Another Tektronix 2215 oscilloscope repair - Page 1
Forum section: electronics / repair

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 02:25:22 am
content: This is a spin-off from this thread, where both Vince and myself had similar issues with the pre-reg board on some Tek 2215 oscilloscopeshttps://www.eevblog.com/forum/repair/tektronix-2215-scope-repair/50/I thought I'd start a new thread as I'm now really scratching my head and thought I'd throw it out to a few others for an opinion.The unit has no trace, however there is a (VERY LOUD ... which is driving me crazy every time I power it!) high pitched whine. Flicking things like the trigger switch etc on the panel sees the LED flick on, and pushing the beam find loads the supply (whine changes). So it wants to work, but doesn't. I thought pre-reg, as that's what failed previously, but I couldn't fault that.As suggested by Vince I am now powering the unit from a bench supply, that takes the pre-reg out of the equation. The unit is drawing approximately 1A @ 42.2V from the supply. The standard test output voltages are all present, but low: TP500 -5.6V (-8.6V); W975 5.6V (+8.6V); W985 4.0V (+5V); W965 20.7V (+30V); W966 71.0V (+100V), (brackets = nominal correct voltages as per manual). The manual provides a typical resistance for those points, and the 5V rail is reading 160 ohms vs book value of 330 ohms, but all other resistance values are close to suggested. I isolated most (if not all) those rails and it didn't seem to make much, if any difference.Next is to check the Head Room voltage at TP952 and confirm/adjust it to +4.3V. Well here is a problem as it was reading +15.3V and could not be adjusted. A relatively easy fault I think so I check the power supply regulator and everything looks ok. Thinking maybe I've missed something I swap Q948 and Q954, but no change. Much head scratching. Everything I check looks ok, but something isn't working quite as it should. So I have written down the DC measurements to see if I'm missing something here. Attached is a photo of a copy of the circuit .I'm also wondering if the circuit is actually fine and I'm chasing a red-herring here? Could it be that there is something (like the HT supply for example) that is loading the output and causing the symptoms I'm seeing?

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 03:02:54 am
content: Before anyone wastes any of their time on this (as I've done the past day, grrrr!), my hunch I was chasing a red-herring was possibly correct. I'd isolated other power supply rails but of course the one I didn't was the one possibly causing the issue. I removed the output of T940 to the CRT high tension coil (U990) and all the voltages came back up to spot on, with the power supply regulator being able to be adjusted to the correct voltage. Rather annoying that was the situation as I thought that one would be ok and not worth disconnecting; Murphy's Law!I will now need to see if I can find out what was actually causing the issue there. It could be bad news for that CRO if the coil is shot, not sure if they are available. I'm of course hoping that it's something further downstream as I still need to diagnose that area, just didn't want anyone to be messed about.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2018, 03:47:41 am
content: It's not a coil it's the PDA multiplier and from -2KV to +8KV makes it a quintupler; 5 stages.They're a string of diodes and caps.Bypass/remove it and check everything again to be sure it's at fault. They draw only mA's but if they're shorted nothing else will work.http://w140.com/tek_2215_service.pdfRelevant schematic page 169/211.

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 04:11:35 am
content: Yeah thanks, after I posted it I saw what I thought was X4 for the designator was actually U990 and the X4 was the voltage multiplication. Therefore I wondered if they'd done it that way but didn't know what was inside so just left it as "coil" in my post. It really doesn't matter as either way it looks like it's shot. Removing R992 isolates it (other than one small cap) and it has the same symptoms so I'm going to say it's rooted. That's a shame as I can't imagine they're easy to get and it probably makes this CRO a boat anchor.Given that I had been working on the power supply I thought it was most likely that was going to be the issue again, so it caused me to waste a ton of time going through the PS when in fact it looks like it was just coincidence and the problem was completely unrelated.

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 04:52:09 am
content: Strangely I can't see that voltage multiplier in either my manual (paper) or the pdf version in order to get the part number to see if they're realistically available. If it can be landed on my door at a realistic price, having gone this far, I may as well throw one in. However I will need to first find out the part number.BTW, from another thread, have a Siglent SDS1202X-E coming at me, not sure when it will arrive as it needs to come from Adelaide.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2018, 05:14:17 am
content: If the quadrupler is stuffed you can build your own without too much fuss and bother.......stay tuned while I hunt out the relevant thread.The meat of it starts here:https://www.eevblog.com/forum/repair/tektronix-465/msg651735/#msg651735There should more than enough guidance there to help you through.However do write it up for others ............

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 05:23:14 am
content: I was wondering the same thing, they should be nothing more than a few diodes and caps. There is a limit to how much time I want to tip into this thing however and don't think I would go down that path. I tend to be a bit obsessive so wasn't going to give up until I found the faultNow I've found it (well pretty sure I have anyway), I have to come back to reality a bit. The original is likely potted (but haven't checked) so probably can't be repaired. It's a shame as there's isn't much to them. It's likely they also rectify the cathode supply in there as well, I can't see an external one, so it all gets to be a bit of a labour of love ... and I have plenty of other luvin to do!!!The part number was marked on the side, very convenient of Mr Tek. It's 152-0791-00 the bad news is the only one I can find is over A$130 landed! That's crazy

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2018, 05:27:04 am
content: Yeah of course, a DC restorer diode need be in there too. Good catch.

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 05:46:27 am
content: Does Tektronix have an office in Australia or who is the master distributer? Sometime these parts can be available, though this is now so old I'm not holding my breath, and by Tek standards was a POS from day one in any case.I tried to ring their "Australian" number and it went to some menu option thing in God knows where, so I didn't waste my time.I suspect the best solution for now is to pack the unit up, push it aside, and create an alert on ebay for when one comes up at something that is less than the replacement price of the thing! All a bit frustrating, that fault took a couple of days to figure out (admittedly not full days, but much longer than I'd wanted to spend), and was on the back of a previous fault I'd repaired in the pre-reg. I expected to have something working at the end of all that!

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2018, 05:47:55 am
content: I wouldn't despair too much about price right now as they sometimes can be had real cheap....this one closed back in April for only $15 +pp.https://www.ebay.ie/itm/Tektronix-152-0791-00-High-Voltage-Module-NEW/173188861235?hash=item2852dc4d33:g:Mm4AAOSwK5tamJAsGrrrr

author: Pete F
author_rank: Regular Contributor
timestamp: July 10, 2018, 06:00:44 am
content: I saw that one and is why I thought I'd sit on my hands for a while. I prefer to just run with things while I'm doing them, coming back to them months or possibly years later is rarely a good idea, but the only other one I can find (also on US ebay) is just too expensive to justify it. Under A$100 and I'd think about it, but not for what they're asking. Indeed looking through the seller's site and the prices being asked I'd suggest they're a bit of a comedian!