Rogers RD Junior FM unit

[dazzlevision]

Rogers RD Junior FM unit – restoration

I've had a first look at this unit, which was recently kindly offered free by “Nigel M” on this Forum.

It looks to be complete, apart from the two front control knobs. The tuning coverage is 87 to 107.5MHz, which is welcome nowadays.

The chassis and front panel are somewhat tarnished, with some rust spots, but nothing too severe.

The only signs of past component replacement are: one of the two ECC85 valves (a Mullard, with the modern version of the classic “shield” logo, introduced in the early 1970s) and a large “R.S.” (Radiospares) 0.5uF @ 500V dc working axial paper dielectric capacitor.

I’ve done some cold dc resistance/continuity checks and the mains transformer windings seem OK. However, the following issues have been found: on-off switch contacts open circuit and a blue wire-wound resistor mounted on the top of the chassis is open circuit (blue coloured, but with no visible markings).

I’ve also checked for low resistances between the lead out wiring to each of the coil cans. They all seem good, except for the inner one of the two cans ganged together, above the two valves in the top right of the photo of the top of the chassis. This is a little worrying, as it is one of the two permeability tuned coils coupled to the tuning control knob, via a reduction drive. So, it will be more difficult to remove for a closer inspection and I’ll have to make a careful note of the positioning of the drive system components, so as not to disturb the alignment.

Apart from the above, one of the 1 Watt Erie ceramic tubed carbon resistors looks to have been overheating; it’s the one right next to the chassis (mid-way) on the left hand side of the underside view photo.

There are two “TCC” wax capacitors and one black “Hunts” “Moldseal” that will almost certainly be very leaky. The Radiospares one will also doubtless be faulty. There is a white powdery deposit on the fixing tags of the HT can, so that may well need replacing. The HT rectifier is an “STC” Selenium type, which will probably be high-resistance by now and need replacing by a Silicon rectifier and series resistor.


The original mains lead has been replaced by a more modern grey PVC type, with the brown and blue core colours.

Fortunately, the unit came with some documentation: sales leaflet and user/installation instructions. Here is a list of the valves and their functions:

ECC85 grounded grid RF amplifier
ECF80 triode oscillator; pentode mixer/IF amplifier
EF80 IF amplifier
EF80 limiter
EB91 Foster-Seely discriminator
EC90 cathode follower (AF output)
ECC85 AFC control valve.

The sales leaflet also contains some reviews of the tuner, which are very favourable (of course!).

I’ve done an initial internet trawl for a circuit diagram for this unit, but to no avail.

I’ll add to this thread as I proceed with the restoration.

[dazzlevision]

Heres a scan of the Rogers brochure:

[Synchrodyne]

The valve line-up and corresponding functions bespeak a circuit that might have been unusual in some respects.

The use of an ECC85 double triode as a grounded grid RF amplifier suggests that it was of the push-pull type rather than the customary single type. More commonly, where a double-triode was being committed wholly to RF amplifier service, it would have been used as a cascode, although that said, the latter was more common in American than in British FM tuner practice.

The ECF80 oscillator-mixer might have been conventional. The description of one section, presumably the pentode, as being both a mixer and IF amplifier might have been a case of claiming the conversion gain as IF gain – perhaps to make it look as if the tuner had a three-stage IF strip.

An ECC85 double-triode as an AFC control valve (presumably a reactance modulator) was unusual. Usually a single triode or pentode (or one part of a multiple) valve was used for this purpose. Possibly one of the triodes was used as a DC amplifier between the discriminator and reactance modulator in order to obtain a wider control range than would otherwise be obtained. That was probably uncommon, but certainly not unknown in FM tuner practice. (Jason used it, albeit with a transistor DC amplifier, in its FMT4 model.)

The Rogers tuner had quite a wide AGC pull-in range, at ±500 kHz. Opinions appeared to have been divided on this. A wide pull-in range could be used to reduce to negligible proportions any likely tuning error caused by inexpert initial tuning, particularly in the absence of a centre-channel tuning indicator, the latter being optional in this case. Thus the AFC would be acting as AFT. An example from the other end of the scale was the Quad FM, which I think had a ±120 kHz AFC range. This was intended to correct for any residual frequency drift after correct initial tuning using the relatively precise twin neon centre-channel tuning indicator, and not so much to assist with initial tuning.

I think that some of the later Rogers FM tuners had a conventional magic eye type tuning indicator that was configured to act both as a signal strength indicator and a centre-channel indicator. This was done by using bias that was effectively the sum of that obtained from a limiter grid (for signal strength) and the modulus of the discriminator off-tune bias via a pair of crystal diodes. I don’t know what sort of precision it achieved – perhaps comparable to the NBS-type circuit used by Leak in its original Troughline, whose precision was comparable to that of the Quad twin-neon indicator. In its early solid-state FM tuners, Rogers used a centre-zero meter as a centre-channel tuning indicator, with no indication of signal strength. On the other hand Leak used a conventional metre configured to show both signal strength and a centre-channel indication using a combined bias.

Schematics for the valved Rogers tuners do seem to be in the unobtainium class, as does good information on the model chronology, although a basic sequence may be derived from the Hi Fi Year Book entries. The only circuit I have seen is that for the Mk II, Series II, which had a conventional, ECC85-based single-valve front end.


Cheers,