen I have had a failure inside my Pyramid PS-4K, unfortunately no "crow-bar" circuit pulled anything low, the power supply was putting out over 15 volts.

I am not that proficient with diagnosing the circuit failures yet.
I had a PC-66 on the bench with most of the caps replaced already (minus the ones in PLL/VCO area)

The radio doesn't work right, I think I fried something when the power supply went high.

Does anybody have experience volting these radios, would you know what to look out for when you go over-board?

Thank You
 
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Tallman said:
Here is the best answer I can give on the power supply issue.
Click to expand...
10-4, I am not too worried about the little Pyramid, I have another PSU here; but thank you.
The damage has been done though, I guess I'll start checking voltages at various places.

Observation: Positive side of C114 has no voltage during TX.
No TX, and RX acting weak (can only hear really close stations).
Public address functions as it should.
 
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If you scope out the schematic, most of the electrolytic caps have a voltage rating. That's the first place to look. Most semiconductors have a voltage range that's listed in the data sheets, so audio amp for example do operate upwards of 18 volts. Even the finals will operate at near 20 volts. The caps is where I would look. Everything else will handle the voltage, run hot yes but not immediate failure. Also , can't recall if it unregulated or not but the high voltage is the average not peak or spikes. So that's another factor when determining the over volt conditions
 
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On my schematic, C114 should show voltage for receive mode only, and should fall to nearly zero when the mike is keyed.

Q16 is the transmit-voltage switch transistor. The center pin should go to around 8 Volts DC when you key the mike, and drop to around zero in receive mode.

Q17 is the 8-Volt regulator. Should have between 8 and 9 Volts DC on the emitter lead.

73
 
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LeapFrog said:
I really do appreciate your experience and explaining where I should check, I was looking for an 8 volt regulator on the schematic with no thing in sight.


I'll look over those areas and post my findings
Click to expand...
About 8.90 volts on the emitter of TR17 (9.55v on the base); TR16 has 8.79V on TX and 63mV on RX.
Input voltage into radio 11.83 and falling. (Lead Acid motorcycle battery)

Thank you Nomad Radio
 
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Well it turns out nothing is wrong with the radio, I changed the last 10v caps, and some of the inductors where way off, so it is alive and transmitting, and let's just say I won't be "twisting knobs and turning screws" late at night anymore, my apologies.

Thank You Tallman, 711 & nomadradio I really appreciate the help!
 
711 said:
If you scope out the schematic, most of the electrolytic caps have a voltage rating. That's the first place to look. Most semiconductors have a voltage range that's listed in the data sheets, so audio amp for example do operate upwards of 18 volts. Even the finals will operate at near 20 volts. The caps is where I would look. Everything else will handle the voltage, run hot yes but not immediate failure. Also , can't recall if it unregulated or not but the high voltage is the average not peak or spikes. So that's another factor when determining the over volt conditions
Click to expand...
Voltage regs have a hard time with over voltage; they heat up more and can fail.
 
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Why would anyone today waste time volting a radio? That is so 1980's! Given the price of rf finals today it is just not a good mod to even play around with. If your soldering skills are good enough to recap the radio you are better off building a rf amp.

Most of the caps in radio's are marginal for stock output all of them are too low in value in terms of voltage for volting. The stock voltage regulators are more of the same marginal at best.

You prob. popped a diode if you blew a regulator or cap. The diode should fail first but seldom do and even when they do usually something else let's go too.

Check all the diodes along that circuit.
 

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