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Old 13th Feb 2017, 1:15 am   #21
SiriusHardware
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Default Re: CB squelch problem

My copy of the GX-19 diagram arrived while I was away over the weekend... I'll try to upload it tomorrow night, if Damo666 doesn't beat me to it.

IC202, the FM demod, is a SIL IC. The number looks like KIA6003S or KIA8003S... maybe Damo666 will be kind enough to read it off the actual IC. The circuit drawing style makes it 90% likely to be a Maxon chassis.

If the FM demod incorporates a noise squelch as suggested by AlanC, it isn't used in this circuit. The FM demod IC isn't involved in the squelch circuit at all. On the face of it, this is good news because the fault isn't going to lie with a hard-to-get IC.

Instead, the squelch is a 'level' squelch. A feed is taken from the secondary of the final I.F. transformer where the signal is rectified to obtain a DC voltage inversely proportional to the signal strength, and this in turn is used as Automatic Gain Control (AGC) back to the receiver input and is also combined with the DC bias provided by the squelch control to turn on / turn off the squelch output transistor, which mutes the audio amplifier IC.

Last edited by SiriusHardware; 13th Feb 2017 at 1:23 am.
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Old 13th Feb 2017, 2:42 am   #22
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Default Re: CB squelch problem

I remember chasing an s-meter (well green LED readout) fault round on a Grandstand Gemini and I narrowed it down to those detector diodes. Tested the particular diode and it seemed good, but tried another one anyway. Turned out to be a bad solder joint in the feed through on the double sided print on the board.
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Old 13th Feb 2017, 9:57 am   #23
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Default Re: CB squelch problem

Some great feedback here, so many thanks for the input.

I'm at work just now, but I expect the schematic I ordered last week to arrive today.

I'll post some images of the schematic, & also some further images of the internals of the CB identifying IC's around the possible offending stage(s).
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Old 13th Feb 2017, 2:29 pm   #24
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Default Re: CB squelch problem

OK, I returned home for dinner & the schematic still isn't here even though the postman arrived today. It should definitely be here tomorrow at the latest.

I opened the CB, and the SIL IC (IC202) is a KIA6003S.

Altering variable resistor RV202 changes the squelch limit, but still no signal can defeat the squelch.

Extremely local stations open the squelch very slightly on the absolute lowest setting (the point where noise and squelch are both on/off intermittently), but the audio is very poor as if it's off frequency.

For what it's worth, here are some more images.
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Old 13th Feb 2017, 6:25 pm   #25
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Default Re: CB squelch problem

My next move would be to check the actual sensitivity of the receiver, if you have the means to do so. This fault appears to be dependent on noise levels in the IF, which could be affected by the performance and correct operation of the IF chip. Distorted audio could be a bit of a clue here. Discriminator problem? The unpopulated three hole component location next to the IF chip looks like it may have been for an IF filter of some description. Intriguing.
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Old 13th Feb 2017, 7:21 pm   #26
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Default Re: CB squelch problem

I'm going to try to scan and upload the diagram later this evening, if the forum interface allows.

I may have to make it a .zip or distribute it to interested parties as email, as the forum pixies will probably reduce the diagram to too small a size / too low a resolution if uploaded as an image.

Some of the circuit lines are already missing on the 'original' so it won't help for it to be degraded any further. I'll see how it goes.
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Old 13th Feb 2017, 8:31 pm   #27
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Default Re: CB squelch problem

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Originally Posted by Biggles View Post
My next move would be to check the actual sensitivity of the receiver, if you have the means to do so. This fault appears to be dependent on noise levels in the IF, which could be affected by the performance and correct operation of the IF chip. Distorted audio could be a bit of a clue here. Discriminator problem? The unpopulated three hole component location next to the IF chip looks like it may have been for an IF filter of some description. Intriguing.
The sensitivity is the same if not slightly better than my friends HF rig, as we compared them just the other day.

With regards the audio, it's only distorted when it's trying to break through the lowest possible squelch setting. When the squelch is off, audio is perfectly fine - so could this still point to a discriminator issue?

I'm going to await the arrival of the schematic I ordered, but in the meantime I'm going to replace the Electrolytic Capacitor just north of RV202 (not sure what it does?), then touch up all joints around the area with a soldering iron.
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Old 13th Feb 2017, 8:35 pm   #28
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Default Re: CB squelch problem

Quote:
Originally Posted by SiriusHardware View Post
I'm going to try to scan and upload the diagram later this evening, if the forum interface allows.

I may have to make it a .zip or distribute it to interested parties as email, as the forum pixies will probably reduce the diagram to too small a size / too low a resolution if uploaded as an image.

