Vintage Airband SQUELCH problem

[M6RDPfly]

Hi, I have a Swinburne Electronics CR600 Airband Monitor which has 6 crystal controlled frequencies. It has suffered years of neglect and whilst still working and receiving fairly well, the squelch is not so great. It activates as you advance it but then it seems so closed that signals will not open it unless you set it EXTREMELY FINELY. The tiniest nudge too far and it stays closed.

I've checked the squelch chip with the help of a radio friend and all the voltage readings seemed OK.

I've paid twice for it to be looked at by radio ham repairers but this problem persists and I'm running out of money!

Any suggestions or pointers would be appreciated. Thanks for reading.

73 Adam

[Cruisin Marine]

Hi M6,
Do you have a manual, or know where to get one please?
I have a blind suggestion, but not knowing the cct. this is a complete stab in the dark.

The Cleartone CH800 marine and PMR handheld used to suffer from this fault, and the answer to reducing it was to insert a 1nF capacitor across the demodulator output pin 9 to earth. This worked because there was TOO MUCH HF noise coming out of the demodulator pin, and it drove the squelch circuitry potty.
Here is a link to the MC3357 data sheet.

http://pdf.datasheetcatalog.com/data...ola/MC3357.pdf

[M6RDPfly]

Hi cruisinradio,

Your reply is extremely helpful as the radio is now operated in an area of horrendous QRM from overhead power lines and solar panels near the antenna and in a building of flats.

I have a circuit diagram of its older brother the DR600 and the CR600 is built on this PCB. Let's see if I can manage to send an attachment with this post. Yep, managed tgat. The squelch chip is IC4011

[Graham G3ZVT]

Airband frequencies have been shifted slightly to comply with the new 8.33kHz channel spacing, so it's likely that your crystals are no longer bang on frequency.

[Cruisin Marine]

Originally Posted by M6RDPfly

Hi cruisinradio,

Your reply is extremely helpful as the radio is now operated in an area of horrendous QRM from overhead power lines and solar panels near the antenna and in a building of flats.

I have a circuit diagram of its older brother the DR600 and the CR600 is built on this PCB. Let's see if I can manage to send an attachment with this post. Yep, managed tgat. The squelch chip is IC4011

Can you upload a larger area of the diagram if possible please showing the demodulator bit if poss, or a link to a manual if poss?

[M6RDPfly]

I obtained the circuit diagrams from this website

http://www.milair.co.uk/airband.html

[SiriusHardware]

TOO MUCH HF noise coming out of the demodulator pin, and it drove the squelch circuitry potty. Here is a link to the MC3357 data sheet

The MC3357 is an FM IF / Demodulator circuit and its squelch is a 'noise squelch' which works by measuring the amount of 'hiss' present in the demodulated signal to determine how weak the incoming RF signal must be. That's why the mod you suggest has an effect.

However, this is an airband radio, so surely AM rather than FM, and it will be likely to use a 'level' squelch which works by detecting the strength of the incoming signal rather than by the amount of noise present.

[Cruisin Marine]

Originally Posted by SiriusHardware

The MC3357 is an FM IF / Demodulator circuit and its squelch is a 'noise squelch' which works by measuring the amount of 'hiss' present in the demodulated signal to determine how weak the incoming RF signal must be. That's why the mod you suggest has an effect.

However, this is an airband radio, so surely AM rather than FM, and it will be likely to use a 'level' squelch which works by detecting the strength of the incoming signal rather than by the amount of noise present.

Good point, I was in auto FM mode, forgot this was Airband.

[Cruisin Marine]

M6, can you take this unit to some other location and try it? Without knowing the levels needed to switch the Squelch, this will be hard, can the guy who looked at it before let you know the levels he measured to open the squelch?
Also make sure the newer channel spacing of 8.333 kHz (stupid idea is not causing you problems) The Xtals may have to be re- adjusted if possible, or, they may not need moving if the Airband frequencies have remained unchanged. I don't know the score on airband unfortunately.
If the receiver and squelch measure OK, then it could be swamping of signals by the high QRM that is doing this.

[M6RDPfly]

Hi again and thanks for for helping. I've got a whole bunch of crystals and some of them our whole frequencies eg 124.750 (ie non 8.33) and others of them are brand new having been manufactured by quartslab lab and they are precisely 8.33khz spaced eg 128.816666

I like your suggestion of trying the radio in a quieter environment. I will have to sort out a 12-volt battery arrangement.

[Radio Wrangler]

The old channels have been kept for compatibility reasons. The 8.333333 kHz spacing is achieved by fitting two new channels in-between the existing ones. The existing ones can be operated either as a 25kHz style with a wide RX bandwidth, or as an 8.33333 kHz style with narrow RX bandwidth. So old crystals will remain useful, but what used to be on that frequency may well have moved.

Marine VHF is FM, Airband is AM

Squelch is an awful lot easier for FM receivers.

In fact many aviation AM radios have an FM IF chip in there just to act as noise-operated squelch. The requirements docs DO-186 and ED-73 are rather stringent about squelch performance and require it to be open by -93dBm, but radios where the squelch flickers on and off drive pilots up the wall. The squelch in a proper, approved RX is a major undertaking.

