Broadcast [Sticky] Decca RU.4011

I remember these at TC - the last time I saw one it was in TC1. I do wonder how close these are to a Decca Bradford in a box; maybe it'd be worth dropping Brian Summers a line?

I'm assuming no-one has got hold of the video level pot and turned it down to zero - and you can tell that's an ex-BBC unit as not only does it show signs of having been fitted with a mains XLR in the past, it also has the audio XLR in BBC convention rather than the normal "signals on pins" convention.

Posted by: @cathovisor

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I'm assuming no-one has got hold of the video level pot and turned it down to zero

If by that you mean the pot accessible via the front panel, that was the first thing I thought of. When I checked, it was at its maximum extent to the right. That made me wonder if the video level had been slowly reducing and someone had compensated. I did send it to the left-most extent in case it was wired differently, but nothing appears on the outputs.

I'll stick it on the to-do pile for now.

@crustytv Given those ICs on the receiver board look like those used in both colour and mono Deccas of that era, it wouldn't be too hard to see if video is coming out of the detector simply by comparing it to, say, a 30-series Bradford.

I have a very similar unit, model no. RU4013, and on seeing your post I dug it out from storage in an unheated but insulated shed. It has suffered from surface rust on tuner and also the on the metal covers. C321 situated on power supply has corroded and it fell off an soon as it was touched. The mains connector on this one is the old XLR type. Not much help I know but this is a photo of the insides.

Hi Jim, other than the VHF/UHF tuner in yours (which must be the model No. difference) that looks to be identical to mine. Mine did have XLR, but someone has hacked it out and installed an IEC socket. Don't suppose you have the cct?

@crustytv It is possible that I have a cct diagram. I will have a look but it might take a while.

@crustytv Might be worthwhile putting a scope probe on the slider of the video level pot. If you have good audio then the IF section is pretty much OK. A case of tracing for video from pin 4 of the MC1330 through to TR302 on the other board. TR302 looks like it may be driving or buffering/matching to the BNC output sockets.

The MC1330 used to fail on the Decca Gypsy quite often. The other smaller chip also failed from time to time. I can't remember the number of that chip anymore and I can't find a Gypsy circuit online to check. ISTR when one of the chips failed it caused a blank raster and shortwave? (possibly) stations to break through on sound.  As the chips plug in, if you have any spares it may be worth trying them as a quick check? 

I tried substituting the MC1330 and the MC1352, still nothing.

I removed the i/f board and found one of the copper cans on the board had a dry joint. Now I have video on the bnc's, but the signal is only about 186mV.

However, nothing appears on Sony PVM. I thought I'd try a TV that has video input, as maybe there's not enough signal for the Sony PVM. Well, that was a good guess, this is what the tv see's on its video input. I tried adjusting the tuning on the RU.4011 but no lock. Sound is rock solid and lots of it.

This is with the RU.4011 AFC on. With AFC off, the screen goes blank. Likewise, with AFC on, if I adjust the video output level correction; DC Zero pot up to max, the screen goes blank.

At no point do I get the colour bar from the PM5509. I'm guessing I've not got enough video level for either the Sony or the TV to resolve, but the TV give more of a clue to what's going on than the Sony.

The video is of course way too low but the scope and the picture on the Realistic monitor show it's inverted, the syncs should be -ve going. I suspect there is nothing on the Sony because as with most Sony sets the screen is blanked unless it's timebases are locked which can be a pain. It's progress though, try scoping back from the BNC sockets and see if you can find a signal with a bit more amplitude, easier said than done without a circuit.

This afternoon, I had another little look at this RU.4011. I started once again by seeing if I could still see a signal on the BNC output, it was still there. I then checked pin 4 of the MC1330 which should have the primary video output, indeed it was there. For good measure, I also checked that pin 5 had the higher video output, and indeed it did, so it looks like the chip is doing its job.

