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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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2nd Oct 2021, 9:56 pm | #1 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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McMichael 802
Hi folks, I've just begun work on a McMichael 802 purchased a few weeks ago from another Forum member. The Broadcaster Supplement for the McMichael 382 Motor-Tuned Nine is the same chassis so I am using that for component data and circuit. I started out by testing all of the valves. The rectifier was good, a VU39, but that is equivalent to a UU5 which has a slightly higher heater current than the specified UU4, so I fitted a UU4 so as to be kinder to the mains transformer. Output valve (AC/5Pen), magic eye (ME41), both double diodes (V914), the frequency changer (AC/TH1), the detector (HL41DD) and the automatic frequency control valve (AC/SP1) all tested very good and so the only valves I really needed to replace were the two AC/VP2 (IF amp and automatic frequency discriminator) one of which had a heater-cathode short and the other heater-cathode leakage of about 25M. The metallized coating on the AC/TH1 was flaking off and since I had a new one I fitted that as well.
Next came the basic resistance checks. Primary winding of the AF output transformer measured 300 Ohms so looks OK. Both HT windings and primary of the mains transformer measured OK. Unfortunately the speaker field coil is open circuit so that will have to be rewound. According to the data sheet it has a dc resistance of 700 Ohms. I have yet to continuity check all of the coils, IFTs and the tuning motor. I pulled out the rectifier and applied mains to verify that the mains transformer remained OK - it did. I noticed that the tuning dial drive wire had come off but I'm not sure yet if it is actually broken or not. Luckily the pointer has been saved in a poly bag. The main HT capacitors are the cardboard box variety. The box has partially disintegrated and is showing evidence of fluid leakage from the internals. The box was hanging off its wires. It was made by Plessey Ilford. I attempted to reform it but, as expected, although it held some charge the leakage was excessive. On the capacitance tester the reservoir measured 18.6uF (should be 8uF) and the other 2 sections (wired in parallel) measured 58.6uF and 6.7uF (together according to circuit should be 28uF). I have made up a small plastic box for a 10uF 450v reservoir cap and a 33uF second smoother. I'll glue cardboard around the plastic and use the same fixings on the speaker baffle-board. A cursory look around shows both electrolytic cathode decouplers appear to have been replaced and stuffed into the second smaller cardboard box. It doesn't look like any of the paper caps have been replaced. Cheers for now |
2nd Oct 2021, 9:58 pm | #2 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Just a couple more pics of the grotty HT capacitor box.
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3rd Oct 2021, 10:32 am | #3 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
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Re: McMichael 802
Interesting(?) aside..... the McM 802 in Radiomuseum is a 10 valve set with an AC/SP1 in the front end rather than just the bandpass dual tuned aerial circuit.
https://www.radiomuseum.org/r/mcmichael_802.html I wonder if there was an export variant?
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....__________ ....|____||__|__\_____ .=.| _---\__|__|_---_|. .........O..Chris....O |
3rd Oct 2021, 5:04 pm | #4 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Hi Chris, yes, you'd think that's the way it appears from the RM description layout, but it isn't. There's a succinct circuit description by Mike Izycky in the Winter 1999 BVWS Bulletin Vol. 24 No. 4 p. 32. I'd forgotten about that article and, although I haven't kept my Bulletins going back that far, when I was kindly reminded of it by David Cochrane after sending him the set's Serial Number, I downloaded the article from the BVWS website. In it Mike says: "The 382 (ditto 802) is essentially a straightforward 4+1 superhet (AC/TH1, AC/VP2, HL41DD, AC/5Pen, UU5 (sic)).... The circuitry is, however, complicated by the addition of the Automatic Tuning Correction circuitry, which adds another 3 valves to the set, viz. side-chain IF amplifier (2nd AC/VP2), Foster-Seeley discriminator (V914) ..... and a reactance valve (AC/SP1)". The Broadcaster Supplement gives further explanation, apart from confusing me by having two V5 valves marked on the circuit diagram, the right-hand one of which should be V6, and V5 marked in the Valve Readings table as AC5P1 not AC/SP1 and, as Mike points out in the article, an HT short circuit from the centre tap of the discriminator transformer L23 to chassis! It goes on to explain that V5 (AC/SP1) is the control valve. The positive or negative voltage produced ....(from the discriminator).... is applied to the suppressor grid of V5.... Components in the V5 circuit are so adjusted that the network tends to act as an inductance and the whole arrangement in parallel with the tuned oscillator circuit of the AC/TH1 triode serves to deliver automatic frequency correction. Re: ten valves, the chassis of the 382 Motor-Tuned Nine is definitely identical so presumably the maker's model name didn't count the magic eye or perhaps the rectifier. Cheers, Jerry
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4th Oct 2021, 8:53 pm | #5 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Completed point-to-point con checks on the inductors today. Apart from the speaker field coil, all were OK including the motor, although I was unable to measure the LW and MW oscillator coils dc resistance remotely from the wavechange switch leads because of the use of padder capacitors in series with the coils located in the depths of the coil box. Removal of the coil box is no small task so I'm not attempting that unless I get problems on LW and MW on test. After some cursing I managed to figure out which coils were which in the IF cans etc. The Broadcaster Supplement error was that the link between the centre tap of the discriminator coil primary (L23 carrying HT) and chassis should, instead, have been drawn to the centre tap of the secondary (L24). There doesn't appear to be an L25, although L26 exists (connected to grid 1 of v5). I removed the speaker field coil ready for rewinding, continued work building the replica HT capacitor box, and removed the contact assembly from the rotor disc. The disk and contacts need a good clean but should be OK. Both mounting rubbers at the rear of the rotor disc and tuning gang have disintegrated and will have to be replaced with grommets, together with one mounting bolt, which is missing. The tuning cord is intact and will have to be restrung. I managed to fit the pointer cord for the tone control indicator. Proving quite a task but well worth persevering. Cheers, Jerry
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4th Oct 2021, 9:30 pm | #6 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Pics of the replica capacitor box, almost ready for the cardboard cover. Also an image of the 10-inch energised speaker, sans energiser coil. Cheers, Jerry
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5th Oct 2021, 11:03 am | #7 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,738
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Re: McMichael 802
Hi Jerry look what I go to bed with every night ,she's a beauty Mick.
