British Relay MR754/756 info

[murphyv310]

Hello.
This is very much a long shot! I was a trainee station engineer with British Relay in the early days and then went back to be a TV engineer.
I was wondering though if anyone has any manuals for the British Relay UHF receivers used in the Relay stations or possibly even one of the receivers.
The 625/UHF receivers were types MR754 & 756 manufactured by Murphy Radio.
I am also looking for info or a 405 line Relay station receiver.
Many thanks


Mods if this in the wrong section please move

[murphyv310]

I have had a chat with a lad in Portsmouth and it looks as if I might be lucky.
Watch this space!

[Tazman1966]

Good news Trevor

[Synchrodyne]

Yes, good news indeed, Trevor. You’ve mentioned these receivers in other threads, including the fact that the MR756 had true synchronous demodulation. That would have made for most interesting circuitry in the pre-IC days. Even in the IC era, there were/are relatively few TV IF/demodulator chips that used true synchronous demodulation, most being quasi-synchronous following the initial Motorola example. I take it that the HF relay systems demodulated the received signals to baseband and then remodulated on to the HF carriers, so needed first-class receivers.

Cheers,

[murphyv310]

Yes indeed they did.

The older Valve receiver the MR754 had an Envelope detector, this was used on the BBC2 service as the 756 was used on the more mainstream channels of BBC1 & ITV, the 754 was still a first class receiver, comparing the two though the 756 was considerably better with accurate resolution of the 2T pulse, the 10T pulse which was used as a guide for chroma shift (2 mhz pulse filled in with chroma subcarrier) was always superior on the 756 also the sync's were cleaner and there was no tendency for the effects of poor low frequency response.

On the 756 we could often see if there was problems in the transmission, The BBC actually would call us for reports on occasions.

I am hoping to either borrow one or purchase one of the receivers as I have about a year of analogue form Divis in Northern Ireland for tests etc, I get a good signal here from NI.

[murphyv310]

Well, I have now tracked down a MR756 one of the early issues which was the type that I had been used to in the early 70's. I have sent off a cheque and hope to have the receiver very soon.
Although our analogue transmissions have stopped here the result of that Divis in Northern Ireland is now very much stronger without the usual patterning from Darvel, so I hope to use the receiver for feeding one aurora for Ulster TV and of course recording their switch off next year. I will post a few pictures once I have the receiver.

[murphyv310]

The Receiver should be here next week, I also should be getting the manual, I know some here are keen to see the synchronous detector circuitry so I will scan it and post it here along with some pictures of the unit. If the signal from Divis is strong enough (which it should be) I will also post up some pictures of the received VIT signal.

[Synchrodyne]

Thanks for the update. I certainly look forward to seeing that synchronous detector circuitry!

Cheers,

[murphyv310]

Hi.
I picked up the receiver this morning from the sorting office. It is in good order but hasn't been used for 24 years so I will be gentle with it!
Unfortunately no Manual but that might just be an omission, my old district engineer possibly has one.
Once I have it I will post it here.
I will once I have slowly wound it up give it a whirl.

[murphyv310]

Hi.
Well the receiver is working perfectly, As its a full VHF/UHF receiver it can believe it or not resolve a 405 line positive going signal albeit negative going, in pic 1, pic 2 is colour bars from th UHF o/p from a Philips PT5418 pattern generator. The other two waveforms are those of the unit off tune either side of the carrier.

[murphyv310]

Hi.
This test card is an image from the test card generator to the Philips PT5418 to the MR756 then into a mono monitor. the Camera doesn't do it justice I am afraid.

Here is the VIT signal as you can see the level of the 2t & 10t pulses are excellent considering my signal source isn't exactly broadcast standard.

[murphyv310]

The signal from Divis is a little weak possibly due to attenuation from heavy cloud base and the wideband response of the 756 is making it difficult to lock up the receiver this I think is the effect of the adjacent digital MUX's from Darvel. BBC1 on channel 31 is fine though even with a fairly low carrier level. Time to improve the aerial! At least I have to 2013 as Divis will be the last UK Analogue service to close in the UK.

[Synchrodyne]

That looks really good. As you say the camera doesn't do it justice, but it looks as if the 5.25 MHz bars on Test Card F are clearly visible.

Cheers.,

[Synchrodyne]

Also interesting is that it would lock to a 405-line positive modulation signal. I have a vague recollection that in another thread that I cannot find right now, you mentioned that the MR756 had a gating system for its synchronous demodulator reference, looking at the vision carrier only during the back porch. The domestic TV vision IF ICs with PLL demodulators, probably designed for N systems, I think usually look at ungated, limited carrier; whether that would be problematical with P systems during the sync pulses I don’t know, although quasi-synchronous seems to work with such.

Cheers,

[murphyv310]

Hi.
This is the sort of signal level from Divis to Kilmarnock (95 miles) UTV is slightly noisy and BBC1 Ulster is noise free the other channels are good with the exception of CH5.
I intend to use the MR756 to feed an aurora and also to record the NI switch off.

[murphyv310]

I have now set everything up and fed the MR756 into an Aurora. The Signal from Divis does vary and I can always remember this was the case when British Relay fed UTV out on the line system in Kilmarnock giving the subscribers 4 channels! The MR756 really needs 500uv + for satisfactory results so I will fit a masthead amp and raise the aerial up somewhat, at the moment it is just 7 feet above ground level, hopefully it will stay dry for this at the weekend.

The manual for the 756 will be sent as soon as possible once the chap I got the receiver from can find it.

[murphyv310]

Here is some photos. one of the set up, one the V320A Murphy showing BBC1 NI via the 756 & Aurora, and lastly the Aerial pointing to Divis, note the aerial is a good few degrees above the horizontal which has improved the signal level.

[dazzlevision]

Hello,

I see the receiver unit was made by "Rank Precision Industries", which might mean it was made by the former Murphy Radio (or Bush Radio) electronics division!

Regards,

Dazzlevision

[murphyv310]

Indeed the receiver was a "Murphy" Hence the MR. The original 405 line receiver had cans that looked as if it had been plucked out of a 310! the later 754, 625 line receiver was a valved type and used E180F and other high grade valves, it was let down by the PC88 & PC86 valves in the tuner though. The 754 was a good and very reliable receiver.

[Synchrodyne]

For the MR756, 500 µV sensitivity for a relatively noise-free picture seems to be quite good. I looked up the 1975 Read/Wireless World design, pertinent page from the article attached, and this, with a Mullard ECL1043 front end, required 700 µV for the “just perceptible” picture noise point, with an optimum in the range 2 to 5 mV.

Re the MR754, would the use of the PC88 and PC86 valves have reflected the use of a “standard” quantity-produced front end, perhaps because a UHF tuner is a specialized component that would have been difficult to develop solely for a low-production special-purpose receiver? The 1980s BBC RC1/511 TV receiver used a Mullard U321 front end, which suggested that this might have been the case. Or was it that unlike the case with RF pentodes, there were not available at the time any special-quality UHF triodes?

I would imagine that the MR754 would have had gated AGC, with the gating pulse probably derived from line sync. Given that valve sync separators usually operate at quite high levels, would there have been a “sidechain” video circuit with sync (video) amplifier to generate the high video level which then fed a sync separator, the line sync output from which in turn timed a gating pulse generation circuit? In solid state I think all of this could be done at low level, and later on within vision IF ICs such as the TDA2640/41, but it looks as if it would have been a bit of a “tour de force” with valves.

I guess that if you also had the Murphy 405-line receiver, you could feed that from the Aurora and then use a studio 405-line monitor.

Cheers,