Ferguson 3840
 

SHMBO asked for a bread maker... upon searching on Freegle for one I could not resist typing in 'TV' in the search box to see what may turn up. A few hours later, instead of a bread maker, I had a nice 14 inch black and white set which is a worker in excellent cosmetic condition ;D. The previous owner bought it new when she got married and used it as a main set for a short while. The code G8-40 on the back (I think) denotes 1978 week 40.

The contrast and brightness sliders are stiff and notchy and got a blast of servisol. From what I could find, this set has the 1690/1 chassis. While I got the back off, what preventative maintenance should I consider? I heard the EHT diode can short but this set for now seem to be OK. No signs of distressed components (bulging electrolytics or toasted resistors) or any repairs can be found which bodes well.

The Mullard tube, an A34-510W appears to be made in Taiwan which was a bit of a surprise. The Sun was setting on the UK TV manufacturing industry it seems by then.

Re: Ferguson 3840
 

A basic clean of the set was done and a check of the tube heater voltage - 10.6 to 10.8v which is not far from the spec of 11v max. The contrast control is flaky - I can't set the contrast right and the set shows flyback lines most of the time. The tube is nice and bright and apart from the contrast, displays a good picture. Would this set use mean level A.G.C which is not working?

Re: Ferguson 3840
 

“The contrast control is flaky”,
if you mean the control is faulty, fix that before delving further into the problem?

Re: Ferguson 3840
 

Philips Taiwan was the designated factory for small screen b/w tubes for all Philips brands from a certain point in time onwards, so that makes sense. It will be stamped with white letters TR (or maybe a single other letter or symbol in which case I would be very interested in a picture of said marking), a date code and a model code somewhere on the cone.

Re: Ferguson 3840
 

Hi Maarten, here is a snap of the white code on the CRT cone which is smudged and somewhat hard to read. The two larger characters appear to be '15'. I moved some wires for a better shot. The contrast slider pot seems to be faulty, probably finished off by all the dust and grit falling into the assembly over the years. I see it is rated at 470K and connected to the main board with spade connectors. I will lash up a spare 500K rotary pot in the meantime to prove the fault. I may attempt a dismantling of the pot to see if it can be revived. I take it these pots were not one of Thorn's successes?

Re: Ferguson 3840
 

I found slider pots from the 70's were not that good and very often went notchy when cleaned.
Definitely prove the pot is faulty, not sure why you would use a 500K pot to test it with when the circuit shows 470ohms or is the circuit incorrect?

Re: Ferguson 3840
 

Well spotted Frank. Thanks for the correction it was a typo, I meant to type 'R' not 'K'.

For those not familiar with this model, The sliders are placed facing upwards on the back of the set and so did not stand much of a chance with dust dropping straight in. I thoroughly brushed and vacuumed the pots followed by a rinse with Servisol. The brightness control is fine but the contrast control did not revive itself. It seems some other models using this chassis have a conventional board mounted pot on the back.

Re: Ferguson 3840
 

I'd just use this. If the EHT diode goes short then it's obvious and a rectifier in the EHT lead will do the job with little effect on performance. Nice little sets that gave little trouble. The sliders did tend to disintegrate when sprayed with cleaner!

Re: Ferguson 3840
 

I'm sure this set is in my 1981 Ferguson catalogue although I haven't seen it for years and I have no idea where it is...

Regards,
Paul

Re: Ferguson 3840
 

I did a post mortem on the 470 ohm failing pot and it appears the resistive carbon track has fractured in the middle. The material looks to be graphite bonded to a paxolin like material. I have nothing to lose in attempting some sort of repair with a conductive filler material of some kind as getting a replacement slider pot is going to be a challenge.

Re: Ferguson 3840
 

Thanks for the pictures! The line A8G in the stamping points to manufacture in july 1978, so that seems plausible give the set was probably manufactured in 1978, week 40. Apparently the letter A was used for Taiwan before the new letter system was introduced roughly a year later. The upper line is less clear because it was stamped badly. It might say XV0 but in any case that line contains the type and revision of the tube.

Re: Ferguson 3840
 

The attempted fix on the pot failed. I have placed a request in the Wanted section here:

https://www.vintage-radio.net/forum/...80#post1128280

Re: Ferguson 3840
 

I managed to obtain some Egen 470R slider pots which fitted exactly and now I have proper contrast with no flyback lines:). The set is now been running for the rest of the evening, the tube is nice and strong. There was a bit of intermittent tearing/rolling but this has since stopped and the set has probably never been on for more that 5 minutes at a time and is probably 'waking up'. I think this telly is a 'keeper'.