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5th Oct 2021, 4:42 pm | #8 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Hi Mick, very nice. Any tips will be welcomed. I think next I'll try and get the tuning assembly fettled and the cord refitted then see if I can get the motor to run. The cap replacements shouldn't be too difficult since it appears the component cluster beneath each valve comes out complete with the valve holder when the nuts are undone. There seem to be more of these sets around than I thought. The only other ones I knew of were in the Vintage Wireless Museum Dulwich and the one owned by Mike Izycky (subject of BVWS Bulletin article mentioned above). Cheers, Jerry
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5th Oct 2021, 4:57 pm | #9 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,738
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Re: McMichael 802
Hi Jerry , its a while since I restored mine poss 20 years ouch! where did that go ,I do remember how fiddley it was replacing some of the capacitors around the valve bases , I do have a service manual , so ask if you come unstuck , Mick.
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6th Oct 2021, 8:52 pm | #10 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
I want to add that the previous owner sold this radio in good faith and was unaware that the field coil was o/c. No criticism was intended or implied by my previous posts - I'm simply reporting the set as I find it so that others might learn and benefit from my diagnostic technique and restoration work done. Cheers, Jerry
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6th Oct 2021, 10:46 pm | #11 |
Heptode
Join Date: May 2003
Location: Devizes, Wiltshire, UK.
Posts: 650
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Re: McMichael 802
Jerry,
Not sure where you bought yours from, but there were TWO of them at RetroTech ! I will get the field coil back to you as soon as I can. Mike...
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Mike Barker. |
7th Oct 2021, 12:01 pm | #12 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Thanks Mike. Mick, I'd be grateful if you would have a look inside the back of your set. I'm struggling with the mounting arrangement for the rear tuning gang frame. See pics. I only have the mounting parts for the right hand side, bolt shown. This has a 2.55mm metal spacer as shown in the pic. I'm not sure if it is original, or if it has been added as an earlier bodge - is there a metal spacer each side on yours (above chassis)? It looks like there was one grommet through the hole in the mounting frame and another in the chassis hole below, and so unless the rubber was originally very thin there is unlikely to be sufficient room for a spacer as well? Cheers, Jerry
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7th Oct 2021, 12:17 pm | #13 | |
Hexode
Join Date: Aug 2009
Location: Blackpool, Lancashire, UK.
Posts: 311
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Re: McMichael 802
Quote:
What more could you ask for !
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If you live, the chassis was dead; if you're dead, the chassis was live !!!! |
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7th Oct 2021, 1:45 pm | #14 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: McMichael 802
Quote:
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7th Oct 2021, 3:48 pm | #15 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,738
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Re: McMichael 802
Hi Jerry just had a look it looks like a grommet but it is poor condition , the parts on yours look original, Mick.
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7th Oct 2021, 5:00 pm | #16 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Thanks Mick. Hi Gabriel. The bracket and chassis holes are about 6mm and I don't have any grommets that small so I have ordered some and will do a trial fit with a pair each side and the spacer in between. I have found a bolt, spacers and washers in the tin that will do to replace the missing parts on the other side. I have cleaned up the rotor disc and contacts. One contact was cracked but I have manage to solder it together. I then spent an hour working out why the push buttons wouldn't work. The motor worked fine when each half of the disc was shorted to chassis. I sprayed plenty of switch cleaner on the right and left switch bank through which each contact is earthed. Eventually it dawned on me.. The wavechange switch was set to SW and the presets only function on LW and MW so there is an extra link to chassis via the wavechange switch. Doh! I replaced crumbling rubber flex to the scale lamps and that'll do for today. Going for a G&T. Cheers, Jerry
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7th Oct 2021, 5:55 pm | #17 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: McMichael 802
If you need any cylindrical spacers 3d printed let me know. I'd just need the dimensions. It won't help with dampening the vibrations though. Enjoy the G and T
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8th Oct 2021, 4:29 pm | #18 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Hi Mick, I could do with a bit more help please. I am struggling with the tuning cord routing. I've attached a sketch with the routing as I'm guessing it should be. If there is a diagram in the service manual, please would you scan it and post it here? If not, then I'd be grateful if you would remove the rear cover on your set and have a look to see if I appear to have got it right or wrong. The main bone of contention is pulley #3. The cord could go from pulley #1 to pulley #3 to pulley #5, instead of the way I've drawn it. The reason I think it should be the way I've drawn it is so as to make it easier to remove the valves on the right-hand side of the chassis without risking breaking the cord. Thanks in anticipation. Jerry
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8th Oct 2021, 5:56 pm | #19 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,738
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Re: McMichael 802
Hi Jerry here is a couple of pictures , excuse the dust twenty years worth , hope they help. Mick.
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8th Oct 2021, 9:01 pm | #20 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,369
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Re: McMichael 802
Hi Mick, that's a massive help, looks like I was on the right track. Than you very much Cheers, Jerry
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