RF power transistor testing
 

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

Re: RF power transistor testing
 

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.

Re: RF power transistor testing
 

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 ?

Re: RF power transistor testing
 

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.

Re: RF power transistor testing
 

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.

Re: RF power transistor testing
 

........hmmm I thought I read on a thread somewhere that somebody else did this, perhaps they had a lot of, or a high concentration, dish wash powder (which is very alkaline and attacks anodizing and aluminium) and it spoiled the nice black anodizing on the heatsink. But you may be ok if the solution is weaker and/or temperature lower and not get any issues. It might be better to do it by hand with a trace of the friendly hand dish wash and a small soft brush.


I found the thread:

https://www.vintage-radio.net/forum/...d.php?t=139689

Re: RF power transistor testing
 

It survived the dishwasher, but has had a coat of satin black anyway

Re: RF power transistor testing
 

......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 ?

Re: RF power transistor testing
 

I haven't, I've only painted the visible elements, and no.

Re: RF power transistor testing
 

rebuild started. Old tantalums and elecs are going. A couple of old 15 ohm carbons have drifted away so will be replaced.

Re: RF power transistor testing
 

AVO on ohms . Junction test/DVM on diode test. if inconclusive, isolate base ( if possible ) for further testing.

Re: RF power transistor testing
 

Pet hate 1. Untidy wiring
Pet hate 2. Non flux removal

Re: RF power transistor testing
 

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

Re: RF power transistor testing
 

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

Re: RF power transistor testing
 

good shout.

the helmet bag was being cleared out anyway but I spotted its potential