"tony sayer" wrote in message
news
In article , Phil Allison
writes

"gary"

My Fm4 has a strange problem, I have replaced the back up battery and
this
is working, if I store a station at 88.10 mhz when I switch it back on
the
frequency comes up at 88.00 mhz for the first 5 - 10 minutes, it then
slowly creeps up to 88.5 then 88.10, it does this on all stations
stored.

Does anyone have any ideas about this


** I take 88.5 should be 88.05 ????

So the tuned frequency drifts up by only about 100 kHz as it warms up -
NOT 500 kHz.

Despite appearances, the FM4 is NOT frequency locked with a digital
frequency synthesiser, like most other tuners with a digital readout.

Tuning frequency is controlled by analogue circuitry via "varicaps " -
so
cannot be entirely drift free.

What is that locked to if anything, or is it free running with AFC?.


The AFC operates on pre-set stations only and is derived from IC10 (4441)
the discriminator. The p.d across pins 6 and 10 is applied to the comparator
of IC14(TL092). The comparator amplifies the difference voltage via a
potential divider and an analogue switch and modifies the ref. voltage and
hence the tune volts.


Doesn't seem to be an FM4 circuit in the web anywhere.....

No...but I have the service manual which does

However, the digital readout shows the ACTUAL operating frequency of the
set
with high ( ie crystal ) accuracy.

There is likely NOTHING wrong with your FM4 - so leave it alone.

There IS something wrong - the set shouldn't drift.
But let's wait for the OP to come back and report on whether removing &
re-inserting the custom programmed microprocessor IC has fixed his problem.
Quad make special mention of this course of action in the service manual -
and suggest it as the first thing to try for faults with the symptoms
described. There are a couple of other components which could cause drift
but let's not go to Plan B until we know Plan A didn't work.