Galaxy 2527 (old 2950) super low am modulation

[CrazyCouillon]

You must first check the voltage at the collector of the final transistors and its driver on TX in AM mode.
You can check this voltage on the board that jumpers the three test pints near the finals.
I suspect that it's going to stay at 12 to 13 Volts. (It should normally go to about 6 to max 8 Volts in AM TX mode) If that is the case, the AM regulating transistor (TR51) is either failed or is being forced to go into full conduction. It should only be in full conduction for SSB modes, and lower the voltage for AM. Check VR13. If it's wide open that could cause your problem.

 

[Enterprise312ok]

I was about to do the voltage check and I noticed vr 12 has no effect

 

[Enterprise312ok]

mirror board voltage stays below 6 volts in all modes. Am regulator does seem to warm up fast but I am not sure if it is faster than normal.

Also no control from vr 12 or 16.
I turned down vr13 to 10 watts and it seems to have dialed my ssb high down to 10 watts.

 

[Handy Andy]

am having trouble finding c295. Is this the correct location?

No SSB power control usually means you've lost Q54 and or it's control signal

 

[Enterprise312ok]

2517 is fine for that. Take signal from R262 C295 junction.

I was looking for c295 on the 2517 for a source of audio to inject into the 2547. I think the diagram may just have a misprint

 

[Handy Andy]

My appologies...

I was looking for c295 on the 2517 f

To help, if VR13 (AM/FM HI-Power) allows you to adjust SSB Power then you've lost the control of Q54 - which is supposed to turn off all audio imposed on the SSB drive voltage.

The VR12 (ALC - Output of Finals sense) and VR16 (output of SSB ALC sense amp that uses VR13's out) and fine trims it - works the Q32 limiter more directly.

So this looks to be more of a "Whatever was done to the SSB side of things on the ALC" so I'd also review the ALC sense amp Q43 circuit both the sense side and the line heading back from it to the Q55/Q31 area too...

If I have the right board...

EPT 2950-12Z/13...

 

[Enterprise312ok]

My appologies...


To help, if VR13 (AM/FM HI-Power) allows you to adjust SSB Power then you've lost the control of Q54 - which is supposed to turn off all audio imposed on the SSB drive voltage.

The VR12 (ALC - Output of Finals sense) and VR16 (output of SSB ALC sense amp that uses VR13's out) and fine trims it - works the Q32 limiter more directly.

So this looks to be more of a "Whatever was done to the SSB side of things on the ALC" so I'd also review the ALC sense amp Q43 circuit both the sense side and the line heading back from it to the Q55/Q31 area too...

If I have the right board...

EPT 2950-12Z/13...

I just got another one of these with the same strange issues, no control over ssb power and only about 15 watts output. No real am modulation but it has been modded with diode resistor at r238 so it swings forward on average on am but very little pep.
I have the voltage chart out, I will not let the 2527 defeat me.

 

[Enterprise312ok]

The only power variables that have any effect are the am high/ low vr 13 and 15. amc and ssb high and low dont do anything. Finals have proper voltage on center leg in each mode. This thing doesent sound bad but I would like to get 30 peak and have my limiters working.

 

[nomadradio]

Does sound like whoever replaced the finals the last few times didn't check all the other stuff connected to the finals. Collateral damage to other parts is a common thing to see.

What's not common is to find a radio that old that still has the factory-original final transistors.



73