Vintage Unknown Radio Information Needed

[ediz1999]

Originally Posted by Edward Huggins

PS I guess we'll all keep on googling and searching our brain-files until we come up with more info. You can re-build it without a circuit diagram or a model type, by replacing the more doubtful components, using a bulb-limiter to boot up, checking tranny windings, valve heaters. There are some good "Stickies" on how to start from scratch on this site. Assume its AC mains, is the chassis isolated?

Yes it runs off AC mains. However there are no valves installed. I got it as it is shown in the picture. Of which is why I'm in need of finding some info on it. I started checking a few caps today. Nearly all are electrolidics but there are some wax capacitors too.

[Cobaltblue]

You can probably take a pretty good stab at the valve types first things first what kind of sockets are in the Chassis, Octal would be a reasonable guess and how many sockets are there?

It will then use either an American style line up or a UK one.

Cheers

Mike T

[ms660]

No valves, that's a shame, any indication on the chassis as to what they should be?

If not it is possible to take a reasonable guess as to what they should be with a bit of reverse engineering, judging from the top caps they are probably octal based.

Start at the rectifier, valve or selenium? Does the receiver have a mains transformer fitted?

Etc, Etc.

A few good pictures might help as well, chassis topside, chassis bottom side.

EDIT: Whoops....post crossed.

Lawrence.

[HamishBoxer]

That is my view they are indeed military caps used mainly in power supplies i.e R1392.

[ediz1999]

Here are the valve sockets from an above look. I'm extremely new to valve equipment and I'm still trying to learn all the terminology. Also attached is an underside view. I've attached a mains cord as the original one was destroyed.

[TonyDuell]

I am pretty sure I had (and maybe still have) a similar set somewhere, also with AMCO on the dial and 5 octal-based valves.

Mine had one valve still in it, a 25A6. I assumed this to be the audio output, and that it was a 300mA series heater string live chassis set (there doesn't appear to be mains transformer in any of the photos of the OP's set).

The mains lead one mine was one of those resistive 'line cord' horrors. I suspect the basic set was designed for 117V (or whatever we call it) mains amd this line cord was to run it off 240V or so.

[Edward Huggins]

Ah, quite revealing! I looks like a 5 valve, 6K8 to 6V6 type of line-up BUT there's the big mains transformer that's missing - and it's a big miss. Are you sure you still want to have a go at this? For starters you are going to need 5 Octal valves, a Double-Wound Mains Tranny, caps, resistors and more.....It only has a small output transformer and a 5" (?) loudspeaker so sound quality may be limited even when fixed. Edward

[ediz1999]

I believe this runs straight off the mains. What was in the missing space were those 3 oddly shaped capacitors in Parallel. The speaker on it tested ok, along with the audio output transformer. Caps resistors and valves are an ok for me. I'm totally alright with taking this on, however I need to know what valves I should order.

[TonyDuell]

I am going to make a lot of guesses here...

You have 5 valveholders, all octal. And I think 3 of them have top cap connectors.

The valves are probably some kind of frequency changer (either pentagrid or triode hexode), an IF amplfier pentode, a double diode triode (detector AGC and audio amplfier), an audio output pentode, and a mains rectifier.

Asuming the 25A6 that I remember in mine is original and that your set uses the same valves, I am going to suggest :

Frequency changer ; Either 6A8 (pentagrid) or 6K8 (triode hexode)
IF amplifier : 6K7
Double Diode Triode : 6Q7
Audio Output : 25A6
Rectifier : 25Z6

Those heaters add up to 69V, so you will need some kind of dropping resistor. Is there anything in the chassis, or does it need a line cord resistor for the heater dropper?

[Station X]

It looks to me like at least one of the smoothing caps within the vertical can has been bridged by an enormous external cap.

I'm not sure that this is a beginner's project, but if you're going to tackle it, the first thing to do is reverse engineer the power supply arrangements. If the valve heaters are wired in parallel, that indicates that there must have been a mains transformer. If in series there'll be a dropper resistor, or the set used a resistive line cord.

[ediz1999]

Originally Posted by TonyDuell

I am going to make a lot of guesses here...

