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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 6th Jun 2017, 4:47 pm   #1
Damo666
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Default Formac 120 CB problem

I've just picked up a decent condition Formac 120 AM/FM CB that's meant to cover 26.965 > 28.305 Mhz on 3 bands of 40, low, medium, and high. However, it's stuck at 26 Mhz.

It uses a PLL02A PLL, the board is a PTBM106AOX as shared with many other models, and it contains a board (PTOS008AOX) that holds 2 Crystals of 20.705 Mhz, and 20.255 for the low and high band switch.

Here is what's happening; On Low, Channels 1 to 6 = 26.260, Channels 7 to 9 = 26.270, Channel 10 to 12 = 26.255, Channels 13 to 19 = 26.260, Ch 20 to 22 = 26.270, Ch 23 to 26 = 26.250, Ch 27 to 34 = 26.260, Ch 35 to 38 = 26.265, Ch 39 to 40 = 26.255.

On Medium (M), Channels 1 to 6 = 26.480 Mhz, Ch 7 to 9 = 26.490, Ch 10 to 12 = 26.475, Ch 13 to 19 = 26.480, Ch 20 to 22 = 26.490, Ch 23 to 26 = 26.475, Ch 27 to 34 = 26.480, Ch 35 to 38 = 26.490, Ch 39 to 40 = 26.480

High (H), Ch 41 to 46 = 26.705, 47 to 49 = 26.715, 50 to 52 = 26.700, 53 to 59 = 26.705, 60 to 62 = 26.715, 63 to 66 = 26.695, 67 to 69 = 26.705, 70 = 26.710, 71 to 74 = 26.705, 75 to 78 = 26.715, 79 to 80 = 26.705.

Can anybody please help diagnose the problem?

Here is a schematic - https://www.google.co.uk/url?sa=t&so...5biHRbg3st3n1Q
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Old 6th Jun 2017, 4:58 pm   #2
Biggles
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Default Re: Formac 120 CB problem

As a first guess I would go for the channel selector switch, particularly if it is the rotary BCD coded click stop type. Maybe you have a cracked print near the switch, or one of the BCD encoding bits is u/s. I haven't looked at the circuit yet. I always go for the obvious mechanical/user interface things first.
Alan.
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Old 6th Jun 2017, 7:14 pm   #3
SiriusHardware
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Default Re: Formac 120 CB problem

For the time being, do all your debugging with the band switch set to 'MID' as this takes the Hi / Low band oscillators out of the equation. On mid band, instead of using a band frequency generated from an extra crystal, the synthesiser gets the required frequency from somewhere else. Once you have it working on Mid, you can look at Hi and Low.

There is a crazy mixture of bad things going on here.

Apart from all band channels being the wrong frequencies and the frequency only changing every few channel steps or so, you also have the (usually) impossible fault of some frequencies in each band ending in '5' and some ending in '0'. Normally the minimum channel interval is preset to 10Khz, in the case of this radio by having the PLL pin 4 (Frequency step size select) permanently high (about 5-6 volts). If this pin is low (around zero volts) the frequency step will instead be 5Khz - so that's the first question - what state is pin 4 of the PLL IC in. Permanently high? Permanently low? Or does it go up and down as you rotate the channel switch?

Second problem (as Biggles says) is that your channel switch may not be feeding the correct progressive binary codes to the P0-P8 binary inputs (pin 15 downwards to pin 7) of the PLL.

Broadly speaking you should see the logic state on PLL pin 15 (bit 0) change state almost every time you rotate the channel switch by one click, the logic state on pin 14 (bit 1) should change roughly every two clicks, the logic state on pin 13 (bit 2) should change roughly every four clicks, the logic state on pin 12 (bit 3) should change roughly every 8 clicks, and so on.