Some of the circuit lines are already missing on the 'original' so it won't help for it to be degraded any further. I'll see how it goes.
Thank you for aiding me in this venture, it's really appreciated.

The Knights website stated that some of the diagrams were copies of copies, but mine still hasn't arrived so I've not seen it yet.
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Old 13th Feb 2017, 9:25 pm   #29
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Default Re: CB squelch problem

The discriminator is probably ok if the audio is fine with a good signal i/p. The receiver sens also sounds ok if it compares with a known good one. That may actually narrow it down quite a bit then. Should be easier once you get the circuit. I would go for a problem around the squelch detector and level trigger circuit, whatever form that takes. Could just be a leaky transistor or capacitor upsetting level controls. As an afterthought...has the radio ever worked? Do you know the history of it? If not, it is remotely possible that it has had a component in the wrong place right from the point of assembly. Stranger things have happened.
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Old 13th Feb 2017, 10:01 pm   #30
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Default Re: CB squelch problem

There you go, gentlemen.

Discuss.

(Damo666, your original, when you get it, will look better than this. I had to compromise a lot to get this down to a file size that the forum would accept).
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Old 14th Feb 2017, 12:45 am   #31
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Default Re: CB squelch problem

Before shotgunning it to look for dry joints it might be worth making some basic measurements now that we have the circuit diagram.
The squelch/AGC diode looks to be the glass diode D103 up by the transformer can near the ceramic IF filter.

If there was no RF signal at all at the final IF transformer then diode D103 should be weakly biased forwards so I'd expect to see a positive voltage here of a few hundred mV. But a healthy radio connected to an antenna will produce some 'healthy' RF noise at the 455kHz transformer due to band signals/noise and the noise from the receiver gain and its noise figure and so I'd expect this diode detector to typically detect this noise energy and begin to pull this voltage down and produce a negative DC voltage at the junction of D103 with R125 with big signals. The stronger the signal the more negative this voltage should go. See the yellow arrow in the image below for the measurement point. You can measure this with a DMM on DCV. Does this voltage pump down negative with big signals? It needs to generate this negative voltage with big signals in order to compete with the squelch threshold set by the squelch control on the front panel.

Whilst you are probing in this area, it's also worth a basic health check of the (switched) receiver supply rail at R120. This is pointed to by the red arrow and I think the voltage here should be a steady voltage in the ballpark of 8V when in receive mode.
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Old 14th Feb 2017, 1:20 am   #32
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Default Re: CB squelch problem

The receive meter also gets its output drive from the secondary of T106 (via D150, 1N4148) so maybe we can assume that if the receive meter is responding normally, the receiver is essentially OK from beginning to end. Damo666 seems confident that the receiver sensitivity is normal, but hasn't mentioned whether the receive meter is also working - I think he would have said so if he thought it wasn't, though.

We know the 'threshold setting' parts of the squelch circuit (R234, RV202, R222, D205, and the Squelch control itself, along with the squelch output transistor Q206) are apparently OK.

Attention now shifts to the other part of the squelch, the components which take the RF signal output from the end of the receiver and convert it into negative going DC which then also goes to the base of the squelch output transistor. The components directly involved here are D103, C117, and R126, so a close visual inspection of these components and their solder joints would be useful. R125 and R127 are also involved and should be checked.

If D103 (1N60) is suspected to be faulty, then one possible dodge is to remove it, put it to one side and remove D151 (same kind of diode) and put it in D103's position to see if that fixes the squelch problem. Removing D151 will (temporarily) disable the TX RF output meter - it can go back where it came from afterwards, the aim is just to find out whether D103 is faulty or not.

Note these 'glass' RF diodes tend to be very sensitive to heat damage so if you do decide to try this, be careful not to heat them up too much, or for too long.
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Old 14th Feb 2017, 1:44 am   #33
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Default Re: CB squelch problem

Damo did say that the s meter appears to be working way back in post #4. That's how I deduced that this is probably a dual diode detector system as used by cybernet.

The radio looks like a curious mishmash of Uniden and Cybernet circuits. The final IF stage with the dual diode detector and the squelch system look to be heavily influenced by Cybernet. But the receiver front end looks more like something from Uniden. Parts of the chassic layout look to be Cybernet inspired and other parts Uniden. So whoever made this radio was heavily influenced by Cybernet and Uniden I think.

