HMV 800 first switch on

[KEITHW]

Hi Peter,

many thanks for all your advice. Is there a reasonably accurate way to test MR1 or is taking it out of circuit as you suggest the best test? I do have another from the second chassis I could sub. it with but not certain
what state it is in. I don't have a diode to hand I'm afraid.

The self ranging meter didn't like making some of the tests and flicked between DC and AC before giving up which would be explained by what you have said. The analogue meter handled it much better.

I am going to study the circuit now to understand what you have told me.

Many thanks,
Keith

[PJL]

I would guess all the old metal rectifiers are likely to leak. Pretty much any diode would do although one with a 100V+ reverse voltage would be preffered.

For the time being just disconnect MR1 - you won't have any AGC but it will stop you over running the valves and will restore gain.

The delayed AGC takes the AC from the last IF stage and rectifies it using MR1. The delay is voltage not time...MR1 is attached to one of two voltages dependent on the waveband (I suspect) so that the AGC ony comes in when the AC voltage has reached that level. Stops the gain being reduced on distant stations...Peter

[KEITHW]

Thanks Peter for that explanation. I have now removed MR1, a Westector. For anyone interested in seeing what's inside (as I was) there's a photo below. Lots of little pellets held under compression with a spring.

This has made some differences to the valve voltages, new readings on right;

V1 VMS4B
Anode = 209v 227v
Grid 1 = 8v 0.4v
Grid 2 = 148v 164v
Cathode = 13v 7v

V2 MH4 catkin
Anode = 9v* had to measure analogue, digital flicked about all over between DC and AC
Grid = -4.25v (negative) -6v
Cathode = 0v 0v

V3 MH4
Anode = 76v 82v
Grid = -124mv -127mv
Cathode = 0v 0v

V4 VMS4B
Anode = 211v 224v
Grid 1 = 9v 2.9v
Grid 2 = 147v 161v
Cathode = 15v 11v

V5 VMS4B
Anode = 210v 223v
Grid 1 = 8v 3.5v
Grid 2 = 148v 161v
Cathode = 33v 31v

Now going to carry out checks as per Steve P's last post.

Keith

[PJL]

Looking much better but there is still a problem with the AGC line which will probably be a faulty valve pulling the AGC up (cooked by the MR1 fault). The grid voltages of V4, V5 & V8 all need to be near zero.

Easiest way to find out which valve is at fault is to remove V4, V5 & V8 one by one and measure the grid. Hopefully, pulling the faulty one out will lower the grid on the others.

It's also worth checking the AGC resistors R27,33,34,26,17,5,1,21 for a start...

Don't worry about the oscillator at the moment as it is working even better now the HT is up a bit - 12V peak to peak on the grid...it's almost certainly your meter getting the anode wrong.

[PJL]

Ahhh, didn't notice MR2...It's on the grid of V8 so can you measure the grid of V8 and if it's higher than V4 & V5 disconnect MR2...Peter

[KEITHW]

Oh dear Peter! 45v on grid 1 of V8. I will disconnect MR2. Might take a few minutes as the wiring is a bit complex round there. Will then test V4, V5 and V8 again.

Keith

[PJL]

That would have killed any signal...it will be very easy to hide a 1N4001 inside the empty MR boxes.

45V is very high and I am not convinced this is all from MR2. If disconnecting MR2 does not fix it then try removing the valve and checking the grid again...

[KEITHW]

V4 grid now measures 2.3v (was 2.9v), V5 grid now 2.83v (was 3.5v) and V8 grid now measures 16mv (was 45v). The anode of V1 has now increased to 250v (was 227v).

Dramatic change to V8 anode, back to checking resistors?!

Keith

[PJL]

Lets focus on V5 as it must be pretty much cut-off with the cathode at 31V. In fact I wonder if VR2 isn't open-circuit also check R19?

If these are OK try swapping V5 with V1 and measure grids and cathode of V5 again.

[KEITHW]

Great hit Peter! R19 is perfectly Ok, but VR2 is open circuit for most of its travel with a very small run of short cicuit at one end.

I will investigate the one on my other chassis and see what that is like. If possible I will do a transplant. Because of the state of the other chassis it has been my intention to restore this chassis I am working on using as many authentic parts as possible using the other as a doner. Depending on what's left of the other, I was going to just use modern parts to get it going. Anyway, that's all in the future!

