Quad FM3 -stereo not working properly
 

Just looking at an FM3. It seems to basically work and sounds excellent EXCEPT that I'm not getting any stereo information. When first switched on if the tuning is moved there is a burst of stereo and the light comes on. It then seems to switch into mono and the light goes out. ( background noise drops at this point)
Once this has happened, no amount of retuning will bring the stereo light back on or any stereo information.
I will admit that I'm using a pretty naff internal dipole aerial to feed it, as it's all I have since my rooftop aerial blew down. Not sure whether the circuitry is arranged to force to mono if the signal strength is low and hence maybe there is no fault ?

Any thoughts welcome.

Andy

Re: Quad FM3 -stereo not working properly
 

Hi Andy,

Do get the service manual from here:
https://www.manualslib.com/manual/407333/Quad-Fm3.html
Depending what is the serial number there are different circuitry.
The latest models do use the MC1310 stereo decoder and CA3089 FM-IF/decoder IC, earlier models use more obscure ICs.
The service manual also gives alignment instructions.

Regards, Peter

Re: Quad FM3 -stereo not working properly
 

OK - there are three generations of the FM3. Which one do you have?

Best alternative is look at the type of decoder chip fitted.

Craig

Re: Quad FM3 -stereo not working properly
 

There was a recent thread on this forum about this issue with a third generation one. The guy had replaced the decoder twice, the IF chip once. A bunch of resistors and capacitors. With zero change - short period of stereo and then bosh - just mono.

It turned out in the end to be the transistor that is used to force mono not being a good transistor any more.

Do a forum search and ferret the thread out.

Craig

Re: Quad FM3 -stereo not working properly
 

I Had one with the same problem. I just changed the electrolytic next to the decoder IC then it was fine again.

Re: Quad FM3 -stereo not working properly
 

You mentioned a naff aerial. Are you sure you have enough signal to make it switch to stereo? Low signal will revert to mono which is how you described it.

Re: Quad FM3 -stereo not working properly
 

I had a Grundig tuner with exactly that problem. A squirt of freezer on the decoder chip would restore stereo for a few seconds. It is was over 25 years ago so I have no idea now what the chip was.

Re: Quad FM3 -stereo not working properly
 

I had a Rotel with these symptoms too, a new decoder ic curing it.

Re: Quad FM3 -stereo not working properly
 

This is the thread Craig is referring to and your symptoms sound quite similar so I would check this first. https://www.vintage-radio.net/forum/...d.php?t=170997

Re: Quad FM3 -stereo not working properly
 

I sold these when new - they weren't the most sensitive of tuners and you do need a good strong signal to get them working at their best .

Re: Quad FM3 -stereo not working properly
 

I appreciate all the helpful ideas from this thread. The difficulty is that I cannot access a decent aerial, so I have no way of knowing whether this is a 'real' fault, or simply the tuner doing as expected. I'm reticent to look for a fault that might not exist !


Andy

Re: Quad FM3 -stereo not working properly
 

Have you tried using an aerial amplifier to see if that improves the stereo switching.

Re: Quad FM3 -stereo not working properly
 

Hmm. The amount of times I have tried using signal boosters to improve FM performance. Usually resulted in more signal + more interference (multipath etc).

Re: Quad FM3 -stereo not working properly
 

Yep , an aerial amplifier just amplifies all the noise and rubbish along with whatever signal might be there - it is no substitute for a better aerial , which is the only way to pull more signal out of the air .

Re: Quad FM3 -stereo not working properly
 

I was suggesting the aerial amplifier as a way of checking whether it switches to stereo. I agree that a proper aerial is always best.