Ferguson 3645 16" portable.

Hi Brian, i have sorted out a slack hand full of various 8Kv pulse caps for you which include a 170pf which should be ok. I will pop them in the post later today, Malc.

Thanks again for the capacitors Malc.

Had another look at the set yesterday. Ignoring the system switch fault I went about seeing why the boost volts remained below that specified with the result it's impossible to adjust Test Card C for a perfect centre circle. I couldn't get more than 620V with the width control set to maximum on a locked picture. The corresponding 625 voltage was the same.

The boost voltages are 670V on 405, 730V on 625 according to the service info.

I'd already replaced the high value resistors but noticed I'd overlooked R132 (3M9). It had risen to over 5M so was confidently replaced, checking the value of the replacement before fitting. I set the width control mid way before switching on.

This made a big difference. The boost voltage was now only 610V at maximum setting of the width control. What on earth is going on?!

Eventually I reverted the width control feed resistor to 330K. The 680K replacement measured perfectly out of circuit. This allowed 630V at maximum width control setting.

What have I missed?

I don't suspect the PY801 or the PL504 as the set warms up in the normal time and the voltage remains relatively stable when fully warmed up. I guess I ought to try new ones just in case. The 30FL1 and 30PL14 have been replaced, the only ones that have needed changing to sort out drafting line hold.

Rather baffling though! 

The set is otherwise working very well.

Cheers,

Btian

What condition is Z4 the stabilisation VDR in?

Posted by: Nuvistor

What condition is Z4 the stabilisation VDR in?

 

I keep forgetting about those flaming things and you were right the last time.

It looks sound but that's no guarantee it's in order as I discovered with the 960 and the height problem!

I'll try fitting a replacement later.

Cheers and thanks

Brian

Posted by: Focus Diode 

.. and you were right the last time.

Grief was I, I don’t remember but it’s nice to know I can get things right at times. ? 

You were right again Frank. Z4 looked perfect but turned out to be defective.

Now I used to find these components generally reliable hence why I concentrate on resistors and capacitors. Perhaps as they get older they're beginning to fail. I'd reverted the width control feed resistor to 680K before testing.

I can only guess the original Z4 was beginning to fail gradually hence why the boost voltage kept dropping.

I got it to 630V before the sticks in the doubler started ticking. I know the sticks will have to be replaced soon. I didn't dare set any higher incase the 51 year old sticks went up in smoke.

Still, it's looking a lot better, still not perfect of course.

They were some slight but irritating height variations so I'll have to replace the height control. The 2M2 pots kindly supplied by Malc will be perfect.

The CRT has dramatically improved since I first got the set working.

Thanks again Frank!

I don't know if I mentioned this earlier, but when I first got the set working the valve heaters were glowing too brightly. I'd already replaced the heater supply rectifier. This turned out to be due to C82, the heater supply decoupler as all was at it should be when it was replaced. I wonder what was happening to the original one for it to act that way?

Cheers,

Brian

That’s good news then, the extra boost voltage and if you change the EHT rectifiers the CRT may look better still.

It would be around 1964-5 or there abouts that VDR stabilisation was introduced by most set makers, I may be a little out with the date but certainly no later.

I never found them unreliable, I don’t think I ever changed one but has you say things start to fail as we/they get older.  

I can understand some of the valves glowing too bright if C82 went leaky but not all of them. Namely V9,10 and 11 so if they were all too bright bright it’s a bit of a mystery. There as to be a reason, don’t know what though.

Hi Frank,

I'm only pleased I cured the fault. The valve heaters looked far too bright which suggested the replacement heater supply rectifier could've been faulty even if the frame locking was perfect. I also checked to ensure the tapping was at the 240V position. It was. The dropper? It's different to the usual ones with the resistor values not printed on it, just "1400". The resistance values were correct of course.

I found the cause of the problem by accident actually. I wanted to rid the set of capacitors known to cause trouble including the one in the C82 position.

The first thing I noticed on replacing it was the heaters no longer lit up like a string of fairy lights.

An interesting one!

Hi Brain, good to see it coming along quite nicely now. I agree with Frank that back in the day you wouldn't have given the VDR another look unless it was physically damaged or something obviously wrong it. Just to note that the same type is used in the boost feed to the height pot and also across the primary of the field o/p TX so if it's an age thing expect trouble here also.

I'm also mystified by the heater brightness problem you had but you seem to be on top of that now but I'm wondering if it may have been something further down the chain and robbing the CRT of the correct heater volts, just a thought now that the emission is looking so much better.

Hi John,

I wonder if replacement VDRs are available? I've only been able to use ones from scrap sets. Z4 came from Z4 on a scrap 1400 panel for example. The same black type was used to replace the green banded one in the 960 set.

I understand it's possible to replace them with ordinary resistors in an emergency, but not sure what value would be suitable. 

As for your thoughts on the heater problem you're probably right. I was under the impression all the heaters glowed far brighter than usual, but could've been a reflection from some of the other valves.

Cheers,

Brian.

At the risk of being completely proved wrong, there are probably very few replacements for those VDR, I never stocked them, they were not something that failed, so I expect there would be few NOS about.

Be ok to be proved wrong though.

Show me a pic of the VDR you need, Malc.

BRC/Thorn 1400 employs 4 VDR's in its circuits 2,3 and 4 are the same Mu01 as detailed below:-

Z1
Type= 37P/W/Spec886
Function = UHF Tuner supply Stabilising

Z2
Type=MU01
Function= Boost HT Stabilising

Z3
Type=MU01
Function= Frame Output Transformer Stabilising

Z4
Type=MU01
Function= Width Stabilising

The MU01

That's the one Chris. I used one of these to replace the faulty green and cream coloured one in the 960 with success too.

Cheers,

Brian

I suspect the MU01 equates to the Philips/Mullard E298CD/A258 but a quick search fails to show any suppliers.

BTW Malc, "Slack Handful" many many years since I last heard that expression. ? 

Posted by: Jayceebee

BTW Malc, "Slack Handful" many many years since I last heard that expression. ? 

I do believe that's where "tight fisted" is derived from. A slack handful would suggest the person's grasp was more open and would contain more, conversely, a tight fisted individual would squeeze his hand as tight as possible, thus containing far less of whatever you were buying by the "handful" be it nails, screws, nuts, peas, beans or whatever! - "Chick peas, handful for a farthing".

Nowadays of course, we have EU rules and HMRC to put a stop to that..........

Hi Marion, for me the saying brought back memories of visits to my grandparents. In the kitchen there was always a glass biscuit jar filled with salted peanuts in the days when they bought hot after being roasted in the store. Any request for said peanuts came with the reply "Go and grab yourself a slack handful". Happy days.

I have found a NOS green/cream VDR. I am sure i used this type in the frame o/p of BRC/Decca mono sets in the 70,s. I will pop it in the post, Malc.

Thanks Malc. It's arrived safely.

Cheers

Brian