Armstrong Radio Tuner and Amplifier Model EXP125/2CP

[Bigears]

Hi this is my first post , I have an early Armstrong Radio Tuner and Amplifier Model EXP/2CP which was built in around 1948. It is in two units, one of the units is the multi-band tuner and the other is the Amplifier. at the Back of the Tuner there is two jacks named "PU" and "AE" can anyone tell me what they are, also can anyone tell me where is the speaker jack is and what speaker I would need.

[paulsherwin]

PU = pickup, an input for a crystal record cartridge with a sensitivity of around 1V and an input impedence of around 1M. You can connect a (mono) line level source to this.

AE = aerial and earth for the radio.

If this gear is unrestored then you shouldn't use it or even power it up without doing some basic maintenance first. At the very least, lots of wax capacitors are likely to need changing.

I don't know this hardware, but there is unlikely to be a 'speaker jack'. It looks as if the speaker connections are the red and black wires soldered to tags on the back of the power amp chassis, but you should be able to trace the wiring from the output transformer secondary. Power amps of that era typically used 15 ohm speakers but there may be transformer taps for lower impedances.

[Nickthedentist]

"EXP" presumably denotes an export model, which may be confirmed if there's no LW band.

Paul's right: don't just plug this in and try it. You may turn an easy restoration into a write-off, and bits can go bang and injure you too. The amp, although mono, may be of reasonable monetary value as it stands, but could be worth almost nothing if you accidently fry it.

[paulsherwin]

There does appear to be a LW tuning scale, but that doesn't mean it wasn't built for export of course.

[Bigears]

I did power it up when I first got it, as I don't know much about it, all the valves light up but no sound as I've no speaker connected also how do I connect a speaker to it?

Thank you very much for you quick reply

You're right paulsherwin it has a LW tuning scale.

[AC/HL]

More photos here: http://kensvintageradios.blogspot.co...model-exp.html

It doesn't appear to be an export model, a good selection of stations marked including BBC ones. Armstrong were suppliers of good quality chassis to the home build market, often advertising in Wireless World and similar magazines.

PS. Remove the rusty springs round the valve holders before attempting to unplug the valves. They're not necessary and are likely to damage the glass envelope.

[Station X]

I can see what looks like a mains transformer on the amplifier chassis, but no sign of an output transformer.

Under chassis views would be useful.

[AC/HL]

This site suggests that it may indeed be an export chassis: http://jcgl.orpheusweb.co.uk/temp/Mo...Radio/337.html

[Mr Moose]

Hello,
http://www.audiomisc.co.uk/Armstrong/general/Pub1.html
Yours, Richard

[Ed_Dinning]

Hi it looks like a reasonable quality unit. Armstrong produced units such as this for people to build into their own cabinets.

It could be test run on a simple speaker, but only for a very short time or you may cause costly damage.

There are people on the forum in your area who may be able to look at it for you and advise.

Ed

[Synchrodyne]

Most of Armstrong’s immediate post-WWII models had EXP-prefixed designations, including those offered for the UK domestic market. Most, but not all had LF coverage as well as MF and HF. I suspect that reflected the fact that the export market was of primary concern, with some production diverted to the domestic market. Of that range, the EXP125, released late in 1947, was the top-of-the-line model. The 1940s range included a small number of AC-DC models, designated with the UNI-prefix.

For or just ahead of Radiolympia 1949, Armstrong revamped its range, changing from circular to slide-rule dials and to miniature valves for the small-signal functions. This revamped range was advertised in Wireless World (WW) for 1949 October:



The revised range was basically of three-tier form, with some variations in the middle tier. The EXP125/2 was the top-tier model in that range. The bottom model, the EXP83/2, was probably the major constituent of domestic sales, but the EXP125/2 appeared to be offered for the luxury end of the domestic market, as suggested in this WW 1950 January advertisement:



Thence the low-tier EXP83/3 was replaced by the EXP73, circa 1951, and the EXP73 by the FC38 in mid-1953. The FC38 did appear to be very much oriented towards the domestic market. I’d say that the “FC” designation denoted the fact that the frequency changer was the first stage. Other and earlier Armstrong models from the middle tier, such as the RF103 series, had “RF” prefix designations, signifying that they had an RF amplifier, important for the export market. The FC38 (as had the EXP73 and EXP83 before it) aligned with the apparent British aversion to RF amplifiers in AM receivers (or “no RF amplifiers please, we’re British”).

The EXP125/2 was superseded by the EXP125/3 in 1951, and the latter by the EXP125A circa 1953.

In 1954, the FC48 replaced the FC38. Amongst others, the IF was changed from 465 kHz to the post-Copenhagen number of 470 kHz, and provision was made to take a signal from and supply HT and LT to an outboard FM tuner, the latter emerging as the FM56, also intended for use with the Armstrong A10 amplifier system released in 1954, and which signified Armstrong’s step into the hi-fi market proper, although it stayed in the radiogram chassis market for quite some time after that. In fact, the last echoes of that era, in the form of the 127 and 127M tuner-amplifiers, lasted until around 1969.

