EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: GoodCrossing on June 17, 2019, 03:21:36 pm
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Hi, I was using my 2215 scope and I left it on for a couple mins while I was soldering a couple things on a project. When I came back, the crt had a sort of oval in the center of the screen what was pulsing, along with a sort of chirping sound. I turned the scope off and later turned it on again. It did the same thing but there was nothing on the screen now. The sound is heard when the high pitch HV should be heard, but it pulses in and out and doesn't stop. I took the scope apart and checked the main voltages. All of them were really low. (for example 100V was abouy 8V, 8.6 was about half a V) Following the troubleshooting guide in the manual, if all voltages are bad the preregulator/inverter is at fault. In that troubleshooting guide, it asks to measure TP940, but I can't find it on my board. The manual has about 4 different revisions of the A10 main board schematic and none of them are like mine. I guess this has to do with the fact that my scope has the preregulator board and my service manual has all of the info about that board in the changes section. However, in that case, I don't know if the preregulator/inverter troubleshooting guide is correct for my scope. This is out of my area of expertise and I don't really want to go deeper without some advice. For now, all the components look fine and there are no obvious leaking caps or anything.
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I recently repaired my preregulator board in the 2215. I replaced the TO-220 power FET I think IRF820. I bought one at All Electronics which is local to me for less than a dollar.
The unit now works great. It had been mounted on a heat sink which I didn't use after the repair. I was concerned but have run the unit for many hours now without problems. I still have the parts in case I have trouble; I bought an extra FET just in case.
The preregulator board is a bit difficult of access. They warn you to note where the wires go before disassembly but of course I didn't. But it was easy to figure what to do.
One thing they advise is to bypass the preregulator and drive the subsequent circuitry with an outboard 43 V power supply. I only had 40V but that was enough; the scope worked when I did that. So try that and if it works you might try a new FET.
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Measuring resistance from main rails to ground, with the A18 prereg board unplugged, it's about 200 ohms except for one that is 1kohm. Perhaps a short down the line? I'm a bit lost because I don't know where I should first troubleshoot. Should I first try to turn the scope on without the prereg board and see if I can still hear the sound?
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Turning on the unit without the preregulator won't make anything work. You need to disconnect the preregulator board, (2 wires) and substitute an external 43 Volt power supply. That will tell you a lot. (I used 40 V and that seemed to be enough.)
If the scope works, the problem is in the preregulator. If not, there are problems farther into the circuit. But you have to take this step to tell you what the right direction should be.
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Current limit the 43 volt power supply to well under 1 amp, as the pre regulator will do the same if there is any fault in the multivibrator power circuit or derived voltage (-8.6, + 8.6, +5, +30 and +100)
There are wire jumpers 975, 965, 985, 966 that can be lifted for down-stream trouble shooting.
Also the HF, HV wire to U990 HV multiplier.
Lift one jumper at a time until the rest V's become normal.
Watch that last one, 2KV at 22 KHz.
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I will try to replace it with a 40 volt supply, but I currently don't have one unfortunately. Just following the voltage, A18 prereg board gets mains 250VAC but seemed to output 0VDC, peaking at 2VDC when I hear the sound. However, that was with one probe on P803 and another in P804. Measuring between P803 and chassis results in -104VDC???? This should be 45V or 43 as you say. And I don't know why it's negative. COM probe was on chassis GND, and P803 is marked as +.
In the midst of all this, while measuring, I suddenly hear the normal HV tone, then see a trace??? :wtf: it was strangely... fine. Then started pulsing just like the first time, and currently is doing the same old thing of not working. I tried checking if the HV multiplier was making the noise, by disconnecting P803 and 4 which I believe will disconnect power to pretty much the whole scope, making A18 and a bit of A10 the only powered parts. When turning it on, it made the same sound, so I believe the HV multiplier is not necessarily it.
Current limit the 43 volt power supply to well under 1 amp, as the pre regulator will do the same if there is any fault in the multivibrator power circuit or derived voltage (-8.6, + 8.6, +5, +30 and +100)
There are wire jumpers 975, 965, 985, 966 that can be lifted for down-stream trouble shooting.
Also the HF, HV wire to U990 HV multiplier.
Lift one jumper at a time until the rest V's become normal.
Watch that last one, 2KV at 22 KHz.
Do you mean W975 and so on? Those look to be resistors, I couldn't find any jumpers.
Sorry for being clueless by the way :-//
I also checked pin 12, Vcc, of U920 in A18. This, according to the datasheet, should be given a max of 41V. It measures at -100V as well
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If they are white ceramic, those are the wire jumpers.
