Jumbo VCO problems?

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sickboy
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Jumbo VCO problems?

Post by sickboy » 18 Apr 2018, 19:01

I have a jumbo which a few weeks ago lost TX & RX. To cut a long story short, I found out that the voltage for setting up the VCO was unreadable. On a digital meter the numbers were moving rapidly between 1 and 5 volts, on a analogue meter the needle was just flicking around at random as well. Turning the slug in the VCO made no difference.
After some help from the nice folk on Cubwolf's page on facebook, I decided to send for a new VCO block from Spectrum. I fitted it today and on ch1 on the low band I was able to set it to the correct 3.6v, on mid band 3.7v and on hi band at 3.8v.
It now transmits on all bands... but only up to around channel 10 or so on each band. When I look at the voltage for the VCO (on the channels it's not TXing on) the voltage is flicking around all over the place again. But if I turn down to below channel 10 on any band the voltage settles no problem and I'm able to TX.
What's causing this rapidly changing voltage on channels above 10 (irrespective of band). Anyone have any ideas?
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cubwolf
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Re: Jumbo VCO problems?

Post by cubwolf » 18 Apr 2018, 19:39

You should have a 10uf capacitor soldered on the back of the board on the vco voltage pin.
Is it there and if it is has it leaked?

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sickboy
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Re: Jumbo VCO problems?

Post by sickboy » 18 Apr 2018, 20:46

No 10uf cap there Dave, but I've just fixed it.. I had emailed Spectrum who I bought the VCO block from to ask if he'd ever seen these symptoms. He suggested it can only really be bad tracks on the channel change pcb, or failing that the PLL chip. I checked the channel change solder joints, then transplanted a pll02a out of an old Tiger 40am and it now all seems to be working fine. All on frequency and even the UK 40 eprom is working.
Happy days :)
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