UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 26th Nov 2008, 8:25 pm   #61
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I finally got hold of the radio chassis yesterday and was disappointed to find another pitch capacitor block, admittedly only containing some of the capacitors. I therefore decided to connect it up and try it.

Amazingly MW came through loud and strong with no replacement to any of the 1931 components - there is no LW yet but I am pleased to have got anything in view of the entombed caps. The only fault on the radio chassis is that the ganged gramophone volume control had gone o/c some time in the 1950s judging by the now separate replacement neatly set into the side of the cabinet.

I think I will get LW working and otherwise leave well alone.

Paul
PaulR is offline  
Old 15th Dec 2008, 7:45 pm   #62
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

LW was actually there, but needed the volume turning up!

I am having a problem with the gram volume control. The original ganged one is o/c and a replacement had already been fitted to the case. The replacement just turned round and round with no end stop so I bought a new 100k log one (the original was 80k). This doesn't reduce the volume to zero at the lower end, and there is a 50hz hum at each extreme end. I have used screened cable and ensured that the metal case of the pot is earthed. The volume is also "all at one end".

I assume that the original was wire wound, but I can't see that would make any diference.

Has anyone got any suggestions, particularly about the dreaded hum, please?

Thanks

Paul
PaulR is offline  
Old 15th Dec 2008, 9:57 pm   #63
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 531 Restoration

The circuit is not at all ordinary! I would guess the VR1 part would have been linear as it varies the cathode bias and VR2 might have been log? The earth wiring seems to be all over the place, so I would start by checking all the inter-chassis connections and screens.
PJL is offline  
Old 16th Dec 2008, 3:04 pm   #64
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

Too right Peter, the diagram and the circuit itself are not the easiest!

The set clearly had not been used for many years and there were various loose wires hanging around which appear to have been soldered to the screens of the leads which originally connected the chassis together and the deck to the chassis. I have replaced the various connecting leads with modern screened cable and tried to rationalise the many earths used originally. Maybe I have missed something - that wouldn't have been difficult!

The radio part works well and there is little hum now when the radio is used so that gives me some hope that I will be able to sort it out eventually. I wonder how many of these remain in existance, or were actually made.

Paul
PaulR is offline  
Old 16th Dec 2008, 7:59 pm   #65
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I really couldn't follow what the circuit was trying to achieve with the pickup connections, so I thought I would try making it a little more conventional.

I earthed one of the leads from the pickup coil and connected the other to terminal 1. On VR2 (gramophone volume control) I removed the lead that eventually went to terminal 2 and earthed that end of the track instead. It now works very much better and the volume is properly controlled. Thers is still some hum from the pickup, but there may always have been some and it isnt intrusive.

I wish I knew how the original circuit was intended to work and what went wrong with it!

Now to put it all back together again.

Paul
PaulR is offline  
Old 17th Dec 2008, 9:11 am   #66
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 531 Restoration

I am working off a RMSE page. The B input to the amp is not referenced from ground. It comes from a resistor in the input valve grid. Putting B to earth will short the resistor out. The resistor is part of a feedback path from the output valve. It is not conventional -ve feedback as it comes off a resistor shared by the output valve cathodes which are individually bypassed. It is there to reduce hum.

According to the diagram the deck chassis is connected to B and not to earth. If you have connected a mains earth to the amp chassis it wouldn't surprise me if the circuit results in hum problems.
PJL is offline  
Old 17th Dec 2008, 11:15 am   #67
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

Thanks Peter, that explains a lot.

I haven't earthed B, and that continues to be used as intended for the radio section. I have eathed both chassis.

When I connected the pickup - I have only got the arm at the moment - there was a lot of hum except at full volume. Also the volume didnt go off completely at the lowest setting of the control - it lost all the treble, though. There was also some distortion when I played a record. I assumed that the hum was being introduced by the circuit connected to the "lower" end of the track so I decided to try earthing the pickup lead that originally went to 2 on the radio chassis, and the lower end of the track.

