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Old 16th Jan 2017, 7:51 pm   #1
Heatercathodeshort
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Default Pilot BM90 AC/DC mains battery portable. 1955.

I have always had a soft spot for valve battery portables from the 1940 period to the introduction of the transistor in the mid to late 1950's. I call them 'Bracklesham Bay' models and you might wonder why.

In the 1930's it was possible to buy around an acre of ground at the seaside at a knock down price typically around £50. The local Council granted permission to allow temporary buildings that could be used as holiday homes in the summer. [They often became permanent during the war when their owners were bombed out of the cities]
These temporary buildings often consisted of an obsolete railway carriage, sometimes two bolted together side by side. Smartened up and painted these provided excellent holiday accommodation for a young family especially in the post war austerity of the 40's and 50's.

Usually there were no services other than a tap for water supply. Cooking and heating was catered for by bottled gas and gas mantles bracket fittings provided the light.
Having no electricity a battery radio was a 'must have' to provide entertainment when rainy days stopped play. Very large number of 'Alldry' portables were sold as second receivers often for intermittent holiday use as battery consumption was quite high and the dry batteries expensive.

I was recently gifted this Pilot BM90 built at the Pilot factory at Park Royal in London.
Presuming it to be one of the larger Vidor models I put it aside for examination later and rediscovering it yesterday, decided to give it a look over. Upon opening the lid I discovered that it was rather a nice looking Pilot AC/DC mains model dating from 1955. A 90v HT battery provided the H.T. and a 7.5v supplied the four valve filaments wired in series. It was very versatile also capable of operation on 110/250v AC/DC mains.
I must admit that I am not too keen on this arrangement with the very delicate valve filaments, a mains dropper together with a live chassis.

The valves employed are the first generation B7G 1.4v 50m/a Brimar range, 1R5 1T4 1S4 3V4. [Mullard DK91 DF91 DAF91 DL94]

This was a very clean example and I was tempted to plug it in and see what happened. I removed the bottom to give access to the 'works' and the first thing I noticed was a horrible bodged extension speaker arrangement. This consisted of a dolls house mini switch connected by thick plastic flex to the primary of the sound output transformer. The soldering was that of a plumber with the whole nasty mess wrapped up in that lovely old 'rubberized insulation tape'. A rather odd arrangement as the output would have been high impedance. The output transformer is mounted on the large speaker with no speaker connecting tags visible. I wonder if it worked? Of course he may have been using another output transformer at the extension speaker end or a 1920's horn. I wonder!

With this junk removed a good examination of the chassis showed the exploded remains of the mains filter capacitor. At least I didn't have to worry about that spectacular component and decided to plug it in. It's failure must have given it's owner a heart attack especially in a restricted space.
Switching to 'Mains MW' a few crackles could be heard but little else. At least this proved that all four valve filaments were active as they are wired in series. No signs of any signals but moving the valves provided violent crackles and pops.

Fearing the worse checked the McMurdo B7G valve holders. My dark thoughts were proved correct. Fumes from an old layer battery presumably left in the receiver when finally retired had rotted most of the base contact forks in the valve holders. I have experienced this many times before and the only cure is to replace the valve holders, or in the case of the excellent quality McMudo types, just the base contact forks.

The well constructed chassis sported a few Hunts brown cockroaches most of which were still OK but as they began to fall apart during the base fork replacement mission I decided to replace them to guarantee reliability. A total of 21 base forks were replaced. The components were unwrapped from the tags as I'm not keen on the chop and stick method.

A check on the fixed resistors proved them all to be well within tolerance so were left undisturbed.
With the mains applied the oscillator burst into action producing some low level signals. A quick check across each valve filament produced 1.2v for the first three and 2.4v for the double filament output pentode, a Brimar 3V4. [DL94] This may well be a fraction low but the 1R5 [DK91] oscillates easily so I would rather the readings be a fraction low to preserve filament life. This was a start but the rest of the repair should have been completed a lot sooner than it was.