At the same time as the FC48 was released, the EXP125C replaced the EXP125A. Again, the IF was changed, and provision was made for connecting an FM tuner. Other nominally export models stayed with the previous 465 kHz IF. So that was an indication that the EXP125 was seen as a domestic as well as an export model. It comes out in this advertisement from WW for 1954 September, wherein the EXP125C was listed under “Other Models” and not under “Export Models”. That latter group included the BS125 bandspread model (released in 1953 as best I can work out) which one might describe as being half a tier above the EXP125.



The unit at interest appears to be an EXP125/2, which was current from late 1949 to sometime in 1951, and was sold as a luxury domestic model as well as an export model. I have seen the “CP” suffix used in connection with other models, but I have never worked out its significance, and it did not appear in any of the Armstrong advertising of the period. That suggests that it was some kind of secondary designator.

Technical information for these Armstrong models appears to be scarce. I have seen schematics for the FC48 and RF103/3, valve lines-up for the EXP125C and BS125, and an IF list, but very little else.


Cheers,

[Bigears]

Originally Posted by Ed_Dinning

There are people on the forum in your area who may be able to look at it for you and advise.

That would be good, if someone lives near me in Croxdale Co/Durham.

Here are some pictures of the inside showing the output transformer.

[TonyDuell]

My guess is that 'OPT' is OutPut Transformer (and 'CH' is CHoke, a smoothing inductor). The secondary winding (loudspeaker side) of the output transformer is, I suspect the 3 wires going to that screw terminal strip on the back of the chassis. One (probably the black wire) is common, the other 2 are for different impedances of speaker, most likely 2 out of 3 Ohm, 8 Ohm, 15 Ohm (I would guess, again, at the first and last of those). You would connect the speaker between the common terminal and the other one which matches the impedance of your speaker.

[Bigears]

Originally Posted by Station X

I can see what looks like a mains transformer on the amplifier chassis, but no sign of an output transformer.

Under chassis views would be useful.

Here's more picture of the chassis.

Thanks Tony

[Station X]

A, B and C are the speaker connections. I'd trim back that braid though in case there's a strand of wire in it to touch the earthed fixing screw.

[Edward Huggins]

If it were were mine I would guess at the speaker connections being A and B is 3 ohms and A and D is 15 ohms. We do not know the OP's speaker type, but we might assume 8 ohms.
I would not recommend switching this on until it is checked over. But if the OP insists, a speaker MUST be connected first. Of course it's possible that some damage might have occured by already running this with no speaker.

[AC/HL]

Looking at photos 12 and 13, the speaker wires appear to be B and D. A is an earth or chassis terminal.
If that speaker is 8 ohms, it'll be OK for testing purposes whether the chassis is 3 or 15 ohms.

[Synchrodyne]

Originally Posted by Edward Huggins

If it were mine I would guess at the speaker connections being A and B is 3 ohms and A and D is 15 ohms.

That “guess” is supported by the available data for the RF103/3, which was a contemporary of the EXP125/2. The schematic shows that output terminal A is grounded, B is for 2.5-ohm speakers, and D is for 15-ohm speakers:



It seems more probable than not that Armstrong adopted the same convention for its other models of the same period, including the EXP125/2.

For the sake of completeness, although I don’t see it as affecting the EXP125/2 question, it may be recorded that later on, Armstrong used a somewhat different arrangement for the FC48, in which terminal D was grounded, terminal B was for 3-ohm speakers and terminal A was for 7.5-ohm speakers:



Something that may be noted is that the EXP125 fell into the top bracket of British broadcast receivers of the time in terms of apparent RF capabilities, being generally comparable to the Chapman S6 and Dynatron T99 for example. (The BS125 would have compared with the Chapman S6BS and the Dynatron T139.) These were not communications receivers, but in HF terms intended for comfortable, hands-off listening to the programme content of SW broadcasts, back in the days when such were a primary source of news and entertainment in the “colonies”, as well as getting the most, quality-wise, from local MF (and where applicable, LF) broadcasts.

In AF terms, the Armstrong receivers of that time, with tetrode outputs, were more an upward extension of domestic receiver practice, this being at a time, before the widespread adoption of ultra-linear output circuits (1954 and up) when triodes and triode-strapped tetrodes were viewed as a being “proper” for hi-fi equipment.

Although the post-WWII Armstrong 125 models, in various iterations, had a reasonably long production run, from 1947 through to c.1961, very few seem to have survived, judging by the infrequency with which they have been the subject of postings on this forum, particularly in comparison with the contemporary Chapman and Dynatron models. Sometime back, there was a thread about a BS125 in Canada in rather sad condition, and originally misidentified as a Heathkit unit. See: https://www.vintage-radio.net/forum/...ad.php?t=94408. But none others come to mind. So, the subject EXP125/2 has some significance both in terms of its type and because of its apparent rarity.


Cheers,

[ms660]

Just an observation, possibly two different wire thicknesses on the secondary.

Lawrence.

[ms660]

Anyone know what type the rectifiers are?

Lawrence.