Ferguson 3660 (Thorn 1400 Chassis) - UK Vintage Radio Repair and Restoration Discussion Forum

sexton_mallard · 16th Aug 2022, 10:10 pm

I recently became the present custodian of a nice Ferguson 3660 purchased from John (unixmanuk). Thanks John, this is a nice set!

Carrying on with a few more jobs on this set, I had it running in 405 line mode for 20 or so minutes after replacing the last remaining black Callins caps and the heater half wave voltage dropper/rectifier diode W10 for a more modern robust 1N5399 diode. (voltage remained just over an indicated 4V on a AVO8 as it was before the replacement).

The sound of a component giving up the ghost was traced to a very hot and bothered 0.3mfd yellow cap C108. The L Lawry-Johns article on the 1400 describes this cap in the 405 line timebase as a common fault.

My question is therefore is this 0.3mfd a critical value? Can I use a 0.22 or a 0.47mfd or parallel in a 0.22 and a 0.1 cap or be patient and order in some 0.3mfd caps of a suitable voltage rating. The bright yellow plastic cased cap in the set (not a Mullard mustard) is rated at 150v. I'd normally stick to 400 or 630v.

dazzlevision · 17th Aug 2022, 8:58 am

That is the 405 S-correction capacitor, in series with the scan coils, to ensure a reasonably linear horizontal scan in wide angle CRTs. Its value is non-standard, as it is a fairly close tolerance component.

If you fit a 0.33uF part, you may notice the left and right hand side of a crosshatch pattern are slightly squeezed or compressed. It may not be noticeable on most programmes.

Your original part is bright yellow, so probably UK made by TCC or Plessey-TCC and have a paper dielectric.

Restoration73 · 17th Aug 2022, 9:53 am

For space saving I would use 2 x 0.15uF in parallel. 5% types are available e.g. Farnell.

Tazman1966 · 17th Aug 2022, 10:53 am

I think I used a 0.33uF in my 1400 (17" Marconiphone 4661) and the only noticeable thing was a very slight difference in the width between 405 and 625 lines. The line linearity was fine with this value.

sexton_mallard · 13th Sep 2022, 9:35 pm

In the end I tried a .22 and .1uf cap in parallel for the craic...If the results were a distorted raster on 405 I'll get 2x 0.15s as suggested. As this rather rough picture shows, the linearity looks ok in this image taken in a mirror. No change was seen after an hour running.

The picture is rough but the geometry looks acceptable to me but is probably the tuner or my Aurora! The 'pull' on the top right is another known issue with the set and will be investigated in due course.

16th Aug 2022, 10:10 pm	#1
sexton_mallard Heptode Join Date: Sep 2013 Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland. Posts: 502	Ferguson 3660 (Thorn 1400 Chassis) I recently became the present custodian of a nice Ferguson 3660 purchased from John (unixmanuk). Thanks John, this is a nice set! Carrying on with a few more jobs on this set, I had it running in 405 line mode for 20 or so minutes after replacing the last remaining black Callins caps and the heater half wave voltage dropper/rectifier diode W10 for a more modern robust 1N5399 diode. (voltage remained just over an indicated 4V on a AVO8 as it was before the replacement). The sound of a component giving up the ghost was traced to a very hot and bothered 0.3mfd yellow cap C108. The L Lawry-Johns article on the 1400 describes this cap in the 405 line timebase as a common fault. My question is therefore is this 0.3mfd a critical value? Can I use a 0.22 or a 0.47mfd or parallel in a 0.22 and a 0.1 cap or be patient and order in some 0.3mfd caps of a suitable voltage rating. The bright yellow plastic cased cap in the set (not a Mullard mustard) is rated at 150v. I'd normally stick to 400 or 630v. __________________ Dom Less snakes...more ladders!

17th Aug 2022, 8:58 am	#2
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,621	Re: Ferguson 3660 (Thorn 1400 Chassis) That is the 405 S-correction capacitor, in series with the scan coils, to ensure a reasonably linear horizontal scan in wide angle CRTs. Its value is non-standard, as it is a fairly close tolerance component. If you fit a 0.33uF part, you may notice the left and right hand side of a crosshatch pattern are slightly squeezed or compressed. It may not be noticeable on most programmes. Your original part is bright yellow, so probably UK made by TCC or Plessey-TCC and have a paper dielectric.

17th Aug 2022, 9:53 am	#3
Restoration73 Nonode Join Date: Jun 2013 Location: Surbiton, SW London, UK. Posts: 2,801	Re: Ferguson 3660 (Thorn 1400 Chassis) For space saving I would use 2 x 0.15uF in parallel. 5% types are available e.g. Farnell.

17th Aug 2022, 10:53 am	#4
Tazman1966 Nonode Join Date: May 2006 Location: St Albans, Herts, UK. Posts: 2,193	Re: Ferguson 3660 (Thorn 1400 Chassis) I think I used a 0.33uF in my 1400 (17" Marconiphone 4661) and the only noticeable thing was a very slight difference in the width between 405 and 625 lines. The line linearity was fine with this value. __________________ All the very best, Tas

13th Sep 2022, 9:35 pm	#5
sexton_mallard Heptode Join Date: Sep 2013 Location: Ellesmere, Shropshire, UK & Co. Cork, Ireland. Posts: 502	Re: Ferguson 3660 (Thorn 1400 Chassis) In the end I tried a .22 and .1uf cap in parallel for the craic...If the results were a distorted raster on 405 I'll get 2x 0.15s as suggested. As this rather rough picture shows, the linearity looks ok in this image taken in a mirror. No change was seen after an hour running. The picture is rough but the geometry looks acceptable to me but is probably the tuner or my Aurora! The 'pull' on the top right is another known issue with the set and will be investigated in due course. Attached Thumbnails __________________ Dom Less snakes...more ladders!