I guess the idea was a hemostat on the ceramic would allow lifting one end without compromising the wire integrity.
The other TP 934 must be used for the 43 volt pre regulator output ground as it is not a chassis ground.
The positive point +43v is the +terminal of C937 (3 terminals toward rear of scope)
My scope squealed like a pig but, on lifting the U990 hv hf wire from the transformer, squealing stopped (sorry, can't hear 20 khz any more) and all dc voltages and CRT filament voltages went normal.
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The chirping noise you describe is likely the power multivibrator trying to power up and drawing over current
and the pre-regulator dropping out.
Normal power-up is one bat chirp as the frequency goes ultrasonic.
The problem is any shorted electrolytic cap in any of the dc power supplies could cause this.
I'm thinking this is why Tek has the huge number of power isolating wire jumpers all over the A10 board.
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List of HF derived low voltages, isolation wire jumpers and electrolytic caps not isolated by wire jumpers:
- 8.6V, W-976, C-972, 976
+ 8.6V, W-975, C-971, 975
+ 5V, W-982, 985, 986, C-985
+ 30V, W-965, 968, C-965
+ 100V, W-964, 966, C-961
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I took the wire jumpers out for each voltage. Every time same exact behavior was seen. Voltage on all lines is much lower than it is supposed to be, and rises slightly when I hear the sound. I think it is quite clearly the prereg board. Keep in mind I'm doing troubleshooting with the prereg board in for now, as I'm trying to find a PS to drive the 40V.
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Any of the caps listed, if shorted, will kill the voltages (pre regulator limits current and keeps 43 volts very low), and multivibrator frequency is way low.
Rather than removing, you can try charging them with batteries, wire jumpers removed.
The two caps listed for +,- 8.6 volts are in parallel through an inductor.
3 volts to charge the +5 cap, 6 volts to charge the +, - 8.6 caps one at a time, 12 or 24 volts to charge the + 30 cap and +100 cap.
If none of that works, disconnect the T 940 wire to U 990 upper solder terminal, don't let it short to the shield, which can be moved out of the way.
Slip a piece of wire insulation over the loose T 940 wire to make sure before re-powering.
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Any of the caps listed, if shorted, will kill the voltages (pre regulator limits current and keeps 43 volts very low), and multivibrator frequency is way low.
Rather than removing, you can try charging them with batteries, wire jumpers removed.
The two caps listed for +,- 8.6 volts are in parallel through an inductor.
3 volts to charge the +5 cap, 6 volts to charge the +, - 8.6 caps one at a time, 12 or 24 volts to charge the + 30 cap and +100 cap.
I believe these are unrelated. While I haven't yet found a suitable PSU, the manual says if all voltages are bad then the prereg board is the culprit. Furthermore, if I disconnect its output, the board's voltage is still bad. I have now diagnosed the board itself and I believe some component in A18 is causing overvoltage, which makes the prereg shut off and when it turns on again, same thing happens. This is supported with the fact that all voltages on the board go from lower than normal -> normal for a split sec -> higher than normal -> 0 for a split sec -> repeat. How could I find that component?
If none of that works, disconnect the T 940 wire to U 990 upper solder terminal, don't let it short to the shield, which can be moved out of the way.
Slip a piece of wire insulation over the loose T 940 wire to make sure before re-powering.
I also did that when I took the jumpers out. Same thing.
After extending the cables delivering input to A18, I can now turn the scope on with the board out of the way and I can confirm the sound originates in board A18, T933.
Here is the chirp, by the way.
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If you have a Variac, you can feed through a 75W incandescent light bulb (current limit ballast) to a full wave bridge, cap, 100 volt, several thousand uf filter to temp supply the 43 volts to P 803 +, and P 804 - (those two fast-on wires) to test the scope.
A lamp dimmer can be used if you have no Variac.
The 75W bulb will brighten full with over-current.
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Good idea, it's 12AM though so I will try it tomorrow. I'll keep you updated. However, I don't understand how there could be a problem with the rest of the scope if the prereg board doesn't output anything (I later realized I needed to check the output voltage with the board's common, indicated in the service manual, that shows the output is 0V, perhaps a few mV)
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Remember it is intended to fail to nearly zero volts if the current demand is to high.
This is why:
Posted by: bob91343
« on: Yesterday at 06:07:04 am » Insert Quote
Turning on the unit without the preregulator won't make anything work. You need to disconnect the preregulator board, (2 wires) and substitute an external 43 Volt power supply. That will tell you a lot. (I used 40 V and that seemed to be enough.)
