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Old 26th Sep 2006, 8:46 pm   #1
ChristianFletcher
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Default R107 Service Receiver

Please could anyone provide any information and a Schematic for a R107 services receiver. I have the circuit printed on the cover but it is very dirty and hard to read. Also is there any problems I should know about before I start with the restoration.

Thanks Christian.
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Old 26th Sep 2006, 9:20 pm   #2
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Arrow Re: R107 Service Reciever

R107: one of the few British Military jobs I haven't had through my hands (yet!) ~ but this might help:

http://bama.edebris.com/manuals/britmil/r107

Good luck!

Al / G8DLH
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Old 26th Sep 2006, 9:54 pm   #3
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Default Re: R107 Service Reciever

Thanks For the link it was spot on information.

Regards Christian Fletcher.
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Old 26th Sep 2006, 10:23 pm   #4
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Default Re: R107 Service Reciever

Yep, a couple of things

1, Book a good chiropractor

2, look out for the three pin triange connector on the front panel - with the set on one of the pins is at ht+ - very ouch!

3, Any form of instability - check the metallising on the EF39 valves - most likely cause of trouble.

Feel free to ask other questions - I restored four of these for various museums over the last couple of years......

Cheers
Sean
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Old 27th Sep 2006, 10:37 pm   #5
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Default Re: R107 Service Reciever

Thanks Sean

Yes there certainly is a live connector. How was this protected. The one I have has no cover but somer rubber hose over the pins.

Thanks Chris.
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Old 28th Sep 2006, 7:26 pm   #6
dave walsh
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Default Re: R107 Service Reciever

Chris there are lots of bits of info on the web. Have you seen the EBP5AGU site? He's just started a restoration. If you go to VMARS [Vin Military and Radio Soc] one of the technical articles kindly made available there is "Bringing An R107 back From The Dead" [Oct 2001] excellent and it can be printed off. May have an original 107 manual in the North. My machine awaits restoration. It's very rough looking but was also very cheap as the guy at Sambrook didn't want to lug it home a number of years ago. Dave
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Old 28th Sep 2006, 7:34 pm   #7
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Default Re: R107 Service Reciever

Hi Christian,

The pins were normally covered by an interconnecting cable that went to the matching transmitter (WS12), however, these plugs are like rocking horse dung!

The bit of sleeving isnt green and about an inch long is it?

Just one of my restoration trademarks.....

The cable was used to mute the R107 during periods of transmitting - this was done by cutting the screen supplies to the RF stages and IF stages - as far as memory serves anyway!

Cheers
Sean
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Old 6th Feb 2007, 10:13 pm   #8
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Default Re: R107 Service Reciever

I have 3 and a half of these great sets,the half is undergoing restoration.
Iv got one totaly original,one i replaced most of the resistors as thay were all out by a fare margin,one is a test bed for various modifications (no cutting or mods that cant be put back to originality!!)
All in all a very good old set,heavy yes but great to work on once you get the individual chassies apart.
Not realy found any thing that goes wrong regulaly, the voltage readings on the test panel can go very low and the set still has plenty of volume.
If valve V1A RF Amp (EF39,ARP34) goes low it will just be a bit deaf to the weaker signals
As with all equipment of this age, the operator/owner must show care,patience (lots) and understanding.
The switches will become noisy and benifit from carefull application of a good switch lube/cleaner (the range change switch is a real pain to get to when set is complete!!!)
One mod i did but dont recommende is replacing the rectifier valve with a solid state bridge rectifier,it put the HT up to 310 volts!!! ok if you got a tired set of valves.
Im working on a solid state vibrator,we"ll see!!!!
Have a fare bit of info on this set along with experience but never found an EMER ( REME workshop manual) is there one?
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Old 6th Feb 2007, 11:08 pm   #9
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Exclamation Re: R107 Service Reciever

Quote:
Originally Posted by glowing antique View Post
One mod I did - but don't recommend - is replacing the rectifier valve with a solid state bridge rectifier. It put the HT up to 310 volts!!! ok if you got a tired set of valves.
Which, of course, it will, since the forward volt-drop across silicon diodes is a lot less than that across a fully-conducting valve rectifier.

