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Old 15th Nov 2017, 9:19 pm   #1
FERNSEH
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Default Ferguson 901-AC.

I've completed the restoration of a Ferguson 901AC radio set.
When first placed on the work bench it was found the set did work but not very well. The 901 was made in 1939 and was one of the first Ferguson radios to shed the American design influences. Three European valves are employed in the receiver chassis, Mullard ECH3, EF9 and EBC3. The separate output and power supply chassis employs a 6V6 and 5Y3. First examination of the chassis units revealed that coupling capacitor between the AF amplifier and output valves is located on the power chassis and as expected it was leaky. Well, it seems that a previous repairer was aware that the capacitor was faulty but instead of replacing it, instead, the cathode bias resistor of the 6V6 was increased to 1000 ohms and the bias bypass capacitor replaced with a great big 32microfarad 500 volt electrolytic capacitor. A very strange solution to effect a "repair".
The offending components have been replaced and the result of these efforts is an improvement of the sound quality.
But the set is still insensitive. The valves are OK and voltage readings are about right for this model. A service sheet was obtained through this website so now full alignment can begin. This is a very well designed receiver and all the necessary adjustments can be carried out from the top of the receiver chassis. Start procedure. Tuning pointer correctly positioned on the scale. Peak up the IF transformers on 470Kc/s. The service notes inform us to start adjusting the short waveband first: tune the signal generator to 16Mc/s to adjust the oscillator aerial trimmers in that order. Signal generator reset to 6Mc/s and adjust the padder trimmer VC3. Recheck tuning scale calibration.
A similar procedure is used on the Medium and Long wavebands. HF end of the waveband adjusted first followed by the padding trimmers VC6 and 9 to adjust the lower frequency end of the MW and LW wavebands.
The attachments show the trimmer positions and frequency adjustments.
Excellent results on the medium waveband. Not so sensitive on the short waveband.

DFWB.
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Last edited by FERNSEH; 15th Nov 2017 at 9:31 pm.
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Old 15th Nov 2017, 11:15 pm   #2
julie_m
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Default Re: Ferguson 901-AC.

Why would someone miss a leaking capacitor? If they knew enough about electronics to realise that the anode current was excessive, and that increasing the cathode resistor would reduce it, surely they would have known that the grid would have to be negative with respect to the cathode.

They must have taken some DC voltage measurements in order to know that the output stage current was excessive, so how could they not suspect the DC blocking capacitor? If somebody just tried diving in, with no circuit diagram and no measurements, what are the odds against them picking the output valve cathode bias components and effecting a sort-of cure? (Assuming nothing else was disturbed.)

The mind really does boggle sometimes .....
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Old 16th Nov 2017, 7:44 am   #3
ColinB
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Default Re: Ferguson 901-AC.

Maybe in those days the right part wasn't available?
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Old 16th Nov 2017, 11:05 am   #4
turretslug
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Default Re: Ferguson 901-AC.

Entertainingly curious, and more into the realms of psychology than electronics. Perhaps someone with a detached, academically-inclined mindset and a tendency to over-focus? If someone had a "physics lesson" appreciation of components such as capacitors without the coal-face understanding of their failure modes, it might not have entered their head that it could be leaky or otherwise prone to failure.

"The output valve is overheating due to excess HT current draw, therefore the negative auto-bias needs to be increased. End of problem."

The components used to effect the wrong solution are overly expensive and over-rated, again suggesting a detached outlook. I've seen the academic/practical conflict in work situations- someone with a string of qualifications will come up with convoluted, fanciful and ultimately misguided reasons for a fault, someone with hardly an exam pass in their life but decades in the trade will say, "Picture shimmy?- change D2437, it's always D2437." The optimum state is somewhere in-between.
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Old 16th Nov 2017, 11:06 am   #5
FERNSEH
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Default Re: Ferguson 901-AC.

Quote:
Originally Posted by ColinB View Post
Maybe in those days the right part wasn't available?
A 0.01microfarad capacitor?

DFWB.
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