RF power transistor testing
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Picked this bargain/money pit up for less than the price of the heatsink. Someones gone to a lot of thought with this then spoilt it by tacking bits on like an afterthought
I'm going to give it a full rebuild. I don't like the rats nest biasing circuits, and I'm thinking of using the LM723 bias circuit as per the Motorola application note. I'm hoping the power transistors aren't fried but I'm working on the assumption that they are. What's the recommended way to test them, just checking for shorts/open circuits and checking the diode junctions, any precautions etc? Thanks |
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Made a start. Cif bathroom cleaner. Never known anything like it to lift dirt.
Also isn't the internet wonderful. Couldnt' find any info on it so googled 140 watt push pull amplifier and found this. |
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Thinking about it after a beer, should I just reassemble it as is first off then give it a sniff of rf and see what it's doing or not ?
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I found what I think is the original auction listing for it - definitely says not working, so another beer and a bit more thinking may be in order.
That amp is probably straight out of one of Motorola's app notes if you can find it. |
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Terry. Over here most radio/cb type equipment is listed as not working to cover the sellers from bad selling experiences. The last cb linear I bought as a non worker was fine when tested.
Stripped it down and these are the components that will be changed as a matter of course. I can now get at it to perform resistance and voltage checks. Nothing appears to have let magic smoke out The heatsink has been snook into the dishwasher. |
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I found the thread: https://www.vintage-radio.net/forum/...d.php?t=139689 |
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It survived the dishwasher, but has had a coat of satin black anyway
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......just a thought possibly not a wonderful idea to spray the surface where the transistors screw down with spray paint, as it is not a wonderful thermal conductor.
Was the anodized finish damaged at all in the dishwasher ? |
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rebuild started. Old tantalums and elecs are going. A couple of old 15 ohm carbons have drifted away so will be replaced.
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Pet hate 1. Untidy wiring
Pet hate 2. Non flux removal |
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It lives.
Haven't tried it at full power levels as I hadn't put thermal compound on the power transistors https://youtu.be/Dy_cqqq89xI I've added a small computer fan to it as there are some parts inside that get warm and aren't heatsinked, I used a riding helmet bag for filter material as it's very breathable It's quiet enough https://youtu.be/WT7t1fq6Kc8 Plan is to use it with the IC-251 which has a Mutek front end. Should make quite a good setup |
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A riding helmet bag sounds expensive.
I normally use a vacuum cleaner motor filter. They are made to be cut to size. I just linked to the first one on a search. Wilco stopped stocking them for some reason. https://www.amazon.co.uk/Universal-V.../dp/B004A8JMSE A cheaper one. https://www.amazon.co.uk/dp/B078VTCW79/ref=dp_cerb_1 |
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the helmet bag was being cleared out anyway but I spotted its potential |
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