Looking for a CRT assembly for a Cenco 71551  - Page 1
Forum section: electronics / general

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 04:07:05 am
content: Hi folks, I'm going out on a limb here....Today I bought a box of old electronics test equipment, including signal generators, capacitor testers, multimeters, and my favorite, a Cenco 71551 VT oscilloscope. The scope doesn't seem to work, and I have reason to believe it's the CRT. All the tubes appear to be in good working order, but there is no display. There is a burn mark on the glass near the neck of the CRT. I'm curious if anyone would happen to have parts to repair this thing? I'm not looking to make any money on it necessarily, it's more of a nostalgia project, for the fun of it.Thanks very much!Matt

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 05:03:18 am
content: Has the tube lost vacuum or are the heaters open? Most likely is an electrode short, which can be removed by application of a large current pulse from a capacitor charged to a few hundred volts between the electrodes.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 01:53:48 pm
content: Quote from: SeanB on September 22, 2013, 05:03:18 amThe tube seems to be in perfect condition--no scuffs, scrapes, breaks, or any other physical sign of damage. I was told the burn mark on the tube is normal, and is most likely not the problem. I'll have to test the electrodes, though.I can post some photos if you like. This CRT is set up much differently from what I'm used to seeing in CRT scopes. I would guess that's because this model predates the other ones I've worked on by 20-30 years.Thanks very much for the suggestion!Regards,Matt

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 02:11:23 pm
content: Then most likely is an open circuit resistor or a broken wire in the CRT area. The tubes themselves are pretty reliable, most faults in them will give some display, but nothing means it is either biased hard off due to a fault or the trtace is deflected way off. Look at the front in a dark room with brightness all the way up and see if the beam is somewhere or at least there is a glow inside, that proves the tube works but is being driven off by something.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 05:09:17 pm
content: Quote from: SeanB on September 22, 2013, 02:11:23 pmI tried this last night (looking at the screen in a dark room with the intensity turned all the way up, and there was absolutely nothing. I could see some of the other tubes inside glowing, but didn't see anything on the CRT.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 05:43:05 pm
content: Ok, so I took this thing apart again and snapped a few photos.Here's the inside of the scope:After looking at the tubes again I noticed one I hadn't noticed before: The rectifier tube has a large white spot at the top. If I remember correctly, this signifies that air got in to the tube:That tube also seems to heat up more quickly than the others, and stays warmer for a longer period of time after the scope is switched off. I have a feeling this rectifier is blown, which I believe would cause the display to not work at all. Am I correct in this assumption?Thanks,Matt

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 05:55:53 pm
content: Maybe I should move this to a different thread? Either that, or does someone in the US have a 5Y3-GT Rectifier tube they'd be interested in selling? If not I'll just check for one at the MIT electronics swap meet in a few weeks.Thanks!Matt

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 06:04:30 pm
content: Simplest is to replace it with a pair of 1N5407 diodes and a 33R to 47R 10W wirewound resistor in series with each diode in the interim.Connect the anodes of the diodes to pins 4 and 6 via the resistors, and the cathodes together and then to pin 2 or 8 depending on the pin used for the positive output. You can safely ignore the heater, 5V of ripple will not be noticed on the HT rail. Note the resistors will run very hot, so space them where they will not touch anything and that if they run hot the cases will have to be classed as live.Look here for the pinout.http://www.radiomuseum.org/tubes/tube_5y3gt.htmlNote if the transformer gives more than 500VAC then use 2 1N5407 diodes in series in each leg, they will be better to handle the voltage drop

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 06:18:06 pm
content: Quote from: SeanB on September 22, 2013, 06:04:30 pmThanks very much SeanB. I actually just found a tube on ebay for $1 (+ $6 shipping) and I went ahead and sprung for it. I figured I should replace it with the right part, just to ensure it will work properly. Besides, I haven't used tubes in a long time, so this is kind of a fun project for meRegards,MattP.S. I did take the rectifier tube out first before ordering the new one, and the glass is indeed cracked. Looks like it will be a simple fix, replacing it.

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 06:25:59 pm
content: Good, hope that this is the only fault, but in any case I would recommend replacing the electrolytics inside and the wax coated capacitors rather soon as they will by now be very far past their use by date and will be leaky. Before powering it on warm it up in an oven at 50c for a few hours to drive moisture out of the components. The big high voltage cap next to the 5Y3 does not look too healthy ant more, and it's cousins under the chassis will be in a similar poor state. Replace them with new ones before they kill something irreplaceable like a transformer or a special resistor or pot.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: September 22, 2013, 06:37:38 pm
content: Quote from: SeanB on September 22, 2013, 06:25:59 pmThanks for the suggestions Sean. I was planning on checking/replacing the caps eventually. Either that or just keeping this thing as a conversation piece. I do have two newer CRT scopes that will actually get some use, I don't think I'd use this one much. I'd just want to know that it can turn on, and I wouldn't need it to run for any real period of time. I'll keep your suggestions in mind though, they're very good to know.Regards,Matt