Another Quad FM4 problem

[chartz2]

Got myself a third one. Now that one makes the fuse melt. Probably the Rifa X2 cap ! Not opened it yet.

[NikPrice]

I got a grey one for a steal on eBay to match my 34 306 set up, and the correct case unit as well. No power when I plugged it in, and a new fuse provided only temporary power until it blew and I noticed the pong. A new X2 and another fuse cured the problem. It needed a new battery, which had leaked, but there was no real damage. A good clean with lemon juice, and a home build perspex box to house the new battery in.

The result is a fully working tuner, which completes my Quad setup. Now to finish up the older tuner and sell it on to recoup some of what I spent on the new one.

[chartz2]

I replaced the suspect RIFA and the tuner fired up right away. Coincidences, right?

The tuner works and is stable (no flicker) but the display is a little weak.

Any cure for that?

[NikPrice]

I’ve not investigated the display circuit as yet as I’ve had no cause. If it’s just dim then my guess is it’s either the display volts low, or the display itself.

I’ll have a nose at the circuit diagram when I get a chance.

[chartz2]

Thanks. And by the way, this example also has the 220 Ω resistor over C12.

[NikPrice]

So has the one I've just purchased. Seems like a common fix for the runaway frequency problem.

Looking at the schematic, I'd start with checking out the 30V rail. My bet is C4 has gone bad, pulling the voltage down.

Fingers crossed.

[chartz2]

My first idea too. C4 new. 32 V measured.

[NikPrice]

How about at pin 10 on IC18 and IC19? If the voltage at pins 10 is less than 32V then look at C46.

Assuming this is all good then I'd check the ground path. This appears to be pin 1 of the display going through a circuit (I'm unsure of it's job) to ground. It can be seen on the schematic to the right of the display.

There is also the the first supply rail from the transformer, 18V (I think), which connects to pin 35 of the display.

That's about all I can see. Hope this helps.

[chartz2]

I think the circuit is partly for a slow start and it appears to settle brightness too. When I bypass it the display becomes legible again but comes on abruptly.

[NikPrice]

Sounds like the fault could be within that circuit then. the transistor ZTX450 appears to be readily available, and the rest of the components are probably already in your war chest.

[NikPrice]

Well, the resto-repair is pretty much finished.

Embarrassingly, I found a track missing which had come away so cleanly from each pad that I had to get the magnfying lamp on it and compare it to a picture to confirm there should have been a track there. It was between the battery neg pad and the leg of C16 that is nearest the front panel of the receiver. This part of the PCB was the most discoloured due to battery leakage. The odd thing is now the tuning range is a little limited - about 88 to 106MHz. I have removed the 220R resistor from the bottom of the board from the anode of D24 to ground, as it is not requited now.

The drift was much better although still not acceptable at the top end. The mod kit to replace the D24 - (Quad Part No QF4DMI1 £27.84 inc VAT and postage) - worked well, almost eliminating upper frequency drift.

All I need to do now is to find out why the tuning range is reduced and the tuner should be finished.

[chartz2]

Good. Minuscule cracks are always an issue and I spend a lot of time inspecting pcbs with a magnifying glass prior to any component testing.
I’ve just finished my third FM4.