Pye MM

[Joe]

I thought it was a TRF? I couldn't make sense of the data either which is why I passed it straight over.

Merry Christmas to you too

- Joe

[AlanBeckett]

Joe,
Yes, it's a TRF as the description says.
Alan

[Joe]

I was confused by the title of the Trader Sheet saying Superhet when even I could tell it wasn't.

Cheers

Joe

[AlanBeckett]

Sorry Col, you fell over the page.
I'm still not entirely sure about the spindle. It feels a bit 'sticky' in places as I turn it, and I would hate to mangle any of those spring fingers.
I hope you're right that this is the home straight.
Alan

[AlanBeckett]

Well, the Home Straight turned out to be a bit further away than expected.
As I reported last time one of the resistors on the Tag Board - R10 according to the Pye info - appeared to be damaged and another one was overheating. Also, the voltage readings I got were nonsense, one even being negative. Clearly something was seriously wrong. I'm of the school which says 'what have I done'. Well, I'd reconnected it all, so I took all my notes and pictures and checked everything. The problem was quite straightforward, I'd swapped over the White and Slate wires. Having put them in the right places the voltages looked much more promising, although a bit high, which is where the Variac comes in, and the smoking stopped. Putting the Detector and Output valves produced a very faint hum, and touching the Gram input produced a healthy buzz. Finally, installing V1 and doing a bit of twiddling produced Radio 4, at good volume. I was quite chuffed

So, a few comments:-
Attached is a marked up Circuit, just in case we have a fire.
I haven't done anything about R10 yet.
The voltages on the diagram are what Pye says they should be. I haven't measured them yet.
The 'Gain' control is a bit odd. The volume increases as the knob is turned clockwise and then decreases as the Pot kicks in - R12 and C20 are ganged. Also V1 screen voltage is probably high which may well affect the 'Reaction' side of things.
The tuning is a bit 'double humped', but it is Long Wave, and there is the R10 problem.
The sound from the speaker is good so the Field coil and the Transformer are fine.
I haven't figured out yet which position of the switch is really LW, MW and Gram.

So, perhaps the Home Straight is in sight - hang on Joe.

Alan

   

[Joe]

Glad to hear it's going well and that your shaft works.

- Joe

[AlanBeckett]

Joe,

I wouldn't go quite that far yet

Alan

[Yorkie]

The circuit suggests that the MM is a simple straighforward set Alan, but the effort you've put into it and the challenges that it has presented proves otherwise! I've built many homebrew TRF sets in the past, but I've never owned or even listened to a commercially produced one such as the MM, so it will be interesting to see how the old girl performs when she has her 'MOT' - 'Moment of Truth'!

[AlanBeckett]

David,

Indeed. In fact if it had been drawn in a more conventional manner, and didn't have that odd 'Gain' control and HT- arrangement, it would be instantly recognisable.

I've had another short play this morning. I keep running it for a bit in order to reform the electrolytics where possible.
So, a few more comments:-
Amazingly I managed to put the Wavechange knob on the shaft in the right place.
The Gram position is essentially LW with the Detector shorted - it doesn't work very well. A quiet area would have to be chosen I think.
Switching to MW I got (local) Radio Derby, loud and clear.
The Gain control doesn't work in the way I thought. Apparently the 'Volume' part works first then the Reaction, I think.
There's some whistles and oscillation as the Reaction kicks in. The Pye info says it's probably C18.
The HT still seems a bit high and the heaters a bit low. It's slightly odd because the info says the electrolytics are rated at 275 but then says there'll be 315 across the reservoir.
The Pilot bulb is O/C. Anyone got a 4v MES they don't want?
Despite the decrepit state of R10, and it's way-out value, V1 Screen voltage is not far out.
Attaching an external Aerial appears to make not one iota of difference - L1 O/C?

So, some more things to investigate and tidy up, but getting there

Alan

[AlanBeckett]

Just a last couple of comments for the night:-
There seems to be a bit more hum. To my shame I haven't yet built David's ESR Meter, even though he sent me a box, panel and PCB I'll have to try substitutes.
There are a lot more stations on MW as expected, but all a bit faint. I really must sort the caps and find out what's going on with the Reaction.
Sending the assembly back to Pye, as the data recommends, really isn't an option

Alan