FORMAC 120 - frequency shifts when modulating.

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FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Does anyone have any ideas why my FORMAC 88 shifts frequency when I speak?

I can't see any leaking caps.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Just to add that it has been modded not sure how much but it does attempt UK forty frequency when the right combination of switches are flicked.

I'm sure it's on frequency but when I speak my frequency counter moves about 10kc up and down.

It was a freebie when I mentioned my new interest in cb radio (rekindled) to a friend, he was able to dig out a couple of of coffin mikes as well although I quite like the powermike!

Id like to get it going for now and I'm not completely incompetent with a multimeter although it looks like the last owner cut tracks and added circuits.
Last edited by GDog on 27 Jul 2020, 12:14, edited 1 time in total.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by MrWeetabix »

post some pictures of the board/insides of the radio, as it may give someone an idea if any mods have been made etc
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Image Image Image
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Re: FORMAC 120 - frequency shifts when modulating.

Post by cubwolf »

They all do to some extent because the fm mod is applied to the 10.695 tx crystal but 10kc seems a lot. So when you speak it goes up a channel?
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Yes it does seem to be about half a channel. I do also have what looks like a ten kc switch on the nb /anl circuit.

If I flick from fm to am then it looks like am low mid high works but even then it's not bang on frequency when I key up it seems to take a while to get there. About a second or so.

If I flick the switch to FM then the HML switch does nothing. I just get forty channels Fm. I am sad about this.

It does when I got it it seemed to be on zeros when I flicked the 10kc switch. Why would someone need zeros?

Can I tune the fm board using the two potentiometer black and white pots? The two pots at the back seemed to turned and cracked / broken by a past owner. You can see them in one of those pictures.

I'm tempted to try and put it back to standard but unsure if it's just a case of removing the diodes and following the circuit diagram. The one I have downloaded is small and difficult to read.

I will upload a better picture when I get home from work. Thanks for the reply, maybe it's not as bad as all that after all but I'm told I am off frequency and unsure how to adjust for the best.

If I could find a rig doctor in my neck of the woods I'd see what it would cost to repair but none here in Cheshire that I know.

Not sure if it is worth repairing but it's a shame I hear they put out 10 watts which would suit me down to the ground. Im not greedy, and I've read the reception on them is quite good.
Last edited by GDog on 27 Jul 2020, 13:33, edited 3 times in total.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

If I could find a rig doctor nearby do you think it's worth repairing?

Would it be possible to expand this set any further? If so it's worth spending some on it I guess.

Would I be right in thinking if I could get low, mid, high, super high Fm with A 10kc shift then I could navigate Uk40 channels on High and Super High?

Also the back plate says FORMAC 88, so could this be an 88 modded with a FORMAC 120 face plate? I'm starting to think so.. The lack of a bleep board being one thing.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by 14CS06 »

Hello

Is your PCB PTBM106AOX ?

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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Yes thats correct, same as Hygain I have read on here. Although which one I am not sure.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by 14CS06 »

Hello


Image

Image

Image

Image

Image

Image

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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Thank you very much indeed Claude!
I think those will help me massively!
I did tweak a couple of pots and now seems on frequency using a powermike set at four I'm not muffled. I need the power Mike at 8 on my major 588 otherwise I'm quiet.

Thabks again I really appreciate those pictures!
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Re: FORMAC 120 - frequency shifts when modulating.

Post by Mitch »

GDog wrote: 27 Jul 2020, 13:15 If I could find a rig doctor nearby do you think it's worth repairing?

Would it be possible to expand this set any further? If so it's worth spending some on it I guess.

Would I be right in thinking if I could get low, mid, high, super high Fm with A 10kc shift then I could navigate Uk40 channels on High and Super High?

Also the back plate says FORMAC 88, so could this be an 88 modded with a FORMAC 120 face plate? I'm starting to think so.. The lack of a bleep board being one thing.
You've only got the single crystal board so it was originally an 88, 80 channel mid + high band, someone's put a 120 front on and epromed it by the looks of it. The 120 has a sel-call plug on the back as well as the bleep board and two band crystals. They're decent radios, I use one myself (an 88), but hardly worth expanding imho. I've just got mids and UK 40 on mine, there's not much doing on extra channels anyway, except sidebanders, so it's enough. I should add that I've got a noise squelch (no pop) and a chaser 5000 roger bleep in it :D
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Ok yes I was guessing as much lol. To know now it's fine as a UK forty rig. Seems clear enough and reception is good. Seems to be giving +50 signal to locals as well.

I'm quite happy with it, now I just need a way of figuring out it's output power stage to see if it will / is already pushing ten watts.
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Re: FORMAC 120 - frequency shifts when modulating.

Post by Mitch »

It's easy enough to measure with a watt meter, but check both ends of the bands to see if it's even, and keep an eye on current comsumption too if you can. If you do need to give it a tweak just be aware that diode D13 will likely go o/c and give you no output, it's an easy replacement but make sure it'll do 2 amps. They will do 15+watts out, but power fade when hot, so 10W is about right. I've changed the 1969 final for a 1307 in mine, nearly 20W out and no fade, lol, just hot, but I've turned it down to 4W for the linear.

Edit: Here's a better schematic -
PTBM106AOX.pdf
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Re: FORMAC 120 - frequency shifts when modulating.

Post by GDog »

Hi Mitch, so should it be 10 watts as standard? Thanks again will look at everything posted today.

I will lift the lid on it again today to take a full board picture and upload here for anyone else who has this set.

It's been 33 years since I was on the air and I'm slowly picking things up again.

I didn't know if I could get this little FORMAC running but I'd like it to be as good as it can be.

20 watts probably too much for me but fifteen watts sounds tempting. Is it an easy mod to do?

I have the 10kc switch but wouldn't it be better to put a shift on the rf-gain... Or maybe I should leave it working as is I.e. If it's not broken don't fix it.
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