HMV 800 Radiogram restoration.

[Boater Sam]

If going down the new transformer for the heaters route, why not go for PL36s, loads around, high anode voltage?

[FERNSEH]

Hi Sam, I'm not so sure if the PL36 is suitable as an audio output valve. In television line output stages the valve operates as a switch, being conductive at the latter part of the scanning stoke.
Two valve socket adaptor plates are on their way to Mathew's address. These have been made in the same manner as those employed in the HMV 801 radiogram.
The attachment shows the plates I made way back in 2012 for the 801

DFWB.

[Heatercathodeshort]

Well done David. King of the adaptor plates.
[You can't make many mistakes with this one.]
Hope all goes well with it. John.

[mister valve]

I, too, am the proud owner of an HMV 800 Radiogram, fully restored and working. It even retains its original EMI K3 Auto changing Turntable unit. (sadly, many of these radiograms were retro fitted with 'modern' 3 or 4 speed auto changers in the 1950's and 1960's).

I have just ordered some new PX25's for the push-pull output stage (costing an arm and a leg. lol) as the originals are on their last legs (one also has one hoop of the filament o/c).

These really are fine machines when working correctly. The tonal quality is beautiful, and the radio is very sensitive and selective if set to 3kHz pass band. If you move the bandwidth switch to the 7 or 8 kHz. you get really good fidelity even from the restricted A.M. Broadcast bands (which is often lost on normal A.M. receivers) I would go as far to say that the sensitivity and selectivity are as good as most communication receivers. Now If I had to find something to moan about.... It would be the weight of the radiogram, it is extremely heavy ! Which makes moving it about quite difficult (even though it has casters on it).

[mister valve]

Re: HMV 800 restoration.... Just to add to my previous post, I forgot to mention that the top cap connectors 'black screen cans' going to the I.F. Valves are connected to the H.T. supply, and NOT the Chassis in the conventional way. Whilst from a signal point of view, the H.T. line and Chassis are virtually the same, from a D.C. potential point of view, they certainly are not. So NEVER hold the chassis and the top cap connectors at the same time when the set is on. Also, make sure that the 'felt' stuck around the inside of the top cap screen cans is there. This is to prevent the possibility of the top cap screen cans (which are at H.T. potential), touching the Valve's metalising which is earthed to chassis.

Now some help needed ! Even though the set is restored and runs well, the so called "Contrast amplifier" doesn't seem to do anything. But because I don't know what to expect from it, I am not really able to fault find. The correct voltages are around the valve 'VMP4' so it could be working o.k. without me knowing it ! Can anyone tell me how to check if it is working or not ? i.e. what to listen for.

P.S. my 'VMP4 contrast valve' is Glass and not 'catkin' as specified. But it shouldn't make any difference should it ?

[Hartley118]

Originally Posted by mister valve

Re: HMV 800 restoration....

Now some help needed ! Even though the set is restored and runs well, the so called "Contrast amplifier" doesn't seem to do anything. But because I don't know what to expect from it, I am not really able to fault find. The correct voltages are around the valve 'VMP4' so it could be working o.k. without me knowing it ! Can anyone tell me how to check if it is working or not ? i.e. what to listen for.

P.S. my 'VMP4 contrast valve' is Glass and not 'catkin' as specified. But it shouldn't make any difference should it ?

Congratulations on restoring a most impressive radiogram!

I do recommend that you download the service information available on this site 'up top', which should answer your questions. It's even a good read for those of us without the privilege of owning an HMV 800, giving an intriguing insight into the minds of the HMV designers of the time. HMV was clearly aiming for a 'no compromise' design.

The service data gives unusually detailed descriptions of the circuit design. The service data goes into detail on the 'Contrast' circuit which, as I read it, is an audio expander circuit, using a variable-mu valve effectively as a damping resistor on the primary of the audio intervalve transformer. It only operates on the 7kHz and 8kHz bandwidth settings.

......and yes, it's OK to replace that Catkin valve with the glass version.

