New patient!

[guitar_199]

From time to time, as I see them, I pick up a few radios, just to put them on the bench and learn with. This is just another in the series.

I got this on an estate auction last night and picked it up today.

It is a Midland 150M 40 Channel AM rig manufactured May 1979. Case and all exterior is in excellent shape. No scratches, no gouges, paint is good all over. Came with original microphone but no power cord which is fine ... it is a standard three pin anyway.

Threw it on the bench just, for now, just to see what it does.

Receive seems to be fine. Fed it Ch 20 modulated ... got good clean tone. Switch radio to 19 or 21 and you don't hear a thing. Nothing!!!!!!! All switches and controls are dirty and cause noise when exercised. Why not? They are just over 41 years old in the unit!

Transmit, is not working at all. Not a glitch at ALL on the radios meter OR my spectrum analyzer. Totally dead.

This is absolutely fine with me as my purpose is to learn to troubleshoot them anyway. The receive works and seems to be tuned fairly well and that right there tells me the PLL and oscillators are (for the most part) working, the audio circuits work. That is a good chunk of the battle right there.

A small "oddity" in receive. After the radio had been on a bit... it developed this little issue of.......had a good clean tone on channel.....then it would go out. I would mess with the CB/PA switch and it would come back...but then go out. Likely the switch .... but it struck me as odd because when it went out....it went out very quietly. No noise.... just .....tone went away. Doesn't phase me.... I'll figure it out.

but when that much of it is working, I think it will just be a good exercise to troubleshoot the transmit problem and fix it.

I really don't think that this thing saw a lot of use. The mic and antenna connectors are still VERY snug. All the channel indicator segments work... I think this is going to give me some fun... as long as I can find parts for it.

 

[guitar_199]

This is weird..... one of the weirdest "mic wirings" I have ever seen and let me tell you why I think so.

The mic socket... only uses three wires/contacts.
Red - Pin 1 - Mic Audio
Black - Pin2 - Shield/Gnd
Brown - Pin 3 - RX switch.
There is no separate wire for TX.

If you look at the switch the "tx/rx switching side" middle contact is shield, the right end is recieve and there IS no wire or connection on the other end pin of the switch.

To my mind this is telling me that with the mic switch RELEASED... the Rcv wire is connected to ground and understood to be in receive mode. The opposite, when you PRESS the MIC SWITCH.... the RCV wire IS NOT connected to ground.... so the radio understands THAT to mean....TRANSMIT.

Okay..... what happens if you turn the power switch on with no mic plugged in? What will the radio do????

Am I right? Opinions?

I realize that you can't see all of the rcv/tx switching logic here.........but.......if all there is ... IS that one brown wire......then..... brown wire grounded means rcv...... brown wire NOT grounded means transmit. Great! What if you don't have an antenna plugged in....and you take the mic off????

"I smell smoke"!!!!!!!!

 

[guitar_199]

Nevermind!!!! I came back to show you.

IN the SAMS schematic.... they broke up the mic switching. I have never seen THAT done before. Usually the mic is in ONE PLACE. NOT in this drawing.

Here is over in the power supply switching.... where the TX voltages are controlled....

Down at the bottom is another little bit of "microphone".....

 

[NZ8N]

As I recall, Midland back in those days had their mic wiring backwards, er, upside down from Cobra/Uniden, in that 1 and 2 were swapped, and 3 and 4 were also swapped, when compared to a Cobra Uniden mic wiring diagram.

1 - Mic audio
2 - Shield Ground
3 - Receive (Speaker Ground)
4 - Transmit

 

[bob85]

Have you looked inside the mic ?
150m's i have seen late 70's early 80's had maxon boards & 4 wire mics like other late 70's maxon board rigs,

1 audio
2 shield
3 rx
4 tx
seems very strange that they would make a version that goes into tx if you remove the mic,

looks too clean to be 1979 it must have been in a box most of its life..

 

[guitar_199]

Right out of the SAMS, your suggested wiring is an exact match. This agrees with the schematic!

As I recall, Midland back in those days had their mic wiring backwards, er, upside down from Cobra/Uniden, in that 1 and 2 were swapped, and 3 and 4 were also swapped, when compared to a Cobra Uniden mic wiring diagram.

1 - Mic audio
2 - Shield Ground
3 - Receive (Speaker Ground)
4 - Transmit

 

[guitar_199]

THanks for replying.
I was terribly mistaken... thats why I left the second message on mics.
Pin 4 is INDEED used.... for some odd reason ... it was just drawn near the power supply.... and neither one one references the other.....
like.....where the normal mic is drawn... they dont have anything hooked to the other pin on the mic switch for transmit. When you go look up the transmit pin 4 diagram near the power supply.... it doesn't show anything at all on the receive pin. It just had me confused for a bit.

But indeed the mic IS wired just like you showed!!!

Thanks much!

Have you looked inside the mic ?
150m's i have seen late 70's early 80's had maxon boards & 4 wire mics like other late 70's maxon board rigs,

1 audio
2 shield
3 rx
4 tx
seems very strange that they would make a version that goes into tx if you remove the mic,

looks too clean to be 1979 it must have been in a box most of its life..