Galaxy 33

[pnut5152]

Hello! I have a 33 here that had a blown final, audio amp IC,
Radio was working fine and now all I have is a carrier. I do have audio through the PA, and if I have an external speaker hooked up, i have modulation on FM only, unhook the speaker and loose the FM audio..its driving me crazy!! I do have a scope, If i'm labeling the audio chip pins correct I have scope signal on Pin 9&10, I have no signal at all on IC4. Any ideas where to start? Thanks in advance

 

[nomadradio]

This radio is not built like a Cobra 29. The audio chip that drives the receiver audio into the speaker is NOT used for transmit.

That model was originally built with bipolar driver and final transistors. Those parts were discontinued around 2006. Ever since then, the driver and final transistors are not really RF type transistors, but a type called IRF520. Designed to be used in switchmode power supplies, but they work halfway well as a CB RF transistor. Biggest reason they are used is price. They sell for a fraction the cost of real RF transistors.

If the final transistor breaks down it can overload the modulator transistor. This is the one bolted to the side rail nearest the power socket.

Have a look at the waveform on the center pin of that modulator transistor. Should be around 6 Volts DC, maybe less depending on where the "power" knob is set. Your mike audio should appear on the DC flat line. If there is no DC output on the center pin, you may observe a lack of white heat-conducting compound on both sides of the insulating washer under it.

And if you can see a visible air gap between the modulator transistor and the heat sink, this frequently causes the transistor to overheat and fail on its own.

The metal tab of the driver transistor should have at least 6 Volts peak-to-peak of RF on it. The RF level on the tab of the final transistor should be twice that or more.

It's always best to identify all the stuff that has failed and replace all of it.

If there are two failures and you fix only one, this could damage the new part.

73

 

[Unit 75]

Radio was working fine and now all I have is a carrier.

You have 3 transistors for your AM mod amp. Use a voltmeter and take some measurements in AM mode on these transistors and compare with what I have below.
TR49 Base 6.10v in tx
Collector 12.10v in tx
Emitter 5.6v in tx

TR50 Base 12.10v in tx
Collector 5.60v in tx
Emitter 12.50v in tx

TR51 Base 12.50v in tx
Collector 5.60v in tx
Emitter 13.0v in tx

If one of these is defective, replacing it should restore your audio.