I say 'roughly' because unfortunately the channel frequencies (and therefore the PLL channel codes) on these radios do not follow a simple incrememental step sequence with each channel frequency 10Khz higher than the last - so you won't get a perfect binary sequence out of the switch. From channel 3 to channel 4 there is a 20KHz jump, so the channel code jumps by two - same when you go from channel 7 to 8, channel 11 to 12, channel 15 to 16, and channel 19 to 20. Around channels 22 to 25 things go even more haywire before finally settling down into a straight 10Khz step sequence with the channel code changing in steps of one from channel 26 up to 40.

Just to add to the confusion, the pin numbering on one of the PLL02 pins on that diagram you linked to looks wrong. The pin they've labelled pin 4 (which should be 'Frequency Step') is wired to provide a DC 'enable' bias to the first stage of the transmitter and so it is clearly not pin 4 as labelled, but actually pin 6, 'Lock Detect'.

On other PLL02 diagrams I see that PLL pin 4 (FS) is either just not mentioned at all or shown as not connected, so from this I would assume that the default state / mode for pin 4 is high=10Khz steps, unless you deliberately tie it low.

Forgot to add: What state is PLL pin 6 (LD) in when all of this is going on? It should normally be high (~ 5V-6V) pulsing momentarily low as you click from one channel to the next.

Last edited by SiriusHardware; 6th Jun 2017 at 7:24 pm.
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Old 6th Jun 2017, 9:09 pm   #4
Damo666
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Default Re: Formac 120 CB problem

Thank you for the replies guys.

This is sounding like it may be a complex problem as opposed to a quick fix, so I'm not sure on whether I should just return this to the seller for a full refund or risk fixing it?

I know if I was selling something to a buyer & I found out he/she had been tinkering around inside it, I wouldn't be best pleased.

I'll get in touch with the seller and see if she'd like me to look at this or just return it I think - I'm really not sure on what to do, whether to risk repair, etc'?
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Old 6th Jun 2017, 10:04 pm   #5
SiriusHardware
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Default Re: Formac 120 CB problem

If it was sold as 'working' then contact the seller first, since it clearly is not working.

If it was sold as 'for parts or repair' or with the familiar story 'working before it was put away years ago but I don't have an aerial / cable to check it now' then the seller did not explicitly claim it was working, and unfortunately then you have bought it 'as seen'.

The worst case scenario is that the channel switch wafers - the ones which generate the channel codes, not the channel number display - are broken, but there are plenty of other potentially fixable causes. Most synth faults can be fixed with a frequency meter and a multimeter.

If you already have the covers off anyway, just do this one test - measure the voltage on the PLL IC pin 6. It should be high (between five and six volts) on every channel on transmit and receive. Is that the case?

If the voltage on that pin is low, it means the synth is not 'in lock'. I didn't consider this at first because the radio should not be able to transmit if the synth is out of lock, but if it really is wired as per the diagram then pin 4 of the PLL (normally high unless pulled low) will be supplying a steady enable voltage out to the transmitter first stage regardless of whether the PLL is in lock. It would be a pretty bad error for them (Cybernet) to make, but not impossible.
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Old 7th Jun 2017, 1:58 am   #6
Damo666
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Default Re: Formac 120 CB problem

The seller is getting shut of numerous rigs for a "silent key" from what I can gather, so I think this was a legitimate mistake as the description didn't state is was defective or "spares or repair".

I've spoken to the seller and asked which route is preferential; IE attempt to repair & send it back for a refund, just send it back for a full refund, and I was told without asking - would a partial refund be OK.
If the partial refund is adequate, I'll keep hold of it & take a punt (with my below par electronics skills) - but if not, I'll get it shipped back.

I have a frequency counter & multimeter here, so when I get a reply I'll come back & update this thread if I'm keeping hold of it.

Cheers again, Sirius.
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Old 11th Jun 2017, 2:35 pm   #7
Damo666
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Default Re: Formac 120 CB problem

I decided to send this back, as if it's an issue with the PLL02A I've found that they're unobtainable.

Nonetheless, as always, I'm extremely grateful for the help offered.

Mods, please feel free to close this now.
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