Home Brew version of HMV904

[ENGLISH VICTOR]

I thought some forum members might be interested in my attempt at replicating an HMV 904 using parts could be made , modified or scrounged. The circuit is almost a direct copy of the original HMV 904. The cabinet has yet to be made. Mazda and Mullard near equivalent valves have been used instead of the original Marconi valve types because of availability problems, the fequency changer is however an X41 as per original. but reflect the types available pre-war quite well.
Pictures attached give some idea of the progress, the testcard is photographed off screen and does not do justice to the real thing. reso is about 2.5Mc/s (MHz.)
More pictures and details if anyone is interested.
Victor.

[Ian B]

Now that's what I call clever! If you ever go into production, I'll buy one!
Seriously though, it's very impressive.
What crt did you use, is it a pre-war type?

In awe,

Ian Blackbourn

[peter_scott]

Very impressive! 'Certainly look forward to seeing the finished article.

Peter

[chipp1968]

Likewise . Fab !
Can you do the cabinet yourself ? The marconi version may be easier to reproduce

[Hunts smoothing bomb]

Absolutely superb!!

A fantastic build and excellent mechanical skills, a very sound job indeed. I am following this with great interest!!
A very deserved Well Done!
Cheers
Lee

[mark pirate]

If the cabinet is finished to the same standard as the chassis, this will
be a fantastic feat of engineering, well done!

mark

[ENGLISH VICTOR]

Thanks for all the ecourageing responses, I will post some better pictures in the near future.
Answering Ian's question: I am using a number of CRTs the one in the pic is a P7 radar tube with a 7" screen, I used it to make photographing easier with my new digital camera( don't know how to use it properly yet). The tube normaly used is a 5AXP4 as recomended by Hugo Holden, it is not quite as good as a 5FP7 but the afterglow with P7 screens is of course far too slow, I could use a blue Wratten filter but I doubt it would look very nice. I have also used a CRM121 but the pictures were OH SO DIM.with 2.5KV. The 5AXP4 is the one that I will likely have to go with unless I can find a 5FP4.
Victor.

[ENGLISH VICTOR]

A few further comments re my 904 copy. The R.F. amp and all the I.F. valves are Mazda SP61s, apart from the heaters they are identical to the 1937 SP41 and use a Mazda Octal base. The sync pulse diode is an EB34 with the two diodes connected in parallel. The EHT rec is an HVR1 whilst the main H.T. rec is U52 or it's equivalent. I have used a 6V6 for audio output and an EBC33 for audio detectors and audio amp, both available pre war. The anode bend video detector is also an SP61. the sync amp line and frame oscillators are KTZ63 direct equivalents, line output stage KT63 and the vertical output stage employs an EL33 because it worked with a vertical output transformer without the need for integrated negative feedback to correct for linearity problems, in fact the circuit only differs in the value of the cathode biasing resistor and the omission of the coupling electro to the vert coils. I intend to revert to the original circuit when I either make or obtain a deflection coil assy with high resistance vert coils. Other changes are the use of a small relay to switch the E.H.T. transformer primary rather than the original arrangement. I erred in my original post by quoting the vision response as 2.5Mc/s it is in fact 2.0 Mc/s (I like Mc/s for old sets).
David B has sent me some excellent details regarding the HMV cabinet even so I must admit the Marconi cabinet does look easier to duplicate. I do prefer the H.M.V. and will possibly regret the fact, however I will try the HMV type first with the help of a friend who is familiar with cabinet making.
I am very impressed with the performance of the radio, we have no L.W. radio stations here in Australia but I have received a Russian station very weakly but no sign of signals on 198Kc from the BBC yet.
Victor.

[yagosaga]

Hi ENGLISH VICTOR,

this is a stunning project, thanks for sharing it with us!

Did you use a focus coil for the CRT and which one did you employ?

What E.H.T. transformer do you use? The original one?

Can you provide photos in larger size?

Kind regards,
Eckhard

[Panrock]

Great work!

Out of interest, HERE is another project on the same lines from Australia.

[ENGLISH VICTOR]

First to comment on Panrock's post. Hugo Holden and his enthuseasm was in part the driving force behind my project, I have seen his work and it is fantastic to say the least. My having owned a 904 in the distant past provided the remaining drive for me to continue with the project.
Eckhardt, the focus coil is home made as are all other wound components except for the audio output transformer and the radio I.F transformers. The line output transformer was a small 110V/ 25V mains transformer that just happened to perform quite well but I have since rewound it to reduce the primary self capacitance. The EHT configuration is temporary in that I have employed a transformer from an earlier project that I wound for a lower voltage and have voltage doubled to provide the correct voltage, this however does not alter the set's performance, when I obtain some thin enough wire I will simply rewind the transformer for 1850V, it is not a priority at this stage.
I will post some higher reso pics in the near future as well as my early digital standards converter, does not perform as well as my Aurora though despite it's dozens of i/cs.
Victor.
P.S. my previous post refered to integrated nfb in the frame output stage, should have been differentiated nfb, anyone notice?