Quad FM4 frequency drift.

[theredhouseinn]

I am monitoring the voltage on the varcap diodes in the front end.
Tune button pressed freq. is 95.1
Temp in the w/s is 13 deg.
At switch on the v is 3.545, after 2 min 3.547 it stayed at this for 18min.
Press bbc1 button 91.55, v is 2.725v it stayed at this for 10 min, no change.
Apply gentle heat for 2 min v went up to 2.728 display went to 91.40.
After 30 min all went back to original state.
I seem to have a temp sensitive component/s.
Talking to my friend, the owner, he stands this on his amp. "O dear"
Question-is this common on this tuner? Has anyone else had this problem?
I will add that I am an analogue man, I try to avoid the digital junk.
John.

[Craig Sawyers]

Haven't seen any drift with my FM4. Stays locked to a station just fine.

Are you saying that with your FM4 the station lock drifts in practice?

Craig

[theredhouseinn]

Hi Craig.
Yes, the station drifts with temp change as the voltage varies on the varicap diodes.
John.

[John_BS]

Using your figures, the tuning ''rate' is around 4.3MHz/V

So each 1mV should result in c 4kHz delta f ? AFC should cope with this easily?

[theredhouseinn]

Hi John.
I think you have hit it, if the voltage varies at the varicaps and changes the freq. then a dc shift at the discriminator should oppose the dc change at the varicaps and pull the osc. back on tune. So I may have an AFC problem here.
There is a manual tune on this thing so I should be able to check this out if I can find it among all the ic's and processors.
John.

[barrymagrec]

I think the AFC doesn`t operate in manual tuning mode.

[John_BS]

Originally Posted by barrymagrec

I think the AFC doesn`t operate in manual tuning mode.

Just had a look: no, you're right.

If the tuning voltage at 'g' isn't changing much, but the diplayed frequency does, that throws suspicion on the R7 /C9 /D12 /L6 area. i.e the local oscillator needs looking at.

[theredhouseinn]

Yes, the AFC does not operate in tune mode Barry.
Selecting BBC3 display reads 91.45, 5min 91.4, 6min 91.35, 25min 91.3, 45min 91.25, 65min 91.25, 185min 91.2.
The station BBC3 remained in tune, the tune mode station, classic fm gone out of tune.
So, the AFC is working, the display is changing, the voltage at 'g' is changing with no signal i/p.
The voltage at 'g' is obtained from an op amp so I assume that with no signal i/p the voltage at 'g' should be constant. Even if the osc. freq. drifted with no signal i/p the voltage at 'g' should not vary as there is no reference to operate the AFC circuit.
My conclusion is that there is a logic fault here. Presumably this thing has a clock in it to operate the logic circuits. As I am a 65 years analogue service man, and I have not been trained in fault finding on digital circuits I am not going poking around in stuff that I am not familiar with.
Unless someone can say yes component designation 'X' is faulty, change it, then I will knock it on the head and return it to my friend.
John

[John_BS]

Before givng up, try resoldering the R7 /C9 /D12 /L6 area, whilst changing C9 for a 2u2 tant.
John

[theredhouseinn]

Thanks John, I have got the tant's on order, I will report back.
John.

[theredhouseinn]

Hi there.
The tant's arrived, went to fit it, no C9 on the pcb.
I traced the track from R7 and there is no hole mounting for a cap.
The serial number is 018671.
Looking at the manual there is one cct with no C9 to earth from R7.
D12 is in the can with L6, perhaps C9 is in the can as well.
I am not going to remove the can from the pcb to check, could do more harm than good, so job finished
John.