FM3 weak

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27-Jan-2020 12:04 AM

QuadOAP

QuadOAP

Posts: 14

I am trying to resurrect an old Quad system. Quad 33/303 and FM3 all of which had a number of faults. I carried out the Dada mods to the 33 and the 303, and after clearing some faults they appear to be OK.

The FM3 is working after a fashion but is generally a bit weak and noisy. The number of stations received is less than it used to be. Radio 2 is reasonable but most are just above the mute threshold. ( I had the same system which I have passed on to my son, so I know the reception area and what to expect). I have replaced C113 and C114 with 1000Mfd/25v and C121 with 470mfd/16v. The +14v rails are at 14.5v and the -14v is reading -14.95v. I disengaged the mute circuit by removing TR11 but it made no difference other than stopping the mute..

The tuning lamps work OK but the Stereo lamp does not come on (bulb OK), but it is a stereo output. I have a USB scope which is a bit limited but it shows both 14v rails to be flat. The 19kHz on pin 10 of the MC1310P chip is sitting at about 18.2Khz, the waveform has flats top and bottom.

I know not to adjust anything on the RF/IF board.

Not sure where to go from here. Any likely culprits or tests I can do to narrow down the fault

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02-Feb-2020 04:13 AM

EJP

EJP

Posts: 1712

18.2KHz is way wrong. Adjust it to 19KHz at the trimpot adjacent to the MC1310, as close as you can get with the aerial disconnected, certainly as close as 19.00KHz. This will improve the stereo and probably illuminate the lamp as well. If it doesn't, replace the lamp.

The only way to fix the sensitivity is via an RF and IF alignment, which requires special equipment. Where are you located?

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05-Feb-2020 03:45 PM

QuadOAP

QuadOAP

Posts: 14


 Thanks for your input.

I adjusted the trim-pot as suggested back to 19Khz as best I could with my equipment. Now fairly sure it is very close to that frequency now. The lamp now goes on when tuned to a station. This made little or no difference to the other fault. Still hissy on some stations. (Fine on Radio 2)

To put you in the picture, the FM3 worked Ok when I got it, a few days of good reception, and comparable to the other FM3 I had at that time. It suddenly went weak with a lot of background noise. It felt like something failed at that point. It didn't sort of drift out of tune. so I feel it is a failing component ??. 

I live in the central belt of Scotland where the reception is first class. On my previous tuner, some stations would partially tune with no aerial connected, the signal is that strong.

Would it be worthwhile to replace some of the transistors and/or coupling capacitors in the earlier stages. Although I am a bit confused with types of capacitors. In an earlier blog, it referred to a quality Poly cap, and there appears to be a number of different types Poly ??.

How about the MC1310P or the IC in the IF stage, could tone of them be the culprit.


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06-Feb-2020 12:53 AM

EJP

EJP

Posts: 1712

It is almost vanishingly unlikely to be any of the ICs. It sounds like RF misalignment to me. I've changed a few of the RF transistors in some FM3s but it rarely helps: they don't seem to fail. No reason why they should, as they are only dealing in micro- or milli-volts. One thing that has helped in the past is resoldering the aerial earth and the big earth solder blob that joins the two PCBs. And of course check the voltage rails for about +14V/-14V at the pink and black leads respectively. If that doesn't help I would send it down to Quad in Huntingdon.

Dada Electronics Australia

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