CB squelch problem

[Damo666]

I've just picked up a Pama GX19 CB, and it works fine on Transmit & Receive - however, there's an issue with the squelch.

The squelch knob completely cuts the background hash as it should, and when the knob is turned back the hash reappears, but once the squelch is on, no signal will defeat it.

I have looked everywhere online for a schematic, but I can't find one - so I'm not sure where to start looking for the issue.

I've attached an image if somebody would please be so kind as to help me where to start looking?

[Biggles]

Does it receive a signal with the squelch defeated. i.e. while the squelch control is turned to give continuous noise, can the set receive a transmission over this noise. If it doesn't then the receiver may be dead or on a different channel than the display says.
Alan.

[SiriusHardware]

I don't have anything by Pama in my collection and I don't don't recognise the chassis, unfortunately.

This site

http://www.shopingathome.com/Circuit...ens%20Band.htm

lists a PAMA GX-19 owner's manual (item #15 as you scroll down the page) which (they say) includes a block diagram and circuit diagram. Unfortunately it's a pay site. I have not used the site myself and can not vouch for it, so proceed with due caution.

[Damo666]

Originally Posted by Biggles

Does it receive a signal with the squelch defeated.

Yes, it receives loud and clear with the squelch set for continuous noise - and only when the squelch is set to turn off noise can nothing whatsoever be heard.

I've even set the squelch at the bare minimum to cut noise, but no incoming signal can defeat it - the S meter shows signals are still coming in, too.

[Damo666]

Originally Posted by SiriusHardware

Unfortunately it's a pay site. I have not used the site myself and can not vouch for it, so proceed with due caution.

Thanks for the heads up.

I can't find any contact information on that site which is a concern for me, and the price of the PDF is more than I'd like to pay as it cost me more than the CB.

If the PDF or paper copy was only about 6 or 7 quid I'd be OK with that, but the asking price is unreasonable to me.

[SiriusHardware]

I thought the website was asking rather a lot for the manual.

This forum post

http://forum.cb-centrum.pl/pama/cb-pama-gx-19/

on a Polish CB forum may possibly have the manual / diagram attached - but you would have to register on the website in order to access it. Again, I have not used this website so proceed with caution.

[Biggles]

Is the set AM or FM? This could help identify the fault as the AM set may use the AGC line to open the squelch. If the S meter is working it may just be some sort of failure between the AGC line and squelch level detector. If it is an FM set this function is normally incorporated in the IF chip. If you could identify the IF chip that may help.
Alan.

[G0HZU_JMR]

Looking at the chassis, I'd have a look to see if it has a PCB model type number stamped on the main board anywhere. It looks a bit like a cybernet clone but with a slightly strange layout. Maybe the best thing to do is to try and find a PCB number stamped somewhere on that main board. Sometimes you can find it tucked away in a corner or on the back of the board. Then try and find a schematic for another brand of CB that uses the same board.

Sometimes the squelch can be adjusted internally to be very strong even on the minimum setting so I'd check the position of the relevant internal trimmer pot to see if it has been set fully on. If you look at the sky blue 455kHz IF filter in the centre of the image and then look vertically down a couple of inches, these two trim pots might be the squelch or the s meter. Is one of them fully rotated? Back in the day, some people would turn up the trimmer fully to allow them to 'squelch out' troublemakers that had a strong signal. The downside of this was that the squelch wculd be very aggressive on the minimum setting on the front panel control. But people often wanted 'more' squelch range for the reason given above.

Cybernet radios tend to use a design with AGC and the squelch and S meter operate from this even on the FM only sets but I don't know about anything made in the last 25-30 years. I am nearly three decades out of touch with CB radio technology

[SiriusHardware]

Originally Posted by Biggles

Is the set AM or FM?

The transformer just north of the Audio amp IC on its heatsink looks very large to be a typical DC supply input choke. It looks almost big enough to a modulation transformer (for AM). But if it was, there would normally be a smaller transformer-like choke near to where the power comes in.

Nor can I see a typical FM I.F. IC, but while I was looking around for service data I did find images of the Pama GX-19 with the CB/27/81 logo on the front panel, so it either is FM, or comes in various international versions all with the same model number.

