Ferguson 626BT restoration

[dazzlevision]

Hello,

I have previously restored one of these early 1960s battery transistor AM/FM table radios, which were (along with other manufacturers) only made for a brief period in the early 1960s during the transition between the valve table radio and fully portable transistor sets.

The set came with its original packing carton, instruction leaflet, guarantee card and sales receipt. However, the chassis was somewhat corroded, so when I had the opportunity to acquire another 626BT with a chassis in better cosmetic condition (i.e. no tarnishing of the metal chassis), I did. The second set has a rough cabinet and a slightly torn speaker cone, so I shall transplant the better chassis into the good cabinet when it is restored.

Externally, the radio is fairly conventional in appearance, with four rotary controls arranged in a row below the glass tuning scale. It looks like a typical valve set of the period. The only clue is a “Transistor” badge underneath the tuning scale (which is missing on the rough looking set with the better chassis). The cabinet is made from veneered plywood. The set is pictured in this thread: https://www.vintage-radio.net/forum/...ighlight=626BT

The radio’s construction is conventional in terms of valve era table radios, with a metal chassis, where the transistors (with the exception of the VHF unit and push-pull output pair) are mounted on top by soldering them to sections of tag strip visible through holes punched in the chassis.

The substantial 30 Ohm impedance elliptical “Elac” loudspeaker is of good quality, with a large magnet. The speaker is coupled to the audio output pair by an autotransformer, with a tap for the external speaker sockets (3 Ohm).

The large dry 9V battery (PP10) is housed in a recessed box, forming part of the back cover, with a hinge down cover secured two large “poppers”.

There is an internal VHF (loaded dipole) and an AM (ferrite rod) aerial.

After dusting down the innards and cabinet, I removed the chassis and connected a 9V power supply (using a linear regulated mains type, to avoid any interference problems on AM) and extension leads to the speaker.

The set worked quite well on MW but not LW; on VHF/FM, it was very insensitive. The black plastic drum, around which the drive cord is wound is a loose fit onto the tuning gang’s shaft, making tuning rather sloppy. This was simply due to the compression spring clip needing to be pushed firmly home. The wave change switch wipers were badly tarnished, so I carefully cleaned them with some contact cleaner/lubricant applied with a cotton bud. This eliminated the intermittent contacts.

I then set about listing the capacitors due for replacement. This will be done in two stages. The first being the replacement of all the Hunts black cased metallised paper “Moldseal” types, which will certainly be faulty. This should improve the set’s sensitivity. There are a lot of those red and black plastic cased “Plessey” electrolytics on the chassis and I shall replace them all.

The most difficult Hunts cap to access was the one in the VHF tuner unit, well hidden behind other components.

More to follow.

Regards,

Dazzlevision

[bobbyball]

I had the "HMV" badged version of this set when a student back in the late 80's

Nicely made and well performing set as I rememberm forget what happened to it sadly!

Looks like the VHF tuner may have originally been intended for use with a valve.

[dazzlevision]

Originally Posted by bobbyball

Looks like the VHF tuner may have originally been intended for use with a valve.

Hello Robert,

Yes, possibly, although the two OC171 transistors are soldered directly to six of the valveholder's tags (with the exception of their screen leads). Maybe they serve as voltage test points, accessible without removing the screening cover and thereby disturbing the VHF unit's alignment?

Regards,

Dazzlevision

[Restoration73]

Do not be disappointed if the FM sensitivity is not up to modern standards. My 626 cost
a mere £1, probably as the o/c AM demodulator diode was not found. Replace the 2 pin
plug with a PP3 snap, and use the 6 x C cell holder e.g. Maplin. I use one to demo my
modulator - people are mystified by a "valve look" set working in a field with no mains.

[bobbyball]

If I remember rightly I actually ran my set on a PP3 for ages as didn't need a high volume!

It looks like the VHF tuner may well have been "adpated" from at least the case used by a valved model and thus the 9 pin socket was probably fitted by default before any components (transistors in this case) were soldered in..

