HMV 2451 stereo system. - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

crackle · 24th Oct 2020, 4:02 pm

Its possibly riddled with Callins capacitors.

Mike

112stereo · 25th Oct 2020, 1:26 pm

Sorry no scope!
Rectifier has 4 single diodes and all check out ok and the smoothkng cap has been substituted now the original one is back in.
My sister had the original speakers so i borrowed them back, yes the sound was slightly inferior but hum still there. Can i fit a second smoothing capacitor to see if that makes any difference if so what value should i try ?

crackle · 25th Oct 2020, 5:16 pm

A photo of the insides showing the layout may help to show if there is any obvious issue with the layout of the wiring.
Is the hum more or less if you switch from gram to radio to tape?
Mike

simpsons · 25th Oct 2020, 5:48 pm

The schematic or some more detail is needed.

The schematic for the Ferguson System 6 is about as close as I can get. But note the transistors are PNP. If your chassis has the positive of the smoothing capacitor connected to the chassis or main part of the printed circuit then read on.

So, whilst it doesn't have magnetic cartridge equalisation, it follows the OP power supply of a single high value smoothing capacitor.
It also highlights the use of a simple passive tone control preceeded by a high impedance pre amp.

First of all it shows the volume control location which follows the pre amp. Set to min there can be no hum induced by proceeding stages.

Secondly, during manufacture the value of the main smoothing capacitor has been increased from 2200uF to 3300uF. The only reason for this would be too much hum! You say that your unit has 5000uF. Help!!

I suggest you double check the grounding on the volume control. This should include making sure the chassis the is also connected to the "earth" side of the smoothing capacitor. There should be zero ohms between the positive of the smoothing capacitor, the earth side of the volume control, that is the outer coax and also the chassis and all other earth points.

There is an earth loop somewhere I'm sure. It is just finding it. If all fails, the photo might help.

Chris

frsimen · 25th Oct 2020, 7:40 pm

The series 74 and 75 are almost identical. The 74 has a magnetic cartridge preamp and is rated for a higher output power with a small number of component values altered to suit compared with the series 75. Other than that the circuits are the same. I wouldn't increase the value of the main reservoir capacitor any more. 5000uF is already a high value, a larger one will increase the strain on the bridge rectifier diodes, which isn't a good plan.

You haven't said whether the hum is 50Hz or 100Hz. If 50Hz, no amount of fiddling with capacitors will help, unfortunately. Look for sillies like mains wiring running close to signal leads and poor chassis earths etc. as others have mentioned. If the problem is 100Hz, hum my suggestions in post 15 are worth a try.

Paula

112stereo · 26th Oct 2020, 12:21 pm

I think i will give up trying to find a fault as we are just going round in circles.
Is there anything i can do to modify the psu to try and stop or at least reduce this hum ?
I have checked all the normal earths connections .
On a scale of 1 to 10 and at minium volume i would put the hum at 3. As tthe volume control is increased the hum stays the same level so any gaps in music speach etc it can be heard. With no input eg on gram with nothing playing the volume control can be put at any level with very little increase in hum level.
As it is 50 cycle hum why is it not on the input ? It only appears on the output !
So if i increase the smoothing for the output i think it should help what do you think ?
I just want to use it without this hum being so annoying.
I am happy to perform any modification if i had a clue what to do.!

Cobaltblue · 26th Oct 2020, 1:16 pm

I think part of the problem is we have not seen inside the actual beast.

It's very likely that none of us has ever actually worked on this exact model but will have worked on many similar sets.

When tackling a fault the repairer uses his eyes, ears and nose as well as his previous experience of that manufacturer and the parts used.

Dry joints venting capacitors, burn't resistors.

As Crackle mentioned a couple of posts back it could well have many parts known to cause faults.

We haven't see inside this beast all of our senses are denied.

Some pictures please it's unusual for a set like this to require an extremely large smoothing capacitor.

Give the guys a chance to help

Picture of the electronics and underside of the deck please.

Cheers

Mike T

Edward Huggins · 26th Oct 2020, 5:02 pm

When you ae trying to trace hum, I'm afraid it often means you do have to go around in circles! Even though your Thorn made unit has a low-to-medium specification, there should be no audible hum on this at low volume settings. The supplied matching speakers have a limited bass extension, so the B&W may have accentuated the problem you certainly have. Why not post a HELP WANTED request on here for a Local Member (and there are many near to you) to come and trace this for you?

