HMV 531 Restoration - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

PaulR · 28th Aug 2008, 8:58 pm

Sorry Peter, yours and my posts crossed as it were.

I should have said that the sound from a CD player gets through fine. Does that affect your output transformer theory?

I replaced both the PX4 cathode resistors.

The volume control is actually on the radio chassis. If I short the input terminals the hum is still there. Does this make the MHL4 look to be more suspect?

The owner has found 4 working PX4s in his box of old valves including what looks like a very old one with a tilted electrode assembly, so he may well have some MHL4s if required!

Paul

PJL · 28th Aug 2008, 9:47 pm

Have you tried swapping the MHL4's?

I'd check the PX4 cathodes just to confirm it's all OK...Peter

PaulR · 28th Aug 2008, 10:20 pm

I only actually have the amp chassis at the moment. The idea is to bring it here bit by bit ending with the case in the hope that SWMBO doesn't notice!

I will go round and get the other MHL4 from the radio section.

PJL · 28th Aug 2008, 10:35 pm

If you measured the h/k resistance when cold then it's probably had it.

PaulR · 29th Aug 2008, 7:37 pm

The instructions on my valve tester (Taylor 45B) are to check the h/k resistance hot, and this shows just over 0.2m. It is the catkin valve I mentioned arlier in this thread.

Can I just confirm that as there is no hum when I earth the transformer side of the audio coupling cap from the anode of the MHL4, the PX4s and associated circuits are ok? I haven't been able to get hold of another MHL4 to try yet.

Thanks

Paul

PJL · 29th Aug 2008, 8:10 pm

Hi Paul, I would still check the PX4 heater DC volts to confirm they are working OK. 45V/35mA if I remember correctly from looking at the sheets. I suspect all is OK though...Peter

PaulR · 30th Aug 2008, 3:21 pm

Success! There is minimal wiring round the MHL4 and I decided to look at it once more. There is a lead from the "negative" input socket that runs to the connection panel for the radio chassis. The radio chassis isn't there at the moment and so the lead terminated in nothing. I have temporarily disconnected this at the input socket end and the hum has gone. I havent got the radio chassis yet or the diagrams I made when I dismantled it all, but I assume it is earthed somewhere on th eradio.

Stand by for the next instalment when I get the radio to do!

Paul

PaulR · 31st Aug 2008, 11:14 am

Having got over the euphoria of getting it workingI notice that there is a 100Hz hum.

Would this be caused by the replacement smoothing choke which has no centre tapping, compared to the original (see earlier in the thread)? The reservoir cap is only 2uf as per the circuit diagram.

Thanks

Paul

PJL · 4th Sep 2008, 12:06 pm

The circuit is complex...the MHL4 is biased off a shared resistor in the output cathode bias chain. I can only guess it is LF negative feedback to reduce hum.

Try a capacitor 0.1 or so across the input to make sure it's not pickup.

Try removing the capacitor from the anode to the intervalve transformer to see if the hum goes. If the hum persists check the output valve cathode DC volts suggested already as a missmatch will result in hum.

PaulR · 5th Sep 2008, 7:21 pm

I find the circuit diagram very difficult to follow due to all the parallel lines on it.

Disconnecting the AF coupling capacitor removes the vast majority of the hum, and a capacitor across the input makes no difference.

The cathode voltages are 54.5 and 57 using a dvm so they are probably in the right region. I don't know whether the difference is significant, but I can try the other PX4s the owner has to get a better match if necessary.

R19 was a little out at 83k rather than 100k as per the ccd, so I have replaced it with a new 100k one. This hasn't actually made any difference. All the others are well within 20% of spec. I had already replaced the PX4 cathode resistors.

It seems to me that we must have eliminated all other sources of hum except the HT itself. I can't see another next step except to have the original centre tapped choke rewound and to bring the circuit back to how it was designed.

Thanks

Paul

PJL · 6th Sep 2008, 10:51 pm

The PX4's seem to be well matched for quiescent current so that's good.

The hum is either from the MHL4 or the anode supply. What capacitor value have you used after C24/C25 (the ones around the tapped choke, CK4)?

PaulR · 7th Sep 2008, 8:47 am

I used the orginal values, that is 2uf before the choke and 4(.7) after. I tried increasing the value of the one before by adding in another 1uf but it made no difference.

phase_splitter · 7th Sep 2008, 10:48 am

Just trying to think a bit laterally, have you measured the hum at the speaker terminals with a DMM? If it's only a few mV, then that might be as good as it gets! Especially as the PX4s will be AC heated.
Also worth measuring the ripple on the HT rail. A push-pull O/P stage will offer some hum cancellation, so ripple on the HT rail is less of an issue than with a single-ended O/P stage.

mickjjo · 7th Sep 2008, 12:02 pm

Another thought, Are you using the original loudspeaker? I'm not familiar with this model but some other HMV grams I've worked on have "hum bucking" coils in the speakers connected in the speech coil circuit to cancel out mains hum, In those days high value electrolytics were not common in smoothing circuits

.

