McMichael 802 - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

cathoderay57 · 9th Oct 2021, 6:55 pm

Yesterday evening I spent over an hour trying to fit the tuning cord

without success. This morning I had another go with the rotor disc at a different angle. Using masking tape to temporarily hold the cord in place around the large tuning gang pulley, with the spring under light tension, the cord went on at the first attempt. The tuning runs nice and smoothly now. The gearbox and bearings were clean but dry and responded well to a few drops of 3 in one oil. Then I started on caps replacement. As I said earlier the component cluster below each valve comes out with the valve holder, but it is first necessary to disconnect all the wires to the other circuitry. Sketches and photos essential. I've done one valve (V2) so 5 more to go. Under V2 the waxies were leaky as expected but I planned to replace all the high voltage decouplers anyway because dismantling is such a pain. I re-used one of the HT decouplers which wasn't leaking badly as the cathode decoupler. Normally I wouldn't bother replacing the cathode decoupler but the one on V2 had gone to very low resistance. The sub-HT line electrolytic C38 wouldn't reform so I disconnected it and fitted a 10uF 450v axial underneath the chassis. Cheers, Jerry

cathoderay57 · 10th Oct 2021, 5:27 pm

Moving steadily on with cap replacements. I discovered that the Broadcaster Supplement table of condensers stops at C39 whereas C40 and C42 appear in the circuit diagram. The component location sketch for V6 shows C40, and C41 (which doesn't seem to exist in the circuit) but no C42

. So back to first principles - spot a waxie, note where it is connected to, compare to circuit. Luckily even the dirtiest waxies scraped clean enough to read their values. I spotted modern wires going to the audio valve cathode decoupling capacitor box and thought maybe the box had been restuffed, but no, only the wires replaced, and the capacitors were well past their use-by date - see pic. I popped some radial replacements inside the box. Cheers, Jerry

cathoderay57 · 15th Oct 2021, 6:58 pm

A bit more progress. I've attached an image of one of the valve holder modules (this one is below the AC/5Pen output valve. An example of a 1930s integrated circuit (except this type is repairable). The grid coupling cap was a Hunts Micamold type and although it measured infinite resistance on the Avo 15v Kilohms range I replaced it regardless as I have seen this type of capacitor go leaky in the past. I have now replaced all necessary (so far) capacitors and restrung all 3 dial cords. Once I get the replacement field coil fitted it will be time to power up, then I'll post another update. Cheers, Jerry

cathoderay57 · 6th Dec 2021, 3:47 pm

Hi folks. Fitted the rewound field coil today and powered up. It works on all bands, lovely rich sound from the massive speaker. Even though I've recapped it I will check the valve voltages against the service sheet as a precaution. I don't think the AFC on motor-tune is working so it looks like I will have to realign the discriminator stage. Jerry

Gabe001 · 6th Dec 2021, 5:31 pm

That's a great outcome. Kudos to the rewinder who I'm sure had his work cut out - I wonder how many metres of copper wire went into that speaker coil.

Is it's performance in keeping with a 9 valve set?

cathoderay57 · 6th Dec 2021, 5:51 pm

Hi Gabriel, thanks for the reply. I haven't really given it a run for its money yet but the results are promising. I've attached the voltage measurements. The HT is 40v high at the rectifier cathode, but the voltages at the valves are, I think, acceptable and so I'm not going to mess with it. I'll have another play tomorrow and check whether the AFC is working or not. The motor tuning spins nicely so that's all good. Cheers, Jerry

cathoderay57 · 7th Dec 2021, 3:01 pm

The alignment was OK and only needed the tiniest tweak on the AFC discriminator trimmers. AFC is working correctly. With the tone control set to "fidelity" it sounds excellent. Thanks to Mike Barker for the field coil rewind. I've attached a few images. Cheers, Jerry

Gabe001 · 7th Dec 2021, 8:11 pm

Is the speaker cloth the original or a replacement? It's in pristine condition.

Sideband · 7th Dec 2021, 9:26 pm

It's a great pity there don't seem to be many McMichaels around. They are superb sets and I would love a 381 table model.

cathoderay57 · 7th Dec 2021, 9:44 pm

Quote:

Originally Posted by Gabe001

Is the speaker cloth the original or a replacement? It's in pristine condition.

Hi Gabriel, the speaker cloth appears to be original. I think keeping the doors closed when not in use probably gave a lot of protection from sunlight fade and smoke/dirt accumulation. Cheers, Jerry

cathoderay57 · 7th Dec 2021, 9:52 pm

Quote:

Originally Posted by Sideband

It's a great pity there don't seem to be many McMichaels around. They are superb sets and I would love a 381 table model.