Pama were a UK based distributor (still are, but they only do things like accessories for mobile phones now) so I would have thought that anything bearing that name here would have been made for the UK market.

Damo666: More images please, especially closer shots of each quarter of the PCB, but a couple of the front panel and one of the rear panel would be helpful as well.

[G0HZU_JMR]

If you look at the image below I think the 455kHz FM chip will be the SIL packaged IC highlighted in light blue near the bottom of the image.

I'm going to guess that the 27MHz receive path is the pink bit. The 10.7MHz IF path is marked in green and the 455kHz is in light blue. It looks like it has a crystal filter fitted in the 10.7MHz IF. I wonder if this is standard or a mod? The first LO path is shown with an orange arrow. But all of this is just a guess.

But the trimmer pointed to by the yellow arrow looks like it might be set fully one way and it might be the squelch pot. So the best thing to do is identify the main board number (and then find a schematic for it) and verify that the pot is the squelch pot.

[Damo666]

Originally Posted by SiriusHardware

This forum post on a Polish CB forum may possibly have the manual / diagram attached - but you would have to register on the website in order to access it. Again, I have not used this website so proceed with caution.

I'm registered on there, but unfortunately the attachments are only images of the CB itself.

Thank you for searching around though.

[Damo666]

I can confirm this is a UK 27/81 FM set, and nobody has been twiddling inside it.

The actual squelch kicks in at just over 1/4 of a revolution of the front panel knob, and at this stage it should be allowing very weak signals to defeat it - in fact, I set the squelch so low that even turning it very fractionally down allows the static continuous noise through.

I'm away from home just now, but I'll post further images tomorrow & look out for a PCB reference to see if it matches any other CB.

[SiriusHardware]

Sorry the Polish lead turned out to be a dud.

If (when you can do so) you can read us the number on the single-inline IC that Jeremy outlined in blue, that would be helpful.

[G0HZU_JMR]

If it definitely hasn't been fiddled with then there isn't much to go wrong on a squelch circuit if you have already proven that the audio can be muted and unmuted with the front panel squelch control.

However, one possible failure could be if the squelch uses a different detector diode to the s meter. This is common on cybernet radios but we don't know who makes this Pama radio.

But looking at the image below the glass diode marked by the white arrow could be the squelch detector diode. This is assuming it really is a diode. The picture is too fuzzy to be certain.

Usually the squelch threshold (on a cheap/basic squelch system) is set by the user to just turn off the audio at a certain signal level. This is done with the front panel control and in order for the squelch to open the receiver has to then receive a signal big enough for the squelch detector diode to override this user threshold. So if the glass diode in the image below is the squelch detector diode and it has failed (or something near it as failed) then the receiver can never open the squelch (even with a huge local signal) because there is no detected signal to override the user threshold. If the s meter uses a different detector diode then the s meter could appear to be perfectly healthy in operation despite the issues with the squelch.

I'm going to guess than the trimmer next to this diode is the squelch range trimmer. The trimmer just above it is probably the rx s meter trimmer and the glass diode above it that sits by the transformer is probably the separate detector diode for the s meter.

Try checking the arrowed diode on a meter to see if it has failed? Maybe have a look to see if it produces a changing voltage with increasing signal strength. Note that these diodes are often set to detect and produce a falling voltage with increasing signal level so don't be surprised to see if the voltage goes 'down'. On quite a few CBs this falling voltage also acts as the AGC voltage for several parts of the receiver strip and it reduces the stage gain of some of them with bigger signals. Of course the squelch diode could also be the one further up by the 455kHz transformer near the 455kHz ceramic filter. If these diodes are all OK then you could check the health of any electrolytic caps that sit on the AGC line that might be connected to the AGC/squelch system to see if they are leaky. Maybe the one near the white arrow? But I'm guessing at all of this because I don't have a schematic.

[SiriusHardware]

What Damo hasn't said (and I forgot to mention) is that this radio, despite its size, has a four-LED meter. The SIL IC might actually turn out to be the bargraph IC, similar to the one in the Cybernet 1000, etc.

[SiriusHardware]

Damo666? Third time lucky?

