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

112stereo · 22nd Oct 2020, 3:57 pm

My dad purchased this in 1971 new and has served him well.
After his death i inherited it to prdvent it going to landfill.
Its been in my office for a little over 4 years znd i thought it was time to use it.
The deck had seized so this was my first job and it was a bit of a job to get it all clean again, but eventually I managed it.
Turning to the rest it all works and sounds very good in my opinion my dad had upgraded the speakers to B&W ..........never heard of them but it does sound good.
The issue i have now is a mains hum which is at a constant level so with the volume to minimum it is quite noticeable.
Substituted the main caps with equivalents but made no difference. Talking to a friend who used to repair appliances for aliving said most of the amps made by the thorn group did hum so it appears to be a design flaw.
Is there any way i can reduce or hopefully remove the hum to a point where its barely audible.?
Any thoughts please.....?add another cap maybe?

Edward Huggins · 22nd Oct 2020, 4:40 pm

Is that the correct HMV model number? I can't trace it. There should be little or no hum on this. Was there hum before the HMV speaker cabinets were changed over to the Bowers and Wilkins units?

112stereo · 23rd Oct 2020, 9:40 am

Yes. They were purchased within the first year.
I do remember when it was first purchased i noticed the hum but dad couldn't hear it so its been there since new. This is the correct model i understsnd its a spin off from the 2354 model with the thorn series 75 chassis.
It has just one smoothing cap c150 5000uf which i thought strange as most of the amps i work on have two with a resistor between them. Could i not do the same, add another smoothing cap ?

Edward Huggins · 23rd Oct 2020, 1:00 pm

Does this mean that the original (Goodmans?) speakers fitted within the Stereogram have been replaced with B&W drive units? If so, I just don't get this as they are unlikely to even fit?

audiomagpie · 23rd Oct 2020, 2:53 pm

The otherwise identical Marconi versions gave you the choice of an all in one radiogram or the same units as separates (other than make of deck - BSR in the 'gram or Garrard as a separate) so maybe that's where the confusion lies?

Edward Huggins · 23rd Oct 2020, 3:21 pm

I'm still confused. Does this mean the OP has the "Seperates" version and ditched the 2 original speakers for two B&Ws? If so, and as I have tried to ascertain earlier, they will certainly accentuate any residual hum on this system!

paulsherwin · 23rd Oct 2020, 3:43 pm

Presumeably the OP has the separates system. There should not be a clearly audible hum so there is a fault somewhere, possibly present when the amp left the factory.

B&W were and still are a very respectable hifi speaker manufacturer. There should be a model number on the back somewhere.

https://en.wikipedia.org/wiki/Bowers_%26_Wilkins

I use a pair of B&W DM560s in my main stereo system.

G6ONEDave · 23rd Oct 2020, 4:19 pm

I have the HMV 2451 listed as the Thorn 74 series. Also service info was released on the ERT service sheets 1925 and 1926. Just a thought but could the hum be a result of the B&W loudspeakers having a lower frequency response than the HMV originals.
Dave

paulsherwin · 23rd Oct 2020, 6:05 pm

It shouldn't be producing obvious hum, though it's true that the speakers are probably overspecified for the rest of the system. I would suspect a power supply problem, possibly a leaky rectifier diode.

112stereo · 23rd Oct 2020, 8:45 pm

After your thoughts on the speakers incidentally they are B&W DM12 I tried another set of speakers and the hum is still there! After substituting the main smoothing I had wondered about the rectifier, but would it have lasted so long ? I could try replacing the rectifier although I would need to purchase some diodes as I don't have anything suitable. I am thinking that it was faulty from new, but where can I go from here ?

AC/HL · 23rd Oct 2020, 11:12 pm

Those speakers will have a much better LF response than the originals, making any residual hum more noticeable.

simpsons · 24th Oct 2020, 9:12 am

Not sure about a design flaw. Thorn engineers were required to design to a price point and target market.

The radiogram in question is spec'd at 3 watts music power per channel. That is record player design.

So, the similar market HMV 2328 has a simple 4700 uF smoothing whilst the up market 23201 has active smoothing using 3 transistors.

The use of a better loudspeaker will highlight just how the engineers squeezed a quart out of the pint pot.

So, to your point changing the bridge rectifier.

