Dynatron FM1 tuner

[roadster541]

I bought this through the forum a couple of weeks ago and there was a post asking for updates from the new owner.
I don't really think of myself as a collector of tuners, but I have acquired quite a few over the years and they do interest me; this one because of the Hacker connection, I use a Mayflower 2 as my kitchen radio and Hacker portables are among the few transistor radios which I tolerate. This tuner will probably end up in a very nice AM only gram which is in my living room. As I listen almost exclusively to BBC R4 and R3, the limited frequency coverage will not be a problem.

This tuner is a later model; it has an ECC85 instead of the earlier ECC81 for local oscillator and AFC.
It is fitted with a B5 plug for the power supply.
As you can see from the photos it does need some caps changing: C22 and C23, using the BBC report circuit diagram (https://www.bbc.com/rd/publications/...tron-fm-type-i) are burnt and most of the other brown Hunts are cracked, so I will replace them all. I will check resistors as I go round and replace as necessary. The BBC report is interesting reading. Access is a bit tight for some of the components so I will have to think about the order in which I work. I will test and reform the elec. caps. I will make a start tomorrow as the Fisher tuner/pre-amp which came from the same advert is now finished and working nicely.
Rod
M0LXY

[Techman]

Yes, it was me that asked for updates, so thanks for that. I actually thought it was someone else that stepped forward to bag it, so well done!

It's interesting to see the under chassis view and the fact that I was correct in my original thoughts that it would have had a separate dedicated power supply and incorrect with what I finally said when I changed my mind on this. For what you're going to use it for I guess you'll build something - there's rather a lot of extra valve heaters there to just plonk onto an existing amplifier or AM gram chassis transformer. Seeing as you don't intend to use it for its original intended purpose as an FM conversion in a period Dynatron radiogram, then it doesn't matter that the ancillary parts for doing this are missing.

It's interesting that it's fitted with a B5 plug for its power connection - the earlier amplifiers have a large octal type for plugging ancillary parts in and interestingly I've got a Dynatron pre-amp with such a pug and socket connection, but the valve has a 6 volt heater whereas the supply from the amplifier would be 4 volts, so wondering if the later 6 volt heater amplifiers used the same plug and socket arrangement, the big question being why would the later amplifiers need a pre-amp?

I've downloaded and saved the BBC info. that you linked to and only had a very quick skim through it, but will take a closer look when I get time - I see it shows the power supply unit that I posted pictures of in that 'offered' thread.

All the ones that I've seen of these tuner units have been removed from radiograms that have been scrapped for their amplifiers and virtually always have the important ancillary parts missing, probably lost or left with the remains of the radiogram cabinet, the thought being that no one would want to retrofit one to an existing gram because everyone breaks them up for the amplifiers.

I still think that the tuner that you have most likely would have been removed from a radiogram sometime in the distant past, unless there's documented evidence with it (or a solid back story) that it actually came from the BBC - do you actually have that information passed on from its previous owner?

Once again, thanks for posting, it's appreciated.

[Synchrodyne]

Originally Posted by Techman

It's interesting that it's fitted with a B5 plug for its power connection - the earlier amplifiers have a large octal type for plugging ancillary parts in and interestingly I've got a Dynatron pre-amp with such a pug and socket connection, but the valve has a 6 volt heater whereas the supply from the amplifier would be 4 volts, so wondering if the later 6 volt heater amplifiers used the same plug and socket arrangement, the big question being why would the later amplifiers need a pre-amp?

Regarding the Dynatron FM1 tuner interconnections, we might derive a clue as to their nature from the arrangements used for the Ether Marshal suite, T139 tuner/control unit and LF613 amplifier, which as best I can determine was the first to be built with provision for the addition of an FM tuner. The Ether Marshal was announced around 1954 September, at the same time as the FM1 tuner and P1 power supply.

As well as an octal socket to provide the main connection with the T139 (HT and LT out, signal in), the LF613 also had a separate 5 pin socket that provided HT and LT to an FM tuner. The T139 had an additional audio input socket to accept a feed from the FM tuner. This went via the input selector (part of the waveband switch) direct to the tone control stage, bypassing the gramophone preamplifier stage.

This resulted in a “triangular” connection, with the FM1 connected directly to both the T139 and the LF613. It seems to be a recipe for a hum loop, but evidently it was satisfactory.

Possibly the 1955 model year version of the Ether Conqueror V suite (T99/LF612) was also modified to accommodate the FM1 in the same way. That seems to have been its final year, with the Ether Pathfinder suite introduced for the 1956 model year. This comprised the T10 AM-FM tuner, TC10 control unit and LF10 amplifier.

It would appear that with the FM1/P1 and FM2/P1 combinations, Dynatron was catering for multiple combinations. The tuner alone could be used with the T139/LF613 combination. But for retrofitting of earlier Dynatron radiograms with the 4 volt heater valves, the P1 unit would also be required. That may also have been the case for the preceding 6.3 volt heater models, unless their power supplies had adequate spare capacity.

The Dynatron RV12 Ether Minstrel FM-only console receiver used the FM2 in combination with the LF12 control unit/power amplifier, with 5 pin interconnection for HT and LT, and a separate AF connection. (I am not sure, but I understand that the LF10 through LF16 power amplifiers were all based broadly upon the Osram 912 circuit, variously without and with the pre-stage. The later LF20 is known to have been based upon the Mullard 5-20).


Cheers,

[stevehertz]

Originally Posted by roadster541

and Hacker portables are among the few transistor radios which I tolerate.
M0LXY

I'm guessing you dislike the weedy, somewhat harsh, 'small speaker' sound from most transistor radios? I can't disagree with that per se, but there are loads of great sounding transistor sets out there from - as you say - Hacker, plus Grundig, Roberts 707, ITT, Tecsun, Panasonic, Bush and others.

[roadster541]

I changed a few caps; those which were showing signs of physical distress.
I connected it to a Heathkit MA-12, which is my bench test amp and to my bench power supply, then gradually brought up the HT. It burst into life and the HT remained a steady 30mA. I was using a indoor folded dipole, it is all that I need as the mast is 2 miles away in direct line of sight. I was able to tune BBC R2, R3 & R4
The tuning method is, to me, unusual; with the AFC switched off, 50Hz, from the heater circuit, is injected and the tuning adjusted until the hum volume is a minimum. I suppose that it is cheaper than a visual tuning indicator.
I can't find my box of valve screening cans, but as soon as I do, I will fit the missing ones, check for any wildly out of tolerance components and check the alignment.
I am pleased with it. There is a small HT power supply in the upcoming VMARS auction which looks similar to the original Dynatron, so I will have a go at that, to use when I fit it in the gram, but who knows what I may find at Wooton Bassett!
Rod

[Synchrodyne]

Originally Posted by roadster541

The tuning method is, to me, unusual; with the AFC switched off, 50Hz, from the heater circuit, is injected and the tuning adjusted until the hum volume is a minimum. I suppose that it is cheaper than a visual tuning indicator.

Another possible reason stems from the fact that the FM1 chassis was intended for hidden mounting within a cabinet. That could make a visual indicator, with flying leads for separate mounting, somewhat cumbersome. Also, when peering into a cabinet to adjust the tuning presets on an internally mounted unit, an aural tuning indicator might make the job easier - no need to turn one's head to look at an indicator.


Cheers,