Alternative EHT system for HMV 902.

[Tractionist]

I do miss Gerry!

[FERNSEH]

The underside of the sound receiver chassis. I only have a limited stock of restuffed EMI made 0.047 and 0.05mfd capacitors which should be used only in those parts of the set which are visible. So for the time being should those polyester capacitors remain or be replaced with common TCC type 343 capacitors? The type of component that would have fitted during the 1950s. Assuming of course a set like this one was still use by then. Next to the Simpson testmeter are the original 1mfd capacitors which were removed from the set twenty years ago.

DFWB.

[peter_scott]

Well I restuffed the caps in my sound receiver but I'm a hypocrite because I didn't recreate the dog bones in my focus chain but left the originals in there beside the modern parts.

Peter

[beery]

Hi all,
this is how I tackled the bleeder chain resistors on my HMV 905.
These resistors are available from one or two sellers on ebay.
I think they were 2W and of course EHT rated. I didn't want to use original resistors in this part of the circuit for safety reasons and of course in David's case here, there is the added incentive to fit something more reliable due to the difficulty of access to the EHT PSU in his set.

Pre-war style resistors are hard to find in good condition and to make it worse EMI even had their own with cast ends.

Cheers
Andy

[davyrocket2]

Hello David the 343s as availability permits .


Andy yours look very neat nice work

[FERNSEH]

"Hello David the 343s as availability permits."
Fitted two EMI 38211DY 0.047mfd capacitors and two TCC 343s.
In addition the two hi-viz 1mfd capacitors have been inserted into the cases of the original EMI ones.
The 38211DY capacitors were chosen because capacitors of the same type exist in the chassis.

DFWB.

[peter_scott]

That looks really beautiful!

Peter

[FERNSEH]

Hi Peter, thank you for the compliment.
The sound receiver is now in the cabinet and the power unit will be next. But before that is done, here's a suggestion that might make future servicing safer. The Baird T5 has a switch to cut off the mains supply to the EHT transformer. Would it not be good idea to do the same for the HMV 901, and possibly all the other first generation EMI sets such as the 900 and 902?

DFWB.

[peter_scott]

Originally Posted by FERNSEH

Would it not be good idea to do the same for the HMV 901, and possibly all the other first generation EMI sets such as the 900 and 902?

DFWB.

Definitely NOT! They already have such a switch that is designed to short the EHT if the rear panel is removed. A horrible safety system that should be disabled or removed entirely.

Peter

p.s. I think this is just a Till wind-up He knows full well about the EMI switch.

[FERNSEH]

Hi Peter, no modifications done to the power unit.
Didn't fit an EHT transformer switch so the "safety" device remains in it's place at the rear of the chassis. Of course what we do is simply bend the lever down so it doesn't spring out and come into action when the back is removed.
The attachment shows the sound receiver and the power unit are now reinstalled in the cabinet.

DFWB.

[peter_scott]

Excellent! I guess if anyone was keen to disable the EHT they could always pull the heat coil out.

I don't really like non-standard additions but if there was one that I might have been tempted by it would be to pull the brightness to zero for switch-on and switch-off. That said, it's just as easy to turn the control down and my pot seems to have survived me doing that hundreds of times and it's probably easier than stretching around the back for a switch.

Peter

p.s. More hypocrisy: I did fit a standards switch to my set that left the factory without one.

[line sync]

David , just admiring your restuffed wax capacitors , very neat job.
How do you seal up the ends ?
Do you use some of the old wax ?
I use a hot melt glue gun which works ok but not the right colour.

Robin

[FERNSEH]

Hi Robin,
The ends of the capacitors were sealed with wax. I was tidying up the garage and found this large block of wax. My Mother used to use the stuff for something to do with dressmaking. Goodness knows what?
Anyway, the wax looks absolutely right for the restuffing the larger capacitors.
There are few Hi-Viz capacitors in the timebase unit. Those will have to come out and replaced with EMI parts. The restuffed capacitors are post-war of the type that was fitted by the EMI service department at the time the sets were recommissioned in 1946/47.

DFWB.

[FERNSEH]

Peter Scott wrote:
"p.s. More hypocrisy: I did fit a standards switch to my set that left the factory without one"
Hi Peter,
I'm considering fitting the standards switch to my Marconi 702. By doing so it'll be possible to demonstrate the awfulness of the 240 line 25 f.p.s. system and superiority of the M-EMI 405 line 50hz interlaced system.

DFWB.

[peter_scott]

My switch location didn't have a removable blanking plate and I couldn't find a switch that would fit in the restricted space anyway so I just mounted it on a rough and ready extension to the bracket where the push about control was located.

Peter

[FERNSEH]

The attachment shows there is only two Hi-vis capacitors in the timebase unit. Also a Philips polyester. These will be replaced today.

DFWB.

[Argus25]

Originally Posted by beery

Hi all,
this is how I tackled the bleeder chain resistors on my HMV 905.
These resistors are available from one or two sellers on ebay.
I think they were 2W and of course EHT rated. I didn't want to use original resistors in this part of the circuit for safety reasons and of course in David's case here, there is the added incentive to fit something more reliable due to the difficulty of access to the EHT PSU in his set.

Pre-war style resistors are hard to find in good condition and to make it worse EMI even had their own with cast ends.

Cheers
Andy

Andy,

Lovely job on the tag panel with the high voltage capable resistors.

I tend to use Philips focus chain resistors, or a type of high voltage resistor that I buy in Japan at the Akihabara markets. Photo attached with some in the upper right hand corner of the image, brown in colour, with other goodies. Also a photo of a typical sales stall there. The Japanese vendors have all sorts of high quality components and they are always kind and helpful.

If you look closely in the upper right hand corner of the image, you will see all sorts of electrolytic can capacitors with clamps.

Just found a resistor photo that shows the relative resistor sizes..

[julie_m]

The pale blue one second from left looks like a Mullard / BC / Vishay / whatever they are called now VR37. Excellent high voltage resistors, although a bit "modern" looking.

[Argus25]

Originally Posted by julie_m

The pale blue one second from left looks like a Mullard / BC / Vishay / whatever they are called now VR37. Excellent high voltage resistors, although a bit "modern" looking.

Yes I agree about the modern look for those Philips/VR37 types ones. One advantage of the Brown Japanese ones is that brown colored components always seem to look at home in vintage equipment, maybe its because they have that overheated look !

[peter_scott]

Below is the original type of dog bones used in my focus chain. I think recreating these with modern components is rather difficult.

Peter