If the scope works, the problem is in the preregulator. If not, there are problems farther into the circuit. But you have to take this step to tell you what the right direction should be.
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The prereg board still outputs 0V when not connected to the rest of the scope, and still chirps (hiccups?). So that's why I'm thinking the problem is in the board itself. Of course that's until I can get the 40V supply or do the lightbulb thing, then I'll let you know ASAP.
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I am voting for a bad IRF820 on the preregulator board. I paid less than a buck for one, over the counter at All Electronics.
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I’ve made the 40V supply as you said. The scope works like that. Now, the failure is definetly on the prereg board. I will try to get the mosfet. Which mouser says is outdated. I think I can get it, but are there some new alternatives? Perhaps my local shop has an equivalent. Otherwise, I’ll try to find the same one.
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Good news! Go to All Electronics and get one that way. Unfortunately their shipping charge nullifies any bargain price but heck, it's for a good cause.
I commend you on your perseverance in this diagnosis. Whether the failure is the FET remains to be seen but they apparently do fail, as in my case.
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I went to my local components store and they've ordered some. Unfortunately minimum order is 5 at 1.50€ each but oh well. I also ordered some replacements for the rifa filter caps, which already looked cracked. (Yes, I have turned the scope on with these out - they weren't the fault but I'm replacing them anyway)
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Glad to hear you've lined it out.
Although the 2215's are a challenge, they have the plus side of large display and light weight, compared to a 453, even though they lack some frills like scale illumination and a pilot light.
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Alright, parts arrived and were replaced, and I'm still facing the same issue. Any other troubleshooting I can do?
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The problem has been localized to the preregulator board. You can troubleshoot it or use a different scheme. In my case I just replaced the FET and all is fine. So you can toss the board and replace with a 43V power supply, or get in there and see what parts need replacing.
Switching regulators are sometimes very sophisticated and a good solution is sometimes just replace parts. Capacitors, especially electrolytic, are suspect. Measure transistors to make sure they are good. Look for signs of overheated parts. Sometimes a fusible resistor has opened. Bad soldering. Loose connectors.
It's not magic. There is something wrong and you just have to get to it.
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Are there any techniques to isolate the problem to a part of the circuit or something? I'm honestly not sure where to continue.
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https://www.ebay.com/itm/233180863827 (https://www.ebay.com/itm/233180863827)
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Good idea, it's 12AM though so I will try it tomorrow. I'll keep you updated. However, I don't understand how there could be a problem with the rest of the scope if the prereg board doesn't output anything (I later realized I needed to check the output voltage with the board's common, indicated in the service manual, that shows the output is 0V, perhaps a few mV)
Was it around 12AM when this started ? It could be the scope is haunted somehow. Anything from the shipping truck killed someone with it onboard, to someone killed at the factory where the screws were made, or someone used it to murder someone, who knows.
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Alright, I've tinkered with it some more and I feel like I am getting close. I measured the caps and checked some for shorts and they seemed fine. The diodes also seem fine. I decided to take a look at the Q933 MOSFET, and decided to check for a short there, as I hadn't checked for a short after the rectifier. Lo and behold, 0 Ohms to ground at the source. Suspicious! I started checking all components going to ground thru the FET, and none of them were shorted after desoldering them. About halfway through this, though, I decided to take a shortcut. (Pardon me.) I fed in 5V limited to 1A with my PSU from the source to ground, and checked which component got hot. And it was T933! Unfortunately I ran out of time so I'll desolder and check the transformer when I get back to the scope on Friday. I also suspect U920 might be dead, so if I can buy a replacement at my local components store I'll get one just in case.
https://www.ebay.com/itm/233180863827 (https://www.ebay.com/itm/233180863827)
I will probably get that board. I wanted to troubleshoot mine a bit more before I did, though.
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Qservice are good people to work with from my experience. I would grab the board for $25, looks like you already have. :-+
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No, I actually didn't buy it; it looks like the seller pulled the listing actually. I was speaking with them about my board, and I asked if they would include the thermistors for a different price, or if they could confirm it was working before removing them; as the listing was classified as for parts/not working. They told me they had all components in the board in stock so it would make more sense to buy those instead of the entire board to repair my scope. I sent them the part number of the transformer, and they haven't replied since. After seeing your post I noticed they pulled the listing, which is kind of a bummer because the only place I've found selling the part I need charges quite a bit for shipping so it ended up being better to buy this one.
