Author Topic: IC-751 PLL weird problem  (Read 313 times)

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Offline VE1IC

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IC-751 PLL weird problem
« on: December 18, 2018, 08:34:07 AM »
I've got an old IC-751 here that has an interesting problem. For some reason the PLL will not go through frequencies correctly, even so the display sez it does.. The HPL and LPL tune proper as per service manual. What is happening is that the PLL will repeat a slot of 10khz 10 times to the next full 100khz through all bands all modes.
For example 10.000.00Mhz WWV. Everything that is receiving in the slot from 10.000.00 to 10.009.99 gets repeated on 10.010.00 to 10.019.99, then again from 10.020.00 to 10.029.99 and so on. It will do this to the next 100khz and it changes and repeats again like this:
So I will hear what is supposed to be between 10.100.00Mhz and 10.109.99Mhz, but that gets repeated every 10khz to the next full 100khz, also 10.200.00Mhz.
The display works fine, HPL and LPL on the PLL board are bang on, the voice module reads the same frequency than the display. The VCO works properly. I can see blips on the data lines for the HPL and the LPL  IC's when I turn the VFO, but it seems that one of the "swallow counters" isn't counting properly and does not allow the tuning voltage to go ahead and allow tuning from 10khz to 99.9khz. Wonder if anyone ran across this before?

Offline KE4EST

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Re: IC-751 PLL weird problem
« Reply #1 on: December 21, 2018, 01:20:55 PM »
Not seen that one before.  That is a strange problem.
Michael, KE4EST

Offline VE1IC

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Re: IC-751 PLL weird problem
« Reply #2 on: December 26, 2018, 11:51:32 AM »
I think it has something to do with the PLL IC's. There is a contraption made called a swallow counter. Problem is I don't know which IC's go for the 10khz.

Offline VE1IC

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Re: IC-751 PLL weird problem
« Reply #3 on: May 05, 2019, 08:53:11 PM »
anyone has a working PLL unit for sale? I am not sure but if the 745 has the same board than the 751 has. It looks the same...