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Old 16th Sep 2018, 1:47 pm   #1
FERNSEH
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Default Ferguson 372 AM/FM receiver 1956.

I'm repairing what is quite a rare radio set, at least I've never seen a Ferguson radio set like this one before.
Unlike most AM/FM sets the VHF tuner employs an ECF82 triode-pentode instead of the more usual ECC85 double-triode. The pentode RF amplifier valve should offer more gain than the grounded grid triode but the noise level might be higher. Also, the VHF tuner appears to be wired among all the other circuits and not inside a separate tuner box assembly. According to the service manual schedule A production receivers employed a PCF80 as the FM RF amplifier and mixer-oscillator.
As stands now there is no response from the RF circuits on AM and FM. An ECH81 frequency changer valve was tried yesterday but still no results.
Fault could be in the waveband switches.

DFWB.
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Old 16th Sep 2018, 9:41 pm   #2
crackle
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Default Re: Ferguson 372 AM/FM receiver 1956.

Hi David
One of these sold on Ebay last year according to one of the photos in the Radio Museum.
Although RM lists it as a 372A, Does your one have an A on the model number?
https://www.radiomuseum.org/r/ferguson_372a372.html

Mike
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Old 16th Sep 2018, 10:59 pm   #3
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Default Re: Ferguson 372 AM/FM receiver 1956.

Hi Mike,
It is indeed a model 372A. A similar chassis was fitted in radiograms 373RG (small bureau), 374RG (console) and 375RG (large bureau).
The radiograms have a ripple cancelling output transformer. In the table model the primary of the output transformer is connected direct to the HT reservoir capacitor, whereas in the radiograms the reservoir capacitor is connected to the tapped primary of the OP transformer.

DFWB.
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Old 17th Sep 2018, 8:44 pm   #4
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Default Re: Ferguson 372 AM/FM receiver 1956.

The AM local oscillator is not working and on FM stations can be tuned in at very low volume. I decided to concentrate my attention on improving the FM reception first. A dramatic improvement came about when a Hunts 0.01microfarad decoupling capacitor in the IF amplifier was replaced.
The original capacitor was found to be open circuit.

Now the FM stations are coming in loud and clear. On the Medium and Long wavebands the background noise level is very high but no stations can be tuned in. No SW reception.

DFWB.
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Old 17th Sep 2018, 9:26 pm   #5
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Default Re: Ferguson 372 AM/FM receiver 1956.

Quote:
Originally Posted by FERNSEH View Post
Unlike most AM/FM sets the VHF tuner employs an ECF82 triode-pentode instead of the more usual ECC85 double-triode. The pentode RF amplifier valve should offer more gain than the grounded grid triode but the noise level might be higher.
Notwithstanding the putative noise advantage of the triode, the pentode was certainly used as an FM RF amplifier by some makers, including in hi-fi tuners. So one could say that the empirical evidence is that there was not a stark difference that clearly favoured the triode as the “go to” valve for the purpose. Rather it was a trade-off, with the choice varying by maker and individual case. Pye provided an interesting example. Its FenMan I AM-FM receiver used a standard ECC85-based front end, whilst the more expensive FenMan II (and its HFT111 hi-fi tuner counterpart) used an EF80. The following Mozart (HFT108) FM tuner also used an EF80, although its HFT109 successor had an ECC85-based front end. I suspect that cost reduction was a factor there. An indeed, the general attraction of the ECC85-based front end was that it required only one valve, was readily available as a proprietary unit from specialist suppliers, and so was a low-cost option. On the other hand, ready-made two-valve FM front ends were probably less available, if at all in the UK, so setmakers had to design their own. In the early UK FM days, even three-gang FM capacitors were apparently scarce, which might be why Pye chose permeability tuning for its three-gang FenMan II/HFT111,

The single-valve FM front end as a separate unit was a late arrival in American practice. For those, setmakers such as GE, who had an apparent strong preference for pentode RF amplifiers, suitable pentode-triodes, such as the 12JN8, were released to enable this to be included in single-valve front ends. (This was despite the fact that there was a plethora of TV triode-pentodes available.)

Those who wanted to take full advantage of the triode possibilities probably opted for the cascode RF amplifier, with its pentode-like behaviour in some respects. But that was not at all common in UK FM practice.

