Quad FM4 Tune Volts

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04-Feb-2020 03:48 PM

Nik Price

Nik Price

Posts: 9

I have a Quad FM4 on my bench that I am trying to repair.  It has suffered the ubiquitous battery meltdown.

I have cleaned the tracks, rebuilding where necessary, replaced the processor and all of the electrolytics and replaced D24 with another 1N823. I have also replaced the faulty battery with another 4.8V Varta battery.

The problem I now have is that REF-L and REF-H are 7.1V and 9.4V respectively.  With D24 removed the readings are 8.9V and 11.1V.  I have briefly replaced IC5 with a TL094 and this appeared to make no real difference.

Has anyone any ideas where to go from here, or any advice?


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20-Feb-2020 07:21 PM

Nik Price

Nik Price

Posts: 9

Saw a picture of another FM4 from the PCB side, and there was a resistor soldered between the anode side of D24 and GND, I copied this via a resistor box and found a 330R stabilised the Voltages and brought them into range.

When powering on the radio, the display raced up to around 115MHz and stayed there, the tuning potentiometer still moved the display, but not to anywhere useful and all the front panel LED's would light and the buttons would be unresponsive.

All these symptoms disappeared upon inserting the resistor and a new battery into the circuit.  The processor now saves the stations into memory and the buttons work fine.

After aligning the receiver I found that the tuning window was too small - from 87.5 to around 105.5 - 106MHz.  I changed the resistor value to 220R and realigned the tuning range and now it is pretty much perfect.

The signal strength/tuning meter was only showing the signal strength (both bars working but as though they were linked together), and saving a frequency to memory had a 0.05MHz discrepancy. Changing out IC14 (TL092) fixed both these problems.

There is still a little frequency drift in the first 5 minutes of powering up the receiver, but this apart the radio appears to work very well,  it sounds very nice connected to my 34/303 set up.

I've changed all the electrolytics and replaced D24 with a 1N815. I've also replaced T11 in an effort to minimize heat related drift.

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21-Feb-2020 07:14 AM

EJP

EJP

Posts: 1742

Interesting. I can't remember seeing an FM4 with that extra resistor, and I don't see why it should be necessary. Considering the voltage divider R21/22/23 alone, the voltages at the intermediate points should be 11.97V and 1.53V, assuming no current is drawn to anywhere else. The LM334 works to reduce the voltage across R22, so I think I would have a good stare at that and its 1K5 programming resistor, and also at C12 if you haven't already. Also of course R21/22/23 themselves. IIRC they don't measure right in-circuit because of the loading.

EJP

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21-Feb-2020 04:15 PM

Nik Price

Nik Price

Posts: 9

Sorry, I should have said that I've changed R20, R21, R22 and R23 (all replaced with 1/4w 1% resistors).  I've also changed C52, R40, R41, R42, R24 and R16 before I changed IC14.  The 220R resistor mod on the rear of the board is still necessary to stop the frequency running away at power on.  Obviously there is still an undiagnosed fault.

I did notice earlier in the fault finding that either R9 or R10 was damaged.  As they are both minuature format, and I had none I replaced them with normal 1/4W 1% resistors and stood them on end.  I've been thinking about this and they must have been minuature for a reason, as every other component in the receiver is standard size.  So this morning I've removed them and replaced them on the underside with 1206 SMD devices also 1%.

From the notes in the service manual regarding the various designs of the L6 circuit leading to it's eventual replacement in ISS5, I'm guessing Quad had stability issues with this circuit.  This leads me to wonder if the frequency agility is not down to the temperature compensation circuit but L6 moving the frequency as the set warms up.


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21-Feb-2020 08:01 PM

Nik Price

Nik Price

Posts: 9

Managed to break the slug in L6.  Not sure if I can obtain another.  I could be in trouble.  I've removed the pieces from the coil, but there's nothing left to rebuild.
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26-Feb-2020 05:32 PM

Nik Price

Nik Price

Posts: 9

Phew!

I managed to get hold of a few variable inductors with slugs that appeared to match the size of the original, they were obviously the wrong mix, but using by using 2 of them together, L6 is now tuned.


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01-Mar-2020 09:26 PM

Nik Price

Nik Price

Posts: 9

Does anyone have a spare set of covers for the PSU enclosure inside the FM4?  The radio I'm working on came without them and I'm wondering if the drift could be caused by the heat generated by the 7812, which seems to be, even with a larger heatsink fitted, hot to the touch.

 Nevertheless I'd like to replace them for completeness of the radio, if I can get them at a reasonable price.

 Many thanks,

Nik.

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02-Mar-2020 05:20 AM

EJP

EJP

Posts: 1742

Do you mean the PSU cage, or the screw-on top cover and the black plastic thing on the bottom that masks the mains connections? Not that I have either, but if it's just the cage or the metal top you could bend one up easily enough out of 1mm steel. Quad UK might be able to help.

The 7812 does run really hot, like 50 degrees C, and it certainly needs a heatsink. Quite soon after I acquired my FM4 in 1987 it damaged the 34 that was sitting on top of it. Curiously enough that has never happened again, despite the 34 still being on top of it for another 30 years.

EJP

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02-Mar-2020 12:58 PM

Nik Price

Nik Price

Posts: 9

Yes, thats them.  I'll give Quad a ring and see if they've any still in stock.  In the mean time I'll fabricate something from cardboard and tinfoil the surfaces,

I've fitted a larger heatsink to the 7812, but it's still going to reach the same temperature as there is nowhere for the heat to dissipate.  The +12 is nice and stable so the heat is not a problem for the regulator. 

 

Nik.

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20-Mar-2020 02:31 AM

Nik Price

Nik Price

Posts: 9

Well, the FM4 is close to being acceptable.

Much to my embarrassment I found a missing section of track between the pads linking the battery neg terminal to one of the legs of C16.  It was right where the corrosion had discolored the PBC and I had to check with a magnifying glass and a photo of a good PCB to ensure that there was indeed a track missing.

When I replaced this section I found that the receiver became much more stable, and that I could remove the resistor I'd added between the anode of D24 and ground to stop the frequency running away.  Now it's stable, and with the D24 replacement PCB mod from Quad it's almost  ready for use.

The odd thing is now the tuning window is reduced to around 88 to 106MHz. This is the same if using D24 (with a measured 6.2V difference across it), or the replacement PCB.

I'd be grateful if anyone has any input on this as it's the last of a good number of problems I've had to repair to get this tuner back to spec.

Thanks for reading all this.

Nik.

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