McMichael Duplex 4 - FORMO multicoupler.

[Keith]

I was recently given this 1932 set and decided to attempt a restoration - more out of interest in its performance than anything else. The original speaker had been replaced (and output transformer added) The replacement had had its cone eaten by mice! Most of the capacitors needed restuffing plus some of the resistors, but eventually it worked - after a fashion.

Even with a new 8" m/c speaker, there was very little bass and the resultant sound was rather "tinny". Having tried all the obvious things I started to suspect the interstage transformers. These went under the name of FORMO multicouplers and seemed to have more connections than the circuit diagram showed. Eventually I resorted to opening one up and discovered an additional R and C inside the can the 30K resitor was fine but the capacitor (1uF measured) was very leaky. This network was connected across the primary so presumably a tone correction arrangement. At some point somone had added another 20K series resistor externally (now over 40K).

I have yet to replace the components (and presuably identical ones in the next stage) but wondered if anyone had come across these items before. There are no markings on the capacitor so I can't tell if my measurement was upset by the leakage.


Any info appreciated.

[Ed_Dinning]

Hi Keith, they were generally a very cheap component with a very small transformer to keep costs down. then Tweeked with a CR network to "improve " performance. Plenty of inflated ads for their performance in the mags of the period

Ed

[Keith]

Hi Ed,


Thanks for your response. I'm just about getting my head around the FORMO arrangement now - and the mod that a previous repairer had done.


It looks like the primary of the transformer was only designed to be capacitively coupled to the previous anode (1uF). I guess that the small core would have saturated if it had to carry much DC (provided by the 30K resistor). I've now discovered that the primary of this transformer is, in fact, o/c. The previous repairer inserted 20K resistor between anode and winding (?) and this clearly restored some coupling. No wonder there is no bass getting through as the interwinding capacitance is only about 7.5nF. The primary looks to be the top winding so I may dismantle further (although I'm not optimistic).

[PJL]

McMichael kit is normally very well made so I assume the FORMO couplers are replacements?

[Herald1360]

You'd lose out on any voltage gain from the transformer, but straightforward R-C coupling would work to get it going.

[Radio Wrangler]

There was another thing from the end of the twenties which relied on massive AF frequency correction, though this one put the price UP a long way. The "Stenode Radiostat" principle had you use a very expensive narrow crystal filter, and then compensate by wildly boosting the high frequency end of the audio stages.

The FORMO multicoupler would have worked, and realised a cost saving at the expense of gain, but suffered from the advertising style of the period. The Stenode system actually went counter to mathematics known at the time, and cost the earth. Joseph Fourier had lived and worked over a century previously and trig identities were old-hat too.

David

[PJL]

This from wireless world 1932-08
"The Formo Multicoupler differs from other parallel -fed transformer units in that all the terminal points are accessible externally, with the result that it may be used as a plain resistance -fed transformer, as a resistance - fed auto- transformer, or, with the addition of a by -pass condenser, as a similar coupling unit with self- contained decoupling resistance ; with this object, the built -in resist- ance is tapped. By making the appro- priate external connections, various ratios of transformation are available. Data relating to the unit shows high and constant amplifi- cation between 5o and 5,000 cycles."

[Ed_Dinning]

Hi Keith, looks as if it could be rewound, or one of the very small I/V transformers fitted in the can. This will reduce the freq response but I doubt if it will be noticable.

Ed

[Keith]

Thank you for all your replies, gentlemen. It looks as if these transformers are original - no sign of any additional holes in the chassis. I've only been able to find one circuit diagram of the Duplex 4 and that is clearly not correct for the version that I have (e.g. mine has a pentode o/p rather than a triode).


I have not proceeded further as yet due to a bout of illness but I will either attempt a rewind of the primary or just reconfigure the FORMO for R/C coupling.


Thanks PJL for the steer to the 1932 WW review of the Multicoupler. I found it on the American Radio History site (https://www.americanradiohistory.com...d_Magazine.htm). Great reading some of the other articles from that era - ARH really is an excellent resource.


I'm not familiar with these "very small I/V transformers", Ed. Do you have any details, I wonder?

[Ed_Dinning]

Hi Keith, I believe RS did them. They were often used not on valves but as transistor set drivers.

Ed