QUAD FM4 small issues and odd repair solution

[chartz]

Hi all,

On my latest example (brown, sn circa 5000), the yellow buttons wouldn't trigger the TMS1000 at all, or erratically so. Sometimes one button would work, just like that, and one moment later it would do nothing.

I realized that when I touched pins 2 and 3 with my finger, while looking for bad solder joints, everything worked as it should. New joints did nothing, unfortunately.

Then I tried to spray a little contact cleaner over those pins and it worked fine until the liquid had evaporated (repeatedly).

I therefore connected two small caps (330 pF) between pin two and GND and pin 3 and GND.

Now stations can be selected and memorized fine as they should.

Every single cap was replaced, new battery. No birdies or anything, strong signal and perfect sound.

As an aside, when cold, the frequency is a bit flickery on the last digit only, then after 30 sec or so, it is dead stable. And yes I've replaced the AFC cap too. All voltages as per SM right after power-up.

Explanation(s) please?

Thank you!

 

[mikmm]

Hi, Chartz,
See my apology and my last post on the original thread.
I did eventually notice the flickering last digit depending on how cold the room was.
regards,
Mick

 

[chartz]

Hi, Chartz,
See my apology and my last post on the original thread.
I did eventually notice the flickering last digit depending on how cold the room was.
regards,
Mick

Hi Mick,

Thank you very much indeed.
That said, I have another FM4 and it will not drift at all, and will always wake up on the last radio selected, or on manual if it was switched off on that mode. The displayed frequency is the correct one with zero flickering of the last digit.

It isn't temperature-sensitive at all. My guess is a so-so TMS1000 perhaps?

 

[ejp]

Very unlikely to be the TMS1000. It just outputs a stored number to a DAC for tuning, and there is no reason it would jitter the number.

My Quad FM4 has been flickering slightly since 1987 and neither the warranty repair guy nor myself as a (now) tuner repair technician have ever been able to cure it completely. It makes no difference to the sound, as there is > 400KHz bandwidth at the detector, and it may only b a display problem rather than actual tuning shift. Adjusting the crystal trimpot for maximum output of the crystal, measured somewhere north (I forget) of the IC above it to avoid loading it, may help.

If it is tuning shift there is a Drift Modification kit available from Quad, or me, which takes the form of a small PCB attached to one of the tuning coils. If you don't have it you have a Zener instead, to the left of that coil I think.

I have found a few of these to be susceptible to dry joints in the area of the TL094.

 

[chartz]

Thank you EJP. Tell me more about the small PCB please: how does it work?
I have it on my second FM4, a later example with potted battery.
However, I don’t have the big coil housing on the old one.

 

[ejp]

It replaces the Zener D24 with a more stable circuit using a TLC431C voltage reference. There are two versions, one for PCB M12615.5/6 mounted to L6, and one for the earlier PCBs that is mounted directly in the holes for D24. Contact me off list if you need one. It's not expensive.

 

[mikmm]

That sounds interesting...
My problems seemed to start when tried to do a better clean up of the mess caused by the leaky battery. I changed several components that had verdigris
on them...
I'll open the set up and have another look at it.
Mick.

 

[chartz]

It replaces the Zener D24 with a more stable circuit using a TLC431C voltage reference. There are two versions, one for PCB M12615.5/6 mounted to L6, and one for the earlier PCBs that is mounted directly in the holes for D24. Contact me off list if you need one. It's not expensive.

Cheers, PM sent.