Quad FM4

[nutteronthebus]

My FM4 tuner has stopped working and it refuses to respond to turn of the tuning pod and/or the presets. It shows 87.20 - flickering to 87.15 - all the time - no matter how the tuning or presets are treated.
I have replaced the battery - allowed it 24 hours of recharcing (power on) and still the same. I have also have an other that the display shows 120.15 but one fault at a time as there is a U-tube video in 7 parts with the same fault I have yet to sit through


Dave

Varicap power supply gone down?

[Michael Maurice]

There are some diodes around the varicap circuit, 1N4148's (I think) replace them.

[nutteronthebus]

I have ordered a lot of caps to replace the old ones (100uf 16v are so small ) plus 1N4148 D21/22 . Watch this space for results

Dave

[nutteronthebus]

I have replaced all of the caps , a lot of the 100uf 6.3v were well out along with the power supply cap next to the heatsink 560uf not 1000uf ( I have 3 to repair 1 humming now ok)

not yet replaced the diodes as waiting to arrive but no change in the tuning


Dave

[nutteronthebus]

Having replaced all the dodgy caps and D21/22 still drifting a lot so I replaced the Zener D24 as the ref was 6.03v now at 6.21 and stable ( 2 down and 2 to go as 1 new fault now has no display ) that other faulty one has no ref voltage ( same either side of T11 that has been replaced iss 4 board )


Dave

[nutteronthebus]

Help. After replacing all the caps on the board and d21/22 the tuner still will not tune ( I have seen several tuners on that site with the same fault ) is this a simple stock fault or has it multiple causes and has any one got a clue.


Dave

[SteveCG]

Were there two versions of the Quad FM4 tuner? An earlier one with brown case, green display and DIN audio connectors; and a later one in blue-grey case, brown display and phono audio connectors. I have a recollection that the exact circuitry used varied depending upon the version. Indeed Quad issued different service information depending upon the serial number of the tuner. Which version/serial number do you have?

[nutteronthebus]

This is the earlier one yellow button FM4 M12615-5 serial No 014135 ref L 1.4v ref H 7.4v . moving the tuning dial it changes from 87.25 to 87.30 across the full range the voltage changes on the output of the pot from 1.7 to 7.4v

[SteveCG]

As I don't have a circuit diagram to hand I speculate now. Do you have the appropriate circuit diagram, if not I can only suggest looking on some of the service manual websites like perhaps hifiengine.

In brief: I think the Quad system is like other 'modern' tuners in that it generates a D.C voltage which is applied to a varactor tuned front-end - both RF and local oscillator. There will be a feed from the local oscillator to a frequency counting section whose display is what we see. The circuit that sets the D.C. voltage to the tuner front-end will an IC that converts the stored frequencies into the D.C. voltage.

If this is the set-up Quad use then a test to do is to measure the voltage going to the tuner front-end and see if it changes when you turn the manual tune dial. I suspect that it is not changing otherwise you would be hearing stations even if the display was not showing the appropriate frequency.

I hope this has not been telling grandmother how to suck eggs...

[KeithsTV]

Check the tuning voltage is reaching the three preset pots in the tuner.

If this is OK there is a small inductor which connects the local oscillator coil to ground and acts as the dc return for the varactor tuning of the oscillator. I've had a dry joint on this coil which caused the frequency display to show a low frequency whatever the position of the tuning control.

I've had several dry joints in my FM4 and 44 which have caused some weird behaviour.

Keith