Midland 77-810UK "Ready Rescue"

[SiriusHardware]

There should be a trimmer capacitor (CT1) very close to the 10.240MHz crystal which is bent over the PLL IC. Adjust that until the TX frequency on any channel is as close to correct as you can get it to be. 300Hz is not terribly far off, as you say the inherent range of error on your frequency meter could be worse.

From what you've said the synth is locked on all channels on RX and TX so no need to do those checks for now.

This pulsing effect on TX: We need a bit more detail. I don't think the radio itself has an S/RF meter does it?

When you are transmitting from that radio and receiving it on another which has an s-meter, do you see the received signal strength jumping / dipping in time with the pulse effect?

If you put a power meter inline between the radio and dummy load and transmit, do you see the indicated output power dipping or jumping in time with the 'pulse' effect?

If the answer to both of those questions is no, mark / make a note of the present setting of the deviation control (RV2, I think) and turn it all the way down to minimum so that there is no TX audio. Does that also make the pulsing sound go away?

[crackle]

Originally Posted by SiriusHardware

When you are transmitting from that radio and receiving it on another which has an s-meter, do you see the received signal strength jumping / dipping in time with the pulse effect?

If you put a power meter inline between the radio and dummy load and transmit, do you see the indicated output power dipping or jumping in time with the 'pulse' effect?

If the answer to both of those questions is no, mark / make a note of the present setting of the deviation control (RV2, I think) and turn it all the way down to minimum so that there is no TX audio. Does that also make the pulsing sound go away?

Yes the display on the adjacent CB pulses up and down.
I may be able to adjust my power meter to give a sensible power reading at less than a watt to check if the RF is going up and down.
Ill try the deviation test as well.

Mike

[SiriusHardware]

No need, if the RF level is jumping then that is what is causing the 'pulsing' sound heard when you listen to it on a receiver.

If the RF level had been steady, the sound may have been originating in the microphone amplifier but that seems not to be the case.

You mentioned you changed the RF driver transistor - why did you feel you needed to do that when the TX had been working a short while before? (What led you to believe the driver was the cause of the subsequent transmitter problem)?

It sounds like you have two problems, one is reduced RF output and the other is interrupted RF output.

Use a meter or scope to look at the state of the PLL 'out of lock' pin (pin 14) when the radio is on any channel in TX. As mentioned before it should be a steady voltage well above 0V. If it is pulsing at the same rate as the sound you hear on a receiver, you have a synth problem.

If the 'out of lock' pin is in a steady high state during TX the next step is probably to use a scope to trace the RF output from the synth output through to the transmitter, to find out where the signal is changing from steady (normal) to interrupted (not normal). Start by looking at the RF output on pin 9 of the AN103 in TX mode on any channel. Is it steady, or interrupted?

Sorry to have to ask, but in your efforts to repair the receiver, did you alter any adjustments, especially coil settings, or are they all as originally found?

[crackle]

Hi
No adjustments were made to any coils or trimmers except after the TX fault happened I did try cleaning the trimmer POTs in case it was one of those that had a dirty wiper etc.
I feel responsible for the fault as I believe the fault on TX has probably happened as a result of an accidental short on the PCB, probably caused by a stray piece of solder on the mat I was working on.
I have recorded the power meter showing the pulsing RF output. (adjust your volume as the video audio is quite loud, the speaker was next to the camera.) https://youtu.be/Rw7hmDR2XWM.
The sensitivity of the power meter is turned up and I cant be sure of the power it is indicating, normally I calibrate the meter using my Yaesu. But considering the current is only about 200mA it may not be much.
There are occasions when the pulsing stops after a few seconds and the power level is steady, but very low.

After the TX fault happened I went through testing the Final - the driver - and the pre-driver. The driver was faulty on the transistor tester so I replaced it with a same type number.
Tests with adjusting the deviation only made the tendency to feedback more as I turned up the deviation, the pulsing was about the same.

I will do the checks on pin 14 of the 20 pin DIL IC.

Thanks for your help
Mike

[crackle]

I may just add that the B figure on the tester when checking the replacement driver was very high, I also tried another 2SC2078 (final OP transistor) in the driver position and the fault with the pulsing was still evident.
Mike

[Techman]

Originally Posted by crackle

After the TX fault happened I went through testing the Final - the driver - and the pre-driver. The driver was faulty on the transistor tester so I replaced it with a same type number.

