Troubleshooting a Colt Excallibur

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29LV145
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Troubleshooting a Colt Excallibur

Post by 29LV145 »

Hey all

Hope someone can point me in the right direction to get this radio back up and running please. Rig has been sitting in the attic for years but decided to bring it down and start using it again when I'm not doing the needful on conference calls for work.

Board is a PTBM125A4X

The rig does not transmit or receive, no change in sound when I adjust the volume, squelch doesn't close as expected. There is no change in received sound when I switch modes.

Someone suggested elsewhere to look at Q36 which might have been the problem so I swapped it out for a TIP120 but still no change in the issue, I measured the volts on the PLL but most of the readings seemed to be around 0.6v

Any pointers on what I could check next please if its something that could be fixed without shipping it over to a rig doctor here in Ireland?

Maybe the VCO has failed? The test point ( TP3 ) next to the Orange VCO showed 5.4V when I metered it last.

Thanks in advance for any help

Cheers Andy 145 Ireland.
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Mitch
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Re: Troubleshooting a Colt Excallibur

Post by Mitch »

Have you got the Cybernet Export Manual? There's an online copy here if you need one. I'd start by checking all the voltages are right, 9v, rx volts, pll volts etc. and check the zener diodes too, it's a bit difficult to know without seeing it though. It could be electrolitic caps gone fubar, or thru board plating fault, or dry joint even. There's a lot that can go wrong, but it could be something simple. Good luck with it.
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29LV145
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Re: Troubleshooting a Colt Excallibur

Post by 29LV145 »

Hi Mitch

Thanks for the link and reply, I found that on my googling around - I'll start checking voltages etc, worse case if its not simple I'll ship it off to someone around here to take a look.
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Re: Troubleshooting a Colt Excallibur

Post by 29LV145 »

Update from my side...

Changed Q36 and D55 - white noise is getting louder when increasing the volume and changes when switching modes. Still no receive and there is some RF on transmit but not having a frequency counter to see where it is actually transmitting isnt helpful.

I'm wondering if the next issue is a bad VCO - pin6 on PLL02A is 2.3V and pin5 is 5.73V - suggesting no lock?

If I tune a HF radio to 19.880/20.105/20.330/20.555 ( offset oscillators? ) for the different bands I can hear a signal, turning off the radio the signal is gone, if I move the tune knob. the signal moves.

Does this sound like a bad VCO?
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Mitch
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Re: Troubleshooting a Colt Excallibur

Post by Mitch »

Have you checked for 10.240Mhz? There's a capacitor there that can go bad. VCO's can go too, Spectrum do a good replacement.
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29LV145
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Re: Troubleshooting a Colt Excallibur

Post by 29LV145 »

Not yet - need to order myself a frequency counter to start sniffing for RF correctly - again thanks for the reply Mitch.
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1968neil
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Re: Troubleshooting a Colt Excallibur

Post by 1968neil »

I had the very same issue on my MK2 excalibur after years in the loft and found that the lead thrus (vias) had corroded on the pcb.
I managed to fix mine by using a small sewing pin and gently probing the vias around the vco and mixer and found two bad ones.

The fix is to drill them out with a pcb drill and insert a vero pin of piece or wire and solder up both sides of the board.
Been alive and well for the past two years :)
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Irishman1127
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Re: Troubleshooting a Colt Excallibur

Post by Irishman1127 »

Ray in Carlow is the man for that job. He'll have it fixed in no time
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29LV145
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Re: Troubleshooting a Colt Excallibur

Post by 29LV145 »

Ended up stripping out the radio and installing an Iler40M kit in place. The Ham Jumbo 3 that sat next to it in the attic is working fine ( so far /touch wood )

Andy
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