I'll have a look tomorrow on that. Thanks for the info. I believe there was a dropping resister but I'll double check tomorrow.

When I searched for the 25A6 audio output valve, I just get referred to a 25A6G. Is 25A6 a replacement?

Thanks again for the feedback.

[TonyDuell]

Yes, sorry. The last letter of an American valve is 'G' for a glass envelope, 'GT' for a tubular glass envelope, etc. With no letter it's a metal envelope. You want 6K7G, 6Q7G, etc.

Now, on all those valves the heaters are pins 2 and 7 I think (but check this). You probably need to trace out the heater circuit. My _guess_ (again) is that one of those pins, on one of the holders is connected to chassis (if not, then look at the dial lamps, if the set has them. If one side of those is connected to the chassis, they are at the bottom end of the heater chain. So start from the other side of those).

The valve that has a heater closest to the chassis is likely to be the 6Q7G. Now trace from the other heater pin to one of the heater pins on another holder, and so on. The order is likely to be the 6Q7G, then the 6K7G and frequency changer in some order, then the 25A6G, then the 25Z6G. What does the remaining pin on the last valve (the 25Z6G) go to? Dial lamps? A resistor? (If so, what is the resistance). Work out the complete heater circuit back to the mains switch/cable.

Since we think we know the heater voltages and current (300mA) of the valves, we can then work out what voltage should appear across the ends of the heater circuit. That will indicate if it needs an line cord or whatever.

[dave walsh]

Looks like you are the serendipity factor Tony. This reverse engineered detective story is better than last nights "Hinterland" on BBC4. I was a bit worried there that ediz might power up without a dropper but clearly that is not going to happen now.
Dave W

[ediz1999]

Originally Posted by TonyDuell

Yes, sorry. The last letter of an American valve is 'G' for a glass envelope, 'GT' for a tubular glass envelope, etc. With no letter it's a metal envelope. You want 6K7G, 6Q7G, etc.

Now, on all those valves the heaters are pins 2 and 7 I think (but check this). You probably need to trace out the heater circuit. My _guess_ (again) is that one of those pins, on one of the holders is connected to chassis (if not, then look at the dial lamps, if the set has them. If one side of those is connected to the chassis, they are at the bottom end of the heater chain. So start from the other side of those).

The valve that has a heater closest to the chassis is likely to be the 6Q7G. Now trace from the other heater pin to one of the heater pins on another holder, and so on. The order is likely to be the 6Q7G, then the 6K7G and frequency changer in some order, then the 25A6G, then the 25Z6G. What does the remaining pin on the last valve (the 25Z6G) go to? Dial lamps? A resistor? (If so, what is the resistance). Work out the complete heater circuit back to the mains switch/cable.

Since we think we know the heater voltages and current (300mA) of the valves, we can then work out what voltage should appear across the ends of the heater circuit. That will indicate if it needs an line cord or whatever.

Ok. All valves ordered. I'll get back to you within a few days once they arrive.

[Station X]

What confidence!

[Cobaltblue]

I am wondering if those big old surplus capacitors were being used as a Capacitive dropper.

6 Microfarad (ISTR they were 2 each and in parallel)

Might work out around correct for a 300mA heater chain.

Have to do the calcs when I get a moment.

Cheers

Mike T

[Cobaltblue]

Had a look now and depending upon the exact drop required its closer to 4.5 Microfarads.

I can see the value on two of the capacitors but not the third but it does look physically smaller.

So I think its possible that's what those three caps were doing.

Can Ediz confirm where they were connected is it to the two yellow wires?

Cheers

Mike T

[Herald1360]

Ahhh...... too late to suggest a 150mA heater chain lineup now- would have made the dropper requirement less onerous. Given the space, maybe a heater transformer would be a way to go.

[paulsherwin]

I agree with Graham, this isn't really a beginner's project. It's an unusual and rare set. My guess would be that it's an AMOCO AA5 intended for use on 120V mains, but fitted with a UK market tuning scale and a resistive mains cord to allow 240V operation.

[Edward Huggins]

Er, was I right about the MANSBRIDGE type of capacitor? Surely there is someone out there older than my 75 years that will know this? Edward