If you look at the squelch circuit it has the same diode in series with the front panel squelch pot as found on the Cybernet 135 chassis. On the Cybernet 134 chassis this diode was fitted but not used if you look at the 134 schematic on terminal posts 7 and 8.
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Old 14th Feb 2017, 1:59 am   #34
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Default Re: CB squelch problem

And to add to the mix, the style (not the circuit, but the way it is drawn) is pure Maxon, including the very thick dashed line denoting the boundary of the main PCB and the words 'Schematic Diagram' in the font/size that only Maxon ever used (I chopped that bit off to lose a few tens of kilobytes from the file size).

It'll be interesting to know if the set has 'Made in Korea' engraved on the back. Maybe by this time, survivors from all three companies were working together? Or maybe they just looked at previous designs from all quarters and picked out the bits they liked?

What's the prime suspect? D103?
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Old 14th Feb 2017, 2:04 am   #35
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Default Re: CB squelch problem

I suppose D103 is going to be a suspect here although none of the parts should really have a hard life here. What is a bit strange is that the PCB hole pattern for D103 and (especially) D202 is too close together. The diode body for D202 is too big for the PCB holes so the legs are bent a lot to get the part to fit. Maybe they changed over to these diodes from silicon diodes or maybe they just made a mistake with the PCB design for these diodes.

I'd suspect either a diode failure or a poor connection somewhere. It's unlikely that any resistors would go high in value with age but I suppose that is another (remote) possibility.

The chassis does look like a Midland/Maxon type so I think it will have made in Korea somewhere on it but I think it has been heavily influenced by earlier designs from Cybernet and Uniden.
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Old 14th Feb 2017, 2:11 am   #36
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Originally Posted by G0HZU_JMR View Post
What is a bit strange is that the PCB hole pattern for D103 and D202 is too close together. The diode body is too big for the PCB holes so the legs are bent a lot to get the part to fit.
Yes, normally they'd mount diodes like that vertically if the holes are too close together. They must have been treated quite roughly to get them to fit horizontally and if D103 turns out to have failed, that may turn out to have something to do with it. (When removed, it may just fall apart into two pieces).
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Old 14th Feb 2017, 2:13 am   #37
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Default Re: CB squelch problem

Some great input there, Sirius & Jeremy, so many thanks indeed.

It's a bit late now & the rig is upstairs, but I'll get the DMM out later today & report some voltages as requested & also give the solder joints a touch up in the vicinity of the suspect area.

By the way - the S meter reads (LED bargraph) fine.
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Old 14th Feb 2017, 6:02 pm   #38
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Originally Posted by G0HZU_JMR View Post
Before shotgunning it to look for dry joints it might be worth making some basic measurements now that we have the circuit diagram.
I managed to do some Voltage checks, and here's what I got, Jeremy;

At the red arrow, R120, it was 7.95v

At the yellow arrow, D103/R125, it was 0.938V with the squelch fully open. With squelch set maximum, the voltage here was 1.208V. The voltage increased linearly as I turned the squelch from open to fully on.

With squelch set mid range, the voltage at D103/R125 is 1.140V, dropping to 1.135 with a weak signal, and dropping to 1.120 with a local strong signal.

If you require any more checks please just ask.

Hope this helps.
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Old 14th Feb 2017, 6:22 pm   #39
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Default Re: CB squelch problem

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Originally Posted by SiriusHardware View Post
Attention now shifts to the other part of the squelch, the components which take the RF signal output from the end of the receiver and convert it into negative going DC which then also goes to the base of the squelch output transistor. The components directly involved here are D103, C117, and R126, so a close visual inspection of these components and their solder joints would be useful. R125 and R127 are also involved and should be checked.

If D103 (1N60) is suspected to be faulty, then one possible dodge is to remove it, put it to one side and remove D151 (same kind of diode) and put it in D103's position to see if that fixes the squelch problem. Removing D151 will (temporarily) disable the TX RF output meter - it can go back where it came from afterwards, the aim is just to find out whether D103 is faulty or not.
I've inspected the mentioned areas (D103, C117, R126, R125, R127) and all looks fine, but I'll touch these solder areas up just to ensure I've not missed anything.

I'm having difficulty locating D151, but I've ordered a batch of 1N60 which will be here imminently - so will replace D103 with one if it concludes that this is the likely suspect.
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Old 14th Feb 2017, 7:23 pm   #40
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Default Re: CB squelch problem

The voltage changes you've measured during your tests (thanks for doing that) seem much smaller than we would expect. Failure of D103 (leaky) could certainly cause a problem like that so it could be worth just changing it anyway.

Replacing D103 (temporarily) with D151 was just an optional way to find out if D103 was faulty without having to buy any 1N60s. As you already have some on the way, you might as well wait a little bit longer.
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