1978 Ferguson 3749

Thorn 9800 Chassis

Model: Ferguson 3749


System:625 Line

Original List Price : £000.00 ( to be ascertained)

Valves: None

Transistors: Yes

Intergrated Circuits: Yes

CRT: A56 611X

From LLJ

The 9800 chassis is a very close relative of the earlier 8800 chassis: it has similar panels and a 90° delta -gun tube. There is one important difference. The 45V supply is derived from the line output transformer instead of from the mains. As a result, a start-up circuit is required. This makes a difference to the fault-finding routine.

Start by listening carefully at the moment of switching on. If the e.h.t. rustles up momentarily or a pulse of sound is heard one knows that the start-up circuit is working. If there’s no noise other than the degaussing buzz, the start-up circuit should receive attention. For this purpose an external source of 24V is most useful.

First check the mains input and make sure that some 200V is present at the anode of the thyristor (via diode W704 which should be carefully checked for condition, soldering etc. if necessary). Then identify socket 4 on the decoder panel and apply the external 24V supply to pin 5 (mauve lead), negative to chassis of course.

If the receiver fires up and continues to work when the external supply is disconnected the start-up circuit is at fault and should be carefully checked. You may well find that R814 (470R) on the right side of the upper left degaussing/start-up panel is open -circuit. Unlike the 8800, the 9800 chassis has a separate line output stage panel with a diode -split line output transformer.

As with the 9000 series chassis, dry -joints in this area can be a problem. Lift the panel to gain access to the connector etc. A dry -joint on connection 851-10 is the usual cause of field collapse.

First Things First, Check The CRT

Oh dear!! Not looking good, I’ll leave it cooking on a raised heater to see if things improve. If not then a clean and balance, failing that maybe a full rejuvenate. Otherwise it will have to be a new A56 120X.

The CRT did not improve being left on for a while or with a higher heater voltage (8.3) to warm up the cathode to boil off the crud exposing a new emmissive layer . Next I tried the clean and balance, this also gave zero improvement. Finally I decided to rejuvenate, this worked.

Leave a Comment

Important Notice: To leave a comment use the box below. You must tick "I'm not a robot" and then complete the puzzle, otherwise the automated comment system will reject your post and treat it as spam. Also as part of GDPR you need to consent to our terms and conditions and site privacy policy.

The following GDPR rules must be read and accepted:
This form collects your name, email and content so that we can keep track of the comments placed on the website. For more info check our privacy policy where you will get more info on where, how and why we store your data.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

We are using cookies on our website

Please confirm, if you accept our tracking cookies. You can also decline the tracking, so you can continue to visit our website though you may find that certain features of Radios-TV website may not function properly without the aid of cookies. Our cookie policy