The result was that there is still some hum, the maximum volume is much higher, there is no distortion and the gram volume pot now controls the volume properly. If possible I would like to return it to the original wiring configuration. All I have done to the gram/radio chassis is:

1 Replaced the pickup leads with individually screeened cable. I originally connected the inner leads to 1 and 2 and the screens to the radio chassis. Now one is connected to 1 and the other earthed.

2 Replaced the non-original gram volume control with a new one and completely disconnected the original which had been left connected when it failed.

I must admit that I cannot follow the circuit around the pickup connections so I cannot tell why it did not work originally. As I said earlier, I would like to put it back to how it was intended but I have reached the limit of my (limited) knowledge!

Paul
PaulR is offline  
Old 27th Jan 2009, 10:59 am   #68
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I am about to put the 'gram back together again and would really like to try to put the pickup circuit back to how it was designed if at all possible. At the moment it is modified so that one side of VR2 and the pickup coil is earthed to the chassis. This has caused some hum as Peter has suggested. It isnt too great but I would like to try to improve on it.

When I replaced VR2 which was o/c with a log 100k pot the volume wouldnt go right off and there was some distortion. Making the modification cured that but it is not now as designed.

I cannot really understand how the circuit around VR2 worked as the track is connected across the pickup coil. How does it get a voltage drop in those circumstances? Also, what else could be faulty so that it doesn't currently work as originally intended? I have replaced the pickup leads with screened cable. S12 seems to be long gone!

Many thanks

Paul
PaulR is offline  
Old 8th May 2009, 1:54 pm   #69
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I hope I am not being a complete pain with this, but I am struggling to reduce the hum which seemed a lot worse when I put the set back together again.

I am probably about to show my complete ignorance, but in every other volume pot I have come across one end of the track has been earthed to produce a potential drop which could be picked up by the wiper. In this case the track is connected across the pickup coil and I cannot understand how this works. Close inspection of the circuit diagram seems to show that the lower pickup lead is earthed to "LS" (loudspeaker?) and there was indeed one disconnected lead coming from the loudspeaker chassis. The speaker was not earthed in any other way.

Peter (PJL) has already told us that the B input should not be directly earthed to the chassis, as this will short out R19 so I cannot see how the bottom end of the volume pot could be earthed without also earthing B. Both chassis are connected via tag 3.

I feel that if I could understand how it is intended to work I could do something about the hum which I am convinced is being picked up from the inputs and any guidance that anyone can give me will be most gratefully received.


Thanks


Paul
PaulR is offline  
Old 10th May 2009, 11:55 am   #70
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 531 Restoration

Paul, the service sheet is not correct. According to the circuit the V6 anode goes via CK2, R8 and R4 to earth! The description suggests that the link between the junction of R14, R13 and R4 to earth should be C16.

V6 is an anode bend detector and is biased almost off by the divider R16 and R17 on the cathode. CK2 and C15 is an RF filter. The anode load is R8 and R4, which is used as a divider for the audio out which I guess is simply to match the output level to the PU. The bias on V6 is critical so they have put in a link between pins 4 and 5 to measure V6 anode current which I guess should be in the low 100's of uA range (not sure what you were supposed to do if it was wrong!).

Then comes the awkward bit. The audio input to the amp is A and B. B is the signal earth and is isolated from the chassis as it forms part of a hum-bucking circuit via C20 and R19. Signal earth (B) is therefore used to shield the signal input A from all other things that might be a source of hum including the deck and LS.

When this set was built there was no such thing as safety earth so this arrangement was fine but it may cause problems for you. Any leakage in the mains transformer must make it's way directly to earth so safety earth must be connected to the chassis. This will leave you with the deck and other exposed metalwork with a DC potential and I suspect subject to other issues related to the amount of RF kicking around in modern houses.
PJL is offline  
Old 10th May 2009, 3:20 pm   #71
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

Thanks Peter

I am probably being particularly dense here, but what do you mean by "signal earth"? I cannot understand how the bottom end of VR2 is earthed without being connected to the radio chassis. The original VR2 was ganged with VR1 and only had a sort of paper track. This had been replaced many years ago by another pot screwed to the side of the case. I replaced this using the original wiring into the chassis but the only way I coul dmake it work was by connecting one end to the radio chassis.