Regards, Mick.

PaulR · 7th Sep 2008, 3:52 pm

Thanks both,

I am assuming that the hum is introduced at the MHL4 and is being passed on to the o/p valves and amplified by them. I am using the original speaker.

I don't think that the amount of hum present would have been acceptable when the set was new, even in 1931, as it was more or less top of the range. The only thing that isnt as it was when original is the centre tapped choke which has been replaced by a "simple" one. Fortunately the original is still in place, and I think I am going to have to send it off to be rewound. If there is any hum left then I think that it must always have been there as I can't think of anything else to do with it.

Thanks

Paul

PJL · 7th Sep 2008, 5:19 pm

Try it without the MHL4 - if that doesn't get rid of it we need to look at the HT.

I would also check the MHL4 LT volts as you may be running it hot without the load of the radio valves.

PaulR · 7th Sep 2008, 7:36 pm

The hum is very much better without the MHL4, but is still there in the background. I thought about h/k leaks but hadn't got another MHL4 to try. It actually works well with an MH4, but the hum remains.

The voltage on the heater is fractionally over 4v, but well within what it should be I think.

PJL · 8th Sep 2008, 10:32 am

HMV seem to come up with odd circuits quite often - I have a 30's radiogram with 'amplified AGC' which is pretty unstable.

I think you will need a scope to investigate exactly what is going on.

PaulR · 9th Sep 2008, 9:17 pm

I have sent the choke to be rewound. It is the only think I can think of now without having a 'scope to investigate further. Ill put a note on here when I re fit it, hopefully reporting that the hum is much better!

I haven't even started on the radio chassis yet (or even got it actually).

Thanks

Paul

PaulR · 18th Sep 2008, 7:30 pm

The choke came back today and I have refitted it. There is still some hum but it is better than before. Having replaced all the caps, checked the resistors and replaced any bad wiring I think that it is as good as it gets (or probably ever was). Unfortunately the mains transformer hums quite a bit itself which adds to that coming from the speaker.

I have tried it with a MS 20i Pearl speaker and it sounds very good and quite powerful. Amazing for something so very old.

Look out for the next post when I get the radio chassis!!

Thanks all

Paul

28th Aug 2008, 8:58 pm	#41
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration Sorry Peter, yours and my posts crossed as it were. I should have said that the sound from a CD player gets through fine. Does that affect your output transformer theory? I replaced both the PX4 cathode resistors. The volume control is actually on the radio chassis. If I short the input terminals the hum is still there. Does this make the MHL4 look to be more suspect? The owner has found 4 working PX4s in his box of old valves including what looks like a very old one with a tilted electrode assembly, so he may well have some MHL4s if required! Paul Last edited by PaulR; 28th Aug 2008 at 9:04 pm.

28th Aug 2008, 9:47 pm	#42
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration Have you tried swapping the MHL4's? I'd check the PX4 cathodes just to confirm it's all OK...Peter

28th Aug 2008, 10:20 pm	#43
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration I only actually have the amp chassis at the moment. The idea is to bring it here bit by bit ending with the case in the hope that SWMBO doesn't notice! I will go round and get the other MHL4 from the radio section.

28th Aug 2008, 10:35 pm	#44
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration If you measured the h/k resistance when cold then it's probably had it.

29th Aug 2008, 7:37 pm	#45
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration The instructions on my valve tester (Taylor 45B) are to check the h/k resistance hot, and this shows just over 0.2m. It is the catkin valve I mentioned arlier in this thread. Can I just confirm that as there is no hum when I earth the transformer side of the audio coupling cap from the anode of the MHL4, the PX4s and associated circuits are ok? I haven't been able to get hold of another MHL4 to try yet. Thanks Paul

29th Aug 2008, 8:10 pm	#46
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration Hi Paul, I would still check the PX4 heater DC volts to confirm they are working OK. 45V/35mA if I remember correctly from looking at the sheets. I suspect all is OK though...Peter

30th Aug 2008, 3:21 pm	#47
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration Success! There is minimal wiring round the MHL4 and I decided to look at it once more. There is a lead from the "negative" input socket that runs to the connection panel for the radio chassis. The radio chassis isn't there at the moment and so the lead terminated in nothing. I have temporarily disconnected this at the input socket end and the hum has gone. I havent got the radio chassis yet or the diagrams I made when I dismantled it all, but I assume it is earthed somewhere on th eradio. Stand by for the next instalment when I get the radio to do! Paul

31st Aug 2008, 11:14 am	#48
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration Having got over the euphoria of getting it workingI notice that there is a 100Hz hum. Would this be caused by the replacement smoothing choke which has no centre tapping, compared to the original (see earlier in the thread)? The reservoir cap is only 2uf as per the circuit diagram. Thanks Paul

4th Sep 2008, 12:06 pm	#49
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration The circuit is complex...the MHL4 is biased off a shared resistor in the output cathode bias chain. I can only guess it is LF negative feedback to reduce hum. Try a capacitor 0.1 or so across the input to make sure it's not pickup. Try removing the capacitor from the anode to the intervalve transformer to see if the hum goes. If the hum persists check the output valve cathode DC volts suggested already as a missmatch will result in hum.