[Herald1360]

In any case the same connections would be required for two triodes in an ECC85 or similar as for two transistors in this version. Probably only the biassing resistors would need to be changed and you still have 3 spare solder tags to play with!

[dazzlevision]

Hello again,

The only Hunts Moldseal replacement that made a significant improvement to reception was C54 (Ferguson service sheet component reference), a 0.01uF which is the AM detector diode’s reservoir. However, they all measured high (capacitance) on my tester and were leaky.

I also found that one of the Plessey electrolytics C59 (8uF/6V) was fitted the wrong way round at the factory! This couples audio from the OC71 AF preamp’s collector to the base of the OC81D driver stage. It measured 3uF. The only other Plessey electrolytic that was really bad was C60 (100uF/6V), a supply line decoupler, which measured 0.14uF.

After Hunts and Plessey capacitor replacement, the set was now working well on AM but rather “deaf” on VHF. I found that by touching a screwdriver on the collector of the RF amplifier transistor (Mullard OC171) in the VHF unit, that reception was better. I measured the transistor’s electrode voltages, which seemed correct, including around 0.2V Vbe drop. I then removed the transistor for testing on my transistor tester, which measures Beta and collector leakage current (measured with the base open circuit). The transistor showed absolutely no Beta value (dc current gain)! The collector leakage current was normal for such a germanium device. Testing the e-b and b-c junctions on my Fluke DMM gave the usual 0.2V junction voltage drop.

Luckily, I have some spare OC171s and tested one of these, which had a Beta or about 150 (typical value according to a Mullard data book spec), so I fitted it and VHF reception improved significantly. The faulty OC171 had a large red paint spot on it, whereas the other OC171 in the VHF tuner unit had a blue paint spot. I suspect the devices were selected on factory test for the specific parameters wanted in these two circuit positions.

However, the VHF sound quality wasn't very good, so I tested the two FM detector diodes (Mullard OA79) using the diode test feature of my DMM. One of them was completely open circuit, so I fitted a replacement and FM audio quality was then very good.

I then investigated the lack of LW reception. In fact, there was very weak LW reception and the local oscillator was working (I used another radio switched to LW to prove this). It looked like the tuned RF stage was at fault and upon close inspection I could see that one of the very fine wires from the LW windings on the ferrite rod aerial was not connected to the solder tag. It looks like a previous repair had been carried out, with a short piece of stiff solid wire soldered to the tag but there was not fine wire from the coil visible.

I unsoldered the LW windings and removed it from the ferrite rod. I then had a closer look using a jeweller’s loupe (eyeglass). I could se a very short length (< 2mm) of wire protruding from the winding. I used a piece of very fine solid core wire to join the end to the solder tag. The LW winding was refitted onto the ferrite rod and LW then worked well.

With the set now working on all wavebands, I set the position of the tuning cursor according to the Ferguson service sheet. On the white painted metal backing panel that the cursor runs along, there are several small notches in the top and these are used for setting the cursor position correctly and during the RF alignment process. The service sheet says the left hand notch (viewed from the front) should be where the cursor rests in it furthest travel to the left and I set it at that. However, with the chassis back in the cabinet, it was apparent that the cursor travel didn’t match the extremes of the tuning scale and I compared the Ferguson sheet with the Trader sheet, which shows the reverse! Looking at the alignment instructions in more detail, the Ferguson sheet is incorrect. I shall run through the RF alignment in order to optimise the scale calibration and to obtain maximum sensitivity.

The next step will be to compare the performance of the two restored sets and if they are the same, fit the best chassis in the best cabinet and that should be that. I shall put the spare set in my loft as a source of spares for the future.

For a moment, I thought I was going to be fixing a TV, as this radio has a black cased double metal recitifier "STC" unit in it (below the volume control, on the underside of the chassis), of the type often found in line flywheel sync phase detector circuitry! In this radio, it is used to stabilise the dc supplies to the IF stages VT4 & VT5 and the audio output stages (VT7, 8 & 9) against reducing battery volatge.

Regards,

Dazzlevision