However, I'm a little puzzled when you say you put in a bridge rectifier. The usual arrangement for valve sets of this vintage is a bi-phase rectifier (two anodes in one envelope with a common cathode) and a centre-tapped transformer to earth, with raw dc taken from the valve's cathode. If you fitted a bridge rectifier across the whole of the transformer sec'y. winding - so that the center-tap of the sec'y. is now not used - you will indeed get a much increased HT rail - probably so high, it'll destroy the smoothing capacitors for sure! And lots of other stuff, too.

Incidentally, when I have replaced valve rectifiers with silicon power diodes, I usually fit a power dropping resistor from the common cathodes to the first filter capacitor - and adjust it's value for the required on-load HT. This also provides the necessary surge-limiting protection for the diodes. Uprating the working voltage of this capacitor at the same time is also a requirement. (The HT is instantly available at mains switch-on before the filaments have warmed up - then HT current flows and the HT voltage falls).

One advantage of silicon power diodes used this way is that the power demand on the transformer is substantially reduced - so the set runs cooler - although I do appreciate that doing this isn't to everyone's taste, since the set will now not be "as original".

Al / Skywave.
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Old 7th Feb 2007, 12:37 am   #10
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Default Re: R107 Service Reciever

Sorry my mistake,the bridge was across one half of the transformer HT out put .Now,not being an expert,would that put an uneven load on the transformer?
The 310 volt reading was off load and the capacitors rated at 450 volt.
Could the 310 volt reading be the caps charging up?
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Old 7th Feb 2007, 7:51 pm   #11
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Arrow Re: R107 Service Reciever

Hi.

Strictly speaking, there will be a slight unbalance in the transformer, since the sec'y. winding is probably wound symmetrically about the core - but it is nothing to worry about.
What's more to the point, however, is that this is a rather inefficient way of using the transformer with silicon diodes - refer to my last post.
My comment on upgrading the voltage-rating of the caps. was really a "safeguard" entry, since, in practice, the smoothing caps. frequently - but not always - fitted will be of adequate voltage rating - as in your case.
Again, I repeat my comment about the importance of fitting a current-limiting resistor - to protect the first cap. and the diodes - (surge current)
As I said in my previous post, your excessive HT was primarily brought about by the simple reason that silicon diodes in this application are far more efficient for rectification than a valve - hence, the 310v off-load. The on-load voltage will be similarly higher, also.

I believe that I have now answered your Qs. in your last post.

Glad to have been of assistance.

Al / Skywave.
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Old 7th Feb 2007, 9:39 pm   #12
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Default Re: R107 Service Reciever

Without knowing the current rating of the mains transformer windings, I am not sure if the following is significant, but ...

Taking the HT current from a bridge rectifier driven by half of the centre-tapped secondary winding will double the current in that winding, as it will be conducting on both half cycles of the supply, which could cause that winding to overheat.

It would be possible to revert to the original centre tapped full wave rectifer configuration whilst still using a bridge rec, provided that the "-" connection of the bridge is isolated from everything and the bridge AC inputs are taken from each end of the secondary, with the centre tap taken to 0V. Then just two diodes in the bridge are used, the other two are permanently reverse biased.

Obviously, everything that Al has said above, about using additional voltage dropping resistors when replacing valve rectifiers with silicon diodes still applies.

Ron
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Old 8th Feb 2007, 9:43 pm   #13
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Thumbs up Re: R107 Service Reciever

Ron -

Re: overheating / overloading the transformer; half of the winding.

Indeed: I hadn't thought of that

Mind you, in my defence, I've never even thought of doing something like that, let alone having tried it! With respect to all concerned, it just seems a clumsy way of going about replacing a hot, inefficient bi-phase valve rect.

Re: the bridge; using half of it.
Yes, if we use one of those 4-terminal enscapulated affairs, then what you've said makes sense. A simple two discrete silicon diodes (1N4007: 800v PIV 1A) bi-phase rect. + details previously referred to, is a far neater & elegant solution.

It's also worth mentioning that the fitment of an RC snubber across the transformer primary to clip spikes on the supply (that could otherwise damage the diodes ) is also worthwhile doing when effecting this mod.

