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Old 26th Apr 2019, 10:44 pm   #141
PJL
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Default Re: Ferguson 382U Firefly not playing ball

I guess you have replaced C45 (4uF) and the +ve is wired to earth? You could check the resistor values in the discriminator but I doubt they would be far enough out to cause problems. To follow the trader or manufacturers FM alignment procedure you need an FM signal generator. Does it look as if anyone has adjusted the cores?

I would not recommend fiddling with FM alignment without a signal generator as you could easily make things worse.
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Old 27th Apr 2019, 7:45 am   #142
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Default Re: Ferguson 382U Firefly not playing ball

Hi PJL,

Yup. Replaced and, definitely wired +ve to ground. As far as I'm aware, nobody has ever adjusted anything in the I.F. since it was purchased.

In your experience, do you feel it's something that does occasionally drift out of alignment, or is it more likely to be associated component failure/out of tolerance?

Ta, Pete
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Old 27th Apr 2019, 9:04 am   #143
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Default Re: Ferguson 382U Firefly not playing ball

May be worth checking high value resistors and capacitors in the agc line.
john
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Old 27th Apr 2019, 11:17 am   #144
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Default Re: Ferguson 382U Firefly not playing ball

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Originally Posted by el gringo View Post
In your experience, do you feel it's something that does occasionally drift out of alignment, or is it more likely to be associated component failure/out of tolerance?
Generally IF alignment is pretty stable. Unless you are unlucky and a parallel tuning cap in the coil fails O/C then usually you won't have a problem. I wonder if the UABC80 you have has unbalanced diodes? Just speculating here but in the days of normal replacements where Mullard or Mazda were used, the tolerance was pretty tight. Now of course we more-or-less have to use what is available which could be any form of import where tolerances were perhaps not as close (and suppliers have also been known to unwittingly sell rejects... simply because they buy apparently new valves from any available source). If I have a chance over my busy weekend (dog-sitting for my son...) I'll have a look amongst my valves and see if I have a new or used UABC80 of known provenance.
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Old 27th Apr 2019, 12:40 pm   #145
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Default Re: Ferguson 382U Firefly not playing ball

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Originally Posted by jonnybear View Post
May be worth checking high value resistors and capacitors in the agc line.
john
Thanks Johnny, just replaced the 'R17' 2M2, with 2x1M (nearest I had! ), are the lower value caps (pF) worth a look as well?

Pete
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Old 27th Apr 2019, 12:41 pm   #146
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Default Re: Ferguson 382U Firefly not playing ball

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Generally IF alignment is pretty stable. Unless you are unlucky and a parallel tuning cap in the coil fails O/C then usually you won't have a problem. I wonder if the UABC80 you have has unbalanced diodes? Just speculating here but in the days of normal replacements where Mullard or Mazda were used, the tolerance was pretty tight. Now of course we more-or-less have to use what is available which could be any form of import where tolerances were perhaps not as close (and suppliers have also been known to unwittingly sell rejects....simply because they buy apparently new valves from any available source). If I have a chance over my busy weekend (dog-sitting for my son....) I'll have a look amongst my valves and see if I have a new or used UABC80 of known provenance.
Hi Sideband, that'd be cool - If you have, just let me know what you want for it..

Pete
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Old 27th Apr 2019, 3:39 pm   #147
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Default Re: Ferguson 382U Firefly not playing ball

Looking at the circuit, it appears I could try swapping the cathodes of V4 b and V4 c ( only 2 wires? ) - Is it worth a go?
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Old 27th Apr 2019, 3:56 pm   #148
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Default Re: Ferguson 382U Firefly not playing ball

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Looking at the circuit, it appears I could try swapping the cathodes of V4 b and V4 c ( only 2 wires? ) - Is it worth a go?
No, both cathodes are connected to chassis.

Be aware that the ratio detector diodes designated as V4a and V4b in the Trader sheet were manufactured with essentially the same characteristics, V4c, the AM detector, has different characteristics to those.

Lawrence.
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Old 27th Apr 2019, 4:02 pm   #149
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Default Re: Ferguson 382U Firefly not playing ball

Ah.. Okay, so not a realistic option.. thanks Lawrence.