I have come across the circuit diagram for the later 532A which seems simpler. It is now available on this site so I can't post the whole thing, but I believe I could post the relevant bits if necessary.

This circuit earths the B input to the chassis and omits the R18, R19 R20 and C20. I am wondering whether I could alter my circuit to match. If I did this would I need to alter the values of R22 and R23? Unfortunately the values for these in the newer circuit are only shown on the diagram for the 532 which I can't find anywhere. Also, would any alterations to the outputs from the radio chassis be needed? My thoughts are that it might work more straightforwardly in a way that I could understand and I could then earth everything to mains earth.

Many thanks for your patience.

Paul.
PaulR is offline  
Old 10th May 2009, 4:45 pm   #72
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 531 Restoration

Most audio equipment has a two wire unbalanced input. The input is supplied as a voltage between the two wires. To reduce pickup you can use one of the wires to shield the other to prevent a voltage difference occuring between the wires. The connection that is being used as the shield I have called signal earth but it probably has a proper name. You may also have heard of balanced input but that's another story.

It would need a little bit of work as the bias for V7 is taken from R20. Leaving R7 cathode connected to R20 would not be a good idea as it is part of the 'clever' design so we need to introduce some auto-bias using a resistor in the cathode with a bypass capacitor to retain the gain.

Remove R19 and C20. Disconnect V7 cathode from R20 and put in a 1K resistor from V7 cathode to earth and a bypass capacitor across the 1K capacitor of say 1uF. Wire B to chassis.
PJL is offline  
Old 10th May 2009, 5:08 pm   #73
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

Very many thanks Peter. I only have the amp chassis at the moment as the radio is in the case which is at the owner's house.

I am away all next week, so might not have a chance to do the mods before then. I will certainly try them as soon as I can and post the results.

Presumably this will solve another problem as the owner would like to be able to connect a CD player to the amp. I assumed that this would not have worked as the amp was originally designed.

Thanks

Paul
PaulR is offline  
Old 10th May 2009, 8:31 pm   #74
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I couldn't wait until next week to try the mods so I did it this afternoon. There is still a slight hum but I wonder whether I am getting paranoid! Next to try it back in the case with the radio.

Very many thanks for your patience and help, Peter

I have uploaded a video of it to YouTube if anyone is interested.

http://www.youtube.com/watch?v=UFLzROvL2NQ
PaulR is offline  
Old 10th May 2009, 10:51 pm   #75
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: HMV 531 Restoration

Much safer with the deck earthed...it would be worth documenting the changes you have made and attaching to the inside of the cabinet.

Try a CD player through it, it should sound pretty good. More than can be said for pre-war record decks and anode bend detectors...
PJL is offline  
Old 11th May 2009, 8:17 am   #76
PaulR
Dekatron
 
PaulR's Avatar
 
Join Date: Oct 2003
Location: Southport Lancashire, UK.
Posts: 3,221
Default Re: HMV 531 Restoration

I am away for a week later this morning, but I certainly have it in mind to try it with a CD and maybe a modern speaker.

The radio actually works very well, although with no AGC you have to have a hand near the volume control whan changing stations.

Sadly the set isn't mine. I saw it in a local auction house a couple of years ago and tentatively mentioned it to SWMBO. The reaction was somewhat frosty to say the least so I told a friend about it and he bought it for £10. It seems to have had a lot of use over the years and has been repaired several times. When it arrived many of the inteconnecting leads were disconnected and all were perished beyond redemption. It also had the dreaded HMV pitch block of caps.

I had already thought that I would print off the relevant parts of this thread to keep with it. I am sure that the owner will keep these safely with the set.

The case is quite scratched and faded but I have hopes that the stain/Danish oil/wax polish treatment will be successful. I will put some photographs of the finished article on here and maybe upload another video to YouTube.

Once again, many thanks


Paul
PaulR is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 5:46 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.