Hacker Mayflower II.

[Nanozeugma]

Evenin' all
Would anyone care to hazard a guess as to why C25 (2uF polarised electrolytic) is rated in the manual at 150v?
(Apart from the possibility that it was all they had in the parts bin.)

I can't see any reason why it would need to be in that position in the ratio detector.

The manual describes it thus:
"C25 is to provide a steady DC potential across the diode loads R21,R22."

It needs replacing - as do an awful lot of other parts - but that's another story.

TIA.

CCT extract attached.

[Uncle Bulgaria]

As a tyro, I'd say because it's between an anode and a cathode, so surely one end is close to HT?

[Nuvistor]

On a strong FM signal you can easily have 40 to 50 volts across that capacitor maybe more. On a Ferranti T1032 on a simple wire dipole behind the set I have as about 35volts. The circuit in that set is slightly different but it does the same task.
The 150volt rating would be one of the preferred values at the time,
The steady voltage it provides is an aid to AM suppression in the ratio detector.

[Nanozeugma]

Thanks for the explanation.
Apropos of nothing in particular, I wonder where Hacker sourced their resistors from? Literally every single one of them is outside tolerance, some WAY outside. I'm having to replace them all. I've had dog bones in 1930's Murphys keep their marked value better.

[stevehertz]

Originally Posted by Nanozeugma

Thanks for the explanation.
Apropos of nothing in particular, I wonder where Hacker sourced their resistors from? Literally every single one of them is outside tolerance, some WAY outside. I'm having to replace them all. I've had dog bones in 1930's Murphys keep their marked value better.

I completely restored my HM2 a few years ago. I measured every resistor and changed any that were greater than 20% out. Less than that makes very little difference. I probably changed around six or so.

[Nickthedentist]

The first one of these I restored needed loads of resistors replacing, as many were >50% high.

Obviously, in some bits of the circuitry, it's more important than in others. There's a thread with a good contribution by Hacker expert Mark Hennesey about this. It refers to the Serenade radiogram, but it's very similar to a Mayflower: https://www.vintage-radio.net/forum/...d.php?t=125398

Nick

[Nanozeugma]

Originally Posted by Nickthedentist

The first one of these I restored needed loads of resistors replacing, as many were >50% high.

Obviously, in some bits of the circuitry, it's more important than in others. There's a thread with a good contribution by Hacker expert Mark Hennesey about this. It refers to the Serenade radiogram, but it's very similar to a Mayflower: https://www.vintage-radio.net/forum/...d.php?t=125398

Nick

Hi Nick, I think my experience with my own Mayflower mirrors yours then.
Thanks for the thread link btw, interesting reading.

N.B. For any who might be interested or care, the Hacker group and it's associated files are now at https://groups.io/g/Hackerradiogroup and not Yahoo.

[Nanozeugma]

V6, one of the pair of ECL86 triode / pentodes has a grounded thin sheet copper shield erected between the pins of the valveholder (image attached.)

Initially, I thought this might have been retrofitted by a repair technician at some point in the past to correct a problem.

A bit of Googling has thrown up an image of another unit however, and you can clearly see the same shield (image attached.)
One must therefore conclude it was fitted during manufacture.

The only reason(s) I can think of that would make this necessary would be to cure instability or hum (?)

I would welcome others perspective on this.

T.I.A.

[stevehertz]

Owners of HM2s may be interested in this thread where I fitted an external 'module' that greatly improves the frequency response of the set given its single speaker (contemporaries had up to three or even four) and single treble tone control.

https://www.vintage-radio.net/forum/...d.php?t=110877

[Synchrodyne]

Originally Posted by Nanozeugma

V6, one of the pair of ECL86 triode / pentodes has a grounded thin sheet copper shield erected between the pins of the valveholder (image attached.)

Initially, I thought this might have been retrofitted by a repair technician at some point in the past to correct a problem.

A bit of Googling has thrown up an image of another unit however, and you can clearly see the same shield (image attached.)
One must therefore conclude it was fitted during manufacture.

The only reason(s) I can think of that would make this necessary would be to cure instability or hum (?)

I would welcome others perspective on this.

A “belt and suspenders” artifice in the aid of stability seems to be at least a possibility.

The amplifier is of “compact” (or “economical”) design, with V6a (triode) directly driving V6b (output), and receiving the main feedback loop at its cathode. V7a (triode) inverts the signal from V6a to drive V7b (output).

So there is a lot going on under the base of V6, such that any stray coupling is probably undesirable. And UL amplifiers apparently have more potential oscillation modes than non-UL types, although that is more connected with output transformer design. V7 would appear to be less critical in that there is no feedback connection to it.

As I understand it, and for comparison, in the Mullard 2 x ECL86 amplifier, each output section was driven by its own triode, and the main feedback loop went to the cathode of the preceding EF86 stage, so there would have been less concatenation of the signal and feedback loop wiring.


Cheers,

[Synchrodyne]

That “compact” push-pull amplifier arrangement in fact appears to have been reasonably common in higher performance radio receivers, where I imagine that valve count was somewhat constrained as compared with audio equipment. But it typically involved four valve envelopes, for example as in the Pye PE80 and FenMan II, and Murphy A188C and TA160. If output stage instability was in fact a potential problem in the Hacker Mayflower, then having that kind of amplifier arranged around just two valve envelopes, with resultant closeness of wiring, may well have been a contributory factor.


Cheers,