5th Sep 2008, 7:21 pm	#50
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration I find the circuit diagram very difficult to follow due to all the parallel lines on it. Disconnecting the AF coupling capacitor removes the vast majority of the hum, and a capacitor across the input makes no difference. The cathode voltages are 54.5 and 57 using a dvm so they are probably in the right region. I don't know whether the difference is significant, but I can try the other PX4s the owner has to get a better match if necessary. R19 was a little out at 83k rather than 100k as per the ccd, so I have replaced it with a new 100k one. This hasn't actually made any difference. All the others are well within 20% of spec. I had already replaced the PX4 cathode resistors. It seems to me that we must have eliminated all other sources of hum except the HT itself. I can't see another next step except to have the original centre tapped choke rewound and to bring the circuit back to how it was designed. Thanks Paul

6th Sep 2008, 10:51 pm	#51
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration The PX4's seem to be well matched for quiescent current so that's good. The hum is either from the MHL4 or the anode supply. What capacitor value have you used after C24/C25 (the ones around the tapped choke, CK4)?

7th Sep 2008, 8:47 am	#52
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration I used the orginal values, that is 2uf before the choke and 4(.7) after. I tried increasing the value of the one before by adding in another 1uf but it made no difference.

7th Sep 2008, 10:48 am	#53
phase_splitter Retired Dormant Member Join Date: Jun 2008 Location: East Sussex Posts: 25	Re: HMV 531 Restoration Just trying to think a bit laterally, have you measured the hum at the speaker terminals with a DMM? If it's only a few mV, then that might be as good as it gets! Especially as the PX4s will be AC heated. Also worth measuring the ripple on the HT rail. A push-pull O/P stage will offer some hum cancellation, so ripple on the HT rail is less of an issue than with a single-ended O/P stage.

7th Sep 2008, 12:02 pm	#54
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: HMV 531 Restoration Another thought, Are you using the original loudspeaker? I'm not familiar with this model but some other HMV grams I've worked on have "hum bucking" coils in the speakers connected in the speech coil circuit to cancel out mains hum, In those days high value electrolytics were not common in smoothing circuits . Regards, Mick.

7th Sep 2008, 3:52 pm	#55
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration Thanks both, I am assuming that the hum is introduced at the MHL4 and is being passed on to the o/p valves and amplified by them. I am using the original speaker. I don't think that the amount of hum present would have been acceptable when the set was new, even in 1931, as it was more or less top of the range. The only thing that isnt as it was when original is the centre tapped choke which has been replaced by a "simple" one. Fortunately the original is still in place, and I think I am going to have to send it off to be rewound. If there is any hum left then I think that it must always have been there as I can't think of anything else to do with it. Thanks Paul

7th Sep 2008, 5:19 pm	#56
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration Try it without the MHL4 - if that doesn't get rid of it we need to look at the HT. I would also check the MHL4 LT volts as you may be running it hot without the load of the radio valves.

7th Sep 2008, 7:36 pm	#57
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration The hum is very much better without the MHL4, but is still there in the background. I thought about h/k leaks but hadn't got another MHL4 to try. It actually works well with an MH4, but the hum remains. The voltage on the heater is fractionally over 4v, but well within what it should be I think.

8th Sep 2008, 10:32 am	#58
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 531 Restoration HMV seem to come up with odd circuits quite often - I have a 30's radiogram with 'amplified AGC' which is pretty unstable. I think you will need a scope to investigate exactly what is going on.

9th Sep 2008, 9:17 pm	#59
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration I have sent the choke to be rewound. It is the only think I can think of now without having a 'scope to investigate further. Ill put a note on here when I re fit it, hopefully reporting that the hum is much better! I haven't even started on the radio chassis yet (or even got it actually). Thanks Paul

18th Sep 2008, 7:30 pm	#60
PaulR Dekatron Join Date: Oct 2003 Location: Southport Lancashire, UK. Posts: 3,237	Re: HMV 531 Restoration The choke came back today and I have refitted it. There is still some hum but it is better than before. Having replaced all the caps, checked the resistors and replaced any bad wiring I think that it is as good as it gets (or probably ever was). Unfortunately the mains transformer hums quite a bit itself which adds to that coming from the speaker. I have tried it with a MS 20i Pearl speaker and it sounds very good and quite powerful. Amazing for something so very old. Look out for the next post when I get the radio chassis!! Thanks all Paul