Pye MM

[pwdrive]

Good to hear you are still at it Alan.

Lawrence.

[Retired]

Hi Alan,

Keep up the good work. I've a feeling that once you sort out the caps and the odd resistor or two you will see a marked improvement.

Most of the hard work is now sorted and once the holidays are out of the way hopefully you can spend a bit more time working on it.

Kind regards, Col.

[AlanBeckett]

Col,

I'm certainly hoping so.

A couple of comments on the mechanical front:-

I think that the Tuning Cap is shorting right up near the fully closed end. As there's very little to listen to up there I thinking of ignoring it after your experience.

I was still worried about whether I'd got the offset - 7 deg. - on the flats in the right direction. I was even more worried when I looked in the top and found that the plates only seemed to be pushing very gently on the contact springs. However, moving the knob very slightly to what would appear to be a more solid contact stops it working, so I have to think it's right.

Alan

[Retired]

Hi Alan,

As the set is pulling stations in I think it likely the new spindle is OK and the problem/s lies elsewhere such as caps or resistors.

If the tuning gang is catching at the very end of travel I think you are wise to ignore it as trying to align these can be a bigger pain than making the spindle was; I ended up snapping part of a vane off on the one I restored and believe me this is incredibly easy to do given the fragile castings involved.

The set possibly has not been run for many years and if so then any number of components could have drifted out of spec or died so I wouldn't take anything at all for granted whilst fault finding.

So near and so far but stick with it and you'll succeed; the only problem then of course is this set will lose interest and you will be onto another project.

Kind regards, Col.

[AlanBeckett]

Onward and upward, at least after I'd waited for the storm to die down a bit.

After removing V1 and V2 I injected a signal into the primary of T1. The output stage voltages look about right and the output is fine, although there is a touch of hum. Tacking a 32u in various places didn't make any real difference so that may have to wait until it's closer to its real configuration. The bandwidth is a bit limited by modern standards - effectively 8kHz, although it sounds fine on a station.

Trying the same ploy with V2 installed was an entirely different matter. The voltages around V2, when switched to Gram, are slightly out - the Anode voltage is a bit high and the Cathode voltage is low, implying perhaps that the valve is a bit weak. However, it isn't the right one although it is a reasonable substitute according to the data. There are two main problems. First, the Volume Control has no effect on the input - see circuit. That's no real problem when using a Signal Generator, but clearly would be with a real Gram. Second, any connection to the Gram input leads to rampant oscillation, at quite a high frequency. I have got long dangly leads kicking about but dressing them in various ways made no difference. Bear in mind that this is without V1. However, as you can see, there's plenty of feedback paths.

That's as far as I've got today. Tomorrow I think it might be best to investigate the first stage as the components in there will affect the feedback.

Alan

[AlanBeckett]

It's bitten me, ungrateful sod

I decided that I'd have to tidy up my shambolic wiring - you'll have noticed that I haven't posted a picture. So, I moved things around and dressed the cables to keep the Aerial wires away from the PSU and Output wiring, and gave it a better Earth. It was then a lot happier, with no hum that I could detect. Injecting an af signal into the Gram Socket via a couple of Caps produced a nice sounding output and monitoring V3 Anode showed a nice clean Sin wave. It's quite sensitive. 100mV RMS in gives a good solid output. What was the Output and Impedance of a 1930s Pickup? In the midst of this, as I was trying the Volume Control to see if it would oscillate, I caught the back of my hand on one of the PSU Caps. That made me jump, but fortunately there was no other damage.

It's all looking a lot better now, so next the rf stage. I measured the Resistance between the Main Chassis and the PSU Negative and it looks OK as the Volume Control is rotated, so next the valve. I hope I don't have to take the Volume Control/Reaction Assembly to bits 'cause that looks to be a bit of a sod. We'll see, but first a new R10 I think, just to see what difference it makes.

By that stage it was coming on to rain, I was hungry and I couldn't feel my feet, so I knocked off.

Alan

[AlanBeckett]

A tiny update, almost embarrassing.
I bought a couple of 3.5v MES torch bulbs and put one in to replace the O/C pilot bulb. It works fine. The heater voltage is a bit low and anyway there's a 6R resistor in the way. So there's a Nice glow on the dial now
Alan

[AlanBeckett]

I hadn't realised how long it's been since I reported. In mitigation it has been a bit brisk in the garage. Things have warmed up a bit - even Col isn't complaining - so ....

Having proved that the af sections work via the Gram sockets I figured it was time to reinstall V1 and see what was what. In a nutshell there were two problems. First there were some classic crackles at various parts of the tuning indicating that the plates were catching somewhere. Second, the Volume Control was still doing the same thing, that is the volume increases until the Reaction kicks in and then it goes bananas.

So, tuning gang first. Actually both problems require the removal of the Left hand cover plate, and as I've done it before it was no big deal. A couple of the split end plates looked a bit close so I bent them carefully, bearing in mind that Col managed to snap one off. That, plus a blast of compressed air and some Servisol, mostly cured it except right at one end. So far so good.

Now the Volume control - see attached. As far as I can tell from the circuit and the description it's supposed to work as follows: R12 and C20 are ganged. Minimum volume is maximum R12 and no Reaction, ie maximum negative bias on V1. As the knob is turned R12 reduces to zero and then stays there - there's a solid piece of track after the wirewound section - and C20 is rotated. As far as I can tell the resistance of R11 plus R12 does just what it says on the tin. The bias on V1 varies from about -9 to -2, which seems reasonable. After that the volume from the speaker drops and there's lots of whistles, indicating I think too much Reaction. One other potential problem was the singed resistor. I believe that it's R10, which should be 10k. However, it reads about 40k. On the other hand the Screen voltage, and the Anode voltage for that matter, on V1 are about right, so I left it for the moment. The data does say that there were various modifications round that area so it may actually be the right value. At that stage I thought I'd just have a look to see what was oscillating. I glanced down at my trusty 455 only to find it blank. The fuses are OK so I've brought it up to the house to thaw out.

The Pye information says that the whistling problem may be caused by a low value C18. Well, at least that's pretty easy to get at so out it came and a modern one was tacked in its place. The best I can say is that it made a marginal improvement. The other interesting factor about it is that whereas the diagram says it should be 25u it's marked 10. Presumably that's big enough. There is a code on it which may imply 1934, which would be OK.

So, it could be the 'Volume Assembly', or at least the Capacitor half of it. It's quite difficult to see exactly what's happening as it's solid dielectric. I've given it a blast of air and Servisol which hasn't made one iota of difference. My enthusiasm for taking it to bits is very low.

I think at this stage the best bet is a careful measurement of all Rs and Cs around that area.

One last comment; I don't know the state of V1. It actually is a Mazda AC/SG which is right - the others aren't - and it's voltages and currents look reasonable, but who knows. I've been looking for a spare but they seem pretty thin on the ground.

The sun is actually shining and my Weather Station says 10.3 outside, but being Sunday I expect I'll be hauled off to 'do things'

Alan

                   

[Skywave]

I certainly admire your persistence with that radio, Alan. I wouldn't dream of getting stuck into something of that vintage and design the way you have. I'm much more comfortable with sets of a later period: things that I can understand how they work and why - and are, generally, much more maintainable.

Al.

[Geaney]

Hi Alan,

I'll add my admiration for your tenacity in getting this set resurrected. For myself I'd have to put it away every now and then to give me time to recharge the batteries and then have another go. I can't wait to see the final result.

Cheers, John