320 Chassis
Model: Philips G17T320
Year:1973
System:625 Line
Original List Price : £—.– (unknown)
Valves: None
Transistors: Yes
Integrated Circuits: Yes
CRT: A44 120
Powered up, no bangs, but PSU is not running as it should.
Well, sort of, 240V into on/off/vol switch, 240V out of switch into PSU no HT whatsoever, there should be 210V at R5637. Need to remove the bridge rectifier to check it, the hunt commences.
The bridge checked out OK, as did the BT100A. After removing and resoldering, It seems to have now given me 230V one side of R5637 and 17V on the other side and 17V on the cathode of BR101 but only 3.3mV on the anode side of B5607. I was just about to check the collector of 5602 when the RCD tripped, plunging the lower section of the workshop into darkness. When I reset, I could see that the 1.6A fuse on the mains input had disintegrated. Just the two stubs in the holder remained, no glass in between! Catastrophic failure, that’s the first time I’ve experienced that, I fitted a new 1.6A, powered up, but that blew as well. I tried a timed delay, that blew. These two did not explode, but did vaporise, leaving a silver deposit on the inside of the fuse.
At this point, I paused, to gather my thoughts on what has occurred, something would appear to have developed a serious short. Upon investigation there is a definite short on the PSU board, I disconnected PLE1/E2 removing the supply to the PSU module, the fuse no longer blows. I find that now the Bridge rectifier has shorted, I wonder if it was in the process of dying when I first had no power then power. I removed it and it now tests bad. Reconnected the PSU to the circuit, with the BR removed ohms test shows no short exists with it out of circuit. No BY179 in stock, but I do have quite an array of bridge rectifiers. An initial rummage, I think two possible subs are BY257 or RS204L, same pinout too, but I found a 2KBP06 which is probably closer to the 179, so in that went.
Investigations revealed a problem around the BT100A thyristor. I have HT to the anode, nothing on the Cathode. Now, I removed the BT100A earlier, and it tested OK on my Thyristor tester. As I’ve now taken live readings and there isn’t any voltage at the gate, it’s not being turned on. I checked the junction of C5624/R5647 for the 1.5V and that is missing too.
As it was getting late, I decided to stop for the day and pick this fault up the next day.
The following day I needed to check the control circuit, the diodes, transistor and especially the switch gate BR101. Hopefully this is where the fault was to be found. A lot of faffing about ensued, it wouldn’t have taken so long If I understood how the triggering worked for the Thyristor. What was throwing me was 227V on the Anode, nothing on the Cathode and initially only 49V on the gate which rapidly fell away to millivolts. After removing diodes, transistors, and the control switch, nothing was found faulty. I’d checked almost all the resistors, again not finding anything untoward. Eventually I found or shall we say stumbled on the culprit, I looked at R5633 a 220K, that was the culprit. It should have been obvious, but sometimes a logic fog descends.
Now the Thyristor could start and the PSU sprang to life. Only four components changes to get this working, and two of those were purely precautionary, only the bridge and 220K had actually failed. I have 16kV and HT1 has been set for 158V as per Philips recommendation.
Then “Dead again”……. Aghhhhhh!
Well, not quite the proverbial “dead parrot”, just the bloody curse of Philips “wire ping”. Back in business and look at the results, as the CRT tester showed, the CRT is superb.
The End…… or until the next fix-on-fail.
