Mending stuff - anyone into radio / electronics
 

Following on from the Mending Stuff thread, is anyone able to take a look at a troublesome circuit and express an opinion on a fix please?


It is the 18Mhz oscillator, feeding the PLL circuit in a rather fancy general coverage radio system. This is the service manual and circuit diagram -


http://www.radiomanual.info/schemi/Y...-8800_serv.pdf


The problem section is the PLL section on page 20, 18Mhz crystal oscillator, which is on the extreme left of the diagram, about halfway down. Q03, Q04, Q05, 18Mhz output of which feeds into Q34 which divides 18Mhz into 4.5Mhz


R18's value seems hypercritical, far more critical than it ought to be, such that the 18Mhz output fails to Q34 a 74LS107. Likewise C20 the decoupling between Q03 and Q04 has several suggestions for its value. No working PLL just causes the display to flash, no reception.



The flashing is an issue reported by many owners and there are as many fixes as there are people with the issue. All of which I have tried unsuccessfully. I have been doing battle with it myself, for several years.

So many blind alleys, so many people suggesting different fixes and so many potentially differing causes :mad:


I made some progress yesterday on this - I managed to get a reliable 4.5Mhz output from Q34, after a bit of tweaking of the value of R18.


I now have reception and audio on part of the VHF band, but it flashes the display to indicate 'no PLL lock' below around 143Mhz all the way down to 160Khz.



I vaguely remember when the flashing display problem first happened, going through the entire PLL section, trying to realign it to fix it - maybe not realising it was the 18 / 4.5Mhz output which was failing, rather than poor PLL alignment.



I now need to go through it all again (long job) and realign it again from scratch - wish me luck ;)

Try switching it off, then switching it on again... ;)