Formac 120 problem

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MiXiN
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Formac 120 problem

Post by MiXiN »

I've just picked up a 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&sourc ... Rbg3st3n1Q
pienari
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Re: Formac 120 problem

Post by pienari »

May be the VCO problem.

Not so easy to repair but it can be fixed.
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superstar
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Re: Formac 120 problem

Post by superstar »

I was tempted by one of the recent boxed formac 120's that were on eBay recently, I think one went for £29 and the other £40, was it any of those ones?
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JollyRoger
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Re: Formac 120 problem

Post by JollyRoger »

Sounds like one or more of the channel selector pins has broke away from the board. or maybe the PLL02A is at fault.
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Mitch
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Re: Formac 120 problem

Post by Mitch »

Have you measured the vco voltage on the channels top to bottom? From the crystal freqs you've got minus 40, mid 40 and +40, does it read 41-80 on high band? You can google the bands but the range is 26.515 Mhz lowest to 27,855 Mhz highest, with the vco voltage on TP 8 (or is it B) between about 4v on lowest to 1.5v or so on highest channel. Adjust L1 for the right voltages, it's the first check I'd do, and the quickest test. Use a plastic trimmer for the core, they're delicate.
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superstar
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Re: Formac 120 problem

Post by superstar »

JollyRoger wrote:maybe the PLL02A is at fault.
I'd go with that, not that I know what I'm doing but I had a ham puma that had the same faults, sent it over to Nick with a formac 88 to raid for the pll02a, if I remember rightly, it was a few years back
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satelitesid
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Re: Formac 120 problem

Post by satelitesid »

Start by checking the binary coding on to the PLL, there is no point fannying around with VCO if it's not getting the correct voltage from the PLL, and if the PLL is not getting the correct chunks of binary, it will not work.
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Otter
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Re: Formac 120 problem

Post by Otter »

It doesn't look like channel switch to me. There's no way you should get a frequency ending in 0 with any channel switch coding.
It looks like it's something to do with the voltage in the PLL circuit / faulty PLL / VCO alignment.
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