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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 31st Mar 2020, 10:01 pm   #1
1980s_john
Tetrode
 
Join Date: Apr 2005
Location: Ipswich, Suffolk, UK.
Posts: 59
Default Quad FM3 repair notes

Hi,
I bought this tuner over 10 years ago and it has been sat next to my desk as an ornament all this time. Having read some of the discussions on here (and with all outside public entertainments closed!) I decided to try it out, the general advice is to first check and replace the two power supply smoothing capacitors, both 1000uF 25V.

I found a service manual on line showing C110/C111, but I didn't check I had the correct one - regulars will know there are several versions of the FM3, I later found this service manual which covers the 3 revisions:

https://www.manualslib.com/download/.../Quad-Fm3.html

My FM3 is a late model (s/n greater than 20000), I should have spotted that the smoothing capacitors changed PCB part numbers to C113/C114. I removed the two caps, one was a Philips 680uF which actually was reasonably OK at 640uF and low ESR, the other was an Erie 1000uF which had failed (measured around 200uF and showed signs of physically leaking). I replaced both with Vishay BC 1000uF 25V, however I would suggest 40V rating would be better as they are run at 21V.

There was a missing bulb so I ordered some from Farnell (14V 70mA LES, part 1139220).

With the two capacitors replaced I measured +11.2V across C113 and -21V across C114. The 14V rail measured 2V so was shorted (actually intermittent). I removed the red wire connecting 14V between the two PCBs and found that 14V to ground measured 23 ohm on the power / audio PCB, and 300kohm on the RF PCB. Further checks showed a short on one of the two bulbs on the tuning indicator. Removing the bulb led to the bulb holder disintegrating, but the short remained. I traced this to a short between C and E on the corresponding bulb driver transistor. As I had the 'wrong' service manual for the original FM3, I thought the driver transistor was a 2N5306 NPN darlington, so I ordered a pair of these. Once they had arrived I removed the faulty one which turned out to be a BC184B (schematic shows BC109B), which had some paint marks presumably as part of gain selection in the factory. I looked more closely at the PCB and saw that the tuning indicator bulbs were driven by two transistors in a darlington formation. I decided to go ahead and replace both TR11 and TR12 with 2N5306 transistors so as to keep both sides balanced.

For the broken bulb holder I was unable to find a LES holder with a clip, so soldered a wire onto the broken holder, soldered this back onto the existing wire, and hot glue gunned the holder and bulb into place with the same spacing as the good bulb in the pair.

Powering on gave a good +14V, and tuning showed the brightness of both tuning bulbs altering as expected. Plugged into my hifi and tuner now working fine again, need to sort out a better aerial to make the most of it.

Quick note on something that is not clear without using the tuner. The muting level control on the rear also affects the behaviour of the tuning indicator bulbs. In between stations, with the muting turned down the tuner outputs a loud hiss and both tuning lights are half on. When the muting is turned up the audio mutes and also both of the tuning bulbs turn off. As a station is gradually tuned in the audio unmutes and one bulb comes on, as the tuning is centred both bulbs light equally and with anything other than a very weak signal stereo decoding kicks in.

Only gripe with this tuner is lack of a mono switch.

Regards,
John
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Old 31st Mar 2020, 11:05 pm   #2
Ted Kendall
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Join Date: Jul 2008
Location: Kington, Herefordshire, UK.
Posts: 3,657
Default Re: Quad FM3 repair notes

Quote:
Originally Posted by 1980s_john View Post
Only gripe with this tuner is lack of a mono switch.
If used with a Quad 33, this handled mono/stereo switching. Nice kit - I have listened to one for many happy hours.
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Old 31st Mar 2020, 11:09 pm   #3
alanworland
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Location: Southend, Essex, UK.
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Default Re: Quad FM3 repair notes

I think it will have a mono output, designed to be used with the 33 which has a mono input on the din socket.
Well done for getting it going good little tuners. Had mine for years and last year had to replace the decoder (plugs in) and while I was in there the power supply caps.

Alan
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Old 31st Mar 2020, 11:40 pm   #4
Ted Kendall
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Default Re: Quad FM3 repair notes

Fair point - forgot about that. I think it's on pin 1...
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