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Old 12th Apr 2019, 5:33 pm   #81
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Hmm.. +1.0V shouldn't the grid of V4 be -ve WRT to the cathode ( which appears to be grounded )?

I didn't have another 6M8 resistor, so just tried a (4M7+1M2) in there. voltage on the grid is still +ve (0.6V).

Pete
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Old 12th Apr 2019, 5:42 pm   #82
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Okay, I've had a look at the socket pins and put the original UABC80 back in and the voltage on pin 8 is now Zero..
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Old 12th Apr 2019, 5:46 pm   #83
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Default Re: Ferguson 382U Firefly not playing ball

Now you have changed the valve can you do a full set of readings on V4 pins 6, 8, 9
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Old 12th Apr 2019, 6:18 pm   #84
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

No problem..

V4 6,8,9 = -0.12V, -0.0V, +150.5V

P.
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Old 12th Apr 2019, 6:53 pm   #85
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Default Re: Ferguson 382U Firefly not playing ball

Unless there's another path from HT to anode, that looks to have the same symptoms that the osc valve had, low emission, low emission would mean higher anode voltage and possibly lower grid voltage, the bias voltage in that circuit is due to grid current, the grid current is very small, hence the high value grid resistor, often some receivers will have a higher value resistor fitted when grid current bias is used, typically 10 meg ohms or sometimes greater, measuring the grid voltage with a meter will reduce the actual operating bias voltage at the grid to an extent depending on the meter's resistance and the value of the grid resistor.

Lawrence.
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Old 12th Apr 2019, 7:42 pm   #86
PJL
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Default Re: Ferguson 382U Firefly not playing ball

Agreed, the HT is around 200V so R25 (220K) has about 50V across it which gives us about 220uA anode current and it should be about 10x that according to the datasheet. The low emission may also affect the diode (V4c) used as the detector.

Can you measure the AC voltage on V4 heater, between pins 4 & 5.
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Old 12th Apr 2019, 7:57 pm   #87
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Default Re: Ferguson 382U Firefly not playing ball

Might also be worth measuring the cathode pin to chassis resistance just in case the path is open circuit or high resistance..

Lawrence.
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Old 12th Apr 2019, 8:01 pm   #88
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Thanks everyone, V4 Heater voltage is = 23.4V
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Old 12th Apr 2019, 8:07 pm   #89
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Default Re: Ferguson 382U Firefly not playing ball

Should be OK with that so I am afraid you need another one.
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Old 12th Apr 2019, 8:57 pm   #90
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Got one on order from eBay £10 inc postage, supposed to be NOS, so letís see..
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Old 17th Apr 2019, 12:32 pm   #91
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Default Re: Ferguson 382U Firefly not playing ball

At last... some progress! With the replacement UABC80, we're getting some good old fashioned hiss and crackles on medium wave.

Moving through the tuning range, it can get very crackly as I pass through a few spots. I have carefully hoovered this out and used a very fin brush - but perhaps there's still some dust?

Now that it seems to be doing stuff at the RF end, where would be a good place to start?

Cheers, Pete
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Old 17th Apr 2019, 12:41 pm   #92
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Default Re: Ferguson 382U Firefly not playing ball

Great news!

Can you get any signal at all on MW? Perhaps use a few feet of wire as an aerial in addition to the built-in aerial.
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Old 17th Apr 2019, 1:47 pm   #93
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Hi Simon, nice big crackle when I plug one end of some long wire in, but no change to discernible signal..

There is the tiniest bit of something maybe on LW, but not definitive..
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Old 17th Apr 2019, 2:02 pm   #94
Simon Gittins
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Default Re: Ferguson 382U Firefly not playing ball

Because of the crackles as you tune, you could try a resistance check of the tuning capacitor with the set unplugged to see if it is shorted over part of the range.
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Old 17th Apr 2019, 2:05 pm   #95
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Default Re: Ferguson 382U Firefly not playing ball

Posts crossed. Is the slight signal on LW in one part of the dial only?
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Old 17th Apr 2019, 2:07 pm   #96
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Simon - Thanks Yup. Found S/C in several places.. Bit of a clean up next I think!
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Old 17th Apr 2019, 2:16 pm   #97
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Default Re: Ferguson 382U Firefly not playing ball

The worse thing to get into a tuning capacitor is wire wool, it makes shorts everywhere.
It's almost impossible to get out of speakers too.

Last edited by Boater Sam; 17th Apr 2019 at 2:18 pm. Reason: added
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Old 17th Apr 2019, 3:59 pm   #98
Mike. Watterson
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Default Re: Ferguson 382U Firefly not playing ball

Card the right thickness.
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Old 17th Apr 2019, 4:02 pm   #99
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Card sounds good..
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Old 18th Apr 2019, 8:47 am   #100
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Default Re: Ferguson 382U Firefly not playing ball

I have heard that a tuning capacitor placed in a dishwasher comes out sparkling clean, just needing lubrication afterwards. Not tried it myself though.
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