What exactly have I just bought?! A 1930s Radiogram

[EF80TVVALVE]

Hi PJL, the condenser is actually very free but it doesn't help, the part of the large drive wheel that is driven by the shaft is really worn down, I've tried quite a lot to strip and rebuild it trying to make it work but it just won't grip enough.

Moving back to the radio for now, (I'll update on the speaker supply once its sorted) I've noticed some capacitors missing originally marked C16 and C17, there is also a valve type MH4 that is not only disconnected from the circuit but also has its own heater supply tags that have no wires to the PSU. I'm not exactly sure why so yet but its interesting and thought I'd share some photos with you. It almost looks as if it was either there as a stand-by or maybe it was a pre amplifier for the MH41 which is used for the output to the amplifier but proved to have too much gain and so was removed. Presumably the absence of C16 & 17 have something to do with this.

[Ed_Dinning]

Hi Gents, according to "Rectifier Engineering" by Westinghouse the oxidation is a complex process. The dis is then coated with a contact material and must be held under the correct pressure. A voltage forming process is also used.
False insulating washers and SMD schottky's may be the way to go

Ed

[EF80TVVALVE]

Hi Ed, thanks for the information, I presumed it would be a complicated process, just forming an oxide layer on the discs sounds suspiciously easy I will leave it in the set for looks. I'll have a look at what I can do with discreet components, there may just be enough space between the transformer and rectifier to hide some, they may be noticible from certain angles but otherwise very well hidden and not in plain sight.

[Herald1360]

That disconnected valve looks to have "AVC(?)" pencilled next to it. The one above in the picture, "TC", tone control perhaps?. Would this make any sense?

[rontech]

Originally Posted by Herald1360

That disconnected valve looks to have "AVC(?)" pencilled next to it. The one above in the picture, "TC", tone control perhaps?. Would this make any sense?

How about top cap?

[EF80TVVALVE]

I'm just having a look now, it looks to say either AVC or AYC. The valve above it in the photo is the output valve (MH41) that supplies the intervalve transformer on the amplifier chassis. Looking up the disconnected one (MH4) it also looks as though these were designed for output use and very similar to the MH41, so it must have a job to do on the LF side of the radio but what exactly I don't know. I wonder if it was supposed to be a push-pull arrangement that was altered.

Some more parts have arrived today and I can begin to re-stuff the capacitors on the radio chassis. I'm just working out the best way to remove the innards from the metal cased Mullard types. The bakelite Telsen ones will be easy enough as they have wax sealed bottoms but the Mullards are fully metal cased/soldered shut. I think any attempt to grind down/desolder and remove the soldered seams can easily cause cosmetic damage to them so I'm considering using a small cutting wheel on my dremel to remove a square section from the bottom, that way the original internals can be heated and removed and once refitted in the radio it will look just as it did before.

[Ernest1]

Fantastic! Well done for being brave enough to take it on! Great to see it.

[Techman]

It's been said that there's a fine line between professional and home construction back then and I'm beginning to think that this gram could well be someone's home build. It's possible that the radio was never finished and has never worked and the unit was just used as a record player. If I were working on that unit I would concentrate on the power supply and amplifier first and getting it playing 78s with that autochanger and leave the radio part to the very last, as it may turn out to be a complete start from scratch job - but of course I could be wrong...it's your call.

[EF80TVVALVE]

Just picking this thread back up - Techman, you could be right but there does seem to be a lot of use in the radio section so I'm currelty holding out hope. I do think that this radio was originally a push pull output design, there is also another hum adjustment potentiometer that is disconnected and it's only causing more confusion!

It seems probable to me that the radio could have been made before the power amplifier and had its own push-pull output, the larger power amplifier may have been added later with a demand for more power and this could have been when the alterations were made. It could be possible that the missing circuitry is part of the hum reduction circuit and was removed as it wasn't needed for the radio section. Strangely both units are of the same construction and of very similar age so its hard to work out exactly which it is.

I'm beginning to think more and more that it is home-built, but are we seeing the works of a small company commissioning these and altering them to suit the needs of their customers? To me that makes more sense as to why these modifications have been made. The other is if the home-builder changed direction of plans through the build, but this would surely have included removal of the second disconnected output valve and this valve itself has definitely done some hours indicating that the radio had some use on its own without the power amplifier.

[EF80TVVALVE]

Hi all,

Although this thread has been pretty quiet these past few weeks I can assure you that the progress of the radiogram has been anything but. Most evenings after work I've been slaving away at it, as you can imagine re-stuffing all of these capacitors is quite time consuming.

Finally the radio section has been re-capped and a few resistors that were o/c have been bridged, leaving the originals in place for historical reference. Ed Dinning has kindly sorted me a good U14 to replace the type that was wrongly fitted in the PSU, along with some replacement pull switches for the meters which are due to be fitted tonight.

I found a good quality 4 way terminal block to distribute the mains around the various power supplies of the radiogram and last night wired up the mains flex and the cables carrying mains to the power switch (which was removed and inside the cabinet upon purchase) and speaker PSU. Hopefully tonight I can power up the speaker PSU and check that everything is okay there and then sort the mains connections for the turntable and main PSU.

