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Old 23rd Sep 2022, 3:29 pm   #1
cathoderay57
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Default HMV 650 Anomaly

Hi folks, I've just started work on a recently acquired HMV 650. Quite a beast. I have removed both chassis and found an anomaly on the PSU/PA chassis. There is a pot and a resistor that don't appear in either the HMV service manual or the Broadcaster Sheet. See photos and sketch attached. It appears to be a means of adjusting the cathode bias voltage on the push-pull output pair, or perhaps more likely the -4.5v bias supply to elsewhere within the set. The type and part number on the pot (identical to the tone pot except this one is a preset with a slotted spindle), the type of fixed resistor, fixing bracket and colour/type of wire all indicate that this is an OEM modification. Anybody else with an HMV 650 that has a similar mod?
The second point concerns R18/R19 the shared cathode bias resistors on the AF output pair. The service data indicate that this should be either 2 x 100 Ohm resistors in series or 2 x 500 Ohms in parallel. Mine has a single wirewound 152 Ohm. When I received the radio it had 2 x 6V6G fitted but, since one of these was extremely low emission, I removed them both and fitted a pair of NOS 6F6G, which are a pretty close equivalent to KT63, the specified valves. Guidance on suitable cathode bias resistors for a pair of KT63 (A.M. Ball) recommends 250 Ohms per valve (when running at Va=250v, Vg2=250v) or, for a pair of 6F6G, a 320 Ohm common resistor (when running at Va=315v, Vg2=285v). The HMV 650 manual states Va=260v, Vg2=275v therefore I will leave the existing 152 Ohm resistor in place for now and see what sort of electrode voltages I get on the 6F6G pair when I power it up. Any comments or advice welcome. Jerry
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Old 23rd Sep 2022, 4:10 pm   #2
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Default Re: HMV 650 Anomaly

Looking at your drawing where does the HT-ve supply for the rest of the receiver come from, only via the pot?

Lawrence.
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Old 23rd Sep 2022, 7:22 pm   #3
cathoderay57
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Default Re: HMV 650 Anomaly

Hi Lawrence, I take your point. I made an error tracing the circuit on the chassis, since the cathodes of the AF output valves are both connected directly to chassis. R18 + R19 therefore also carry the remaining HT current from the rest of the set back to the HT winding centre tap via the HT fuse. R24 & R25 form a potential divider across R18 + R19. Cheers, Jerry

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Old 23rd Sep 2022, 10:03 pm   #4
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Default Re: HMV 650 Anomaly

I think its known as back biassing. Its sometimes used as the bias for the output valves but more commonly for deriving a negative supply for the AGC

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Old 24th Sep 2022, 7:27 am   #5
cathoderay57
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Default Re: HMV 650 Anomaly

Thanks Joe. Looking more closely, the -4.5v bias supply is connected to g1 of the triode-connected Z63/KTZ63/6J7G AF driver. Phase splitting for the output pair is achieved using a driver transformer. Once I get to the power-up stage (probably a few days yet) it will be interesting to see exactly what effects this mystery pot has. Cheers, Jerry
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Old 24th Sep 2022, 11:21 am   #6
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Default Re: HMV 650 Anomaly

There's a couple of HMV 655's here that look to have the same:

https://www.vintage-radio.net/forum/...d.php?t=150391

https://www.vintage-radio.net/forum/...ad.php?t=50793

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Old 24th Sep 2022, 11:59 am   #7
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Default Re: HMV 650 Anomaly

That pot is part of an inter station muting circuit, it is connected to a pair of contacts operated by the main tuning shaft via a slipping clutch. The contacts can be seen just behind the dial on the receiver chassis. It was a modification fitted to later sets. I have seen this fitted a few times, but often disconnected as it doesn’t work.
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Old 24th Sep 2022, 12:14 pm   #8
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Default Re: HMV 650 Anomaly

In here with drawing:

https://www.vintage-radio.net/forum/....php?p=1276732

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Old 24th Sep 2022, 2:58 pm   #9
cathoderay57
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Default Re: HMV 650 Anomaly

Many thanks folks, all is revealed, makes sense now. If all goes well I shall post a success story in due course - if not, I'll be back for more advice.....

Cheers, Jerry
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Old 24th Sep 2022, 4:13 pm   #10
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Default Re: HMV 650 Anomaly

The silent tuning control is described in the instructions for the HMV 660 radiogram which has the same chassis as the HMV 650.

Steve
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Old 24th Sep 2022, 8:01 pm   #11
cathoderay57
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Thanks Steve.
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Old 25th Sep 2022, 9:49 pm   #12
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Default Re: HMV 650 Anomaly

I have a 650 which has been sporadically prodded over the last 20 years but has yet to be completely finished-pretty sure it doesn't have that inter station muting but am happy to check over the coming days if it helps.
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Old 26th Sep 2022, 8:08 am   #13
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Default Re: HMV 650 Anomaly

Mine has the pot and I just turn it fully anticlockwise as the set just seems to perform better like that. Otherwise spending time listening to interstation noise is something that I tend not to do, it doesn't bother me and in some ways is all part of the 'fun' of listening to a vintage radio. But seriously as I see it there are two choices, adjust the control to reduce interstation noise, or for best 'on station' performance.
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Old 26th Sep 2022, 8:45 am   #14
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Default Re: HMV 650 Anomaly

