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Old 5th May 2019, 2:35 pm   #161
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Update: Replacement UABC80 (Thanks Rich! ) in place, ( germanium diodes removed etc. ) seems quiet and voltages are as follows:

pins
6 = -0.3V
8 = -0.4V
9 = +68V

Any ideas?

Pete
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Old 7th May 2019, 3:50 pm   #162
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Default Re: Ferguson 382U Firefly not playing ball

Just wondering if anyone has any ideas on why my anode voltage is now low?

ta, Pete
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Old 7th May 2019, 6:27 pm   #163
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Default Re: Ferguson 382U Firefly not playing ball

Is the anode load resistor OK? Usually a high-ish value like 100K or 220K. It's very common for them to go high and thus lower the anode volts.....I hope it's not the valve I sent you....it worked in a Bush radio very well.
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Old 8th May 2019, 7:15 am   #164
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Default Re: Ferguson 382U Firefly not playing ball

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Originally Posted by Sideband View Post
Is the anode load resistor OK? Usually a high-ish value like 100K or 220K. It's very common for them to go high and thus lower the anode volts.....I hope it's not the valve I sent you....it worked in a Bush radio very well.
Hi Mike - Thanks again for the valve by the way, the anode resistor is measuring 221k, so I'm wondering if another passive component has failed/drifted somewhere?

Thanks,
Pete
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Old 8th May 2019, 8:51 am   #165
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Default Re: Ferguson 382U Firefly not playing ball

Does the anode voltage change when you switch from AM to FM?

I take it that C50 manufacturer's sheet, C52 Trader sheet has been changed?
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Old 8th May 2019, 9:37 am   #166
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Default Re: Ferguson 382U Firefly not playing ball

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Originally Posted by Station X View Post
Does the anode voltage change when you switch from AM to FM?

I take it that C50 manufacturer's sheet, C52 Trader sheet has been changed?
Hi - yes it has been replaced.. Measuring DC resistance in situ, its about 680k, out of circuit, no reading!


Thanks,
Pete
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Old 8th May 2019, 9:51 am   #167
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Default Re: Ferguson 382U Firefly not playing ball

So long as there's no positive voltage on V5 pin 2 that cap is OK.
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Old 8th May 2019, 10:31 am   #168
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Default Re: Ferguson 382U Firefly not playing ball

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So long as there's no positive voltage on V5 pin 2 that cap is OK.
Hi thanks - Just checked and there's about 8mV?

Pete
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Old 8th May 2019, 10:33 am   #169
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Default Re: Ferguson 382U Firefly not playing ball

That's fine.

Does the anode voltage change when you switch from AM to FM?
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Old 8th May 2019, 12:16 pm   #170
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Default Re: Ferguson 382U Firefly not playing ball

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That's fine.

Does the anode voltage change when you switch from AM to FM?
Hi we get:-

AM = 71V
FM = 68V

thanks,

Pete
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Old 8th May 2019, 12:40 pm   #171
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Default Re: Ferguson 382U Firefly not playing ball

I'm thinking the anode resistor is dropping 92V, so the current flowing presumably, is I=V/R. i.e. = 92/220k about 0.4mA? does that seem a bit low?

Pete
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Old 8th May 2019, 1:15 pm   #172
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Default Re: Ferguson 382U Firefly not playing ball

Quote:
Originally Posted by el gringo View Post
Hi - yes it has been replaced.. Measuring DC resistance in situ, its about 680k, out of circuit, no reading!
When you measure resistance in circuit you need to look at other circuit paths that link the ends of the capacitor. In this case R25 + R26 + electrolytic leakage.

The first place to look is the valve datasheet http://www.r-type.org/pdfs/uabc80.pdf. According to this your anode voltage is high not low as the anode current for 70V is higher than 0.4mA!

The problem here is the design uses a 6.8M grid resistor that may add some bias using grid current but there is no specification for grid current so no guarantee valves will behave the same way, however, this is not all that important as the bias conditions are not static as any signal on the grid will be rectified and drive the grid negative.
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Old 8th May 2019, 2:26 pm   #173
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Default Re: Ferguson 382U Firefly not playing ball

Quote:
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Quote:
Originally Posted by el gringo View Post
Hi - yes it has been replaced.. Measuring DC resistance in situ, its about 680k, out of circuit, no reading!
When you measure resistance in circuit you need to look at other circuit paths that link the ends of the capacitor. In this case R25 + R26 + electrolytic leakage.

