Vintage Airband SQUELCH problem

[M6RDPfly]

Hopefully I have successfully uploaded a photo of this section of the circuit diagram

[SiriusHardware]

The diagram is welcome but we can't use that alone to know which of the resistors on the diagram is your suspect resistor lurking behind the volume control. Ideally, we need to know which resistor on the diagram corresponds to this resistor.

If you are sure it is open circuit that is reason enough to investigate it further. If you can read resistors, can you say what value it is supposed to be, if not, what colours are the bands on it and does it have a circuit reference number (Rxxx) screen printed next to it on the PCB?

[M6RDPfly]

Thanks Sirius. The resistor is the one I've circled, a 16k. I've checked other resistors on the board and they are all reading more or less within their tolerance but this 16k one just reads "1"

[M6RDPfly]

It's banded brown blue orange gold and fits right underneath the volume pot. It's not immediately clear from the circuit diagram that there is another component in there but I think these are just design drawings by the company. Looking at the PCB tracing, the resistor iseems to sit between the centre pin of the volume pot and a transistor BC237

[SiriusHardware]

If the circuit is as drawn then the job of the transistor is to mute the audio regardless of the setting of the volume control by shorting the audio to GND - so logically that transistor is going to be the output end of the squelch circuit.

Setting that aside for one moment, desolder one end of the 16K resistor and then measure the resistance of the resistor only. If it still seems open circuit then try changing it - if you don't have that relatively uncommon value, something like 12K / 15K / 18K would do just to see if it makes a difference to your problem.

[M6RDPfly]

Sirius,

Thanks for the reply. I have this morning ordered a 1 mm soldering tip as my 2.3 mm is a bit wide. It's a nasty place to get to, right underneath the POT. I'll do what you suggest, remove one side from the PCB and re-measure resistance. Ive ordered half a dozen 16k resistors, so if when I've done that it still looks bad we'll replace it and see where we're at. Be great if we're on the right track. I will be sure to report back. 73 Adam

[buggies]

The detected audio comes out of pin 8 (top left) and then via the volume control and the circled resistor to the audio amplifier input on pin 9 (bottom left).
If the resistor were open circuit then you would not get any audio at all...

[SiriusHardware]

That was my thinking, which is why I asked M6 to definitely verify the resistor value. If it really is open circuit then we're going to have to think of some other path the audio could be taking..?

This might be a good time to ask if the meter M6RDPfly is using is auto-ranging or has switched, fixed ohms ranges.

[M6RDPfly]

Hi, ok I've double checked the resistor value and it is definitely 16k ( brown blue orange gold see photo). But you're right it is not infinite resistance. My digital multimeter with set ranges was reading "1" at the 20K range. But I have just put it up to the next range, 200K, and it is reading 23.8 which I assume is too high for things to work correctly.

Replacements are on the way along with a finer tipped soldering iron as it is a xxxxx of a place to get to.

[M6RDPfly]

Here's s the resistor

[SiriusHardware]

Well, you were quite unlucky there, it was perfectly reasonable to expect to be able to read a 16K resistor on the 20K range - 23K is quite a long way out.

When the new resistors arrive see what value your meter says they are. If they also read ~23K you may actually have a problem with your meter. (If you have any other resistors, try measuring the value of those).

If the meter looks OK, definitely replace that original resistor. It's making me wonder if the value of all of the resistors might be significantly out, if they all came from the same source / factory originally.

[Cruisin Marine]

Hi MR6D, apologies for replying late, just seen it all.
I suspect your resistor is fine, as there would be no audio if it was open cct. The AUDIO path is from the wiper of the volume control to the input of the top chip (I.F and Audio I.C.) via the 18K resistor, the chip side of the resistor has a BC237 across the audio , and the BC 237 decides whether audio is passed or not.
In a nutshell what happens is:
The output of the 4011 (Quad, 2 input nand gate) goes via the 39 k resistor to the base of the BC 237 transistor.

The BC 237 acts as on on-off switch that effectively shorts the audio out when the squelch is CLOSED (= NO AUDIO).

When the squelch is OPEN the BC 237 becomes effectivly transparent, and the audio flows through the 18 K resistor without hindrance.

Now, what you have to look for is the switching voltage that controls the BC 237.
So, place your multimeter on the base of the BC237 and see whether it varies between approx 0.6v (squelch CLOSED) and approx. 0v Squelch OPEN.

I cannot say what the variance in the BC 237 collector is exactly, but it will be low volts when the squelch is closed, and higher volts when the squelch is OPEN (when audio is getting through). The difference may not be a lot.
I hope that helps a bit.

[Sparky67]

Bit late reading this...but is that 16k actually an 18k?

[M6RDPfly]

Cruisin, Sparky, Sirius

Great to have you guys helping me out. I really appreciate it. I'm sure we'll get there in the end. I wondered if it might have said "18K". Whatever it is, the banding code is 16K and it reads 23.8 and I've checked other resistors on the radio and they all read within their tolerances, so when I get a few spare hours later in the week I'll whip it out and replace it. Mightn't make any difference but it'll be good practice. I'll run the checks you suggest on BC237 at the same time.

I've repaired a few scanners before, simple things like backlights, signal meters, the blue striplight on the Realistic 2006, so have a little experience at PCB soldering. But health problems make a steady hand a rarity. But I'm a patient fellow and determined not to send it away for any more "we've had a look at it mate but the parts are obsolete now. That'll be fifty quid plus postage please." I've already bought the radio twice over that way. One chap even told me it didn't receive, which is rubbish. It receives extremely well. Who are these unregulated "radio repairers"??!!!

Thank God for this forum! Cheers Adam

[SiriusHardware]

I think Sparky was looking at the image in #30 and thinking the second band looks grey (8) rather than blue (6). Have to admit I thought the same.

Even if your 16K(?) resistor has drifted all the way up to 23K I don't think that alone would cause the problem you are experiencing but it usually helps if the values are as they were originally intended.

[M6RDPfly]

Ah, now that's my unfamiliarity with resistor colours. Sorry. They say a little bit of knowledge.......

[ms660]

It looks like 18k in the schematic and in the photo.

Lawrence.

[SiriusHardware]

The top half of the eight, if it is an eight, is a bit deformed so I can see why M6RDPfly read it as a '6'. 16K would have been an unusual value to pick when the 'standard' values at the time were 10K, 12K, 15K, 18K, 22K...etc.

[M6RDPfly]

OK.... BC237 reads 0.0v squelch open and 0.64v squelch closed, just as you said.

I take it that means the 18k resistor maybe doesn't need replacing, is that right?

[M6RDPfly]

I've removed the 18K resistor and replaced it with a new 18K one. But the squelch is no better. The old one read 19.9 out of the circuit but the new one 18.05 out of circuit. Obviously not enough drift to make any difference.