Discharge CRT  - Page 1
Forum section: electronics / repair

author: CryptLordGR
author_rank: Contributor
timestamp: September 07, 2016, 05:23:17 pm
content: As I always wanted my own oscilloscope, I followed Dave's advice and I was hunting for cheap analog oscilloscopes. And I finally scored a Tektronix 2215. Whooping 60MHz for 40 GBP!The seller was saying it was working. Arrived today. Plugged the power and the magic smoke escaped!!!I opened the case and as I can smell it is from the power supply. Something may have short circuited from the transportation. So I am thinking of opening the power supply to check it out.What's the way to discharge the capacitors so that I can work freely? In the service manual, it says to ground the anode of the CRT and the output of the high voltage multiplier. Ok for the anode, its simple. But I cannot find any reference in the manual about the other. So afterwards all capacitor will be discharged?Any comments?

author: Yansi
author_rank: Super Contributor
timestamp: September 07, 2016, 05:26:09 pm
content: Verify the HV capacitor voltages with a multimeter (if they need a treatment), grab a power resistor of an adequate resistance, discharge the caps manually, done :-)

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2016, 06:57:09 am
content: You may have just fried the line input capacitor and blown the fuse.There is no need to worry about discharging the capacitors; they will do that themselves in a few minutes.There are two very different versions of the power supply; one uses an TRAIC preregulator and MOSFET current limiter while the other uses a MOSFET switching preregulator.

author: CJay
author_rank: Super Contributor
timestamp: September 08, 2016, 08:58:19 am
content: Quote from: David Hess on September 08, 2016, 06:57:09 amUmm, no, that's a rather dangerous assumption*ALWAYS* discharge them yourself with a suitably insulated resistor and flying leads.In the case of a faulty power supply or piece of HV equipment it's in no way guaranteed that the capacitors will discharge themselves.It's likely they will, but I would not bet my life on it.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2016, 03:52:36 pm
content: Quote from: CJay on September 08, 2016, 08:58:19 amThe 2215 design includes bleed resistors and there is no safe way to manually discharge the capacitors anyway do to access issues. The CRT high voltages are well insulated even when accessing the power supply. From page 6-12 the manual:To avoid an electric-shot hazard, observe the following precautions before attempting any soldering: turn the instrument off, disconnect it from the AC power source, and allow approximately three minutes for the power-supply capacitors to discharge.There is a separate warning about discharging the CRT cathode and anode circuits which does not apply here.

author: CJay
author_rank: Super Contributor
timestamp: September 08, 2016, 04:13:07 pm
content: Quote from: David Hess on September 08, 2016, 03:52:36 pmI've seen enough reservoir capacitors that have lethal charge on them when a PSU has failed that I would *never* rely on a service manual telling me it's safe, I much prefer to check for myself.Bleed resistors go open circuit or high, it's a fact, and one that's caught out many service techs, including me on one or two occasions.

author: CryptLordGR
author_rank: Contributor
timestamp: September 08, 2016, 07:05:21 pm
content: I am waiting the torx kit to disassemble the power supply. I will discharge them in any case.The fuse didnt break, it has a back side removable fuse and checked it and it's ok...I notified the seller with the exact problem and even if it was not his problem, he offered refund immediately. What a nice guy!If in the end it works, I scored a 15 pound(the shipping) oscilloscope, beauty!

author: CryptLordGR
author_rank: Contributor
timestamp: September 23, 2016, 09:24:34 pm
content: changed the capacitors of the power supply and it works perfectly. they were cracked and dry.What a score

author: CJay
author_rank: Super Contributor
timestamp: September 26, 2016, 08:31:41 am
content: Brilliant result, I like a bargain like that.