Midland 5001 z PTT squeak.

[Uncle Ronnie 336]

I just happened upon a 5001 z and it transmits and receives but there is a bit of a squeak with the PTT going in and out of transmit mode. And I have a hunch that another one of the same model that I have on the shelf over here which doesn't receive or transmit at all has the more advanced symptoms of this issue. On the radio that still works, should I swap some electrolytic capacitors? I have a schematic printed out

 

[sp5it]

Does it squeal when you short TX pins with wire instead of microphone?
Does it happen on dummy load or on antenna?

 

[Uncle Ronnie 336]

dummy load, tried a couple of mics

 

[Uncle Ronnie 336]

It's not a long squeal. Just a quick squeak.

 

[Uncle Ronnie 336]

Dove back into this radio after I took a look at it through a magnifying glass and found some very odd scorched parts so I replaced everything that looked like it might have burnt up and it definitely sounds good on the audio but I'm still getting that little bit of a squeak when I hit the mic PTT. Doesn't matter what Mike it is. I feel like there is something hanging up in the process of going from RX to TX.

 

[Handy Andy]

You may not be able to do much about the "squeak" - it is a power supply feeder line issue that causes it.

• It's the "ramp" voltage from the tuning shift it does, when the radio goes from TX to RX - the IF the PLL generates, changes - it shifts, drifts actually - 455kHz so the AM IF on RX lines up to hear - but then shifts, ramps back up on TX to put the IF back to the approximate 16MHz signal.

So what you are hearing is a "de-coupling" artifact - not by shift itself, but the PLL's power draw used to make the Varactor change it's capacitance and the PLL "tracking it" is what causes the noise.

You don't always hear it on RX because of the switchover - others hear it because of the Mic and Audio Chip amplify the noise in TX - in older radios they did several different methods to improve the noise "chirp" caused by the PLL tracking thru the process of seeing the Frequency IF it needs - so they put in smoothing capacitors and even used high-quality "computer caps" the ones rated to 105C versus the factory standard of 85C temperature rating.

In the 5001Z though - the PLL is vastly different and it's power supply needs are quite complex, as it uses the on-board for channel display not the encoder, BCD switch of old - so the design is even different - to guide you to fix the problem? There's really not much that can be done.

Mike had a vid similar to this but makes the noise appear thru showing leaky caps - so if you have some newer caps to try and replace but the noise may not go away - this is a design flaw.

And in these radios, there is a LOT of them...


But there is a frequency mod...

 

[Uncle Ronnie 336]

Right on I will go through and recap everything. This radio has a history of being in a fuel hauling truck so it has seen some work environment which I feel plays a factor in things acting wonky.

 

[Uncle Ronnie 336]

I'm also looking at l16 like there's some power loss going there because I've never seen an output coil encased in metal

 

[Uncle Ronnie 336]

In the process of recapping c38 which is a 33 UF cap got swapped to a 4.7 UF. Looking at the schematic that is what appears to be the limiter release.

 

[Uncle Ronnie 336]

Just noticed rv2 on the schematic is 10K ohm and in the radio it is 2K. Could this be the reason why the audio is clamping at around 11w pep with it wide Open?

 

[cralt]

It's not a long squeal. Just a quick squeak.

When its squeaking do you see it on the radios meter?

I have a 5001z that doesn't always go to transmit. This was new out of the box. It seems really sensitive changes in the antenna system. A friend of mine got one also at the same time that acts the same.

When it fails to go to transmit the meter stays low, quickly twitching and it transmits a squeal with almost no power. When its on the edge of doing it it will squeak and then the meter will jump up and it transmits like normal with good audio. It is like the PLL is falling out of lock or is on the edge of always falling out.

I can mask the problem by tweeking 1 of the cans in the final stage of the transmit circuit to "match" the antenna but as soon as you change antennas it happens again.


It shows up bigly with the channel mod once you start to get away from the normal band.

 

[Uncle Ronnie 336]

When its squeaking do you see it on the radios meter?

I have a 5001z that doesn't always go to transmit. This was new out of the box. It seems really sensitive changes in the antenna system. A friend of mine got one also at the same time that acts the same.

When it fails to go to transmit the meter stays low, quickly twitching and it transmits a squeal with almost no power. When its on the edge of doing it it will squeak and then the meter will jump up and it transmits like normal with good audio. It is like the PLL is falling out of lock or is on the edge of always falling out.

I can mask the problem by tweeking 1 of the cans in the final stage of the transmit circuit to "match" the antenna but as soon as you change antennas it happens again.
View attachment 46965

It shows up bigly with the channel mod once you start to get away from the normal band.

It doesn't really show up on the meter but it is audible on another receiver no matter what I use for an antenna or dummy load. I'm going to do the channel mod now to confirm if it is worse on the outer edges. I really feel like this radio could be way cooler than it is with the right mods to it

 

[cralt]

Ya the case is really well built heavy gauge metal and the mic is a decent build and weight. Bigger speaker then a cobra 25. I hope I can get them straightened out someday.

I wired my Chanel mod to the ch9 switch.

 

[cralt]

I found the alignment procedure for the 5001z on the FCC application of all places.
https://fccid.io/MMA5001Z


So I cleaned out the glue and figured out what was what on the board vs the schematic. And what I thought was a can in the final stage was actually the lock voltage. Now I have the attached PDF I was able to set it correctly and it seems stable now. No more squeaks or buzzes and it keys everytime.

and fyi for the searchers in the future
rv2 - amc
rv1 - squelch adjust
rv3 - s meter
rv4 - rf meter
also attached the schematic

 

[midnight special]

tighten the main board screws to chassis,,,