The hum level was very high despite having eliminated the smoothing and heater decoupling capacitor. Valve voltages were spot on. Some odd symptoms surrounded the DAF91 detector/preamp. Being very high impedance this was very sensitive to hum pick up, just placing your hand a couple of inches away produced a loud 50c/s hum that was not effected by the volume control. There was also a tendency for the valve to go unstable producing a howl that woke up the local farm dogs..

After checking all the obvious I did what I should have done in the first place, that is check the value of the volume control itself! Yes it read 22m instead of the 1m quoted on the circuit. This explained everything. The control grid of the DAF91 was virtually floating willing to grab anything that came it's way, hence the hum level and the instability around the controls center setting.
Replacing this removed the hum and produced a better level of signal but it was still well below what these battery portables are capable of. It was time to check the alignment.

The model employs an early ferrite rod aerial. The coils were well secured so attention was paid to the I.F.'s. With the signal generator set to 470kc/s they were discovered to be spot on, almost a let down. Confidence was restored when the front end alignment was checked. The RF and oscillator trimmer bank on the rear of the chassis was well out and soon corrected.
This produced a very high output from many stations on the MW despite my workshop being almost entirely screened with foil covered polystyrene type insulation. The LW performance is outstanding due to the use of full RF tuning coils not just a cheap loading coil as can be found in many of these portables. The loudspeaker is a large ELAC elliptical unit and gives a nice mellow tone.

Finally the case was lightly cleaned and the lid retainers brought back to a dull shine with a fine rotary brush and a coat of clear varnish that should protect them from future corrosion.

A lot of work for what some would consider a lowly radio but interesting and a satisfactory result. It is very stylish but maybe just a little bit dated for 1955. The trade was soon to suffer the chill of change. The build quality is excellent. Regards, John.
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Old 16th Jan 2017, 8:21 pm   #2
G6Tanuki
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

That's really rather nice - it's truly the sort of radio I can imagine my parents taking along when driving in their Austin A70 from London out to Epsom Downs on a balmy summer-weekday afternoon to enjoy a picnic-tea, relax, and watch the horses training.
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Old 16th Jan 2017, 8:22 pm   #3
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Nice receiver!

I've always liked the 'All Dry' portables and have a few (mostly Vidor) myself, including a battery/mains model that has a double-wound mains transfomer and uses an EZ40 valve as the rectifier (produces the 90V HT and supplies the heaters via a dropping resistor). Also another Vidor with MW, LW, and VHF (FM). That one uses a couple of germanium diodes in the detector stage but is otherwise valved. Non-Vidor models I have include a Marconiphone P20B and a Pye which is built on a PCB.

For testing on the bench I have a home-made PSU consisting of a 30V transformer feeding a voltage doubler. Gives about 85V which is near enough for the 90V HT . LT either from my modern bench supply or a single alkaline D cell.

I generally assume that any second-hand ome I find will have all the filaments burnt out due to the previous owner trying various batteries. So far I have been right almost every time...

When I started being interested in them about 25 years ago, nobody wanted them. Now, fortunately, they are being collected.
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Old 17th Jan 2017, 10:11 pm   #4
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Hi John,
I've always had a soft spot for Pilot radios, particularly the pre-war models. The pre-war and early post-war sets were very much influenced by American designs. Nothing wrong with that of course because the sets were always of a high standard of construction.
From the mid fifties the companies radios and TVs were becoming more English in design, the BM90 is an example.
Brave man replacing all those valveholder pins, there wouldn't be many service technicians willing to do that task. That kind of job sorted the men from the boys.
How is the mains voltage adjusted? as I can't see anywhere on the chassis where an adjustment selector is located.

DFWB.
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Old 18th Jan 2017, 10:12 am   #5
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

There are tappings on the mains dropper David. I think fitting a customer adjustable mains adjuster 110v/250v would be too tempting for the knob twiddlers..
I have not checked the selling price of this Pilot but I can't believe they made their fortunes from it. Regards, John.
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Old 18th Jan 2017, 1:29 pm   #6
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Nice work, John. I also enjoy working on battery valve portables and have a small collection of Vidor, Ever Ready and Ekco sets.

