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Old 12th Nov 2021, 5:53 pm   #1
murphyv310
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Default HMV 2807 Restoration

Hi.
I recently acquired an HMV 2807. Initially it was intended to donate the CRT to a Marconiphone console variant with Radio. It was found that one of the heater pins was snapped off so that plan was scrapped. In fact after some careful filing and a quick dab with a very hot soldering iron I got a connection, a covering with epoxy and I was able to test the CRT. Like the vast majority of these 10 inch Emiscope tubes emission is zero.
So what to do with the set? We all know the reputation of these sets but I do like a challenge. So I've now decided to have a go especially as the cabinet is pretty good.
There is definite signs of damp storage but it looks more recent than old. The chassis was covered in a layer of rust but not deeply and the underside had this white powered deposit that looks like corrosion, the main smoother has the obligatory hole and corroded connections. So I have set about cleaning off the rust and have started work on the set on replacing the faulty caps which were very badly leaky and off value. The electrolytic caps are also looking very tired.
I've done a couple of videos here:

https://youtu.be/ex0igqWgZ0Y

https://youtu.be/B6VbEgLgI64
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Old 12th Nov 2021, 6:30 pm   #2
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Default Re: HMV 2807 Restoration

Hi Trevor this set will be a great contender for the New Years Countdown
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Old 12th Nov 2021, 9:22 pm   #3
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Default Re: HMV 2807 Restoration

Hi Trevor,
with the much better components available there is no reason why an HMV 1807 or 2807 shouldn't become a reliable receiver.
My Marconi VRC74 has been in my possession for almost thirty years and is still entirely usable. Or, at least was when it was last used.
A real nuisance component was the B36 double triode, replace it with the American or Brimar 12SN7GT and that's the end of timebase frequency drift.
And for all the set's bad reputation some components are very reliable, the main dropper resistor and the line output transformer for example.
The U31 efficiency diode can be replaced with a PY33.
It's the 10" CRT that is the real problem with these sets. Wish I had bought some of those surplus Cossor 108K CRTs there were going cheap way back in the sixties.

DFWB.
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Old 12th Nov 2021, 10:12 pm   #4
murphyv310
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Default Re: HMV 2807 Restoration

Thanks David.
I'll get a 12SN7, I had one but can I put my hands on it.... No chance!
The set has a low component count so its a shame it was so unreliable. The slider controls in this one are duff but there should be sufficient room for rotary controls.
The CRT is likely to be a difficult one, a member here has one but its unknown if its any good or not. My plan is to pick it up in a week or two.
In the meantime I'll test the set using a 7BP7 radar CRT fed from a separate heater tx and an HT supply for the A1 off the boost line.
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Old 12th Nov 2021, 10:35 pm   #5
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Default Re: HMV 2807 Restoration

Hi Trevor,
some 12" models were supplied with a Mullard MW31-16 in lieu of the EMI tube.
The extra volts for the CRT first anode was derived from the flyback pulse on the anode of the frame oscillator valve. The extra components needed are W4 rectifier and C58 (0.015mfd)
According to the service notes, "all models using the Mullard tube may have an Emiscope 3/18 tube fitted when replacement becomes necessary".

DFWB.

Last edited by FERNSEH; 12th Nov 2021 at 10:47 pm.
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Old 13th Nov 2021, 12:24 am   #6
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Default Re: HMV 2807 Restoration

Do you have a JW Big Bertha tube tickler you could try on the emiscope? I can't remember if I dreamed it or not but did heatercathodeshort manage to get emission back on one?
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Old 13th Nov 2021, 10:24 am   #7
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Default Re: HMV 2807 Restoration

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Originally Posted by murphyv310 View Post
Thanks David.

I'll get a 12SN7, I had one but can I put my hands on it.... No chance!
I have a 12SN7GT pull. If it's likely to be any use to you I can test it. Postage only, so £3.00. I have a lot of pulled TV valves (condition unknown) if you're interested. All FOC. I could send quite a few for £3.00.
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Old 13th Nov 2021, 10:59 am   #8
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Quote:
Originally Posted by murphyv310 View Post
Thanks David.

I'll get a 12SN7, I had one but can I put my hands on it.... No chance!
I have a 12SN7GT pull. If it's likely to be any use to you I can test it. Postage only, so £3.00. I have a lot of pulled TV valves (condition unknown) if you're interested. All FOC. I could send quite a few for £3.00.
Thank you Graham.
PM on its way
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Old 13th Nov 2021, 4:10 pm   #9
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Default Re: HMV 2807 Restoration

Quote:
Originally Posted by McMurdo View Post
Do you have a JW Big Bertha tube tickler you could try on the emiscope? I can't remember if I dreamed it or not but did heatercathodeshort manage to get emission back on one?
Yes Kevin, you can sometimes be successful with the 3/16 but not very often. Bertha relies on some emission to start with and this can sometimes be obtained with the 15W pygmy bulb method and finished off with Bertha.
She dealt with a 9" CRM93 yesterday at the museum. It was a difficult one but managed to get a very good picture with hardly any reading on the meter.

