Heathkit Oscilloscope CRT Help - Page 1
Forum section: electronics / repair

author: CrowdedOuthouse
author_rank: Newbie
timestamp: June 30, 2017, 09:40:43 pm
content: Hello!I got a Heathkit OR-1 Oscilloscope recently and have been working on repairing it.I replaced all the power supply filter capacitors, and that went well. I tested all the voltages and they more or less all match the schematic (http://www.vintage-radio.info/download.php?id=392)Before I did this there was some output on the screen, just a dot in the center. Now there's no output at all.I think it's because the CRT heater isn't working, because it doesn't get warm, and I can't see any glow. I tested the transformer secondary and that's good, and I also tested the DC resistance of the heater and it's about 3 ohms -- not open.Does this mean that my CRT is blown? or could it be because of something else? What could I do to troubleshoot this? I'm not too familiar with these.Thanks!

author: orbanp
author_rank: Regular Contributor
timestamp: June 30, 2017, 10:41:30 pm
content: If the heater is fine on the CRT, then chances are that the CRT itself is all right.You only had a point on the screen as there was no deflection. Now it is possible that the point, or the line (if the sweep generator is running), is off the screen.Start by checking the DC voltages on the CRT and on the final tubes, V3, V8. Luckily the schematics gives all the proper voltages.Be careful, the highest voltage is 1.6kV!Another scope would be useful to see if the sweep generator multivibrator is running.Good luck, Peter

author: CrowdedOuthouse
author_rank: Newbie
timestamp: July 01, 2017, 01:29:20 am
content: I checked all the voltages and they're all within 10%.The thing is, I can't really tell if the CRT heater is working.It isn't an open circuit, and it's getting 6.3v AC. It also has ~-1600v DC from the Gate #1 as the schematic details.Looking into the glass, I can't tell if it's heating up or not. It doesn't feel warm after it's been on a while. I did take the tube out of it's shield when I was rebuilding the power supply. Could this have broken it somehow? It came out without much effort, so I don't think I was really rough with it but I'm not sure.Also I have been testing it without the little transparent - green sheet that goes in front of the tube. Is that a necessary part to get any output on the display?Anyway is there any easy way to test the tube? Or are the heaters kind of tough to tell that they're running or something?I tested the sweep generator with the frequency detector on my multimeter and it was working.Any ideas would help.Thanks

author: FlyingHacker
author_rank: Frequent Contributor
timestamp: July 01, 2017, 02:11:12 am
content: You checked the deflection voltages at the pins on the tube? 7,8,10,11Check all the wires that go up to the tube.Check the intensity and focus voltages and the pots.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: July 01, 2017, 04:24:26 am
content: After you check the deflection voltages, take a close look at the tube socket. I've had those break way inside where it isn't immediately obvious it's broken. While I don't know that particular tube I think you should see something, and it should certainly feel warm near the base after 5 minutes.Minor items: No the green plastic graticule does nothing electrical. 3 ohms sounds about right for a filament.

author: voltz
author_rank: Frequent Contributor
timestamp: July 01, 2017, 09:04:25 am
content: As already mentioned, the tube sounds like its fine. 3 ohms is correct.If its still not heating up, and remember not all tubes show an actual orange glow or feel warm, then check the CRT base holder.These contacts can break over time.With the scope off, disconnect one side of the heater to the transformer. Check you still have 3 ohms at the CRT holder. In effect checking the contacts are working. If everything checks out, contacts working, transformer giving 6.3v etc.. then its likely the fault is somewhere else and not the heaters. A broken heater would be o/c for sure.

author: tautech
author_rank: Super Contributor
timestamp: July 03, 2017, 10:12:04 pm
content: Quote from: CrowdedOuthouse on June 30, 2017, 09:40:43 pmThis pic of assembly of Tek CRT's shows the heater element is inserted into the cathode assembly in a manner that it's probably not possible to view the heater when powered and hence not able to see any glow.Resistance therefore is your only check.https://youtu.be/jHGAnJjnNY0?t=835

author: Nemo1956
author_rank: Contributor
timestamp: July 06, 2017, 05:33:58 am
content: 3 ohms sounds about right to me as well. You can put a multi meter set to AC current in the feed wire from the filament winding of the transformer. If on current no heater but I bet it's ok (But be careful as the cathode volts are very high so keep your finger out from harms way). Also you can try putting a 6.3 volt (check the heater voltage before you do this on the web as it could be 5V? I can't remember now) transformer directly onto the heater pins of the tube but first remove the base connector and just run this and again with a AC current meter in circuit. Its been a very long time since I have worked on one of these.

author: Nemo1956
author_rank: Contributor
timestamp: July 06, 2017, 05:52:23 am
content: Just letting you know the tube runs on 6.3 volts for the heater I just looked it up in my records