Thanks again for your replies. I'm clear now on how the muting control is supposed to work. I have started the refurb having completed the PSU/amp chassis yesterday which was straightforward - transformers and chokes all OK apart from needing to re-sleeve rotted and crumbling rubber-insulated wires, and replacing one HT cap that wouldn't reform. Interestingly I found a pencilled signature under the chassis from a long lost repairman dated 24/6/46 which is perhaps when some of the old radiospares replacement caps were fitted. The HMV manual specifies a 500mA HT fuse (Broadcaster Sheet says 1A ). I have fitted a slow-blow 250mA instead and will see how that goes. I only have quick-blow 100mA which I would guess might not survive switch-on surge. Jerry
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Old 26th Sep 2022, 10:26 am   #15
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Default Re: HMV 650 Anomaly

Quote:
Originally Posted by cathoderay57 View Post
Thanks again for your replies. I'm clear now on how the muting control is supposed to work. I have started the refurb having completed the PSU/amp chassis yesterday which was straightforward - transformers and chokes all OK apart from needing to re-sleeve rotted and crumbling rubber-insulated wires, and replacing one HT cap that wouldn't reform. Interestingly I found a pencilled signature under the chassis from a long lost repairman dated 24/6/46 which is perhaps when some of the old radiospares replacement caps were fitted. The HMV manual specifies a 500mA HT fuse (Broadcaster Sheet says 1A ). I have fitted a slow-blow 250mA instead and will see how that goes. I only have quick-blow 100mA which I would guess might not survive switch-on surge. Jerry
Is that the fuse that sits right at the back of the top chassis? ie nearest to you when the back is off? The holder for mine has got contorted ends and they don't grip the fuse too well. It's proving very difficult to bend them back into shape without having to remove the top chassis in order to gain access to the underside of the holder - and that's a whole different ball game. But it's fine for now until I eventually take the set apart. For now I've just tested the valves, changed a few of them and cleaned up their holders, and the set works absolutely fine. I'm not saying that it can't be improved, I don't know, but perhaps a good re-alignment would do something. And that's quite a procedure according to the BVWS mag write up of the sister Marconi set a few years ago.
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Old 26th Sep 2022, 7:09 pm   #16
cathoderay57
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Default Re: HMV 650 Anomaly

Hi Steve,
Quote:
Is that the fuse that sits right at the back of the top chassis?
That's correct. A word to the wise: I spotted crumbling rubber insulation on the wires going to the AF driver transformer which is inside a low round can front left (when viewed from the rear) of the radio chassis. I could possibly have got away with these, by leaving them undisturbed. I strongly recommend that you leave yours alone if the set is working. Unfortunately I jumped in and disconnected the wires, pulled them through and unscrewed the transformer only to find that it is potted in tar so unless the tar is melted out it is impossible to access the ends of the wires at the transformer to replace them. I am now going to have to sleeve the wires where they enter the cable duct into the pitch, and seal/pot the sleeves with silicone sealant, then solder in replacement extension wires covering the joins with heatshrink. I'm sure it will be fine, but if you haven't disturbed the wires, then don't. Cheers, Jerry

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Old 27th Sep 2022, 5:26 am   #17
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Default Re: HMV 650 Anomaly

Gary Tempest described how he dealt with the crumbling insulation on his Marconiphone 561 on page 7 of this BVWS bulletin: https://www.bvws.org.uk/publications...letin_30_2.pdf

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Old 27th Sep 2022, 7:55 am   #18
cathoderay57
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Default Re: HMV 650 Anomaly

Thanks Steve, lots of useful tips in the article. I'm hoping I can get away with sleeving because only the yellow and yellow/black wires have crumbled badly at the point where they enter the cable duct into the base of the transformer, and so the risk of shorts is low. I've seen in many other sets that it always seems to be yellow and red rubber insulation that rot worse than all the other colours. Maybe something to do with the dye used. Cheers, Jerry
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Old 27th Sep 2022, 7:42 pm   #19
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Default Re: HMV 650 Anomaly

Quote:
Originally Posted by steve102 View Post
Gary Tempest described how he dealt with the crumbling insulation on his Marconiphone 561 on page 7 of this BVWS bulletin: https://www.bvws.org.uk/publications...letin_30_2.pdf

Steve
In Autumn 2010 (Volume 35, Number 3) there was another article by Gary Tempest of the full restoration of a 650.

Unfortunately, the PDFs of downloadable back issues stops at 2005 Winter 2005.

I'm not sure if BVWS can supply a back issue:

https://www.bvws.org.uk/publications...olume35number3

Page 8: Restoring an HMV 650 Gary Tempest (Date Code P/2 = 1937)

Incidentally, Gary created the artwork for a replica 650 dial and had a batch of high quality screen printed, which he sold on to other restorers.

Several 650s have been sold recently, on eBay and at auctions, one of which I noticed in the listing said it was 'unrestored, but fitted with a Gary Tempest dial'.

I wonder in that's the one that Jerry (cathoderay57 bought, which is the subject of this thread?
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Old 27th Sep 2022, 8:39 pm   #20
cathoderay57
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Default Re: HMV 650 Anomaly

Hi David, yes, that's the same radio, from eBay. I am a BVWS member so hopefully I still have the Autumn 2010 Bulletin somewhere. Over the years I have donated and loaned a few Bulletins but it might still be here! I am progressing reasonably well so far. The sleeving and silicone sealant on the AF driver transformer seems to have worked. I still have a few paper caps to replace. Even some of the old grey Radiospares replacements haven't stood the test of time and need replacing again. Just discovered that one section of the high wattage, chassis mounted HT dropper is o/c, the other section already bridged (recently) with a 6k 10W porcelain resistor. The realignment procedure sounds challenging so I'm rather hoping it won't be essential.... Cheers, Jerry
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