Galaxy 29hp no modulation

[Mk121]

Ok working on a radio for a friend. Its in good shape and no mods done to it that I can see. Problem is I'm getting a full blown carrier with zero modulation. I cannot adjust carrier with vr pots and it doesn't matter if it's in high or low power. I already changed tr51. Took that out of circuit and just get a very small maybe 2 watt carrier. Any help is appreciated.

 

[ExitThirteen]

Try removing TR50 and see if everything drops to 0, if it doesn't, it would appear that there's a shorted trace somewhere, if it DOES drop to 0, that means TR50 has gone short. Replace TR50 and TR51. Replace TR50 with a TIP42C, and replace TR51 with a TIP36C.


Let us know how you get on.

~Cheers~

 

[sonoma]

I have a 29HP I bought from a guy with the same problem of no audio. I found a cold solder joint by the audio IC. after cleaning the joint and putting new solder on it the radio works just fine now. have had it about a year and works every time I try it out.

 

[nomadradio]

If you have a way to test the transistor you removed, I suspect you'll find that Exit13 is right, and the transistor is shorted. A close look at the way the transistor was mounted to the heat sink surface may explain why. The back surface if the transistor, and both faces of the insulating washer should have a layer of the white heat-conducting compound on them. Good chance they don't. The compound improves the transfer of heat to the heat sink. The factory tends to be stingy with it. You don't need to apply a lot of it, just enough to see a tiny bead squeeze out around the edges. This tells you that the stuff has filled any tiny air gaps between the flat surfaces.

Always replace both transistors, TR50 and TR51 at the same time. When one of them fails, it puts an overload on the other one.

It's common to see a tiny dark air gap under this part in a lot of Galaxy radios.

That will cause the transistor to overheat and fail prematurely. We always replace both TR50 and TR51 at the same time when this happens.

73

 

[Mk121]

Try removing TR50 and see if everything drops to 0, if it doesn't, it would appear that there's a shorted trace somewhere, if it DOES drop to 0, that means TR50 has gone short. Replace TR50 and TR51. Replace TR50 with a TIP42C, and replace TR51 with a TIP36C.


Let us know how you get on.

~Cheers~

Ok so leave tr51 out of circuit as well or just take tr50 out? I already replaced tr51 , tr50 and tr49. When you say drop to "zero" I'm guessing you mean voltage and not watts? Thanks

 

[ExitThirteen]

Both, actually. With 0 volts present, you should not get any deadkey wattage, as there shouldn't be voltage present on the TP7/TP8/TP9 rail. TR50/TR51 is designed to drop and regulate the voltage going to the Source pins on your driver and final (center leg), usually to the tune of 4-7V, depending on where your deadkey VR control is set.

The thing to do is take both TR50 and TR51 out of circuit, and then check for voltage at TP7/TP8/TP9, there shouldn't be any voltage present. If there is, there's a bad trace, or a bad solder joint somewhere that's dropping voltage to that rail. Check for any burnt traces or bad solder joints, as these radios are notorious for bad solder joints and bad traces. If everything looks good, reinstall new transistors at TR50 and TR51, and double check voltage at TP7/TP8/TP9, you should have 4-7V roughly on that rail. If it checks 12.5V+, there's a fault in the regulator circuit.


~Cheers~

 

[Mk121]

Sounds like a plan. I'll get to it later today and report back. Thanks for pointing me in the right direction.

 

[Mk121]

Well I took out tr50 and tr51 and voltage dropped to nothing at tp7 ,8,9. They are already new transistors tip31 and a tip36. They both tested fine on my atlas semiconductor tester as well. So any thoughts? I have a scope as well if there are any tests I can do with that to hunt down the gremlin.

 

[Mk121]

I just put tr51 and 50 back in and now voltage is Over 13v at tp7,8,9. Also voltage readings are high on tr51 and 50 according to the voltage chart.

 

[sonoma]

I have never seen any one put a tip31 in the tr50 slot. a tip42c is normal for that location. the tip36c works fine for tr51 location

 

[Mk121]

Ok thanks Sonoma for that. I swapped back in the original for tr50 and also replaced c212. I now have modulation!! But now no carrier, nothing showing up on wattmeter. I can hear myself finally though on my monitor radio. Also seems like all the voltages fell back into place.

 

[sonoma]

the original tr50 may be bad . just a guess.

 

[Mk121]

It tested good so that's why I put it back in.

 

[sonoma]

measure the voltage on each leg with the radio keyed and see what you are getting.

 

[Mk121]

13v, 3.1, 12.5 (b,c,e)