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There are a lot of the 22XX series scopes out there, though maybe not in Spain. I would look to see if your board was used in another of the 2XXX series scopes, that would give you more to choose from looking for a donor scope. I have a 2225 and found a parts scope for $25 that I could pickup without shipping. Even if you found a board that was similar you may be able to rob some components.
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I had a 2215 with something loose inside the CRT. Occasionally it would sit there chirping refusing to start if that loose bit landed in the wrong place.
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https://www.ebay.com/itm/323857754896 (https://www.ebay.com/itm/323857754896) Posts to UK ONLY.!
I don't mind buying it and forwarding it to GoodCrossing in Spain.
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Oh, that'd be great. I'll be contacting the seller to make sure it works and will get back to you.
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Well I thought I would do a final update after getting the board.
Replacing the board obviously fixed the issue but I took a moment to compare some measurements on the new and old boards, which pretty much confirmed T933 was internally shorted to ground. Still, replacing the board was easier and cheaper than replacing that component as it was more expensive than the board @pbarton found. So, the noise was probably caused by the board resetting after detecting the short to ground, and the transformer turning off and on again quickly.
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Really good luck to find a reasonably priced, working A18 board.
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Good to know that the replacement board fixed the problem. Well done.
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That's surprising, I don't recall ever encountering a bad SMPS transformer. You might look closely and see if you can find a short between the windings and the core or something where the leads come out.
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It does seem that the T-933 ferrite core is very likely non-conductive and not likely to allow a short to ground from any leads.
CR 913 feeds back "run power" to the PWM operation, check it is not opened or shorted
R 933, 0.2 ohm, feeds back current to the PWM, not to exceed 1 amp (200 mV across R 933).
Crow bar Q 935 (SCR), R 934 (3 ohm) and VR 935 (51 volt zener diode) trip and cause excess current, putting the circuit into a repeat start (chirp) mode if 51 volts is produced or the zener diode or Q 935 are shorted.
I've just refurbed one of these with the FET (Q 933) gate-to-source shorted.
Replaced the FET and R 934 (burned with no voltage regulation) and it now runs a 90W incandescent bulb at 40.6 volts through a variac.
It will repeatedly chirp, trying to start if the variac is less than 70% of 120VAC.
These can be easily tested out of their scope, attach a small piece of Aluminum plate to Q 933 for a heatsink.
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Got one that chirps the same way, flex the A 18 board corner and normal operation resumes.
Flex/relax problem repeats, start trying to narrow it down, 1 amp fuse blows, remove A 18 board, test and find gate-source leakage in Q 933 and crow bar fires when AC voltage in is raised above 60.
Checking T 933 winding 9,10, the one that is negative line bridge rectifier (pseudo ground) tied to one side and earth ground through C 932.
That resistance measures 0.4 ohms, whereas the similar (in wire gauge) 6, 7 winding shows 2.2 ohms.
The winding is not used as the grounded shield is installed and there is no C 932 installed.
I have other 2215's and will check that winding resistance on at least one.
If it turns out to be shorted, I have two options, since all tektronix 120-1439-00 seem to have been exhausted:
1) dis-assemble, count turns, measure wire gauge, rewind, eliminating the 9, 10 open circuit winding as not needed.
2) got two of these cheap, NEW MAGNETEK TRIAD TRANSFORMER F-40X 115V VOLT 26.8V.C.T@1A AMP,
Wire the 120 VAC in parallel fed by P1, P2 to A18 one full secondary plus the second to the center tap to
fast-on 1, 2 on the A 18 board, a 10 ohm resistor or choke (AC Power Choke, ( D-37-N) 0,6 H 0,51 А, made in USSR , NOS. Lot 1pcs) between T 933, 1 and Q 933, D.
Already checked the scope with a 40 volt bench supply for several days.
I would leave the crow bar circuit and 1 amp fuse to blow if the supply goes over-voltage.
this arrangement can also have the primaries wired in series for 240 volt operation.
Internally mounted, this would add about 4 pounds to the weight, but nowhere near the 12 pound weight of the Sorensen 40-0.75 supply, not to mention only a single lift.
Anyone with T 933 winding turn-count /wire gauge would be greatly appreciated.
Keep the old dogs running!
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Tested T 933 winding resistances on a working A-18 board:
1-2, 2.2 ohms
4-5, 1.8 ohms
6-7, 2.2 ohms
9-10, 0.4 ohms
Did find numerous cracked/poor looking solder joints so re-soldered all and cleaned flux with IPA.
I had already replaced all of the Rifa type and power supply electrolytic caps.
Scope now works fine.
All low voltage power supply values in spec'.