The Pilot must be relatively rare compared with most other makes.

Why didn't you post this in Success Stories? It's a success in anyone's book!
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Old 18th Jan 2017, 2:10 pm   #7
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Quote:
Originally Posted by Heatercathodeshort View Post
I have not checked the selling price of this Pilot but I can't believe they made their fortunes from it. Regards, John.
A nice set in every respect, and an interesting write-up with some social history thrown in!

The BM90 went on the market in June 1954 - a month before I left school, and when food and other rationing ended. Priced at £13.19s.9d plus Purchase Tax, so about £20.00 all in I guess, plus batteries if needed. That equates to about £500 in today's terms - not cheap alongside today's offerings, but a fairly typical price for a radio back then. A couple of months later I started work as an apprentice on £2.7.6 a week (£2.35) and recall rushing home excitedly to say that the chap I was working with had earned £10 the week before with overtime.

http://inflation.stephenmorley.org/

1955 - 60 was certainly the golden era of UK attaché case 4-valve portables, often marketed by the likes of Every Ready and Vidor in support of their battery sales, but not for much longer - the coming of transistors was the death knell. It 1959, along with two workmates, we spent 3 weeks cycling around Switzerland, and we each brought back an Hitachi pocket portable in a leather case, which cost us £8.00 apiece. A PP3 battery lasted for weeks.

Such radios soon put paid to the likes of 'Lady Margaret', 'Lady Anne', Pye 'Jewel Case' and a plethora of other valved attaché case portables. The Bush TR82 transistor set prospered because many transistor radios were considered 'tinny' due to their small speakers and lack of bass. The TR82, with its larger speaker and tone control, sounded like a 'proper' radio (ie, valved), which people had been accustomed to. (They still do sound good).
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Old 18th Jan 2017, 2:43 pm   #8
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Nice to see you back here with a radio, John, and great work.

This is a type of radio I've yet to dabble with. The nearest I've got is a handful of Roberts RMBs: similar electrically, I suppose, but culturally quite different!

Where on earth did you source all those valveholder forks from? A real labour of love to change, especially as you did it "properly".

Quote:
Originally Posted by Heatercathodeshort View Post
I call them 'Bracklesham Bay' models and you might wonder why.

In the 1930's it was possible to buy around an acre of ground at the seaside at a knock down price typically around £50. The local Council granted permission to allow temporary buildings that could be used as holiday homes in the summer. [They often became permanent during the war when their owners were bombed out of the cities]

These temporary buildings often consisted of an obsolete railway carriage, sometimes two bolted together side by side. Smartened up and painted these provided excellent holiday accommodation for a young family especially in the post war austerity of the 40's and 50's.

Usually there were no services other than a tap for water supply. Cooking and heating was catered for by bottled gas and gas mantles bracket fittings provided the light.
I believe these areas were known as Plotlands, and the residents as Plotlanders. There's lots on the internet, e.g. this site which has lots of interesting photos: http://www.roselake.co.uk/ There are some rather poignant photos of an elderly couple re-visiting all that remains of their pride and joy half a century later here: http://www.roselake.co.uk/main/revisit.htm

Nick.
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Old 18th Jan 2017, 4:26 pm   #9
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Default Re: Pilot BM90 AC/DC mains battery portable. 1955.

Thanks for your encouraging comments lads, my nurse will be pleased.

Yes Nick a bit of a drudge but the McMurdo pin retainers can be easily removed from a good holder and fitted into the faulty one.
I was tempted to cut and stick but it would have looked nasty. Once you start to unwrap all the connections, it is difficult to stop, rather like opening a packet of wine gums.
I could have replaced the entire valve holders but that would have entailed removing all the connections in one go, a recipe for disaster and frustration. One 'fork' at a time is enough!

I can't believe the undistorted output of the DL94! 240 milliwatts at 90V. It honestly sounds more like 1.5 watts from that large speaker and output transformer. [More like an ECL80]

I have far too many of these little valve portables..They keep popping up everywhere.
Regards, John.
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