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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 11th Jan 2022, 5:46 pm   #1
theredhouseinn
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Default Quad FM2

Hi folks.
This has been in the damp for about 35 years.
Stood it in front of the fan heater for a week.
Changed all the red hunts, all leaky from 2k upwards. Brought up the mains tx on the variac over a few days. ECC81 faulty, replaced.
replaced all the black electrolytics, leaky from about 25k up.
Radio working fine.
Using sig gen and muliplex stereo modulator.
Stereo decoder, replaced the 3 red hunts and electrlytics, the hunts .004uf, replaced these with 3900pf suflex type.
Folowed the alignment proceedure in the SM but cannot get better than 25dB left channel only or right channel only. SM says 35dB.
Using ac millivolt meter and scope on the o/p I managed 27dB. Fine tweeking.
Looking at the scope the channel not modulated seems to contain residual 38kHz mush, both channels are the same. Tuning the coils and adjusting the pre-set I can minimise the signwave in the unmodulated channels.
Has anyone had this problem with Quad tuners?
I can achieve 32 to 34 dB on Leak Trough line, Stereofetic, and delta tuners.
This is my first Quad tuner, am I asking to much?
John.

Last edited by theredhouseinn; 11th Jan 2022 at 5:47 pm. Reason: Spelling error
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Old 12th Jan 2022, 2:51 pm   #2
Herald1360
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Default Re: Quad FM2

dB relative to what

Is this S/N at a specified input rf level or something else?
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Old 12th Jan 2022, 5:23 pm   #3
theredhouseinn
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Default Re: Quad FM2

It is the ratio of one channel modulated at 1kHz and the other channel not modulated.
i.e. on the multiplex modulator one channel only is selected, either L or R.
The SM spec says better than 35dB, at the o/p sockets.
John.
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Old 12th Jan 2022, 5:29 pm   #4
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Default Re: Quad FM2

20dB is more than enough, if it sounds OK I would stop worrying.
 
Old 13th Jan 2022, 9:42 am   #5
Ted Kendall
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Default Re: Quad FM2

You may need to filter above 15kHz to meet the specification figure. In any case, 20dB is enough for decent stereo. Keith Snook describes a modification to the decoder which may allow you to trim out some more of the 38kHz residual, but if the thing is working and stable I'm not sure I'd bother.
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Old 13th Jan 2022, 10:08 am   #6
John Caswell
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Default Re: Quad FM2

Done a few of these lately, in FM tuner check the Rs especially the 82K in the IF strip and the 220K around the detector, tend to go high/out of balance. Similarly in the decoder check all Rs change they value drastically, replace with MRS25 or similar.and diodes (use 1N41418). I forgot did you use ceramic discs in the IF strip decouplings Cs? They are the best types for that job.

John

Last edited by John Caswell; 13th Jan 2022 at 10:10 am. Reason: Added inf
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Old 13th Jan 2022, 10:09 am   #7
theredhouseinn
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Default Re: Quad FM2

Thanks guys.
I am not going to modify it, if it ain't broke, dont fix it.
John.
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