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  1. Yaesu-FT-980
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Replacement trimmer for TC4001 (PLL/VCO Unit)

 

Anyone know where to source these parts in both the correct capacitance range and temperature coefficient?

Over Christmas break I repaired several PLL/VCO Units whose OEM trimmers had become intermittent. After initial adjustment, my 20-40-75M '980 has primarily been used on 40-75 (LSB) and when I went to USB last night the VCO wouldn't lock.

No amount of adjustment will bring the oscillator into lock range for USB/CW, although LSB works fine. This leads me to believe that the part has drifted in value and/or is of the wrong temperature coefficient for this use.

FWIW, the replacements were obtained from Surplus Sales and look to be a high-quality component.

73 - Fred, N8YX
 

I purchased an assortment of NOS Russian trimmer caps at Dayton.? A good quality ceramic trimmer *should* be NP0 :) I was lucky, and I found one that dropped in any worked perfectly. The range was "close enough".

Note: It may be possible that it is one of the other parallel-(C07)/Series-(C08) capacitors that has drifted. If you worried, you can always replace all with your own set of offsetting capacitors :)

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-Mat Breton, N8TW
 

Mat,

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I replaced the trimmer with a nice ceramic one that Paul W6XA found in my first 980.? Problems with the trimmer and the vcv is well documented in the 1983 and 1984 Fox Tango newsletter.? I guess Yaesu corrected that after 1984 as my other 980's did not have the problem.? Still you need to check the sound of upper sideband and lower sideband to make sure they sound the same.? My first trimmer fell apart as I tried to adjust it.

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Good you found a trimmer that was a good replacement.

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73 Dave K4JRB.

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-----Original Message-----
From: <[email protected]>
Sent: Mar 23, 2022 9:30 PM
To: <[email protected]>
Subject: Re: [Yaesu-FT-980] Replacement trimmer for TC4001 (PLL/VCO Unit)

?

I purchased an assortment of NOS Russian trimmer caps at Dayton.? A good quality ceramic trimmer *should* be NP0 :) I was lucky, and I found one that dropped in any worked perfectly. The range was "close enough".

Note: It may be possible that it is one of the other parallel-(C07)/Series-(C08) capacitors that has drifted. If you worried, you can always replace all with your own set of offsetting capacitors :)

--
-Mat Breton, N8TW

?
 

This gets better.

I pulled the rig out of line, put it on my bench (in a cooler environment) and the next day I had a go at adjusting the 30MHz reference and VCO trimmers.

Exactly, perfectly on frequency. And stays that way as long as the rig doesn't get hot. FWIW, there's an auxiliary fan (a Drake FA-7) on my operating table, set to blow on the PSU area of the rear panel. Evidently, this isn't enough to keep the rig cool...so I switched to Plan B and ordered a prototype accessory fan plate (which I described in another thread).

Once I have the extra fans installed I'll see how hot the rig runs when back in its original position. The heat can't be good for it, so I figured I'd address the matter before looking into changing VCO caps.

73 - Fred, N8YX
 

Note: My 30MHz reference had drifted so much over the years (courtesy of the attached heater I'm sure) that I couldn't pull it into range with the existing trimmer. I ended up changing the trimmer and pulling capacitors to different values, and now I should have enough "play" for a lot more years. The xtal continues to drift at about 10Hz year, but that is workable.

VCO: I have a feeling that it may not be the capacitors. Do you have any cold spray and/or a hair dryer? Any chance you can localize the issue to one physical area?

--
-Mat Breton, N8TW

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