Just checking in and saying hi....

[guitar_199]

I'm the nut that does this, once in a while, for retirement "fun".

Putting a radio on the bench to see what is up with it.... (one of the many radios I bought JUST TO SEE if it needed anything fixed... or maybe just an alignment!).

So... there may ACTUALLY be nothing wrong with it... in which case... I will just align it and put it back in the box.

The patient is a Midland 150M.

Don't know anything about it.... but I will soon. (yep.. that is the microphone right behind it!)

 

[guitar_199]

okay... it needs fixing!!!!!!!!

Both RCV and TX (carrier only) work for 2 to 3 minutes.
WHILE it is working....
modulation audio does NOT work,
RCV audio DOES work great,
PA does NOT WORK EVER.

Something is happening after a few minutes that shuts of BOTH RCV and TX.
I also notice that the combination TX/MOD LED quits lighting when I key down AFTER the failure. It DOES LIGHT UP before failure.

If you turn it off for 3 to 5 minutes or so...... it all works again... for a couple of minutes.... then dies again.... repeat ad nauseum.

So Iam thinking two problems here:

1. Something in PLL/VCO/Frequency Generation is "croaking" that disables the radio in both RCV & TX, and

2. RCV audio works but MOD/PA don't: something is wrong in microphone or mic audio path up to the audio amp itself where it joins the RCV audio.

Got the SAMS on it.... so I am good to go!!!!!!!

(not looking for help so much as.... sharing my insanity!)

 

[guitar_199]

Recently repaired mic tester.... says mic is NOT producing audio out. Don't know the reason yet.... (wiring or element????). But I'm pretty sure the mic is the problem on the MOD & PA issue.

 

[guitar_199]

I think I have a big part of this figured out.

Q207 is a PNP (MPS9681T) that switches on a pair of TX only power sources.

The radio switches fine when you first turn it on. After about 3 minutes time.... Q207 stops turning on and the base pulls up to the same voltage as the emitter.... but the collector does not turn on anymore.... only shows about 1.28 vdc all the time.

When you turn the radio off for about 2 minutes.... then turn it back on.... it works again...but only for about 45 seconds or so...then quits again.

I am thinking Q207 has gone thermal and is shorting from base to emitter which would answer for that base being pulled up so high. Normally... when you key... the base pulls down to a little over 7v which is enough to turn Q207 on. When it hits the failure point... the base pulls up to about .02v less than the emitter and never will output the TX voltages.

Now.... if I can get a replacement transistor. The original P/N is still available in places.....

Onward I go......

 

[brandon7861]

Now.... if I can get a replacement transistor. The original P/N is still available in places.....

stick a 2N3906 in there. Could probably use just about any pnp transistor salvaged from any radio (assuming you check pinouts because the 2SA and 2SB PNPs are different from the 2Nxxxx variety)

 

[secret squirrel]

Nothing technical about this idea. You are right on there. I love the sound of the old Midlands. No digital processing. Pick up that mic and instantly you are transported back to a different world, "you got a copy on me Pig Pen?" Keep on resuscitating old radios. Lost pieces of art from yesterday.