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Old 12th Oct 2018, 1:30 pm   #41
Welsh Anorak
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Default Re: A pair of Rediffusion monos....

The last 1500s were nice sets, with varicap tuning, slider controls and all the modifications present. And nice tried and tested circuitry, a LOPT that never failed, and a far better picture than the 1615 with its imported tube. I never recall anyone worrying about the slightly longer warm-up time.
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Old 15th Oct 2018, 9:09 pm   #42
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Default Re: A pair of Rediffusion monos....

Having parked up the Mk13 chassis set whilst a UHF tuner module is sourced, I've had a look a the MK10.

Initial tests revealed a short circuit boost cap - one of the blue and white moulded plastic types. I have no service info on the MK10 and unfortunately the value is partially obliterated- It's either 0.047uf or 0.47uf at 1000v. Does anyone have this information?

Ta!
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Old 15th Oct 2018, 9:50 pm   #43
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Default Re: A pair of Rediffusion monos....

Do you have a picture of it? 0.47uF at 1000v will be a rather large physical size .

John.
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Old 15th Oct 2018, 10:44 pm   #44
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Default Re: A pair of Rediffusion monos....

Good point John- a 0.47uf of that vintage would be quite a size. I think it must be a 0.047uf. For some reason it's encased in a clear sleeve, which I've peeled back. I think the mangled figure was done during the manufacturing process.

Steve
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Old 15th Oct 2018, 11:24 pm   #45
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Default Re: A pair of Rediffusion monos....

Hi Steve, I remembered when you and rob had gone home that I have the proper service manual for this chassis which you're welcome to if you want it.


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Old 15th Oct 2018, 11:39 pm   #46
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Default Re: A pair of Rediffusion monos....

Hi Steve, C507 is a 0.047 1000v according to the manual.


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Old 16th Oct 2018, 8:33 am   #47
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Default Re: A pair of Rediffusion monos....

Hi, when i first started working for Rediffusion w/shop Newton Aycliffe in 1975 i was on grade F, which was mono only. I spent hundreds of hours on the Mk 10 test rig. The stab boards had to have had stand off valve bases fitted, a 10k safety resistor was fitted in series with the VDR and the 330k boost feed resistor was changed to a 220k. The 0.01 boost decoupling cap was replaced with a disc type. Early chassis needed the tag strip replacing due to heat damage and one of the w/w resistors was removed from it an mounted on the top of the chassis on a seperate tag strip. The s/f t/b panel was reliable, Bfy52 and 15v zener diode caused hum bars, early boards used a BLY 48 frame o/p instead of the more reliable 2N3055. The Mk10 chassis ran for years, Malc.
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Old 16th Oct 2018, 7:24 pm   #48
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Default Re: A pair of Rediffusion monos....

Hi Neil

Thanks for confirming the cap is a 0.047uf and for the offer of a manual. I'll take you up on that!

Hi Malc

You may be able to solve one mystery on this chassis- the presets for the two line holds and the vertical hold and mounted vertically behind a metal 'barrier' which seems to be there specifically to stop them being adjusted! Theres no room to insert even the smallest screwdriver. On any other set of the era these are user adjustable but on this set they don't even appear to want an engineer to get at them. Any reason? The linearity and height are mounted horizontally next to them and can be adjusted with ease.

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Meanwhile, replacement of the s/c boost cap and the o/c 1 meg resistor in series with it has brought forth EHT (hooray!) and a fairly dim raster, which could be down to the fact it seems suspiciously wide and tall, although the line structure at 405 is visible. There's also a distinct whiff of ozone from the LOPT. Hmm.

Steve
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Old 16th Oct 2018, 8:40 pm   #49
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Default Re: A pair of Rediffusion monos....

I connected up the Rediffusion translator box and into that fed a VCR running a 405 line VHS tape, so at least I had a picture to look at.

Unfortunately the EHT drops off fairly rapidly a few minutes after switch on, dropping down to the point where the picture disappears. The overwind on the LOPT gets pretty warm too.

Not looking good unfotunately!
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Old 16th Oct 2018, 9:10 pm   #50
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Default Re: A pair of Rediffusion monos....

Oh dear that's a shame, the lopt has an RBM A774 look about it so maybe not a complete surprise. The overwind looks to be a potted type so passing current through to drive out any moisture won't help I suspect.

Is it plug in by BTW. There looks to be an edge connector on the main winding?

John.
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Old 16th Oct 2018, 9:42 pm   #51
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Default Re: A pair of Rediffusion monos....

Hi John

Yes it's rather disappointing although not too surprising I suppose. If left on, the overwind gets really quite hot.

The LOPT can be removed very easily;pop off the EHT connector from the crt, remove the edge connector, move a metal spring clip and it lifts straight out. The entire set is very pluggable (or unpluggable).

The last MK10 I had suffered from a duff LOPT. It went to Malc, so I wonder if he had any luck with it...

Steve

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Old 17th Oct 2018, 12:09 am   #52
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Default Re: A pair of Rediffusion monos....

Hi Steve, the vertical mounted pre-sets can be adjusted with your finger. The faulty loptx we had at Rediffusion were all sent to our Rochdale factory and were returned triplerised. The faulty overwinds removed and a tripler fitted. They worked ok, but the eht regulation was never as good as a valve eht rec.
Try drying the loptx out for a week of so in a dry, warm place, Malc.
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Old 17th Oct 2018, 6:49 pm   #53
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Default Re: A pair of Rediffusion monos....

Worth a try Malc. I'll do that and report back.

Steve.
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Old 19th Oct 2018, 10:47 pm   #54
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Default Re: A pair of Rediffusion monos....

Well, some unexpected good news. I ran 30v through the (potted) overwind for a couple of nights, so a good 16 hours worth of gentle cooking.

Popping it back in the set and the EHT now holds steady. The set previously only managed about 3 minutes before the EHT died off.

Just a shame the tube is pretty clapped out


Steve
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Old 19th Oct 2018, 10:57 pm   #55
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Default Re: A pair of Rediffusion monos....

No, hang on - it's all gone pear-shaped! Although the picture showed absolutely no signs of distress, a pungent pong after about ten minutes suggested all was not well. Time to power off. Sure enough the overwind was getting very hot again.

I don't think this one wants to live....

Steve
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Old 25th Oct 2018, 5:56 pm   #56
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Default Re: A pair of Rediffusion monos....

I imagine you could fit a doubler or tripler? You'd need to experiment with the tuning/EHT however but it might be worth it. Then you'd just need a tube...
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Old 25th Oct 2018, 7:22 pm   #57
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Default Re: A pair of Rediffusion monos....

Yes a tripler would appear to be the way to go Glyn. Maybe the poorly LOPT is making the crt look worse than it is. It may even wake up a bit with prolonged use.

Steve
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