Go to the website of Knights CB (They have been going for absolutely years, since the year dot).

http://www.kcb.co.uk

On the left side menu, go to

Spares and Tools \ Service Manuals

In the box headed 'Circuit diagrams and service manuals' go down to the link 'list Of diagrams' and click on it. The circuit diagrams listed include five Pama models including the GX-19. I don't think you'll object to the price they quote for the diagram only. I might even buy one myself for my collection.

As an aside, they also seem to have a pretty good range of typical CB spare components including RF output transistors, all the usual suspects.

I had been wondering where to get them from ever since the original Grandata, - who used to be the go-to-people for Japanese (2SA / 2SB / 2SC) transistors - disappeared. So my determination to find you a diagram has actually been of direct benefit to me as well.

[Damo666]

Sorry about the late reply, guys. My intention was to post further images & try to look into this, but I've been rushed off my feet all day.

Sirius - that's a nice find, so many thanks. I've ordered the GX19 circuit diagram so will upload an image when it arrives.

If you or anybody else wants a copy, feel free to ask and I'll eMail or send it via normal post.

[G0HZU_JMR]

In the box headed 'Circuit diagrams and service manuals' go down to the link 'list Of diagrams' and click on it. The circuit diagrams listed include five Pama models including the GX-19.

Good spot. That should help a lot.

The reason I think the SIL IC is the FM chip is because it has a tunable inductor (455kHz resonator?) in a metal can next to it and a mixture of ceramic (RF) caps and those shiny green (AF) caps around it. So those three clues add up to some form of demodulator chip.

I'd expect the LED s meter chip to either be near the edge of the main PCB or maybe even in the front panel PCB. Either way it is going to have to provide a path for 4 signal lines to the LEDs. So this is best done from the edge of the PCB.

I'm not sure how it does the squelch detection because I'd expect to see the detector diode up near the final 455kHz IF transformer near the ceramic filter. So maybe the one I arrowed in white does something else.

This radio looks to be newer than anything I've ever worked on so I'm going to guess it is post 1990. It's therefore difficult for me to guess who made it but the chassis looks quite cheap in terms of construction. A bit like the old Maxon/Midland (made in Korea?) radios but these typically had a MC3357 IF stage. It would be interesting to see the circuit diagram. I suspect that the squelch and s meter will be derived via a diode detector but I can't see enough detail in the image to see if it might have a dual diode detector (with AGC) system like the old cybernet FM radios. Could there be another diode hiding up somewhere near that final 455kHz transformer? It would also be worth inspecting the solder joints in this area to see if something has a poor connection.

[SiriusHardware]

Originally Posted by G0HZU_JMR

The reason I think the SIL IC is the FM chip is because it has a tunable inductor (resonator?) in a metal can next to it and a mixture of ceramic (RF) caps and those shiny green (AF) caps around it. So those three clues add up to some form of demodulator chip.

For all the reasons you've stated, I agree. After all, if that isn't the FM / IF / demod ...then what is?

Originally Posted by G0HZU_JMR

I'm not sure how it does the squelch detection because I'd expect to see...

Hopefully when we get sight of the diagram all will become clear.

Originally Posted by G0HZU_JMR

A bit like the old Maxon/Midland (made in Korea?) radios but these typically had a MC3357 IF stage.

Funny you should say that, I found a couple of images of the back ends of Pama radios (not necessarily the GX-19) and they had 'Made In Korea' on them, although not with the classic Maxon date stamp and red 'Inspected' sticker. That doesn't mean all Pamas are Maxons, or even that those particular radios are Maxons.

Notice there's a GX-25 and a GX-29 as well - most likely the same PCB with the missing components fitted to provide RF gain, Mic gain as additional (mostly unnecessary) features on the higher models.

[AlanC]

Normally with these types of FM IF system chip (the Motorola chips like MC3357/59/61 etc being classic examples) the squelch is a 'noise squelch' that utilises the characteristic reduction in noise level as signal increases- what we call quieting. A sample of the discriminator audio output is fed to a diode rectifier via a RC high pass filter with a cutoff of about 5khz to ensure that it does not contain speech components, and the resulting DC is proportional to the degree of quieting. This is applied to an op amp in the chip together with a DC reference level from the squelch pot, when this latter is exceeded the op amp output changes state and enables the audio path.

Assuming this rig is like the above, given your symptoms it's likely that one of the filter components or the noise rectifier diode is faulty. But we will see!

Best wishes
Alan