It's a weakpoint in many hobbyist receivers.

David

[M6RDPfly]

That's very interesting to read especially about the commercial radios on board aircraft having an FM squelch to match the stringent requirements.

I will have a careful look with a Jewellers loupe at the components around the squelch. I can't remember if I said but this area of the PCB suffered some damage and when I found it the squelch potentiometer was hanging off. It has been replaced with a matching value but possibly a transistor diode or capacitor in that area might be damaged despite there being no visible signs of such.

The headphone socket is also missing so it might be worth replacing that and seeing if it makes any difference.

I'll keep working away at it and any updates will come back to the forum.

Adam

[Cruisin Marine]

The problem with AM squelch is no doubt caused by the fact that in most cases the signal; level is probably driving a comparator, if so this suffers from hysteresis, that is where a set high level, say 10 uV switches the squelch open, but it won't close until that level drops down by a considerable amount, that may be 1uV or so.
That is a large difference of 10 dB. This a desriable trait though, because you don't want the end of the message to suddenly disappear.

There are special comparator OP-AMPS that make this better like the LM319, but from memory they are not pin for pin compatible with a "normal" op-amp, they have open collector output without looking it up.
They do work very well though.
All this said, it doesn't help fix the fault- that needs a signal generator and a bit of measuring, but you can eradicate local QRM being the problem by taking the scanner to a remote quiter QRM location and testing it there of course.

[Radio Wrangler]

Noise based squelch uses the fact that intercarrier noise from a limiter IF strip as used for FM gives noise with a flat frequency characteristic. Speech has a reducing level with frequency, so comparing the power in two slices of the audio spectrum gives one of the more reliable ways of discriminating between speech and noise. FM IF chips were cheap, so it was easy to include one in receivers with 10.7 or 21.4MHz IFs, two audio filters, two rectifiers and into a comparator.

Modern receivers just digitise their IF and throw DSP at the problem, but the same principle works well.

David

[agardiner]

Originally Posted by M6RDPfly

I can't remember if I said but this area of the PCB suffered some damage and when I found it the squelch potentiometer was hanging off. It has been replaced with a matching value but possibly a transistor diode or capacitor in that area might be damaged despite there being no visible signs of such.

The headphone socket is also missing so it might be worth replacing that and seeing if it makes any difference.

I'll keep working away at it and any updates will come back to the forum.

Adam

Trying to look for a simple solution - just a thought - but is the new control a Linear pot? If a log one has been fitted it would make it a lot more sensitive to adjustment.

[M6RDPfly]

Cruisin Marine,

Hi again. Thanks for this information. It's interesting learning about my radio in this way.

I remembered after a while that I took the radio to show my Dad at a remote farmhouse in Cornwall. RF would have ben extremely low there, compared to the flats, solar panels and urban noise here. The squelch was still not working there. HOWEVER I do think a reduction in noise (being picked up almost certainly by the antenna from radiated power supply units in the nearby flats) would help enormously as I could back the squelch off a tiny fraction but still keep it closed. These fractional nudges make all the difference to its operation. The noise is picked up by everything in the shack whether mains or battery powered, from LF right up to VHF. You know you can use QRM Eliminators for HF. Can one do anything about this noise at VHF?

Thanks. Adam

PS The Uniden 9000XLT (triple conversion I think) has much greater immunity.

PPS RE: last question about lin/log POTS. I did this particular part of the repair some years ago and am mistaken, the squelch POT is the original, it was the Volume POT that I replaced, linear with linear.

[Cruisin Marine]

PPS RE: last question about lin/log POTS. I did this particular part of the repair some years ago and am mistaken, the squelch POT is the original, it was the Volume POT that I replaced, linear with linear.
Yesterday 8:45 pm

Just a qucik note:
The VOLUME pot should ba a A type (just think of A for audio) this would have a logarithmic taper.
The SQUELCH pot is normally a B type - this signifies a LINEAR taper.

To find out, centre the potentiometer, as close mid range as you can get and measure the resistance. On a LINEAR or B type this should be approx. the same either side of the centre wiper on the pot (best measured disconnected from the rest of the cct. so as to avoid loading by other components.

[M6RDPfly]

I'll check that over the weekend. Could replacing the volume with a linear pot be responsible for the problem do you suppose?

[Cruisin Marine]

Originally Posted by M6RDPfly

I'll check that over the weekend. Could replacing the volume with a linear pot be responsible for the problem do you suppose?

No, a linear volume pot will not cause the squelch problem.

[M6RDPfly]

Cruisin Marine,

Can you remember we were discussing this squelch problem last week? Well I decided to check the 16k resistor behind the volume pot because it was this volume pot that got ripped from the circuit board years ago and I figured maybe some surrounding components might have got damaged. This resistor does indeed appear to be damaged as I am getting infinite resistance on each of the settings.

Reading infinite resistance and not a short do you suppose the adjacent transistor will have escaped damage or ought I replace that as a matter of course?

Thanks for helping. 73 Adam