As usual on the BNC output, the video signal is so low the TV monitor cannot lock. I decided to dispense with the services of the PM5509 and use the Tektronix TSG95. Hooked it up to an external modulator and connected it to the RU.4011. First, I verified the TSG95 into the modulator was outputting a colour bar, set the TV to RF, connected the modulator output to the TV and tuned in the signal, all good.

Next, I connected the modulator output to the RF in, on the RU.4011, connected the video out of the RU.4011 to the TV, and set the TV to video input and tuned the RU.4011 to the modulators signal. The signal was found, it appears the output of the TSG95 is higher than the PM5509. Even so, as you can see below, the signal being output on the RU.4011 BNC outputs is still insufficient. At least this does confirm the RU.4011 is doing what it is designed for but with a video amplification signal fault.

Scoping the signal on pin 4 of the MC1330 I have around 320mV, at the BNC's this is reduced down to 216mV. I left the device running and connected to the scope, whilst I nipped off to make a cuppa. Upon my return, I could see the poor colour bar I previously had, was now gone. Glancing at the scope equally confirmed the previous 320mV signal at that I had on pin 4 of the MC1330 was now a mere 34mV and the larger signal that was on pin 5 was now also greatly reduced.

I was then left wondering was the MC1330 at fault all along, or has something killed it? I then remembered installing a new MC1330 on day one, finding it made no difference so reinstalled the original, so I'm more inclined to think something else is going on here further back, prior to the signal reaching the MC1330. It's a devil of a job without a circuit.

I've had no luck in finding the circuit diagram that I thought, and still think, I may have. I have replaced the 2 caps in the power supply that had corroded on the one I've got. I powered it up and fed in a white from pattern generator on rf with a test tone. I tuned in the tuner and got a picture and audio. I had time to check the video test point on front of unit and could adjust it with the video level control up to about 2.2 volts peak to peak and down to about 0.75 volts peak to peak. As I was doing this I could smell something getting hot and then the audio started motor boating and both video and audio faded away. I think I have a fault on power supply/ audio/video amplifier board. That's as far I've got apart from looking for obvious problems on the boards and doing cold checks. I did power up again later and same problem occurred.

Hi Jim,

Like yours, mine is also objecting to being powered up. As mentioned, not only have I've appeared to have lost the MC1330 output levels, the audio tone from the generator is still giving tone but now accompanied by a lot of background hiss, where the was none. Before it was a solid-locked audio tone with no hiss whatsoever. Oh, and the video test point on mine is -1.5V regardless of how you adjust the video level.

The receiver section on this sounds similar to that fitted to the Decca 'Gypsy' portables, might be worth looking at the circuit of those. However, a glance at the data sheet for the MC1330 shows that the detected video on pin 4 should be in the volts range, not millivolts.

I've found the service manual for the RU4013 version. I won't be able to scan it for a while but I hope the pictures of circuit diagram are useful.

@jcdaze

Hi Jim, You're a star, that'll tide me over until the full one is scanned, really appreciate your efforts on digging this out.

Chris F aka @the_teleman and I, have weekly get together sometimes his workshop, sometimes mine. Today it was a crusty workshop and the RU.4011 was on the bench, I thought I'd run this by him for a second opinion. Without a cct I'd been going by the datasheets for the MV1330 and MCF1352, but that assumes a typical build. We pretty much spent the entire time probing around, trying various tests. It's looking like there are possibly two faults in this unit, there are some wildly off voltages, especially on the MC1352 where we have 25V on 7,8 especially when the datasheets suggest vCC of 18V, which is dropped by a 220R into a centre tapped TX, out to 7 & 8 so quite how we had 25+V left us flummoxed. Now I can study the cct from those photos and see how it's hung together in reality. 👍 

A bit more info from manual.

I noticed in the footnotes of the RU. 4013 circuit that there is an additional bit of circuitry for the RU. 4011 (viz. a phase corrector) so it would seem that maybe the circuits are broadly similar? 

Posted by: @cathovisor

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an additional bit of circuitry for the RU. 4011 (viz. a phase corrector)

That'll be this then, I wondered about that as it seemed an afterthought and very flimsily attached.