Looking at "rectifier trouble shooting", an open circuit diode will mean 50Hz hum will superimpose itself on the HT. If the rectifier is encapsulated then unless you have a scope to see what is going on, all you can do is change it and see. Should the 4 diodes be accessible then using a test meter to check them is an easy matter and my best advice, if necessary,is to search the web for further details.

Chris

paulsherwin · 24th Oct 2020, 9:46 am

The DM12s are bookshelf speakers and don't have a very extended bass response. They are very good speakers though, and vastly better than the rest of the system. If you google you will find lots of info about them.

Quite why your dad spent all that money on them for this setup is a mystery.

Radio Wrangler · 24th Oct 2020, 10:10 am

If the bridge rectifier and the smoothing capacitors check out, then you are at a fork on the road.

In one direction you fix the hum in the same way it was fixed in the original system... fit some substantially worse speakers so you can't hear it.

In the other direction you spend time modifying the radiogram chassis to reduce the hum. It's likely that as you reduce one way it gets in, you'll discover others so a fair amount of effort will be needed.

There is a third way: give up on the radiogram chassis. There are plenty of decent little hifi amplifiers out there offering low hum done the proper way, and they'll be much more capable of driving your B&Ws to what they can do.

Do you want to preserve the radiogram as an antique, or do you just want to listen to some records?

So many radiograms have been chopped up and burned that seeing one with a preserved cabinet housing a 1980s tuner-amp isn't too upsetting. Different people have different views, but it's your set so it's your choice.

Bowers and Wilkins, B&W have always made decent speakers. They are considered quite classy nowadays... just have a look at the prices of their latest offerings!

David

frsimen · 24th Oct 2020, 10:44 am

Looking at the circuit of the Series 75 chassis, does the hum level change by adjusting the bass control?

If it does, have a look at C155 (300uF). If the bass control makes no difference, you could try increasing the value of C134 and C135 (both 0.22uF) which smooth the supply to the bases of VT24 and VT25.

Paula

simpsons · 24th Oct 2020, 11:58 am

From G6ONEDave the radiogram is a Ferguson Series 74. The audio stage schematic can be found by searching for just that - under images, it's all there listed by "bytes, bolts and leaves.

I can't attach it, for some reason, but at least it can be seen for what it is and I'm not sure what more I can add to the advice given.

Chris

112stereo · 24th Oct 2020, 12:54 pm

Thank you to everyone's help. Its given me plenty to try out. The speakers are not a problem i have two other makes which i have tried and they make no difference to the hum but sound quite different i will fit the ones that sound the best to me. I will tackle the psu next week and hopefully reduce the hum to improve things.
Have uploaded a picture of the unit, hopefully you can all see it and maybe you will get why i want to keep it and get it working properly.
Will let you know how i get on next week.

music-centre · 24th Oct 2020, 2:14 pm

Personally I dont think it is the better speakers showing up deficiencies in it, I had the Marconiphone badged version that I used for around 10 years in our dining room with a reasonable pair of large bookshelf speakers and never noticed any hum.
It was a reasonable quality unit with a magnetic cartridge fitted, sounded good.
Steve.

https://www.vintage-radio.net/forum/...d.php?t=155136

Edward Huggins · 24th Oct 2020, 2:25 pm

It was good to see the photo at last in Post #17. It would have saved much interrogation of the OP if we had had this earlier. It may be worth checking the earth bonding of the record deck-plate on the tag card underneath. It will sound different on other speakers, but the use of those B&Ws is a mystery to me as they are not a natural partner for this.

paulsherwin · 24th Oct 2020, 2:43 pm

It's a bit of a sheep in wolf's clothing, but it does look nice and has sentimental value. Do you have an oscilloscope? If so, scope the DC rails and post the images here. You can tell a lot about hum from a scope trace.

I still think it left the factory with a manufacturing fault which quality control didn't pick up. It wouldn't be the first time that had happened.

22nd Oct 2020, 4:40 pm	#2
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,326	Re: HMV 2451 stereo xystem Is that the correct HMV model number? I can't trace it. There should be little or no hum on this. Was there hum before the HMV speaker cabinets were changed over to the Bowers and Wilkins units? __________________ Edward.

23rd Oct 2020, 9:40 am	#3
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo xystem Yes. They were purchased within the first year. I do remember when it was first purchased i noticed the hum but dad couldn't hear it so its been there since new. This is the correct model i understsnd its a spin off from the 2354 model with the thorn series 75 chassis. It has just one smoothing cap c150 5000uf which i thought strange as most of the amps i work on have two with a resistor between them. Could i not do the same, add another smoothing cap ?