112stereo · 27th Oct 2020, 11:58 am

I have checked everything from the rectifier diodes, smoothing cap and earth connections Andv can't fault any of them. I would just like to be able to use it without this annoying hum.
Can anyone suggest how to modify it to reduce it please ?
In answer to other questions none of the controls have any effect on it. Volume from minimum to maximum hardly alters the level the hum is at. It just seem to be constant on the output transistors.
Can I try fitting another cap anywhere in that power line and if so what sort of value could I try ?

paulsherwin · 27th Oct 2020, 12:10 pm

You can't simply modify it to remove the hum, because there is a fault. The only way to cure the hum is to identify the fault and fix it. I appreciate that it can be a frustrating business, but there's really no other way forward.

ms660 · 27th Oct 2020, 12:14 pm

Quiescent current?

I don't have a schematic.

Lawrence.

112stereo · 27th Oct 2020, 12:20 pm

Uploaded a few pictures, hope it helps.

112stereo · 27th Oct 2020, 2:08 pm

A friend has seen my post to you guys, he follows a lot of posts as he was in the business of repairs in the 60s, he wont mind me telling you he's 92 !
He said he will come round and take a look at it and he has a scope !
He remembers this model as being popular in his shop and sold wuite a few, he also thinks there was some modifications for it but cant remember what they were. This set must have been a very early one as according to a vintage site i was looking at said it came out in 1973, but this was purchased christmas 1971 as thats when i got married and he was showing it off at our Christmas dinner and i have the origional receipt for it !

crackle · 27th Oct 2020, 6:00 pm

Quote:

Originally Posted by 112stereo

I have checked everything from the rectifier diodes, smoothing cap and earth connections Andv can't fault any of them. I would just like to be able to use it without this annoying hum.
Can anyone suggest how to modify it to reduce it please ?
In answer to other questions none of the controls have any effect on it. Volume from minimum to maximum hardly alters the level the hum is at. It just seem to be constant on the output transistors.
Can I try fitting another cap anywhere in that power line and if so what sort of value could I try ?

Paula (frsimen) made some very specific suggestions way back in post #15 which you appear to have not acknowledged.

Mike

ben · 27th Oct 2020, 7:19 pm

Hum that is present and largely/totally unaffected by the volume control, suggests the rectifier to me. Change the diodes, they're cheap enough.

kirstyd · 27th Oct 2020, 7:44 pm

Quote:

Originally Posted by 112stereo

Uploaded a few pictures, hope it helps.

that looks suspiciously like the innards of the HMV 2354 stereogram .it also has two separate PCbs

crackle · 27th Oct 2020, 8:55 pm

The Thorn 74 series service information is on my site here http://www.kbmuseum.org.uk/other_ser...es74_r&tvs.pdf

I wonder if the caps across each rectifier diode are OK.
If this unit has tin plated plugs and sockets connecting the modules together try spraying with switch cleaner and pushing them in and out a few times.
You never know, might just be as simple as a dirty earth or other connection.
Mike

112stereo · 29th Oct 2020, 9:22 am

My apologies to Paula i thouht i had answered her post.
The answer is no the bass control doesn't alter the hum level, but it is an area i will look at as i have just noticed the shape button doesn't work. It might be the button its self.
You might think i am spending a lot of time on this, but in the present situation i have so much time on my hands what else is there to do, there's only so much cleaning you can do !

simpsons · 29th Oct 2020, 10:49 am

With the schematic solving the problem should be easier.

Go back to first principles.

Ground the input to the power amplifier C132 or C133 channel to earth. This should be done on both the main board and tuner/pre amp just in case something silly is going on.

Should there be no change then it can only be the power supply.

Should the hum go then it is in the preceding stages and this can then be addressed.

Keep up the good work and thank goodness we all have the schematic to work from.

Chris

112stereo · 31st Oct 2020, 2:23 pm

Shorted out the main input to the power amp, no hum !!
As i said before the shape button doesn't work, ive tested it and its totally open circuit in every way.
Shorted out the two pins as if the switch is off, no change, shorted out the two pins as if the switch is on no hum ?
The switch is part of a bank so not sure what to do i might just short it out as thoigh it's on for now and make more use of the bass and treble controls it will make it more useable.