Al / Skywave.
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Old 9th Feb 2007, 12:04 pm   #14
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Default Re: R107 Service Reciever

Regarding the instant avalability of HT at switch on.
Would it be a good idear to put a switch in the HT supply,thereby letting the heaters warm up before hitting them with the ht,i read some were that if this is not done it causes electron space charge stripping and damage to thorium cathodes.
I did this on my test R107 and give 30 seconds warm up time before switching on the HT.
As iv said im not an expert(i still have to plough through all the maths/books to find answers to things in this hobby) but have one hell of a book collection on all things thermonic with a little silicone mixed in.
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Old 10th Feb 2007, 11:13 am   #15
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Default Re: R107 Service Reciever

I'm no expert either but this was certainly standard practise on a number of valved communications receivers HRO etc. Years ago I had an Eddystone 358x set [also plug in coils] with HT switch. It was not exclusive to sets without bandswitching but probaly more essentia!.
By chance I was looking through a RSGB Bulletin Mag from the fifties yesterday which contained an article on creating a mechanical bandspread dial for the R107 around the main tuning knob. It used some existing holes but involved drilling others so I doubt if it would appeal to anyone doing a modern day "historical" restoration Job. Dave
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Old 10th Feb 2007, 6:36 pm   #16
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Default Re: R107 Service Reciever

Hmm, I wonder why anyone needs to mess about with the R107 at all - 6x5 rectifiers are not exactly rare, nor for that matter are EF39s......

As for a bandspread.........

Blimey, there was some vandalism during thre 50s!

Cheers
Sean
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Old 10th Feb 2007, 7:40 pm   #17
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Default Re: R107 Service Reciever

Looks like vandalism to us but especially with the first SSB experiments, being able to spread an amateur band over perhaps a couple of inches would be seen to be worth it! Modifying gear was the cheap [and only] option for most people in those days. Put together with self build in the twenties and thirties it explains a continueing reluctance to spend too much among experimenters/restorers today. I remember at one stage you could buy complete R107 front end [effectively a seperate unit] and use them as a front end convertor. By the time I saved the money for one they had sold out-tragedy. Dave
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Old 10th Feb 2007, 8:14 pm   #18
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Quote:
Originally Posted by glowing antique View Post
Regarding the instant avalability of HT at switch on.
Would it be a good idea to put a switch in the HT supply,thereby letting the heaters warm up before hitting them with the HT ?
Yes, quite a common facility on early Eddystone receivers (S640, for example) - and as Dave Walsh has mentioned (358X) - and in addition to the reason you suggested, the HT on / off switch often had a pair of contacts wired to the rear panel backdrop to control an external relay for switching on a transmitter and operating the aerial change-over relay.

As regards damage to the valves running heater voltage but no HT, this is what is done in the Racal RA-17 comms. receiver when it is switched to stand-by - I don't think Racal of all people would have done this if any harm was likely to develop to the valves by doing this. Agreed, a dummy load resistor is connected across the HT source to compensate in stand-by - but in this Topic, that's not the point.

Al / Skywave.
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Old 10th Feb 2007, 8:18 pm   #19
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Default Re: R107 Service Reciever

Quote:
Originally Posted by Sean Williams View Post
Hmm, I wonder why anyone needs to mess about with the R107 at all - 6x5 rectifiers are not exactly rare, nor for that matter are EF39s......

As for a bandspread.........

Blimey, there was some vandalism during thre 50s!

Cheers
Sean
Well, some people would argue that replacing valve rectifiers with power silicon diodes prolongs the reliability and life of the set by reducing the heat generated within the cabinet. It's not a case of rarity of 6X5's.

Al / Skywave
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Old 10th Feb 2007, 8:54 pm   #20
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Default Re: R107 Service Reciever

Back to the HT switching. We have now largely forgotten the slow warm up experience of domestic gear in former years. I remember how impressive it seemed to me-to be able to go into instant reception with the 358 [especially as it took an interminable amount of time to rev up]. You threw the switch from Standby into instant response!
In that respect it was superior to any thing in the house ie radio/record player /tv. Would have been much the same with the R107. I've never heard any complaints about valves being damaged when run without load. I think that when they ran the early valve computers "continuous running" was seen as less damaging than switching the heaters on and off. Dave
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