The first place to look is the valve datasheet http://www.r-type.org/pdfs/uabc80.pdf. According to this your anode voltage is high not low as the anode current for 70V is higher than 0.4mA!

The problem here is the design uses a 6.8M grid resistor that may add some bias using grid current but there is no specification for grid current so no guarantee valves will behave the same way, however, this is not all that important as the bias conditions are not static as any signal on the grid will be rectified and drive the grid negative.

Yes completely agree, the "in situ" and "out of circuit" measurements allow us to see the overall effective amount of resistance at DC, that the DVM reads between the two points?

This UABC80 reads 68V, yet interestingly, the previous UABC80, reads very slightly higher Anode voltage of around 71-72V.

The "what's changed" break/fix methodology is baffling me right now - as the only thing that I can see that's different from a couple of days ago, is that I've gone from:-

a) UABC80 with suspect rectification? ( FM works, distorted sound )
b) Germanium diodes wired in and the valve As+Ks disconnected ( FM louder, better sensitivity, but still a bit distorted )
c) Replacement UABC80 substituted ( no sound )
d) Prior UABC80 plugged back in, now no sound as well?

Cheers, Pete
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Old 8th May 2019, 2:44 pm   #174
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Default Re: Ferguson 382U Firefly not playing ball

Have you checked that you've reconnected the valves diode connections correctly?

Lawrence.
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Old 8th May 2019, 3:11 pm   #175
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Default Re: Ferguson 382U Firefly not playing ball

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Have you checked that you've reconnected the valves diode connections correctly?

Lawrence.
Hi - That's exactly what I thought!! Just double checked and it seems alright - and, we know that AM is working fine, so I'm guessing that the issue would appear to be reasonably localised to the FM stage?

Pete
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Old 8th May 2019, 3:41 pm   #176
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Default Re: Ferguson 382U Firefly not playing ball

If the set were mine I'd get the signal generator out and work backwards from the FM detector.

In the absence of a signal generator all I can suggest is checking the voltages on all valves and cleaning the contacts of the wave change switch.
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Old 8th May 2019, 4:11 pm   #177
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Default Re: Ferguson 382U Firefly not playing ball

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Originally Posted by Station X View Post
If the set were mine I'd get the signal generator out and work backwards from the FM detector.

In the absence of a signal generator all I can suggest is checking the voltages on all valves and cleaning the contacts of the wave change switch.
Hi - Good call checking around the wave change switch - blinking dry joint on C47 !!

FM Back on (HOORAY) and with replacement UABC80, it's a bit clearer in my book. I am now wondering about this notion of "very strong" FM signals being too much for the older sets? It's certainly more noticeable with Radio 1 and Eagle Radio (my local station), which appear way louder than the other stations I'm able to receive?

Thanks, Pete
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Old 8th May 2019, 4:42 pm   #178
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Default Re: Ferguson 382U Firefly not playing ball

Keep in mind that these sets were also designed before stereo was being transmitted but compression on popular radio stations is also a problem. FM is optimised for car drivers and not hi-fi these days.
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Old 8th May 2019, 5:34 pm   #179
el gringo
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Default Re: Ferguson 382U Firefly not playing ball

Quote:
Originally Posted by PJL View Post
Keep in mind that these sets were also designed before stereo was being transmitted but compression on popular radio stations is also a problem. FM is optimised for car drivers and not hi-fi these days.
Very interesting, will do a bit more research!

Thanks again everyone for all your help !!

Pete
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Old 8th May 2019, 7:47 pm   #180
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Default Re: Ferguson 382U Firefly not playing ball

Just done a quick and dirty test - put a trimpot across L12 and dialled the gain down to where it was 'still there', but def clearer for the louder stations..

If I get the urge, I may see If I can get to the other FM IF transformer, L16 and try something similar!

Pete
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