Tek 466 was working and suddenly stopped - Page 1
Forum section: electronics / repair

author: YadiYada
author_rank: Contributor
timestamp: October 15, 2020, 02:54:28 pm
content: Actually my 466 was sometimes blinking and I could also hear the fan slowing down for a short moment. This week it did that for the last time. Came up, display fainted, lights went off fan stopped.Open the case and checked voltages. all way too low. (140 -> 84, 15 -> 5, 8 -> 3) Obviously something is shorting but all rails? Checked for signs of burning, nothing. No funny smells either. Started to check tants, none found defective so far. Any ideas from anyone would be much appreciated... HELP!

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 15, 2020, 05:39:36 pm
content: A clue could be the Fan Motor Board ( along with a lot of other boards)is supplied from the +15 V supply on Power Supply and Distribution board ( Diagram 11)Do you have the Instruction Manual?rectifier cr 1761 and 5500 uF electrolytic C1751, series pass op amp U1724B (MC1458)Edit: 140 V low too: That is on a separate winding. Could be the main transformer or the switching on the primary

author: YadiYada
author_rank: Contributor
timestamp: October 15, 2020, 07:17:46 pm
content: Thanks, I have the service manual and will have a look at Fan board and primary switch as suggested.Fan Board looks OK. Tant not shorted and transistor fine.Measuring resistance on +15v rail shows 14 Ohm and dropping which looks like a cap is involved.

author: YadiYada
author_rank: Contributor
timestamp: October 15, 2020, 09:07:43 pm
content: All 5 Rectifier bridge check out.EL Caps also look ok

author: m k
author_rank: Super Contributor
timestamp: October 15, 2020, 10:19:50 pm
content: It's like partial mains is missing.I have 465 but I've never seen that AC selector.

author: mag_therm
author_rank: Frequent Contributor
timestamp: October 15, 2020, 11:03:56 pm
content: I think you are close to finding it.Probe all rectifiers to check even 100 Hz ripple, to check no shorted diodes.One shorted diode may put DC saturation on the core, pulling all supplies down if F1701 is wrong value or does not open.Check that the Fuse F1701 is correct.

author: YadiYada
author_rank: Contributor
timestamp: October 17, 2020, 05:21:56 am
content: Search continues... Most diodes and caps tested, no major discoveries so far. Hell I'm patient! Have to pause for a few days, I'll be back! (In the Terminator voice)

author: andy3055
author_rank: Super Contributor
timestamp: October 17, 2020, 05:05:17 pm
content: Is it possible for you to isolate each secondary load one at a time and observe if the rest of the supplies will come up to spec? Once you know which one is pulling it all down, you can concentrate on it to pin-point where the trouble is.

author: YadiYada
author_rank: Contributor
timestamp: October 18, 2020, 06:01:58 am
content: Thanks for the suggestion. The issue with isolating the load is that its all board based (no cable you can really disconnect to isolate board by board). Meanwhile I found an open 9V (TC) Zener in the 140V section (VR1726 ). 1N937B seems to be in the un-obtainium category...Any ideas where to find this or a decent replacement?

author: andy3055
author_rank: Super Contributor
timestamp: October 18, 2020, 04:22:02 pm
content: 1N937B:https://www.mouser.com/Search/Refine?Keyword=1N937BAs for checking the secondary side, I would pull out all the plug-in boards and check the voltages on the test points/main board. Then plug them one by one until the voltages start dropping.