23rd Oct 2020, 1:00 pm	#4
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,326	Re: HMV 2451 stereo xystem Does this mean that the original (Goodmans?) speakers fitted within the Stereogram have been replaced with B&W drive units? If so, I just don't get this as they are unlikely to even fit? __________________ Edward.

23rd Oct 2020, 2:53 pm	#5
audiomagpie Heptode Join Date: Apr 2003 Location: Rugby, Warwickshire, UK. Posts: 822	Re: HMV 2451 stereo system. The otherwise identical Marconi versions gave you the choice of an all in one radiogram or the same units as separates (other than make of deck - BSR in the 'gram or Garrard as a separate) so maybe that's where the confusion lies? Attached Thumbnails __________________ Greg BVWS committee chairman

23rd Oct 2020, 3:21 pm	#6
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,326	Re: HMV 2451 stereo system. I'm still confused. Does this mean the OP has the "Seperates" version and ditched the 2 original speakers for two B&Ws? If so, and as I have tried to ascertain earlier, they will certainly accentuate any residual hum on this system! __________________ Edward.

22nd Oct 2020, 3:57 pm	#1
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	HMV 2451 stereo system. My dad purchased this in 1971 new and has served him well. After his death i inherited it to prdvent it going to landfill. Its been in my office for a little over 4 years znd i thought it was time to use it. The deck had seized so this was my first job and it was a bit of a job to get it all clean again, but eventually I managed it. Turning to the rest it all works and sounds very good in my opinion my dad had upgraded the speakers to B&W ..........never heard of them but it does sound good. The issue i have now is a mains hum which is at a constant level so with the volume to minimum it is quite noticeable. Substituted the main caps with equivalents but made no difference. Talking to a friend who used to repair appliances for aliving said most of the amps made by the thorn group did hum so it appears to be a design flaw. Is there any way i can reduce or hopefully remove the hum to a point where its barely audible.? Any thoughts please.....?add another cap maybe?

23rd Oct 2020, 3:43 pm	#7
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: HMV 2451 stereo system. Presumeably the OP has the separates system. There should not be a clearly audible hum so there is a fault somewhere, possibly present when the amp left the factory. B&W were and still are a very respectable hifi speaker manufacturer. There should be a model number on the back somewhere. https://en.wikipedia.org/wiki/Bowers_%26_Wilkins I use a pair of B&W DM560s in my main stereo system.

23rd Oct 2020, 4:19 pm	#8
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,700	Re: HMV 2451 stereo system. I have the HMV 2451 listed as the Thorn 74 series. Also service info was released on the ERT service sheets 1925 and 1926. Just a thought but could the hum be a result of the B&W loudspeakers having a lower frequency response than the HMV originals. Dave

23rd Oct 2020, 6:05 pm	#9
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: HMV 2451 stereo system. It shouldn't be producing obvious hum, though it's true that the speakers are probably overspecified for the rest of the system. I would suspect a power supply problem, possibly a leaky rectifier diode.

23rd Oct 2020, 8:45 pm	#10
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. After your thoughts on the speakers incidentally they are B&W DM12 I tried another set of speakers and the hum is still there! After substituting the main smoothing I had wondered about the rectifier, but would it have lasted so long ? I could try replacing the rectifier although I would need to purchase some diodes as I don't have anything suitable. I am thinking that it was faulty from new, but where can I go from here ?

23rd Oct 2020, 11:12 pm	#11
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: HMV 2451 stereo system. Those speakers will have a much better LF response than the originals, making any residual hum more noticeable.

24th Oct 2020, 9:12 am	#12
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: HMV 2451 stereo system. Not sure about a design flaw. Thorn engineers were required to design to a price point and target market. The radiogram in question is spec'd at 3 watts music power per channel. That is record player design. So, the similar market HMV 2328 has a simple 4700 uF smoothing whilst the up market 23201 has active smoothing using 3 transistors. The use of a better loudspeaker will highlight just how the engineers squeezed a quart out of the pint pot. So, to your point changing the bridge rectifier. Looking at "rectifier trouble shooting", an open circuit diode will mean 50Hz hum will superimpose itself on the HT. If the rectifier is encapsulated then unless you have a scope to see what is going on, all you can do is change it and see. Should the 4 diodes be accessible then using a test meter to check them is an easy matter and my best advice, if necessary,is to search the web for further details. Chris

24th Oct 2020, 9:46 am	#13
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: HMV 2451 stereo system. The DM12s are bookshelf speakers and don't have a very extended bass response. They are very good speakers though, and vastly better than the rest of the system. If you google you will find lots of info about them. Quite why your dad spent all that money on them for this setup is a mystery.

