50's car radio repair. Smith and Sons 200X.

[barrymagrec]

Depending on the model you have some of these Smiths / HMV / Radiomobile radios had the IF stage in the power unit, the head unit was only the rf tuner. You needed a special lead between the two units.

[PaulR]

According to the Radio Servicing book the control units of all this series contained the stages up to and including the 1st AF amplifier so the power unit should work with another source. It might be easiest simply to try it with the existing head unit, though.

[TonyDuell]

I have a similar set, same control unit as far as I can see but a 'posher' amplifier with a pair of EL84s in push-pull. With that one the PSU/amplifier will certainly work on its own, I have used it as such as a test amplifier on my bench.

As regards powering it, note that some computer SMPSUs require a load on the main output (often the 5V one) to get the other outputs to be of the right voltages and to be able to supply significant current. You may find such a supply is not suitable as a 12V-only source. RS do a range of SMPSU units from Meanwell which seem to work OK and are not that expensive. There are certainly 12V-only units, I bought a 12V 1A-ish one (not large enough for this radio) for <£15 a short time back. You'd have to put it in a box, the mains input is on an open barrier strip, but that's the only issue.

[barrymagrec]

Interesting, there must have been more models than I thought, I have only seen the version with 2 N78s in push pull.

[Techman]

Yes, I didn't think the push-pull version of this set used EL84 valves either.

It sounds like the OP is experienced with 12 volt lead acid batteries and the current available if they're shorted out, so I don't think we need to worry about that any more - and he's using it in the shed and not the house anyway. Batteries like that made in the last decade or so are almost sealed for life, so you don't have the same risk from accidental spillage and possible hydrogen gas explosion from spark ignition from connections at the terminals like you used to with the old ones, although it's still there to an extent. What I would say is that if it's an old battery that was removed from service due to partial failure, then it may have a bad cell or two, so worth keeping an eye on the terminal voltage while in use, as strange faults can occur when the voltage drops below 10 volts or so.

I fully agree that old PC supplies (not laptop types as these are 19 volts) are very useful for this sort of thing. A couple of connections on the multiway plug need to be linked together to start the supply up, but other than that nothing else needs doing. However, seeing as he's got his car battery sorted for the job I think we can put the lid on that one for the time being.

[TonyDuell]

Maybe the valves in mine shouldn't be EL84s...

I got this set in a clearout at school about 40 years ago. It was mising the covers and also the 3 valves from the amplifier unit (the vibrator and metal rectifier were there). The 4 valves in the tuner/control unit were present and correct.

Anyway, a quick look at the connections to said missing valves (all on B9A bases) suggested that they were an ECC83 and a pair of EL84s. I had those, fitted them, and after replacing the open-circuit smoothing resistor it worked. I've had it running for seveal hours like that with no ill effects.

The N78 is on a B7G base according to the data I have, that certainly wouldn't fit my amplifier unit.

[Techman]

That's interesting, Tony. I wonder whether it had been modified to take the different valves - you could probably tell if it had.

[TonyDuell]

I don't think there's any evidence of modifications. Let me find the unit and I will check and also see if there are any identifying nameplates, etc, on it

[PaulR]

According to the service data I have the push pull version used 6AQ5s

[robreddog]

my radio is as per the 200x schematic with a 6X4 and a 6AQ5.

[AlanC]

I was given an example of the one with the IF stage in the power supply box in the 70's. The head had two valves in it, an X78 and W77 IIRC. It had a fault which was that the N78 valveholder was arcing between its pins; I was able to file away the carbonised bit and it was then working fine. I remember it most for being one of the best medium wave receivers I've ever had, late at night you could hear plenty of DX including American stations. Mind you this was before the days of BBC and independent local stations on every other channel, and no switch mode power supplies either! Unfortunately it went the way of many radios that came my way in my youth, it got cannibalised for spares to feed my radio building habit! I used an old car battery to run it, charged using my dad's trickle charger.

[TonyDuell]

I've worked out what mine is.

Strictly it's a coach radio, not a car radio. If you have the latest DVD from this site, look up the Radiomobile 240x manufacturer's manual. I don't have the microphone preamplifier, just the radio unit (which is much the same as the 200x one) and the amplifier + on/off switch control unit.

And I guessed right about the valves. The output stage is a pair of EL84s in push-pull.

[robreddog]

the rewound transformer has arrived, (thanks to ed) a good looking job,aquired the main components now so going to start re assembling the power unit and see if i can get some output from it.

[robreddog]

Components fitted and wiring checked,supplied with 12v dc from a battery (via a 5amp fuse) and checked fingers crossed.
300v ac on each leg of the transformer secondary and and 223v dc and 238v dc
on the two smoothing capacitors so very happy so far.
a question or two if anyone knows... tuner shows a fuse in the dc supply but i cannot find on the chassis was this a inline fuse outside the chassis and i also cannot find reference to neg or pos earth for the tuner is this because the power unit can be neg or pos ??.
and a friend of mine made a great job on the facia.

[robreddog]

progress.
First thing remember is to switch off fluorescent light and note ht drops if dc supply volts gets much below 12,spent an hour checking why until i realised the battery charger was off.
7 days ago i fired up the tuner with the power unit and a lot of hash and very faint vocals on radio 4, good the pilot lamp works.
checked voltages and was getting low volts (2-6)dc on the grid of the valves and high voltage (188) on the anode of v4 6at6,scheme says 75v.
replaced caps c5,c11,c18,c23,c27 this cleared up the dc leakage but still had the high voltage on v4.
ordered a valve off ebay which arrived 2 days later !!.
fitted the valve and now 66vdc on v4 anode, may change the resistor.
better but still hash/noise.
found a wire hanging off and the only place i could see it came from was the screen of wire to the volume control and as the wire went to earth could see no harm connecting it.
clear sound now of the shipping forecast at 6.
the tone control switch s3 works.
now to move on to medium wave.

[PaulR]

Good progress.

Regarding +ve or -ve earth look for electrolytic capacitors connected the chassis of the power supply and ensure that they are the right way round. I forgot this when I did a similar set and was rewarded by a bang from one the wrong way round. Everything after the vibrator/transformer primary shouldn't "see" the car's polarity.

[robreddog]

mw is working but what to expect ?
local stations derby,nottm are good also gold and talksport.
maybe i will experiment with the aerial got a piece of wire out of the shed door at the moment, although back in the day seem to remember usually only having a metal coat hanger !!
Does anyone know how the presetting works there is one LW and 4 MW
pushbuttons,that also don't sit very evenly so maybe a problem there.
Now i have a reasonable working set i will change the remaining wax caps and check resistors.

[Radio Wrangler]

If I remember. Pull a pushbutton out, It will now stick out beyond the others. Tune around for what you want, push the protruding button all the way in. That button now stores that tuning position. Do the others as you wish, any order, Just remember one is long wave.

David

[PaulR]

There will probably be an aerial trimmer which might help with reception

[robreddog]

david...correct pull out and push back in till you feel the pushbutton latch (no better way to describe)had to fiddle with the mechanism to get it to work though.
paul... yes there is a trimmer which pulled in the stations a little stronger.
So i think i have the stations now that i can expect on MW and LW and compares pretty well with my merc radio LW and MW,considering it's age.

Next step is to add an aux or bluetooth connection, i have been reading the various threads on this subject and am i correct in thinking to put a c/o switch in the line to the volume control, i have room enough in the power box but that would mean cabling between that and the tuner what are your thoughts all.
attched is a pic of it driving a grundig speaker,i have also obtained and awaiting the correct speaker plug.