Last edited by el gringo; 27th Apr 2019 at 4:26 pm.
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Old 27th Apr 2019, 4:25 pm   #150
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Default Re: Ferguson 382U Firefly not playing ball

You can replace the UABC80 FM detector diodes with silicon diodes, 1N4148 will do, to prove the point.
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Old 27th Apr 2019, 4:47 pm   #151
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Default Re: Ferguson 382U Firefly not playing ball

Quote:
Originally Posted by el gringo View Post
Quote:
Originally Posted by jonnybear View Post
May be worth checking high value resistors and capacitors in the agc line.
john
Thanks Johnny, just replaced the 'R17' 2M2, with 2x1M (nearest I had! ), are the lower value caps (pF) worth a look as well?

Pete
No low value caps are normally reliable, R6 & 17 = 2.2m ohm, R1 = 680k,R16 = 100k, C23 = .1uf, you can test these with a meter to see if they are in range, you may have to remove one leg though.R7 C21 also worth a check.
John
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Old 27th Apr 2019, 7:10 pm   #152
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Default Re: Ferguson 382U Firefly not playing ball

C21 = not s/c at DC
C23 = already replaced
R1 = inaccesible?
R6 = replaced
R7 = 50.6k (47k value)
R15 = 78 ohms (68 ohms value),
R16 = 110k
R17 = replaced

ta, Pete
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Old 28th Apr 2019, 11:06 am   #153
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Default Re: Ferguson 382U Firefly not playing ball

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You can replace the UABC80 FM detector diodes with silicon diodes, 1N4148 will do, to prove the point.
Hi Sam - Thanks for the suggestion - ( I only had 1N4002s ) It does work, although interestingly not as well as the UABC80 diodes.

Regards, Pete
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Old 28th Apr 2019, 12:32 pm   #154
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Default Re: Ferguson 382U Firefly not playing ball

1N4002's are not signal diodes so probably will not bias on correctly. Actually I've found a good s/h Mullard UABC80 that you can have. Even if it doesn't cure the problem at least it will eliminate your replacement valve as being the cause.

Let me have your address by PM and I'll post it during the week.
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Old 28th Apr 2019, 3:23 pm   #155
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Default Re: Ferguson 382U Firefly not playing ball

I suspect the forward voltage of a silicon diode will also be a problem.
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Old 28th Apr 2019, 7:15 pm   #156
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Default Re: Ferguson 382U Firefly not playing ball

Germaniums might be OK for test. I've done that before when checking an AGC diode in a PEN4DD
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Old 29th Apr 2019, 9:11 am   #157
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Default Re: Ferguson 382U Firefly not playing ball

Forward volts only reduces AM sensitivity. However power diodes are really slow, the 1N4007 are REALLY slow, so much that you can use them as PIN diode replacements.

Buy a 100x 1N4148 and 50x 1N60 (there are modern ones and they are best for AM sensitivity in a crystal radio, beating any Schottky signal diode).

Even the 1N4148 is fine in a superhet detector, but the 1N60 good for RF sniffer peak detectors etc.

I buy 1N4007 rather than any other 1N400x as they are slower and thus don't need snubbers. No good for higher voltage SMPSU, you need Ultra Fast types. The actual Schottky power diodes mostly are under 20V rating, I use 1N5819. Schottky signal diodes are more application specific and expensive, so I design for 1N4148 or 1N60 and only use them for replacements.
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Old 29th Apr 2019, 1:55 pm   #158
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Default Re: Ferguson 382U Firefly not playing ball

Thanks for the suggestions Mike!

I am indeed wondering ( as suggested earlier ) whether some GEX34 Germanium Diodes might sub for a temporary fix?

( my goodness .. I was looking around this morning for a miniature 10uF cap for another project and have only just seen the cost of components has absolutely plummeted? - < Although I suspect it was prob. 1982 when I was buying anything regularly LOL > )
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Old 29th Apr 2019, 4:11 pm   #159
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Default Re: Ferguson 382U Firefly not playing ball

Update : Tried with a Mullard OA5 & OA10 - and its def clearer and louder..
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Old 29th Apr 2019, 5:06 pm   #160
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Default Re: Ferguson 382U Firefly not playing ball

GEX34 should be fine. I had a full carton of plastic flat packs of these when I last moved, around 2000 diode, I binned them! Fool!
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