Let you know what happens with VR2 (hopefully soon).

Keith

[PJL]

Hopefully that will finally sort the AGC line out - well it won't actually work anymore but at least it won't be pulling all the grids high.

If it's still quiet I suggest we start working backwards from the last stage. To make this as safe as possible I suggest you find a couple of meters of insulated wire and use this as an 'aerial' to inject some noise into the grids.

The trouble is the circuit is soooo complicated. Try it on grid of V10 first then V9 as I can't work out what V11 does

OK...it's difficult without component values but it looks like V11 is some kind of noise suppression - if you don't get any hum from V10 grid then try with V11 removed.

[KEITHW]

End of the line for today I'm afraid. I have tried VR2 and VR1 (both 14k) from the 'donor' chassis and all suffer from a similar problem, they all have a break at some point in the wirewound track. I have dismantled VR2 from the good chassis and might see about trying to rewind it otherwise I'll have to look for suitable replacements. I hope VR1, VR3 and VR4 are not all the same!

V11 is a VMP4 catkin valve, it acts as a contrast amplifier.

If you are interested Peter, and Brianc is agreeable (he sent me it) I can Email the manual to you as a PDF, it might help! I'll send Brian a PM.

Many thanks for your guidance today, Peter, it's very much appreciated, you've given Steve P the day off! I'm now going to tidy up.

Keith

[PJL]

Hi Keith, you don't need to thank me it's nice to have someone else hold the iron! I have had a similar problem with these 14K pots on a Marconi 296 and a 287, I used a standard 25K lin from maplin with a parallel resistor as they don't need to be anything special.

So you can order all the parts at once I suggest you work around the problem.

1. Short out VR2.
2. Check continuity of other pots.
3. If it still doesn't work try working backwards through the stages V10 V9 etc. to locate where the signal is lost.

All the best...Peter

[PJL]

Before I forget...the 45V on V8 grid seems to high for delayed AGC which suggests a fault in the divider chain R66-R70.

[PaulR]

Keith

As someone who is about to start on a 1931 HMV console set, I am very impressed by your restoration. What did you use to do that immaculate lacing?

Paul

[KEITHW]

Hi Peter,
I'm going to be busy for the next couple of days, so will not have the opportunity to get back to investigating 'til after this, but thanks for your further thoughts. I have checked the other pots and VR1 is also open circuit, the remainder seem fine. If you would like a copy of the manual please PM me your Email address, it's quite an interesting read!

Hi Paul,
thanks for your comments. For the lacing I used 1 mm waxed cotton thonging. It is available on Ebay in handy 10 metre lengths where it is sold for hippy http://www.feebleminds-gifs.com/smiley-faces-71.gifnecklaces and the like! Just the job. For anyone who wants to have ago, attached are the best instructions I have seen.

Good luck with your restoration.

Regards,
Keith

[cavemuscle]

Hey Keith! Hows the restoration coming along? Curious minds want to know!

Brock

[KEITHW]

Hi Brock,

Thanks for your interest. Things haven't gone too well I'm afraid.

My work bench and work room are actually our dining table and dining room so I had to clear everything out of the way just before Christmas ready for the festivities. Unfortunately, the Sunday before Christmas I was trying to get something down from the loft, I over reached myself and managed to break a rib in the process It has been extremely painful and consequently there has been no lugging heavy radiogram chassis about!

I am well on the way to recovery now but it is still painful if I bend suddenly. I have spent my time restoring a Roberts P5A and doing another hobby of mine, family history.

On a plus note, I have obtained replacements for VR1 and VR2. They are NOS 15K wire wound potentiometers made by Colvern Ltd and they are just the job. The spindles are insulated, unlike the the originals, so I will have to earth their center tags to chassis.

I have also been experimenting with fitting a diode inside the Westector rectifier but due to its construction I don't think that is going to work. I will therefore mount the diode under the empty shell.

Hopefully I can get everything out again in a couple of weeks and get back to business.

Best wishes,
Keith

[Steve_P]

Sorry to hear that you've been under the weather. Make sure you are fully back to normal before you lift anything heavy. That set is 70 years old, so another few weeks won't matter!

Cheers,

Steve P.

[KEITHW]

Hi Steve,

thanks for that, hopefully it won't be too long before I'm back, pestering for help

I am determined that this year I will bring it back to life! Will have to be more careful going in to lofts though.

Best wishes,
Keith