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Power supply malfunction

Jhawk

Jhawk

New Member
Oct 17, 2017
8
1
3
California
I am a newcomer to worldwidedx.
I received a used power supply as a gift from a friend and I turned it on and tested the voltage and it was around 13v I connected my radio and noticed a slight squeal while transmitting for a radio check I also noticed the meter on the power supply stopped moving. So I disconnected the radio and tested the voltage again this time it was at 24v yikes. My question is how would I track down which component (s) would most likely be damaged? My radio hr2510 receives well but tx is down to less than 1w.
 

This is why I don't use linear PSUs because if the voltage regulator fails you get the full 24V put on the rails whereas with a switch mode PSU it just turns off.

Unfortunately it has likely fried the HR2510.
 
Bad news would be that the rare and expensive final-amplifier transistor is the most-likely victim of a runaway power supply.

Turning off the radio's power switch doesn't protect it. The final stage is always connected to the power cord. Saved them money to use a tiny power switch that could never handle the radio's full current draw on transmit. Power switch only shuts off the receiver and transmit/receive switching circuits. They get away with this by turning the final transistor on and off with the bias voltage that's only active while transmitting.

The MRF477 can still be had from some Ebay suppliers. Totally-bogus fakes and counterfeits are also widespread. If you can find it listed under "Polida" or "UTSource" it's probably authentic. Other suppliers could be shady or clueless about the difference.

You could try testing the transistor. Having a look-see with an oscilloscope for drive power feeding into it would be a better way to see if it's gone on a permanent vacation. The drive transistor that feeds into it is also vulnerable. But the final is the most-likely victim.

Higher-quality linear-regulated power supplies include a sensing circuit for excess voltage. Triggers a shutdown circuit called a "crowbar". No joke.

73
 
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Thank you for the heads up on the final. I will test it and replace if needed. I will post results.
Ebay has a Uniden MRF477 Rfparts has a Motorola MRF497 replacement for MRF477. About $10 difference in price. Either one better to use than the other?
 
I would certainly trust RF Parts, before anyone on Ebay unless they have a STELLAR feedback rating (and some due) but never gotten anything not as described from RF Parts...
As per direct replacement(MRF497>MRF477) NOMAD certainly would be more versed than I.
All the Best
Gary
 
Jhawk said:
I am a newcomer to worldwidedx.
I received a used power supply as a gift from a friend and I turned it on and tested the voltage and it was around 13v I connected my radio and noticed a slight squeal while transmitting for a radio check I also noticed the meter on the power supply stopped moving. So I disconnected the radio and tested the voltage again this time it was at 24v yikes. My question is how would I track down which component (s) would most likely be damaged? My radio hr2510 receives well but tx is down to less than 1w.
Click to expand...
Get a new power supply from MegaWatt. I have used and abused them for years.
 
M0GVZ said:
This is why I don't use linear PSUs because if the voltage regulator fails you get the full 24V put on the rails whereas with a switch mode PSU it just turns off.

Unfortunately it has likely fried the HR2510.
Click to expand...

Actually it would require the series pass transistors to fail and not just the regulator chip. When my regulator failed many years ago the voltage dropped to about 1.5 volts. My linear PSU has over voltage protection and crowbars at about 16 volts.
 
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Shorted pass transistors are the most-common failure I have seen. They get all the stress and tend to fail before low-power components like the regulator chip itself.

The MRF497 is a 60-Watt rated "big brother" to the MRF477. But simply dropping it in place of the original part won't get you more transmit power. Tends to get you less, as a rule.

The reason for this is the impedance-matching circuits, both feeding into the final and out of it, into the antenna socket. The circuit is optimized for the circuit specs of the smaller part. Making the 497 work properly requires modifications.

There was a poorly-researched article in a popular magazine called "73 Amateur Radio" back in the 80s. Showed how to upgrade the HR2510 to the '497. RF Parts would sell you the 'kit' of parts to do this mod. Later on they would only pack a reprint of the article. The guy who wrote the article didn't research or develop the modification properly, but it would 'sorta' work. The added power was not enough to improve your transmit range in any meaningful way, but it appealed to wattmeter worshippers.

The original 477 part is simply less work to use. RF Parts ran out of that one years ago. I have no idea how the "Polida" and "UTSource" guys get them. But I have obtained long-discontinued chips from them in the past, so they're probably the real thing if they have that one.

73
 
I received an email from UTsource last night stating they have changed their shipping charges. very funny Nomad that you brought them up and I received the email. not many use this parts supplier. I will see if I can post the link to them here for info if needed.
 
nomadradio said:
Shorted pass transistors are the most-common failure I have seen. They get all the stress and tend to fail before low-power components like the regulator chip itself.

The MRF497 is a 60-Watt rated "big brother" to the MRF477. But simply dropping it in place of the original part won't get you more transmit power. Tends to get you less, as a rule.

The reason for this is the impedance-matching circuits, both feeding into the final and out of it, into the antenna socket. The circuit is optimized for the circuit specs of the smaller part. Making the 497 work properly requires modifications.

There was a poorly-researched article in a popular magazine called "73 Amateur Radio" back in the 80s. Showed how to upgrade the HR2510 to the '497. RF Parts would sell you the 'kit' of parts to do this mod. Later on they would only pack a reprint of the article. The guy who wrote the article didn't research or develop the modification properly, but it would 'sorta' work. The added power was not enough to improve your transmit range in any meaningful way, but it appealed to wattmeter worshippers.

The original 477 part is simply less work to use. RF Parts ran out of that one years ago. I have no idea how the "Polida" and "UTSource" guys get them. But I have obtained long-discontinued chips from them in the past, so they're probably the real thing if they have that one.

73
Click to expand...
Thank you Nomad. I did read about the 497 in a few other mod articles and i remember that it would need additional mods done to work well. I agree with you i would rather restore the 2510 to what it was. The price range is drastic there is a uniden mrf477 on ebay for $53 the motorola from polida is $12
Ebay sells the motorola for as cheap as $1. Confusing. I dont want to be cheap but i dont want to waste money either. There is a motorola in usa = fast shipping for $20. Any ideas? You already mentioned polida which is a reasonable price 5 weeks shipping though. Thanks again for all the good info. I hope others will find it useful. I had a hard time finding any threads about radio damage from linear psus. The threads i did find just said the radio was fried but didnt mention an approach to repairing. 73
 
Ebay doesn't "sell" that part. An Ebay member does. The risk with any discontinued part is fakes and counterfeits. The more rare and expensive a part becomes after they quit making it, the more incentive there is to sell you a bogus imitation.

That was the reason to recommend UTSource and Polida. There are probably other honest sources out there, but plenty of shady fakers, too.

73
 

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