I always use an AVO for testing RF transistors, I never use a transistor tester. There's a lot of fakes out there, so unless the replacement transistor was either from known previously working equipment or from a (very) trusted source, then I would be suspicious. Transistors like this bought 'on-line' can often be any old thing such as audio devices re-labeled as RF types, in other words they may test as a transistor, but may not be quite the transistor you think they are, in fact sometimes you're lucky if they're a transistor at all.

[crackle]

The replacement driver (and spare final) transistor was taken out of a TEAM Euro 3100UK CB radio handed to me in pieces, not known for sure if it was working because the radio had been modified and there were quite a few wires loose, but it was original to the radio. I had already robbed the regulator transistor to repair the owners Audioline home base CB.
There are plenty of more spares if anyone needs them.

OK here is the result of putting the scope on pin 14 of the of PLL synth, starts with steady state then transmit, - starts off as pulsing then goes steady, but still low on power.
https://youtu.be/oD6bLGatOZk

Mike

[SiriusHardware]

Are you certain that was pin 14? The reason I ask is that there should be a DC voltage there rather than an RF signal which is what appears to be there.

Although there is RF going to the base of the pre-driver Q6, it should be mostly taken out by the 4.7K resistor going up from the base and then the 4.7uF electrolytic which goes from pin 14 to GND. I'd normally only expect to see a steady DC state on that pin.

In this radio (and most other CB radios) the 'out of lock' pin is wired into the bias feed for the pre-driver transmitter. When the synth goes out of lock for any reason the 'out of lock' pin goes to 0V and removes the bias from the pre-driver transistor, effectively switching it off. This is to stop the radio from transmitting anything when the synth is not in lock.

[SiriusHardware]

If it was pin 14 you looked at, I wonder if this is just a question of scale...

Try looking at pin 14 with the scope again but this time

-Have the scope probe (if switchable) set to X1,
-Set the Volt/Division range to '1'.
-Set the Time/Div control to 2 ms
-Set the input mode to 'DC'
-Use the Y-Pos control to move the trace down to the bottom line on the scope screen

Then try observing pin 14 of the PLL in TX mode again.

[crackle]

OH dear sorry, I am must be loosing my touch, I counted the pins wrong, that previous video was pin 17.

This one is taken on pin 14. https://youtu.be/8GOjBEiWtHM

Mike

[SiriusHardware]

Oh, that looks bad.

For some reason, your synth is going in and out of lock all the time in TX mode and that is why the transmitter is also switching on and off. Your peak output power may be higher than you think it is, but because the carrier is off at least as much as it is on, the average power indicated by a power meter will be lower.

Looking with a meter, can you tell me the DC voltage on the PLL IC pin 17 when on

-Channel 1 in RX mode
-Channel 1 in TX mode
-Channel 40 in RX mode
-Channel 40 in TX mode

Incidentally, I think it is more likely that your measurement in #27 was taken from pin 19, which is where some of the RF from the VCO is taken back into the PLL IC.

[G6Tanuki]

I'm wondering if the 'pulsing' thing is some sort of RF-feedback issue: can you perhaps rig-up a 'microphone' lead with the audio-lines shorted?

Are you running it into a decently-shielded RF dummy-load? And is your power-supply known to be RF-immune?

If all of these are true, then look at the decoupling around the PLL: RF from the output-stages leaking back into the PLL and being inadvertently rectified can easily throw the PLL out of lock - at which point the RF output goes-away, then the PLL manages to re-lock, so you get RF-out again, which perturbs the PLL so it goes out-of-lock, and the RF output goes-away.... wash rinse repeat as we used to say.

[crackle]

Originally Posted by G6Tanuki

I'm wondering if the 'pulsing' thing is some sort of RF-feedback issue: can you perhaps rig-up a 'microphone' lead with the audio-lines shorted?

Are you running it into a decently-shielded RF dummy-load? And is your power-supply known to be RF-immune?

If all of these are true, then look at the decoupling around the PLL: RF from the output-stages leaking back into the PLL and being inadvertently rectified can easily throw the PLL out of lock - at which point the RF output goes-away, then the PLL manages to re-lock, so you get RF-out again, which perturbs the PLL so it goes out-of-lock, and the RF output goes-away.... wash rinse repeat as we used to say.

Nothing has really changed other than some accidental damage which has caused the TX to fail like this.

I will check again but I feel the measurement in post #27 was definitely pin 17.

