1977 Ferguson 3734 ColourStar: Thorn 9600 Chassis

Result, found one, tested and its OK. Glad I have some in stock, the prices online are ridiculous, cheapest I found was £9.59. 

Perfect. Back to fault-finding the blowing 34V fuse.

I'm certain everyone here knows the following test procedure.

However it occurred to me new starters to the hobby may not know how to check a bridge rectifier. The following is aimed at them and how I tested a bridge rectifier with a mutlimeter on diode test. The case example above, my bridge rectifier failed on the centre A.C  ~ tests, I had readings here when there should be none. The new diode tested perfect as per the diagram.

Chris,

would it be possible to upload the 24V regulator circuit for those that want to 'Sing Along With Max' in due course?

Yes Mike I'll get that done now, back shortly( Oooo bad pun)  ? 

24V regulated circuit as requested

So I can see what's what, the filthy scanning module need a clean off. Pretty serious heat-sink there!

For the FS11 blowing I would try removing the 24V supply to the video/IF board (PL4).

I had lots of blue tants fail on my 9000s but instead of blowing the fuse (1.6A in the 9000) they cooked the 3.9ohm (R735) resistor instead.

Actually... this could be a good exercise for using the recording ability of your new meter, Chris. Putting it in series with the fuse on a suitable current range and recording what happens will give you a good idea of how serious the short is.

A bit of me is inclined to do a quick meter test across the field output pair (VT807/809) just for the sake of it before returning to fault-finding proper. Likewise examine C821, although what you mentioned earlier about ohmmeter readings would suggest it is blameless.

Posted by: Mikey66

 

I had lots of blue tants fail on my 9000s but instead of blowing the fuse (1.6A in the 9000) they cooked the 3.9ohm (R735) resistor instead.

'Beware the blue tants......' The ghost of LLJ is nearby!!

The memories are coming back now, I'd forgotten how "busy" that field output stage was. Added complexity due to the toroidal scan coils and the requirement of both north-south and east-west pincushion correction, that being said it was pretty reliable probably helped by that huge heatsink.

Lots of feeds also into the trip circuit which could make tripping faults challenging.

The most common fault on the 9600 series was the lower diode in the E-W modulator circuit, I think originally a BY198 then upgraded to a BY298 then BY398.

John.

Hi John,

You might be interested, I've made the servicing article from Television August 1977, it extensively details the 9600 and its improvements over the 9000. Might jog some of those grey cells.

https://www.radios-tv.co.uk/thorn-9600/

In case you're wondering what's been happening after replacing the bridge rectifier. Well I didn't proceed with another power on, something was nagging in the back of my mind, something was not sitting right. I had visions of powering on again only to fry yet another fairly expensive diode.

I'm working on an unknown set, with unknown faults and no idea what if anything has been done by a previous engineer. What if I have a bag of faults. I cast my mind back to this mornings events and whilst pondering them read the article I mentioned above about the 9000/9600 series.  It explained what I found earlier this morning but didn't report in the thread, with regards to F1, that being it was totally shattered. I've never encountered a fault where the fuse it totally destroyed and I mean destroyed all that was left was the two cap ends!

Now prior to reading this article I just thought someone had been heavy handed with its insertion, so replaced it with a new one. That's where the thread picked up earlier today, the set was powering up with the new F1 perfectly fine, but as you recall F511 was found to be open and I was attempting to track problem on the 24V regulated rail. Obviously a little latter F1 started blowing (not shattering I hasten to add) when I powered up and that's when I found the bridge rectifier W525 had decided to fail. 

I could have just assumed that now the bridge rectifier was replaced I could install a new F1 and away we go, but as I said above, something was nagging away at me.

Now what was revealed in reading this article?

I find that a shattered F1 is a common fault. It stated that if F1 is found to be shattered you need to check the 1uF filter unit, the h.t. rectifier diode W701, the continuity of the two 3R surge limiting resistors R701/702, and the condition of the 400uF h.t. reservoir capacitor C702, any one of these can result in catastrophic F1 failure.

 So I need to check all this lot out before I power up again. In fact there's a whole host of potential other things to check, a very good article indeed.

Ah yes, the three legged filter capacitor, similar to those used in some makes of washing machines. Now, there was one which had to be changed on sight, I think it may have been the yellow or black ITT type but I'm not longer certain.

Did you notice the author of the 9600 article, S(simple) Simon? one of LLJs nom de plumes.

John

Posted by: Jayceebee

Ah yes, the three legged filter capacitor, similar to those used in some makes of washing machines.

John

Why three legs? How on earth do you test those? 

If faulty can you still get them? I'll have to check my stores

What can you replace them with if not?

Edit: Re testing; I have a spares in stock, from what I can tell you can read either outer pins to centre to get the 1uF but not between outers. Still don't understand why the three pins though.

Look up delta suppression capacitor and all will be revealed. 

I do have them in stock

The three pins became clear when I noticed the little symbol imprinted on the top. The one shown below is from my stock. I'll now get on with testing all the suspects detailed last night for the F1 destruction.

For those like me, unfamiliar with these three legged filter caps  and to save folk searching, he's what I found after Mikes tip off.  ? 

These small filters are popular devices that can be supplied with a wide range of X and Y values to suit most purposes.

In a Delta Suppressor, capacitors are connected between the Live, Neutral and Earth terminals. As the frequency of the R.F.I. rise, the impedance of these capacitors falls thus attenuating the R.F.I. at the mains terminals. These filters are constructed using Class X and Y capacitors with a resistor to discharge the device.

And yes you can still get them.

Posted by: Chris

And yes you can still get them.

Indeed you can - in fact, I got some of those above a couple of weeks ago to replace the filter assembly in an Armstrong 526 I'm supposed to be doing up for someone.

For the record, Chris - is your three-legged 1µF friend C530 on the posted circuit on page 1?

Posted by: Cathovisor 

For the record, Chris - is your three-legged 1µF friend C530 on the posted circuit on page 1?

 

The PSU circuit on page 1 looks more like a preliminary version of the manual or lifted from the circuit description, probably pre-production as no plug/socket numbers or degauss details shown. I'm only aware of one version in production which may have had minor changes to components and values, certainly no problems with interchangeability though.

BTW C521 was the disc ceramic I mentioned earlier.

John.