She also managed to test a massive Mullard 21" MW 53-20 and luckily it appears to be OK. It's the one pictured in the Museum Friends Newsletter, the HMV 1848.

She will need a rest for a day or two. She is 65 years old. John.
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Old 13th Nov 2021, 4:22 pm   #10
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Default Re: HMV 2807 Restoration

Make her a cuppa and a biscuit John and she'll be fine.
I'd like to know what makes her tick.
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Old 13th Nov 2021, 4:56 pm   #11
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Default Re: HMV 2807 Restoration

The 12SN7GT can be replaced by the 12AU7. The characteristics of the two valves are almost identical. Also, it's an opportunity to use an adaptor plate.
The two filaments in the 12AU7 can be connected in series and a 42ohm resistor fitted across pins 4 and 5 to bypass 0.15amps.

DFWB.
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Old 13th Nov 2021, 6:53 pm   #12
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Default Re: HMV 2807 Restoration

84 ohms?
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Old 13th Nov 2021, 7:08 pm   #13
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Default Re: HMV 2807 Restoration

That's right, I calculated for 6.3volts. Resistor need only be a two watt component but 5 watts would be better just in case the valve goes OC heater.

DFWB.
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Old 15th Nov 2021, 10:40 pm   #14
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Default Re: HMV 2807 Restoration

Tomorrow I'll replace the caps above chassis and do those chocolate hunts caps in the tuner/IF deck, check resistors and then give it some power. Hoping that I may get some results before the end of the week...... We will see.
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Old 16th Nov 2021, 1:19 pm   #15
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Default Re: HMV 2807 Restoration

Quote:
Originally Posted by FERNSEH View Post
The 12SN7GT can be replaced by the 12AU7. The characteristics of the two valves are almost identical. Also, it's an opportunity to use an adaptor plate.
The two filaments in the 12AU7 can be connected in series and a 42ohm resistor fitted across pins 4 and 5 to bypass 0.15amps.

DFWB.
I'm puzzled. The ECC82 has the option of 6.3v at .3 amp or 12.6 at .15 amp. Why wire it for .15amp and then fit a resistor to correct the current difference?
I must be going through a funny moment. Please help. John.
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Old 16th Nov 2021, 7:36 pm   #16
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Default Re: HMV 2807 Restoration

Hi.
Further progress today after being busy with other things over the last couple of days.
Today I recapped the Tuner and IF deck, I used the last of my 1nf Iskra ceramics, very good capacitors indeed and had to use Mullard mustards too. The top is now done.
I'm awaiting a 12SN7 from Graham and once fitted we will slowly power the set up. If the line and frame run I'll dig out a 7BP7 to see what we have on screen.
I've done a further video here: https://youtu.be/FWenEtliWuk

I will upload some stills after powering the set up in a day or two.
So far so good.
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Old 16th Nov 2021, 7:39 pm   #17
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Default Re: HMV 2807 Restoration

Quote:
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Quote:
Originally Posted by FERNSEH View Post
The 12SN7GT can be replaced by the 12AU7. The characteristics of the two valves are almost identical. Also, it's an opportunity to use an adaptor plate.
The two filaments in the 12AU7 can be connected in series and a 42ohm resistor fitted across pins 4 and 5 to bypass 0.15amps.

DFWB.
I'm puzzled. The ECC82 has the option of 6.3v at .3 amp or 12.6 at .15 amp. Why wire it for .15amp and then fit a resistor to correct the current difference?
I must be going through a funny moment. Please help. John.
Hi John.
I don't think you are going through a funny moment, I also wondered why David would wire it that way.
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Old 16th Nov 2021, 9:04 pm   #18
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Default Re: HMV 2807 Restoration

Hi Trevor,
why do it the easy way when a more complicated solution can be employed?
There's a 1950s Baird TV receiver which employs a 12AT7 double-triode as the Band 1 frequency changer. Like my circuit suggestion the two heaters in the valve are in series but this set doesn't use a resistor to bypass 0.15amp. Instead, a capacitor is used.
It just means that if the set is connected to DC mains the valves' heater will be over-run.

DFWB.
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Old 16th Nov 2021, 10:43 pm   #19
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Default Re: HMV 2807 Restoration

Nice one David.
Unlike you I prefer the simple approach, I'm a minimalist and an extra resistor or capacitor is an extra component to fail and with respect of the resistor a source of heat.
Each to their own though.
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Old 17th Nov 2021, 6:55 pm   #20
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Default Re: HMV 2807 Restoration

Hi.
Today the set got some power less the CRT. I gradually invereased the voltage from the variac and was rewarded with both line and frame running and EHT.
Next was to dig out an 7BP7 CRT out of the loft. This was cobbled in with 6.3v from the lab pack as it has a 600ma heater and HT for the G2 from the set. On running up there was nothing even with EHT present. Discovered there was no G1 volts caused by the line hind slider being open circuit. This was bridged temporarily with a 47k. We now have a raster. Later I found two chokes open in the video amp.
Still work to do but getting there.

https://youtu.be/ycLd1UqIJLE
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