24th Oct 2020, 10:10 am	#14
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,858	Re: HMV 2451 stereo system. If the bridge rectifier and the smoothing capacitors check out, then you are at a fork on the road. In one direction you fix the hum in the same way it was fixed in the original system... fit some substantially worse speakers so you can't hear it. In the other direction you spend time modifying the radiogram chassis to reduce the hum. It's likely that as you reduce one way it gets in, you'll discover others so a fair amount of effort will be needed. There is a third way: give up on the radiogram chassis. There are plenty of decent little hifi amplifiers out there offering low hum done the proper way, and they'll be much more capable of driving your B&Ws to what they can do. Do you want to preserve the radiogram as an antique, or do you just want to listen to some records? So many radiograms have been chopped up and burned that seeing one with a preserved cabinet housing a 1980s tuner-amp isn't too upsetting. Different people have different views, but it's your set so it's your choice. Bowers and Wilkins, B&W have always made decent speakers. They are considered quite classy nowadays... just have a look at the prices of their latest offerings! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

24th Oct 2020, 10:44 am	#15
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 772	Re: HMV 2451 stereo system. Looking at the circuit of the Series 75 chassis, does the hum level change by adjusting the bass control? If it does, have a look at C155 (300uF). If the bass control makes no difference, you could try increasing the value of C134 and C135 (both 0.22uF) which smooth the supply to the bases of VT24 and VT25. Paula

24th Oct 2020, 11:58 am	#16
simpsons Octode Join Date: Feb 2007 Location: Harrow, London, UK. Posts: 1,493	Re: HMV 2451 stereo system. From G6ONEDave the radiogram is a Ferguson Series 74. The audio stage schematic can be found by searching for just that - under images, it's all there listed by "bytes, bolts and leaves. I can't attach it, for some reason, but at least it can be seen for what it is and I'm not sure what more I can add to the advice given. Chris

24th Oct 2020, 12:54 pm	#17
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: HMV 2451 stereo system. Thank you to everyone's help. Its given me plenty to try out. The speakers are not a problem i have two other makes which i have tried and they make no difference to the hum but sound quite different i will fit the ones that sound the best to me. I will tackle the psu next week and hopefully reduce the hum to improve things. Have uploaded a picture of the unit, hopefully you can all see it and maybe you will get why i want to keep it and get it working properly. Will let you know how i get on next week. Attached Thumbnails

24th Oct 2020, 2:14 pm	#18
music-centre Heptode Join Date: Sep 2007 Location: Brentwood, Essex, UK. Posts: 687	Re: HMV 2451 stereo system. Personally I dont think it is the better speakers showing up deficiencies in it, I had the Marconiphone badged version that I used for around 10 years in our dining room with a reasonable pair of large bookshelf speakers and never noticed any hum. It was a reasonable quality unit with a magnetic cartridge fitted, sounded good. Steve. https://www.vintage-radio.net/forum/...d.php?t=155136

24th Oct 2020, 2:25 pm	#19
Edward Huggins Dekatron Join Date: Sep 2015 Location: Southwold, Suffolk, UK. Posts: 8,326	Re: HMV 2451 stereo system. It was good to see the photo at last in Post #17. It would have saved much interrogation of the OP if we had had this earlier. It may be worth checking the earth bonding of the record deck-plate on the tag card underneath. It will sound different on other speakers, but the use of those B&Ws is a mystery to me as they are not a natural partner for this. __________________ Edward.

24th Oct 2020, 2:43 pm	#20
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,934	Re: HMV 2451 stereo system. It's a bit of a sheep in wolf's clothing, but it does look nice and has sentimental value. Do you have an oscilloscope? If so, scope the DC rails and post the images here. You can tell a lot about hum from a scope trace. I still think it left the factory with a manufacturing fault which quality control didn't pick up. It wouldn't be the first time that had happened.