The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

I have had this thing on my shelf awhile. was pretty dirty when i got it, and frequency didn't just drift, it rolled. I removed the extras from it, like extra crystal switch and restored the original in its place. then i replaced the pll but the frequency is still rolling. on the tester it shows the voltage is rolling too.

well, aside from a very large digital clock, any other ideas i might try?

PC-497 AB is the board,

I cant say for sure what else is going right, like receive, transmit or modulation with this thing all over the place, it wont even register on my meters.

Have you checked the Diodes around the PLL area.
There should be several diodes between the PLL and
where your wires go to the channel selector..One or
more of these diodes could be O/S

I would go through and check all of the voltages first, in case the root cause is a bad regulator, you could be chasing the symptoms not the cause.

Check Pin 9 on the PLL, it should be 8 vdc, if ok check C90 and C92.

so far I quickly tested pin 9, not sure which is 9, but tested all 4 corners to see what i got... on the back side left, i get a voltage rolling between 1.5 and 3.25 (roughly) the other three corners are also erratic but i get the negative along with a 0.10 area voltage.

as with the frequency, all erratic.

trying to set the vco gives a similar result. it wont lock on anything.

I will give a look at the caps and diodes as well but thought I would update you guys so far

It sounds like IC4 (MB3756) regulator has gone south, check Pin 1 it should be at 8.0 vdc, if not replace IC4.

FYI pin 1 is identified by either a dot at that corner, or it is the bottom left pin when the notched end is at the left (first pin when going anti-clockwise from the notch), so just go from there and count.

-thanks guys, gives me someplace to start. making my christmas components list now

Can you test this radio with another frequency counter?

If your radio has the same problem with an external Freq. Counter you have to take a look for a crack or bad solder connection in the crystals.But if your radio is stable in frequency with an external Freq. Counter you have to test the D528 in the Freq. Counter Module.The D528 is a 5.7 Zener Diode and is a very common problem in this type of radio.

thanks, and yes, its off on an external as well as far as i know, it wont send out enough power to test on my external really. will double check those

did you ever get this issues solved... i had one do the same thing... lower the mike gain... no more problem