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.

 

[guitar_199]

I'm going to have to order a transistor to try it. I'm not doing this as a business... I am just an "electronics trained nerd" that is waking up (in retirement!) from a career in operating system internals and software development!!! I don't have much in the way of "parts stock". I'm just doing this to keep my mind busy. I will let you know.

(however, if I do happen to find a suitable PNP transistor around here I will probably throw it in there as it IS just a PNP switch more or less.....)

 

[secret squirrel]

Totally understandable. My A+ certification test was on Windows 3.1 and MS-DOS 6.2- totally dating myself. I have much more parts along the lines of resurrecting old PCs. My soldiering skills leave a lot to be desired. I really should pick up that skill. I never got into programming- just fixing the machines. I

 

[guitar_199]

I did just find that I DO have a 2N3906 available... and several have told me that this should do the job..... just to be mindful and get the pins right. I will likely stick that in and see what happens.....

 

[guitar_199]

The good news..... a 2N3906 worked just fine in place of the MPS9681T.
The bad news... the radio still behaves the same was as it did......

Meaning that there SEEMS to be a thermal problem in this circuit that allows the circuit to work for about 2 to 2 1//2 minutes if the radio is stone cold... THEN the TX voltages (#8 and #9 in the attached image) DIE and won't come back until you power off and let it cool down.

I THINK I have it figured out though........ and it centers on D209 just BELOW Q207.

I took a bunch of voltage measurements this morning in both "WORKING" and "FAILED" conditions. Bear in mind.... when you key the mic for TX... that puts a DC ground on the bottom of that RF choke up through the resistor R316 (10K) and up to the cathode of D209.

When it WORKS there is a drop of 0.58 volts across D209.... and the base pulls down to about 7.3-ish volts.

In FAILURE mode... the drop across D209 falls to 0.08 volts... and the base is just about the same voltage as the emitter on Q207 (which is OFF).

In BOTH CASES the PTT on the mic is grounding the bottom of that circuit which should result in a forward bias condition on D209. I would say that the "0.58 v when it works" condition looks good........ but after the FAILURE hits... the drop is only 0.08 volts which is way low for a forward biased diode.

More bad news..... I don't have a diode that will suffice to test my theory.
I have a 1N4007 but the leads don't even come close to going in the hole. A small glass diode is in there now... MA150... just a general purpose silicon diode good for 100 mA or so.

So.... back to needing to order some appropriate diodes.

Wheeeeeeeeeeeeeeeee!!!!!!!!!!

 

[brandon7861]

I don't think D209 is the problem. 0v at the base of a known good transistor means .7v (or less) at the emitter, and that means the regulator is likely not outputting. Does the regulator output drop too?

 

[guitar_199]

I don't think D209 is the problem. 0v there means .7v at the emitter, and that means the regulator is likely not outputting. Does the regulator output drop too?

No sir. Not one little bit. Everything ABOVE Q207 stays solid.

Only the power sources fed by Q207 fail. When they fail... the only thing that will bring them back is to power off for a minute or so. When you do this.... they come back on for about 45 seconds to a minute... then they fail again.

By the way.... when the radio is stone cold..... on the FIRST power up..... it will stay working for 2 1/2 minutes before it fails. After that.....30 seconds to a minute turn off..... will get you 45 seconds to a minute... then it fails again.

Seems like... SOMETHING is thermal......

the only solid state devices in the affected circuit are Q207 (replaced and problem stayed) and D209. There are no electrolytics that I see IN the affected area. I can't figure anything else that would act so "thermal".

Thanks for thinking about this !!!!!

Bob

 

[brandon7861]

I misunderstood you. I was thinking the voltage at (not across) the diode. Ok that makes more sense.

 

[brandon7861]

D209 makes sense. I guess a quick test would be to let the radio cool but heat the diode with the soldering iron and then turn it on and see what happens.