Over the years I accumulated quite a substantial collection of 1930s McMichael sets including Twin Supervox, 135, 137, 138 (all 3 on cabriole legged stands not dissimilar to the 802), 380, 381, 435, 362 table model, 364 and quite a few later models. The trouble is they take up rather a lot of space so now I only have the 802 and 435. I agree they are fabulous sets if you like 1930s woodies!

Cheers, Jerry

9th Oct 2021, 6:55 pm	#21
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 Yesterday evening I spent over an hour trying to fit the tuning cord without success. This morning I had another go with the rotor disc at a different angle. Using masking tape to temporarily hold the cord in place around the large tuning gang pulley, with the spring under light tension, the cord went on at the first attempt. The tuning runs nice and smoothly now. The gearbox and bearings were clean but dry and responded well to a few drops of 3 in one oil. Then I started on caps replacement. As I said earlier the component cluster below each valve comes out with the valve holder, but it is first necessary to disconnect all the wires to the other circuitry. Sketches and photos essential. I've done one valve (V2) so 5 more to go. Under V2 the waxies were leaky as expected but I planned to replace all the high voltage decouplers anyway because dismantling is such a pain. I re-used one of the HT decouplers which wasn't leaking badly as the cathode decoupler. Normally I wouldn't bother replacing the cathode decoupler but the one on V2 had gone to very low resistance. The sub-HT line electrolytic C38 wouldn't reform so I disconnected it and fitted a 10uF 450v axial underneath the chassis. Cheers, Jerry

10th Oct 2021, 5:27 pm	#22
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 Moving steadily on with cap replacements. I discovered that the Broadcaster Supplement table of condensers stops at C39 whereas C40 and C42 appear in the circuit diagram. The component location sketch for V6 shows C40, and C41 (which doesn't seem to exist in the circuit) but no C42 . So back to first principles - spot a waxie, note where it is connected to, compare to circuit. Luckily even the dirtiest waxies scraped clean enough to read their values. I spotted modern wires going to the audio valve cathode decoupling capacitor box and thought maybe the box had been restuffed, but no, only the wires replaced, and the capacitors were well past their use-by date - see pic. I popped some radial replacements inside the box. Cheers, Jerry Attached Thumbnails

15th Oct 2021, 6:58 pm	#23
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 A bit more progress. I've attached an image of one of the valve holder modules (this one is below the AC/5Pen output valve. An example of a 1930s integrated circuit (except this type is repairable). The grid coupling cap was a Hunts Micamold type and although it measured infinite resistance on the Avo 15v Kilohms range I replaced it regardless as I have seen this type of capacitor go leaky in the past. I have now replaced all necessary (so far) capacitors and restrung all 3 dial cords. Once I get the replacement field coil fitted it will be time to power up, then I'll post another update. Cheers, Jerry Attached Thumbnails

6th Dec 2021, 3:47 pm	#24
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 Hi folks. Fitted the rewound field coil today and powered up. It works on all bands, lovely rich sound from the massive speaker. Even though I've recapped it I will check the valve voltages against the service sheet as a precaution. I don't think the AFC on motor-tune is working so it looks like I will have to realign the discriminator stage. Jerry

6th Dec 2021, 5:31 pm	#25
Gabe001 Octode Join Date: Oct 2019 Location: Stockport, Greater Manchester, UK. Posts: 1,639	Re: McMichael 802 That's a great outcome. Kudos to the rewinder who I'm sure had his work cut out - I wonder how many metres of copper wire went into that speaker coil. Is it's performance in keeping with a 9 valve set?

6th Dec 2021, 5:51 pm	#26
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 Hi Gabriel, thanks for the reply. I haven't really given it a run for its money yet but the results are promising. I've attached the voltage measurements. The HT is 40v high at the rectifier cathode, but the voltages at the valves are, I think, acceptable and so I'm not going to mess with it. I'll have another play tomorrow and check whether the AFC is working or not. The motor tuning spins nicely so that's all good. Cheers, Jerry Attached Thumbnails

7th Dec 2021, 3:01 pm	#27
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,384	Re: McMichael 802 The alignment was OK and only needed the tiniest tweak on the AFC discriminator trimmers. AFC is working correctly. With the tone control set to "fidelity" it sounds excellent. Thanks to Mike Barker for the field coil rewind. I've attached a few images. Cheers, Jerry Attached Thumbnails

7th Dec 2021, 8:11 pm	#28
Gabe001 Octode Join Date: Oct 2019 Location: Stockport, Greater Manchester, UK. Posts: 1,639	Re: McMichael 802 Is the speaker cloth the original or a replacement? It's in pristine condition.

7th Dec 2021, 9:26 pm	#29
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	Re: McMichael 802 It's a great pity there don't seem to be many McMichaels around. They are superb sets and I would love a 381 table model. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....