24th Oct 2020, 4:02 pm	#21
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 2451 stereo system. Its possibly riddled with Callins capacitors. Mike

25th Oct 2020, 1:26 pm	#22
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. Sorry no scope! Rectifier has 4 single diodes and all check out ok and the smoothkng cap has been substituted now the original one is back in. My sister had the original speakers so i borrowed them back, yes the sound was slightly inferior but hum still there. Can i fit a second smoothing capacitor to see if that makes any difference if so what value should i try ?

25th Oct 2020, 5:16 pm	#23
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 2451 stereo system. A photo of the insides showing the layout may help to show if there is any obvious issue with the layout of the wiring. Is the hum more or less if you switch from gram to radio to tape? Mike

25th Oct 2020, 5:48 pm	#24
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: HMV 2451 stereo system. The schematic or some more detail is needed. The schematic for the Ferguson System 6 is about as close as I can get. But note the transistors are PNP. If your chassis has the positive of the smoothing capacitor connected to the chassis or main part of the printed circuit then read on. So, whilst it doesn't have magnetic cartridge equalisation, it follows the OP power supply of a single high value smoothing capacitor. It also highlights the use of a simple passive tone control preceeded by a high impedance pre amp. First of all it shows the volume control location which follows the pre amp. Set to min there can be no hum induced by proceeding stages. Secondly, during manufacture the value of the main smoothing capacitor has been increased from 2200uF to 3300uF. The only reason for this would be too much hum! You say that your unit has 5000uF. Help!! I suggest you double check the grounding on the volume control. This should include making sure the chassis the is also connected to the "earth" side of the smoothing capacitor. There should be zero ohms between the positive of the smoothing capacitor, the earth side of the volume control, that is the outer coax and also the chassis and all other earth points. There is an earth loop somewhere I'm sure. It is just finding it. If all fails, the photo might help. Chris Last edited by simpsons; 25th Oct 2020 at 6:05 pm.

25th Oct 2020, 7:40 pm	#25
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: HMV 2451 stereo system. The series 74 and 75 are almost identical. The 74 has a magnetic cartridge preamp and is rated for a higher output power with a small number of component values altered to suit compared with the series 75. Other than that the circuits are the same. I wouldn't increase the value of the main reservoir capacitor any more. 5000uF is already a high value, a larger one will increase the strain on the bridge rectifier diodes, which isn't a good plan. You haven't said whether the hum is 50Hz or 100Hz. If 50Hz, no amount of fiddling with capacitors will help, unfortunately. Look for sillies like mains wiring running close to signal leads and poor chassis earths etc. as others have mentioned. If the problem is 100Hz, hum my suggestions in post 15 are worth a try. Paula

26th Oct 2020, 12:21 pm	#26
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. I think i will give up trying to find a fault as we are just going round in circles. Is there anything i can do to modify the psu to try and stop or at least reduce this hum ? I have checked all the normal earths connections . On a scale of 1 to 10 and at minium volume i would put the hum at 3. As tthe volume control is increased the hum stays the same level so any gaps in music speach etc it can be heard. With no input eg on gram with nothing playing the volume control can be put at any level with very little increase in hum level. As it is 50 cycle hum why is it not on the input ? It only appears on the output ! So if i increase the smoothing for the output i think it should help what do you think ? I just want to use it without this hum being so annoying. I am happy to perform any modification if i had a clue what to do.!

26th Oct 2020, 1:16 pm	#27
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,869	Re: HMV 2451 stereo system. I think part of the problem is we have not seen inside the actual beast. It's very likely that none of us has ever actually worked on this exact model but will have worked on many similar sets. When tackling a fault the repairer uses his eyes, ears and nose as well as his previous experience of that manufacturer and the parts used. Dry joints venting capacitors, burn't resistors. As Crackle mentioned a couple of posts back it could well have many parts known to cause faults. We haven't see inside this beast all of our senses are denied. Some pictures please it's unusual for a set like this to require an extremely large smoothing capacitor. Give the guys a chance to help Picture of the electronics and underside of the deck please. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

26th Oct 2020, 5:02 pm	#28
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,331	Re: HMV 2451 stereo system. When you ae trying to trace hum, I'm afraid it often means you do have to go around in circles! Even though your Thorn made unit has a low-to-medium specification, there should be no audible hum on this at low volume settings. The supplied matching speakers have a limited bass extension, so the B&W may have accentuated the problem you certainly have. Why not post a HELP WANTED request on here for a Local Member (and there are many near to you) to come and trace this for you? __________________ Edward.

