3rd Jan 2019, 7:31 pm | #1 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
Posts: 487
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Quad FM4
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
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3rd Jan 2019, 7:59 pm | #2 |
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Re: Quad FM4
Varicap power supply gone down?
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5th Jan 2019, 12:12 am | #3 |
Moderator
Join Date: Feb 2006
Location: Wembley, Middlesex
Posts: 7,225
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Re: Quad FM4
There are some diodes around the varicap circuit, 1N4148's (I think) replace them.
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6th Jan 2019, 8:39 pm | #4 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
Posts: 487
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Re: Quad FM4
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
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11th Jan 2019, 10:19 am | #5 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
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Re: Quad FM4
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
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20th Jan 2019, 11:26 am | #6 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
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Re: Quad FM4
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
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4th Feb 2019, 10:14 am | #7 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
Posts: 487
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Re: Quad FM4
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
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4th Feb 2019, 1:45 pm | #8 |
Nonode
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
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Re: Quad FM4
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?
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5th Feb 2019, 11:22 am | #9 |
Hexode
Join Date: Jun 2011
Location: Glossop, Derbyshire, UK.
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Re: Quad FM4
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
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5th Feb 2019, 2:17 pm | #10 |
Nonode
Join Date: Jun 2009
Location: Herefordshire, UK.
Posts: 2,495
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Re: Quad FM4
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... Last edited by SteveCG; 5th Feb 2019 at 2:23 pm. Reason: added sentence |
5th Feb 2019, 5:44 pm | #11 |
Nonode
Join Date: Aug 2007
Location: South Bradford, West Yorkshire, UK.
Posts: 2,573
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Re: Quad FM4
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 |