Some useful comment on the triode vs. pentode RF amplifier debate was provided by F.H. Beaumont of R.N. Fitton (Ambassador Radio) in his IET paper on the design development of the BBC VHF AM-FM Comparator receiver. Here is a salient paragraph:

“At frequencies above 100 Mc/s, the triode is generally superior to the pentode as regards noise. At such frequencies thermal noise due to the first circuits becomes of less importance than valve noise. Between 100 Mc/s and approximately 40 Mc/s there is little to choose when one of the miniature pentodes, such as the Mullard EF42, and latterly more improved types such as the Mazda 6F1 and Mullard EF80, with multiple cathode connections, are used. The Mullard EF42 has an equivalent noise resistance of only 750 Ω. A typical grounded grid amplifier was set up with a bandwidth of 7.5 Mc/s from 87.5 Mc/s to 95 Mc/s, and it gave a noise factor (N) of 9 for a stage gain of 12 db. On the face of it this was quite good, but unfortunately, owing to the extremely low aerial circuit gain resulting from the low input impedance (1800 for half ECC81) the effective overall noise factor is likely to deteriorate. Also, the bandwidth is affected—the skirts of the r.f. response curve are flattened and an extra tuned circuit would be needed to give the required protection against spurious response excitation of the mixer stage. Turning to the pentode quoted, improvements are noted on aerial circuit gain and selectivity. The input impedance with the signal applied to grid 1 is slightly over 2 kΩ at 90 Mc/s. The noise factor with the appropriate circuit conditions proved to be 13 for a stage gain of 18 db. The deterioration, in noise factor, is not unduly serious while a useful doubling of gain is obtained. Subsequent work with the newer pentodes with multiple cathode connections shows that these figures can be much improved upon, but they were not available at the time.”

The author went on to state pentode advantages along other vectors, but quoting more than I have might be pushing the fair use boundary. (The comparator receiver used an EF42 as RF amplifier, being designed before the EF80 and 6F1 became available.)

Anyway, we could reasonably infer that someone at Ferguson favoured the pentode as FM RF amplifier, but that economics required a single-valve FM front end, hence the unusual – but well within its capabilities - deployment of the ECF82.


Cheers,
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Old 17th Sep 2018, 11:19 pm   #6
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Default Re: Ferguson 372 AM/FM receiver 1956.

There's no doubt the Ferguson 372A performs very well on FM, sensitivity seems to be better than receivers fitted with the more usual ECC85 double-triode. Noise performance is pretty good as well.

With regards to the AM fault, my worst fears have been realised, failure of the insulation between the AM/FM HT switch and the local oscillator SW-MW-LW waveband switch. I've disconnected all the wiring to the HT switch so now the AM local oscillator is supplied with HT irrespective of the waveband switch position. Likewise, the FM tuner is active at all times.
The receiver performs very well on the AM wavebands, very lively on the short waveband.

DFWB.
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Old 18th Sep 2018, 9:23 am   #7
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Default Re: Ferguson 372 AM/FM receiver 1956.

David

It is possible to add another wafer to the wavechange switch on some of these Fergusons, to get rid of the HT leakage problem. I did this on a Ferguson 401RG and it worked very well. The wavechange switch is quite long and will accommodate another wafer and the AM/FM HT switching can be achieved with a single pole 12 way wafer as the switch function is simple, rather than requiring a complex shape rotor (i.e. as used for LW/MW/SW selection).

https://www.vintage-radio.net/forum/...d.php?t=121343

Ron

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Old 18th Sep 2018, 10:41 am   #8
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Default Re: Ferguson 372 AM/FM receiver 1956.

Hi Ron,
It might be possible to add another switch wafer and that's something I'm considering for this set. For the time being the set seems to be working OK without the HT switch.
It was never good practise to have waveband switches operating with HT potentials on the same wafer as the signal and local oscillator switches.
Some Ekco TVs had a slider TV/FM switch which was prone to breakdown between connections with HT and other switches operating at lower potentials. The model TMB272 for example.

DFWB.
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Old 18th Sep 2018, 1:29 pm   #9
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Default Re: Ferguson 372 AM/FM receiver 1956.

I have arranged a microswitch and mechanical link to switch the HT on several sets, seemed to be the easy way.
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Old 18th Sep 2018, 2:07 pm   #10
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Default Re: Ferguson 372 AM/FM receiver 1956.

I would leave the HT permanently connected. It will be more reliable like that, and the additional valve wear is unlikely to be significant unless the radio sees very heavy FM only use.
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Old 19th Sep 2018, 7:14 pm   #11
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Default Re: Ferguson 372 AM/FM receiver 1956.

Agree. J.
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Old Yesterday, 12:08 pm   #12
FERNSEH
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Default Re: Ferguson 372 AM/FM receiver 1956.

It's most likely the set will be used on FM so there is no problem about the permanent HT supply to the VHF tuner. The AM local oscillator will be inactive on FM which results in a slight increase of anode current in the triode section of the ECH81 AM frequency changer valve.

Just two more jobs to do the set. The FM tuner drive cord has become detached and the cabinet needs attention.

The attachment shows the method of moving the tuning core in the FM tuner.

DFWB.
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