Gould Advance OS3000 - Page 1
Forum section: electronics / repair

author: jbchild
author_rank: Newbie
timestamp: October 20, 2024, 09:36:27 am
content: Hi there - hoping someone can help. I saw some stuff posted about 10 years ago regarding this OS3000. I'm having some difficulty with the EHT section and looking for some assistance. At some point TR813, 814 and 815 suffered a catastrophic meltdown. They have been replaced, along with the electrolytics and the main power supply electrolytics.At this point the EHT and the scope are working, but it seems the EHT oscillator is drawing too much current. I have isolated this EHT oscillator from the driver ("Z") and am powering it with a separate DC supply at 12v. Current draw is 350ma, and the two osc transistors are running at 100c plus. +/-15v and 150v are in spec.Vk is a bit low -1350v. EHT is hard to measure - I get different results with different meters. I've isolated the 3 EHT supplies individually at A,B and C but the current draw remains the same.Any ideas most welcome,thanks

author: coromonadalix
author_rank: Super Contributor
timestamp: October 20, 2024, 11:06:54 am
content: flybacks at some point where notorious to crack, you could hear them hisssing and smell some odor,you had ones who could short on their windings and get bad overtime, consuming more current until they short or open and damage the power output transistor (tr813)if you say it is working in a sense, it could be signs of future failures ... if you had any chemical capacitors changed and they are all ok, no burned resistors etc ...it could be a sign .... as replacement part, do some checks if it is available or not, flyback transformers are getting obsolete in many thingsto testnormally you remove it and do lcr tests but you need the good values first to compare, ohms tests are not enough.In the day, Sencore had a wonderful tests equipment to do theses kind of test, pulses / ring tests to check if the flyback is okayFor high voltage tests you need a voltage divider probe, and yes some meters will hate the frequency, that's why old needle meters where best for thisbe careful isolating B could create problems since it has a feedback line for the oscillator, the focus is a feedback ...

author: jbchild
author_rank: Newbie
timestamp: October 20, 2024, 09:10:27 pm
content: Thank you!An attempt at repair by a previous owner was unsuccessful. I replaced R814, the three HT drivers, TR816, C809, 817, 812, 813, 814, 816,. I suspect that C817 failed causing TR813 to go full on, resulting in the meltdown. Isolation testing was done with a separate power supply for the HT, therefore the feedback was isolated as well.The 350ma current draw of the osc. seems to be well within the capability of the BFY50's, so possibly the heat sink is inadequate?

author: Gyro
author_rank: Super Contributor
timestamp: October 20, 2024, 09:52:40 pm
content: This probably isn't going to be helpful, but back in the early '80s I scored a free OS3300 from the Calibration dept as BER (apart from rubbed front panel markings, they possibly couldn't get a spare transformer or tripler even back then). The 3300 is the same as the 3000 but slightly higher bandwidth and a potted EHT trippler. It was suffering the same problem as yours, overheating and failing BFY50s but otherwise working.Being very green in those days, I replaced them a couple of times and then fitted a couple of uprated TO126 package BD-something-or-other, heatsinking them to the EHT unit box, like TR813. It ran for a few weeks like that and then, when I had the lid off, I noticed that the EHT box was running cool and remained that way for many years, until I passed it on.I've often wondered what the cause was, I think I rationalized it as a failed capacitor in the potted tripler self healing but I was never sure. Nothing quite rang true, there was no apparent drop in EHT or CRT cathode bias, focus etc. Likewise the transformer, a shorted turn would probably have caused more severe symptoms. So I'm really not sure in hindsight.I'm not suggesting that you do the same thing without further diagnostics, but it doesn't appear to be an isolated problem. I think they do run the BFY50s a bit too hard. A shorted TR814 or 815 would almost certainly take out TR813 as collateral damage.As coromonadalix says, be very careful with the focus supply (B) as it provides the feedback that the EHT unit uses for voltage regulation, causing it to overload, as would an open circuit EHT adjust pot or connection. It would be worth checking the track resistance of the focus pot too.

author: coromonadalix
author_rank: Super Contributor
timestamp: October 21, 2024, 12:18:25 am
content: yes the focus control could be a problem too if the flyback is okay ...