There is still a capacitor to restuff in the PSU, one of the two Ferranti metal casted-pitch filled paper types. These are quite difficult as with heating it is impossible to break the bonds so they require the bottom cutting away before heating and restuffing. The metal cased mullards are much easier as with expansion of the pitch upon heating they often break their own bonds and the lids can just be removed easily.

Hopefully over the next few days we will start to see some life from this thing after many years of slumber. As you can imagine it's becoming much more exciting in these final steps and I can't wait for the moment the auto-changer drops and plays its first record. I'll try and update with more photos tonight.

[Techman]

Great stuff!

There was another large floor standing radio of similar design and date in the 'radio' section of this forum a few weeks ago - you never, ever see one and then two come along at once. They say that things come in 3s, so I wonder what the next one to turn up will be - luckily the one mentioned didn't contain any power triodes, otherwise its future may have been limited. It didn't contain a record deck, but I think there was some discussion regarding a mystery switch that could have had something to do with the possible addition of a deck.

[EF80TVVALVE]

Hi Techman,

I may have seen the thread but I can't be certain so I'll take a look, it is strange how often these things do crawl out of the woodwork and it makes you think how many more are hiding far away from the hands of collectors!

Progress has been made this evening but unfortunately no photos as I just haven't had the time. The speaker section is now working and I've had it playing some music. It is quite an efficient speaker and just using the headphone output of my old phone gave just enough power to drive it at a low but listenable volume

It still needs some fettling in this section as I now seem to be getting a few volts of AC after the rectifier and smoothing makes little difference. It may be one of the diodes giving up as they've had a lot of heat from soldering/desoldering/resoldering during experiments but I have plenty more so I'll get this resolved before anything else.

Speaker quality seems to be very good and I'm sure when it's driven by the amplifier it will sound very nice indeed.

[sparkymike]

Probably too late in the day now, but when re-stuffing the metal cased caps, could they be held in a lathe and the ends just turned out enough to remove old cap material, leaving most of the casing intact?

Mike.

[Station X]

I've tried that in the past with aluminium can capacitors. The tool digs in and the case gets screwed up like a cork screw. Might be OK if the cans are steel. Might work with a drill.

I've found the best method is to use a pipe cutter and conceal the joint under the fixing clamp.

[Peter.N.]

Looks like a nice piece of engineering. I've never seen anything like it and I started in the trade in 1954. Not come across those valves either, it really is a nice piece of history.

Peter

[EF80TVVALVE]

Hi all,
So it's been a while since I last updated the thread but work has still been underway on the radiogram. This evening I finally applied power to the amplifier and radio units and encountered some faults. Firstly the amplifier goes into oscillation as we get close to full mains, but seems fine otherwise being ran at lower power so I'm sure it's nothing major and I'll start by checking over the circuitry and voltages.

The radio section seems less promising at the moment, I'm sure once I have the pick up connected it will play records okay but there just seems to be quite a lot of hum when switched to radio. I haven't tried to pick up any stations as of yet, I'll get the amplifier working nicely and then focus on the radio section but so far it looks as though things are going relatively well and obviously the PSU is working correctly which is a good start.

[EF80TVVALVE]

Hi All
I did more testing last night and it seems the oscillation is being caused by the grid resistors/intervalve transformer. The type of transformer used is a Ferranti AF5C (centre tapped secondary). I posted a rough and ready draw up of the circuit in a previous post. Anyway, whilst I was poking around measuring voltages I noticed that my DMM cured the oscillation when measuring at the outputs of the secondary to earth. I wonder if the transformer has a fault and is causing the oscillations.

I was scouring the Internet last night to find some data for the AF5 series transformer and found that the primary has a small capacitor fitted across the winding, presumably if this has became faulty it could cause problems so I'll have to investigate.

For now I just wondered if anyone had any data, mainly DC resistances of the AF5 transformers. From memory I have about 16k on the secondary measuring from each end to centre tap and approx 2.6k on the primary.

[Ed_Dinning]

Hi Bren, from the Ferranti book it looks as if the internal cap across the pri is about 700pF

Ed

[PJL]

You have checked all the transformers as I have found quite a few of the Ferranti ones open circuit. They are rough wound piles on a dedicated former so it is possible to rewind individual sections.

[EF80TVVALVE]

I've checked the Txs for any shorts or O/C and they are all fine in that respect. Swapping a few emails with Ed he suggested fitting 1 megs from grid to earth on each PP5/400 and lowering the resistance of the 100k stopper resistors. I've done that and powering back up it was free from oscillation and working well.

I've had a few 78s playing this evening which is quite a milestone but it is still a long way off yet. The autochanger still needs work and the radio section certainly needs more attention but it's really put a smile on my face hearing it play away after all of the hard work I've done on it. It's also given me the motivation to crack on and keep at it.

As many members can probably understand, spending most evenings working on something and taking an age to see any results can be difficult and it's been a real task to keep at it some days. I'm sure it will be a wonderful bit of kit once it's finished and I'm quite excited to get it there.