RCA WO-33A Oscilloscope repair - short circuit - Bad transformer ?? - Page 1
Forum section: electronics / repair

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 13, 2018, 09:08:37 am
content: http://rhetoricity.com/misc/tubes/oscope/RCA-Oscilliscope-WO33A.pdfI have this really cool vacuum tube oscilloscope, it's had a big fall at some point and broke the PCB completely in 1/2 and broke 3 resistors. Broke some chips/dust loose in the CRT and bend the chaise.So since Saturday I've repaired all 14 parallel traces with jumpers. Recapped, even saved the resistors, just chipped off the enamel around the lead hole and exposed some metal so solder onto. And I used a microwave oven capacitor for the CRT. Rewired the 6X4 socket for an old soviet equivalent tube. Wired on a new cord and fuse.So with NO VACUUM TUBES plugged into it, and in series with a 100W light bulb, I turned it on and the bulb lights brightly. The voltage across the mains terminals winding goes to 15.5VAC (120V in) and nothing on the secondaries. I measure 16Ohm from across the plug, and its not shorted to the chasis. From chasis ground I get 37Ohm and 60 Ohm to 6X4 plates, and 47 and 49 on the 6C4 HV plate/heater winding. The other heater winding's read like 0.5, 1, and 2. The pilot incandescent bulb doesn't light.Whats going on

author: stj
author_rank: Super Contributor
timestamp: March 13, 2018, 02:29:39 pm
content: maybe a shorted cap, check c36 and c37

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 13, 2018, 04:09:06 pm
content: No without the "low" voltage rectifier 6X4 and HV rec. 6C4 plugged in, or any other of the 4 tubes, nothing at all should be past the transformer, but it's shorting. I'll have to take apart all those new caps I just put in to get the trans. out.The ohm's reading sounds about right for if it was fine from my limited experience checking the transformer's I've salvaged. Except I should be reading more at the HV end.There's 2.8 and 1.8VAC on where the plates of LV rec. go, and 0.5-6VAC on the HV end, but 15VAC on the primary.This thing is only rated for 50W normally.Oh well, got to pull it apart.

author: Rog520
author_rank: Regular Contributor
timestamp: March 13, 2018, 04:58:56 pm
content: Have you unplugged the CRT? It has an AC filament, connected directly to the transformer.

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 13, 2018, 06:50:35 pm
content: No thats out too, no reading on the heater windings either

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 13, 2018, 09:18:54 pm
content: Did you actually check those resistors? Carbon resistors are bad enough, let alone after being crunched and soldered internally.Are you sure there's no caps across the line?Check the replacement cap for C42. If it suffered infant mortality and R75A is turned all the way down it will short out the filament winding.EDIT: Don't turn R75A, if there is a short it will fry! Just disconnect and check the cap.

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 24, 2018, 06:31:25 pm
content: Well I took out transformer off, and yeah it's the s.c., the primary mains winding drops to 6V with a 40W light bulb limiting it. It reads 8 Ohm. IDK what ohm's the HV winding should read, but I only have 0.5V on it to gnd, but I get 2V on the +/-350V.The terrible thing now is the transf. is potted in a steel case with 5 spot welds. Trying to cut down through the potting around the case mostly works, but I don't have a long enough razor blade. So I might have to try cutting the little 1mm spot welds with a dremelThat's my only option I can think of for near future, until I get a metal drill bit

author: Cyberdragon
author_rank: Super Contributor
timestamp: March 24, 2018, 08:13:19 pm
content: The question would be how you're going to reassemble it if you cut it open.

author: lordvader88
author_rank: Frequent Contributor
timestamp: March 24, 2018, 10:43:51 pm
content: After a little grinding and some hammering I managed to get it out. And then a lot of hammering, I took it completely apart, all the E plates, 1st time I've done that. Bad thing is though I broke about 15-20 super fine windings from the HV loop I guess, when some paper cracked after the coils were out.The thing smells like ashes from a wood stove. A lot of the E plates have some rust too. It is from the 1960s tho, so IDK, just need so sanding. I bent a handful, but I have steel plates to press them flat again.Now it try and unwind it and see whats up.