27th Oct 2020, 11:58 am	#29
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. I have checked everything from the rectifier diodes, smoothing cap and earth connections Andv can't fault any of them. I would just like to be able to use it without this annoying hum. Can anyone suggest how to modify it to reduce it please ? In answer to other questions none of the controls have any effect on it. Volume from minimum to maximum hardly alters the level the hum is at. It just seem to be constant on the output transistors. Can I try fitting another cap anywhere in that power line and if so what sort of value could I try ?

27th Oct 2020, 12:10 pm	#30
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,949	Re: HMV 2451 stereo system. You can't simply modify it to remove the hum, because there is a fault. The only way to cure the hum is to identify the fault and fix it. I appreciate that it can be a frustrating business, but there's really no other way forward.

27th Oct 2020, 12:14 pm	#31
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: HMV 2451 stereo system. Quiescent current? I don't have a schematic. Lawrence.

27th Oct 2020, 12:20 pm	#32
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. Uploaded a few pictures, hope it helps. Attached Thumbnails Last edited by 112stereo; 27th Oct 2020 at 12:26 pm.

27th Oct 2020, 2:08 pm	#33
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. A friend has seen my post to you guys, he follows a lot of posts as he was in the business of repairs in the 60s, he wont mind me telling you he's 92 ! He said he will come round and take a look at it and he has a scope ! He remembers this model as being popular in his shop and sold wuite a few, he also thinks there was some modifications for it but cant remember what they were. This set must have been a very early one as according to a vintage site i was looking at said it came out in 1973, but this was purchased christmas 1971 as thats when i got married and he was showing it off at our Christmas dinner and i have the origional receipt for it ! Attached Thumbnails

27th Oct 2020, 7:19 pm	#35
ben Dekatron Join Date: Aug 2006 Location: Madrid, Spain / Wirral, UK Posts: 7,498	Re: HMV 2451 stereo system. Hum that is present and largely/totally unaffected by the volume control, suggests the rectifier to me. Change the diodes, they're cheap enough. __________________ Regards, Ben.

27th Oct 2020, 8:55 pm	#37
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: HMV 2451 stereo system. The Thorn 74 series service information is on my site here http://www.kbmuseum.org.uk/other_ser...es74_r&tvs.pdf I wonder if the caps across each rectifier diode are OK. If this unit has tin plated plugs and sockets connecting the modules together try spraying with switch cleaner and pushing them in and out a few times. You never know, might just be as simple as a dirty earth or other connection. Mike Last edited by crackle; 27th Oct 2020 at 9:11 pm.

29th Oct 2020, 9:22 am	#38
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. My apologies to Paula i thouht i had answered her post. The answer is no the bass control doesn't alter the hum level, but it is an area i will look at as i have just noticed the shape button doesn't work. It might be the button its self. You might think i am spending a lot of time on this, but in the present situation i have so much time on my hands what else is there to do, there's only so much cleaning you can do !

29th Oct 2020, 10:49 am	#39
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: HMV 2451 stereo system. With the schematic solving the problem should be easier. Go back to first principles. Ground the input to the power amplifier C132 or C133 channel to earth. This should be done on both the main board and tuner/pre amp just in case something silly is going on. Should there be no change then it can only be the power supply. Should the hum go then it is in the preceding stages and this can then be addressed. Keep up the good work and thank goodness we all have the schematic to work from. Chris

31st Oct 2020, 2:23 pm	#40
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. Shorted out the main input to the power amp, no hum !! As i said before the shape button doesn't work, ive tested it and its totally open circuit in every way. Shorted out the two pins as if the switch is off, no change, shorted out the two pins as if the switch is on no hum ? The switch is part of a bank so not sure what to do i might just short it out as thoigh it's on for now and make more use of the bass and treble controls it will make it more useable.