Thorn 9800 Chassis
Model: Ferguson 3749
Original List Price : £000.00 ( to be ascertained)
Intergrated Circuits: Yes
CRT: A56 611X
THORN 9800 CHASSIS
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.