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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 31st Dec 2017, 4:06 pm   #21
Edward Huggins
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP/2CP

Possibly a 5Z4 as per the circuit above?
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Old 31st Dec 2017, 11:56 pm   #22
Synchrodyne
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP125/2CP

The valve complement (from 1957) that I have for the later EXP125C shows a UU7 pair as the rectifiers.

We could read this back through the EXP125A, EXP125/3 and thence to the EXP125/2, with some probability of being accurate.

As Edward has noted, the schematic for the RF103/3 shows a 5Z4 rectifier. But not included on the portion that I posted was a footnote to the effect that the 5Z4 was replaced by the UU7 effective 1950 November 01.

Reasonably possible then is that the EXP125/2 started life with a pair of 5Z4, but was then changed to a pair of UU7 at about the same time as the corresponding change was made for the RF103/3.

The full valve complement for the EXP125C was:

6C9
3 x 6F15
6LD20
6F13
4 x 6P25
EBC41
2 x UU7
6M1

No functions were assigned in the information that I have, but for the most part they may be deduced.

If one compares this with the valve line-up of the RF103/3, namely 6F15 RF amplifier, 6C9 oscillator-mixer, 2 x 6F15 IF amplifiers, 6LD20 demodulator, AGC rectifier, and AF amplifier, 6LD20 (triode) phase splitter, 2 x 6P25 output, then the EXP125C list is also plausible for the EXP125/2.

Apart from a second pair of output valves and a second rectifier, the EXP125C added another RF pentode (the 6F13), and another double diode-triode as compared with the RF103/3. Given that it had one RF and two IF stages, it is possible that the extra RF pentode was used for an AGC side-chain IF. Although one cannot discount the possibility that the 6F13 was employed as an AF amplifier. But for the AF side within the tuner-control unit section, I’d be inclined to pick a triode (probably from the 6LD20) amplifier stage followed by a Voigt passive tone control (treble and bass), a form that was favoured by Armstrong.

In the above list, I’d assume that the EBC41 was in the power amplifier and power supply section, and that only its triode was used, as a phase splitter. Why an EBC41 in one unit and a 6LD20 in the other is hard to fathom, but from c.1954 Armstrong started using some Mullard valves, and perhaps wanted to share the available business between Mullard and Mazda. Probably a 6LD20 was originally used in the power amplifier. And the third photograph of the unit in the original post shows what could be a 6LD20.

The photographs also show that on the tuning dial, the LW and MW bands were marked with both frequency and wavelength, perhaps indicative that this unit was aimed at both the domestic and export markets.


Cheers,
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Old 1st Jan 2018, 11:26 am   #23
ms660
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP125/2CP

Thanks for the UU7 info, looked like the main candidate, pin 2 as an anchor point for the limiting resistor, pins 3 and 5 strapped together.

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Old 2nd Jan 2018, 8:42 am   #24
Synchrodyne
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP125/2CP

At this site - http://ukhhsoc.torrens.org/makers/Ar...ams/index.html - there is a schematic for the Armstrong BS125/4, dated 1954 May 07. For now, this is probably the closest approximation to the EXP125 circuit available.

The BS125/4 power amplifier/power supply section, with 1 x 6LD20, 4 x 6P25 and 2 x UU7 – is likely to have been quite similar to that of the EXP125 series.

The RF section up until the final IF stage, with 6F15, 6C9, 2 x 6F15, might have been generally similar to that of the EXP125 series, apart from waveband switching. Note though that the BS125/4 had AGC applied to the RF and 1st IF stage only, whereas the EXP125/2 was described (at: http://www.audiomisc.co.uk/Armstrong/general/Pub1.html) as having AGC applied to the RF amplifier and both IF amplifiers.

The BS125/4 had an EBC41 demodulator, AGC rectifier and 1st AF stage. The last-mentioned triode drove a Baxandall tone control network using a 6F13 as the anode follower. From that was taken the feed to the power amplifier. An interesting aspect was that the main feedback loop for the power amplifier went back to the cathode of the 6F13 in the tuner/control unit part. So, the two units were interdependent. The BS125/4 also had a 6D2 noise limiter, which feature I think originated with the EXP119 in 1950, and was not used on other Armstrong models.

Clearly the EXP125/2 predated the arrival of the Baxandall tone control (whose details were first published in 1952 September). And at the above-mentioned audiomisc site, it was described as having switched treble and bass controls. It may well have had a 6LD20 as demodulator, AGC rectifier and 1st audio stage, but the following circuitry would have been different.

The BS125 was the bandspread model in the 125 series, approximately at least combining the bandspread RF section of the EXP119 with the AF section of the EXP125. As best I can determine it was released in 1953.


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Old 4th Jan 2018, 2:24 pm   #25
Bigears
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP125/2CP

Thank you very much for all your input, I don't feel I have the confidence or the ability or know how to even start any restoration work on this radio, as I am only just entering the world of electronics, hope you understand, but I will hold on to it as it is very rare. I have made a Blog about it on blogger, and read up on Armstrong it was a very sad story.

Regards to you all and I take this opportunity to wish you all a very Happy New Year, Ken
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Old 6th Jan 2018, 10:58 pm   #26
Synchrodyne
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Default Re: Armstrong Radio Tuner and Amplifier Model EXP125/2CP

Thanks for letting us know the situation, and pleased to hear that you will hold on to it.

By the way, here is what I think is the likely valve line-up for your unit:

Tuner-Control Unit:

6F15 RF amplifier
6C9 Frequency changer
6F15 1st IF amplifier
6F15 2nd IF amplifier
6LD20 Detector, AGC rectifier and 1st AF amplifier
6F13 (?) 2nd AF amplifier
6M1 Tuning indicator

Amplifier-Power Supply Unit:

6LD20 (triode section only) Phase splitter
6P25 x 4 Parallel push-pull output
UU7 x 2 Rectifiers

I am not sure about the 6F13 as 2nd AF amplifier, but from the photographs, the valve looks like a screened pentode. And it was used in the later EXP125C. At first glance, the 6F13, a high-slope sharp cutoff pentode, was an unusual choice. It was primarily a TV valve, intended for use as RF amplifier, mixer, IF amplifier and video amplifier in Band I TV receivers. But it was part of the Mazda post-WWII domestic receiving valve range announced in 1947, and that range did not include an AF pentode, leaving the 6F13 as a reasonable choice where such was required. Interestingly, Baxandall recommended the use of a high-slope pentode for his tone control circuit, and the published version (Wireless World 1952 October) used a Mazda SP61, which was the predecessor to the 6F13.


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