Will do the tests
-Channel 1 in RX mode
-Channel 1 in TX mode
-Channel 40 in RX mode
-Channel 40 in TX mode

Thanks
Mike

[SiriusHardware]

This is one case where I don't think pick up in the microphone lead will be an issue. The set doesn't have a traditional corded microphone, just a large PTT 'bar' on the front panel rather like an old-school desk intercom. The actual microphone element is inside the unit. It's really more of a walkie-talkie in a modified form-factor.

I'm wondering if the VCO adjustment is just out of whack. In these sets the VCO runs at half TX frequency on transmit, partly so that any deviation applied to the VCO is doubled when the VCO frequency is doubled to get the final TX frequency. It means you don't have to wobble the VCO quite so hard in order to get the required FM deviation. It also places the RX VCO and TX VCO frequencies a lot closer together, only about 2.5-3MHz apart.

In receive mode the VCO runs at 27MHz - 10.695MHz (~16MHz). The DC control voltage alone can't provide that great a shift in VCO frequency, so the 'natural' free-run frequency of the VCO is 16MHz, and to get it to shift down to ~13.5MHz, transistor Q10 switches an extra capacitor into the VCO tank circuit in TX mode only.

On channel 20 RX the VCO coil should usually be adjusted so that the VCO control voltage is around 3V (or half of the synth supply voltage) so it has room to drive the VCO frequency down as far as channel 1 and up as far as channel 40.

The switched parallel capacitance is usually adjustable and conventionally you'd adjust that so the VCO control voltage on channel 20 TX is also about 3V. Here they've used a fixed value shift capacitor so the VCO control voltages for channel 20 RX and channel 20 TX won't be exactly the same.

The setting for the VCO T1 core will be a compromise which keeps the highest and lowest VCO control voltages in RX and TX mode as far as possible from the upper and lower limits.

One possibility is that the 'TX shift' circuit is not working properly, but it's also possible that the VCO coil (T1) just needs adjusting to shift the VCO control voltage more towards the middle of its swing range in TX mode.

[Techman]

Originally Posted by SiriusHardware

I'm wondering if the VCO adjustment is just out of whack.

I agree. I've had this pulsing effect on transmit when the VCO has just been slightly out - on one just recently in fact.

[crackle]

Originally Posted by SiriusHardware

One possibility is that the 'TX shift' circuit is not working properly, but it's also possible that the VCO coil (T1) just needs adjusting to shift the VCO control voltage more towards the middle of its swing range in TX mode.

Hmm, but no coils have been moved since the TX worked.

Anyway here are the results with a meter measuring the DC voltage on the PLL IC pin 17 when on

-Channel 1 in RX mode = 1.649v
-Channel 1 in TX mode = 0.939v
-Channel 40 in RX mode = 2.867v
-Channel 40 in TX mode = 1.786v

Do you think there is damage to the PLL chip, they have some at "Thunderpole".

Mike

[SiriusHardware]

Those voltages are a bit on the low side. Are you up for a bit of tweaking?

Set the radio to channel 20, RX mode, and gently / carefully adjust the VCO coil (T1) until the voltage on pin 17 is about 3.50V. Then see how the radio behaves on all channels on RX and TX.

Don't worry about trying this adjustment, if it doesn't work and you want to set it back to where it was afterwards, just re-adjust the VCO coil until the voltage on pin 17 in RX mode, channel 1, is 1.649V as per your results in #36.

[crackle]

OK but I need to identify T1, The coil with the exposed slug appears to be T1, so I suppose the one with the wax seal is T2. is there any other way to tell.
T2 is directly connected to pin 6 of the 9 pin IC, is that correct?

Mike

[SiriusHardware]

On the diagram, T2 is the one which is directly connected to the AN103 pin 6.

T3 is directly connected to the AN103 pin 9.

I suspect T1 is the one which is bewaxed, as that was a common method used to deaden the physical sensitivity of circuit areas which might be vibration-sensitive or 'microphonic' otherwise. Especially VCO circuits.

(did you realise you can zoom in on the diagram, as it's a PDF?. Still not perfect but you may find it easier to read some of the component numbers if you do that).

[SiriusHardware]

Regarding T3 - the narrow area shown by your picture in #39 does not cover it but I suspect the grey coax next to the north end of the AN103 carries the signal from the AN103 pin 9 to T3, wherever it is.

On your photo, I believe AN103 pin 1 is at the bottom end of the IC and AN103 pin 9 is at the top end of the IC. The pin 1 end usually has a chamfered corner on the package to identify it.