Pilot BM109 Poppet

[Mike Watterson]

Arrgh. Horrible.

Three attempts to get the tuner fixed.

It's not easily taken apart as there is a soldered on spring rod to earth the rotor at the front, so I didn't. Soldering the spring back in under tension is probably even harder.

There is a long boring story, but eventually it checked out on the continuity tester. However you could hear it shorting. So off with the connections to RF and LO coils. Put 32V current limited to 2A and see where it sparks! Sand and prod plates till it's OK.

Now stick in chassis after sanding end of shaft and reaming the knob a little so it doesn't need a hydraulic press for reassembly.

Re-align 1500 kHz and 600 kHz (trimmers UNDERNEATH tuning cap accessible via holes in the chassis. The isolation transformer is great. The LO coil is behind the 1R5/DK91 so if you have a not too tall and not too short trimmer you can access it from the top. Also accessible from underneath, but even then my plastic trimmer is too long. An old Philips' one with a brass tip and various lengths of handle fits.

Re-align 1500 kHz RF trimmer. The BBC R4 LW is maybe a little weaker, but tracking is now much improved with RTE1 LW very strong. MW seems sensitive based on signal generator level, but doesn't pick up Spirit from North Cavan. Amazingly the Decca TP60 with the GEC transistors does.

Date stamp on the back cover is 25th February 1958. I suppose opting for the louder 3V4/DL94 rather than the DL96 meant all 50 mA types, was a 1R5 so much cheaper than a DK92 in 1958, though no SW anyway. A badly laid out version of a 1946 radio with horrible access and even for 1950s a poor design from a safety point of view?

Just needs cleaned a bit now and see what it's like from dusk on MW.

I thought maybe it might need half a dozen capacitors changed and it would be a simple one. Maybe I've used up all the simple ones.

[Crackle]

Has your camera broke Mike, its always nice to see photos of what you are talking about. I would have liked to see the resistors packed as waxie, and the radio as well.

Mike

[Mike Watterson]

No, I have photos actually at each stage. I need to copy from the phone camera, crop, adjust and resize. The capacitors are not exciting.

[Mike Watterson]

Here is under the removed chassis as received.

Loose Mains wire is off adjacent tag to soldered one. It goes via wavechange switch and volume control switch to chassis. The other wire goes to the rectifier, direct. The large grey RS cap is thus across the mains when the set is on and on mains.
   
It's no doubt a replacement of a waxie. We should really call them paper dielectric capacitors as not all are card tubes coated with paraffin wax. The right proportions of Petroleum Jelly and candle wax (both paraffins) would likely be identical. The paper is impregnated with a lighter paraffin oil except in the most expensive military capacitors that use a different oil. Plastic and metal packaging was used. Some PIO only use paraffin and rely on hermetic seals. Plastic film are better.

The medium grey resistor is the 1800 Ohms filament chain ballast. One side is either the battery voltage or mains derived 85V and the other side connects via the wavechange switch to the filament chain on mains settings.

I'm thinking I should measure current and voltage on the ballast and dropper resistors as they likely higher resistance when hot. Depends on what sort of wire is used. You can see seven original paper caps and the dubious RS part. The ninth paper cap is on top of the chassis in series with LW contacts for battery and mains to the LO section of the tuning capacitor.

The bright yellow sleeved 100uF Plessey capacitor was bad. I often find the two part plastic low voltage caps are dried out. The low voltage cap I've most often replaced is on the FM detector, which are not all discriminators, but almost always are + to chassis.

[Mike Watterson]

Here is the chassis rear view before removal.
   

The capacitor above the ferrite rod is a ceramic type connected between an RF coil tap and the aerial socket. The otherwise more complete Trader 1331 doesn't show it.

There are two tags to park the battery plugs as otherwise they are a hazard only being partially disconnected (LT+ and HT-) on mains.
Mad cloth straps to tie in the batteries, though I've seen that on a few other models.

I might have bought this off Crackle a while ago.

The alleged DC voltage taps are just intermediate taps. It feeds DC mains via the rectifier as if AC; no doubt it's polarity protection.

Perhaps I'll put a US plug on the cable and set the jumper to 110V?

[ppppenguin]

The bright yellow sleeved 100uF Plessey capacitor was bad. I often find the two part plastic low voltage caps are dried out. The low voltage cap I've most often replaced is on the FM detector, which are not all discriminators, but almost always are + to chassis.

Along with many other low voltage electrolytics, these are replace on sight.  Big HT cans seem to survive better, possibly because they have more space for electrolyte and don't dry out so readily. The only small electrolytics I know with a reasonable survival rate are the blue Mullard ones.

[Murphyv310]

I've loads of NOS small electrolytic caps some are 40 odd years old. A few months ago I did a 10% check on them and found only a few to be faulty and they were genuine Japanese Panasonic!!!!!!

[Crackle]

Sorry Mike I was a bit slow there, I took you at your word when you talked of resistors packed like capacitors. They intrigued me that's why I wanted to see them, I understand now. There were no waxy resistors, just capacitors acting like they were resistors. Am I dim or what!

Mike

[Mike Watterson]

Too early or too late. They might make do as noisy grid leak resistors with a built in decoupler for a common grid amplier? They do burn well with excessive current. :D

I've cleaned the coal fire muck and the possible blood stain as well as possible, so photos soon.

Measuring the dropper cold (room temperature) and just after disconnecting power after five minutes (cold, hot) on the 250v tap:
279, 295 (12.1)
1546, 1622 (73.2)
630, 645 (29.5)
790, 822 (36)

The 250V tap is just a parking position; you get the same effect simply removing the jumper selection plug. Numbers in brackets are voltages with a 210V supply, though it's on the 250V tap. The shaver isolation transformer gives 220V on no load for 230V mains. The same current flows in each section if on the 250V tap. The lower voltage taps progressively short out starting at the 790 Ohms, which feeds HT and filament ballast resistor. The current is thus 46mA on the 250V tap with a 210 V supply. It's then 52mA at the next tap position, shorting out the 790 Ohms, at which point the set starts working (1R5/DK91 filament going from 0.91V to 1.1V). Between 1V and 1.1V is about the lower limit for any of the B7G 1.4V battery valve "pentagrid" mixer/oscillators. It's badly worn if it needs more than 1.1V.

I put 2 x 39K in parallel across the 1546 section and replaced the 180 Ohm on the 1R5 filament with 180 Ohms, because though 190 Ohms is in theory within spec, the choice of 180 Ohms is marginal because with the 190 Ohms the voltage was 1.42V when the total LT was 6.6V.

[Mike Watterson]

I've loads of NOS small electrolytic caps some are 40 odd years old. A few months ago I did a 10% check on them and found only a few to be faulty and they were genuine Japanese Panasonic!!!!!!

Philips/Mullard 60 year old ones are also mostly OK, but not Dublier, Plessey or Daly low voltage ones.