Martin

[mister valve]

Well my brand new matched pair of "KR" PX25 valves arrived and have now replaced the aging and miss-matched pair. The sound quality is now much 'cleaner'; and as these valves are made to a higher quality standard than the original mass produced ones, hopefully they will outlive me !

All I have to sort out now is the 'Contrast valve circuit' to find out why it is not working as it should. I found out how it should work from some paperwork I have got, so I may have a go at tackling it.

[MurphyNut]

I have a friend in Edinburgh who owns HMV 800 Radiogram, unfortunately he doesn't have the technical radio knowledge to get it going again, it sits in his living room as a very large ornament. At least it's safe and dry.

[Edward Huggins]

Originally Posted by mister valve

All I have to sort out now is the 'Contrast valve circuit' to find out why it is not working as it should. I found out how it should work from some paperwork I have got, so I may have a go at tackling it.

This was better known via my Father as a "Contrast Expansion" circuit. He was an HMV Dealer before the war. As I remember it, it will "open up" the higher frequencies of the wideband AM circuits. It is very roughly similar the the Philips "Presence Control" on their better 1970s transistor amplifiers. Also it is not to be confused with a "Loudness Control".

[mister valve]

This was better known via my Father as a "Contrast Expansion" circuit. He was an HMV Dealer before the war. As I remember it, it will "open up" the higher frequencies of the wideband AM circuits. It is very roughly similar the the Philips "Presence Control" on their better 1970s transistor amplifiers. Also it is not to be confused with a "Loudness Control".

To put it simply and according to the paperwork I have; the circuit leaves the loud notes alone, but quietens the soft notes - thus giving a bigger and more realistic dynamic response. So it seems that even in the 1930's they used 'compression' in both Broadcasting and recording Gramophone Records.

On my 'HMV 800' the 'Contrast control' (when switched on) just seems to lower the overall level of the audio - regardless of whether they are loud or soft notes. So there is no "Contrast Expansion" at all. But at least it does have some effect (even if it is not quite the correct one !)

I'm sure I will eventually discover where the problem is, with time.

[mister valve]

Originally Posted by MurphyNut

I have a friend in Edinburgh who owns HMV 800 Radiogram, unfortunately he doesn't have the technical radio knowledge to get it going again, it sits in his living room as a very large ornament. At least it's safe and dry.

With such a fine instrument as the "HMV 800" it might be worth his while getting it professionally restored.

This will take a long time and lots of money, but you do end up with a 'Top-of-the-range' Radio-gramophone from the 1930's that could easily compete with all but the tip top modern 'Hi-Fi' !

These instruments sold for 110 Gns. (£115-10s-0d) back in the mid 30's, and according to an 'inflation calculator' is the equivalent of nearly £6,000 at todays prices !!

[Trifocaltrev]

Hi,

How is the restoration going, it seems a long time since you posted any further progress on it?

Regards
Trevor.

[skodajag]

That was my thought too Trevor. I've tried to contact him but with no reply.

[mister valve]

Hello "Trifocaltrev" and "skodajag",

Were you asking about my restoration of my "HMV 800" ? If so, I still can't get the 'Contrast control' circuit to work properly.... I have changed the valves in the circuit concerned, measured the voltages around it, checked and checked again - the capacitors and resistor values and just can't find anything wrong anywhere !! Yet there obviously is something amiss somewhere I did find a 2.3M ohm resistor had gone high (nearly 4M ohms) I changed this for a new 2.2M ohm resistor, but it made no difference at all.

Maybe if I heard one working properly, I would then know what to expect mine to be like. But as the likelihood of seeing another HMV 800 in working order is pretty remote, I can only guess what it should sound like.

The rest of the Radiogram including the 'K3' autochanger are working beautifully.

[mister valve]

Here's a picture of my "His Master's Voice, No. 800, Radiogramophone" I have restored....

[mister valve]

Don't know what's happened.... The pictures won't load - Sorry about that !

[Herald1360]

You could perhaps measure whether that contrast expansion system was doing anything by feeding in different signal levels from a sig gen and checking whether the gain changed with level.