Author Topic: Tektronix 2215, pulsing/chirping noise, no trace.  (Read 1928 times)

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Offline GoodCrossing

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Tektronix 2215, pulsing/chirping noise, no trace.
« on: June 17, 2019, 03:21:36 pm »
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.
 

Offline bob91343

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #1 on: June 17, 2019, 03:53:05 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #2 on: June 17, 2019, 07:04:44 pm »
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?
 

Offline bob91343

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #3 on: June 17, 2019, 08:07:04 pm »
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.
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #4 on: June 17, 2019, 09:31:20 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #5 on: June 17, 2019, 11:01:29 pm »
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
« Last Edit: June 17, 2019, 11:03:57 pm by GoodCrossing »
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #6 on: June 18, 2019, 12:27:54 am »
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.
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #7 on: June 18, 2019, 01:01:32 am »
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.
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #8 on: June 18, 2019, 11:37:34 am »
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
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #9 on: June 18, 2019, 02:33:51 pm »
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.
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #10 on: June 18, 2019, 08:58:19 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #11 on: June 18, 2019, 10:02:46 pm »
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.
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #12 on: June 18, 2019, 10:24:47 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #13 on: June 18, 2019, 10:30:08 pm »
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)
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #14 on: June 18, 2019, 11:48:00 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #15 on: June 19, 2019, 12:11:50 am »
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.
 

Offline bob91343

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #16 on: June 19, 2019, 04:38:39 am »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #17 on: June 19, 2019, 01:10:30 pm »
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.
« Last Edit: June 19, 2019, 01:13:01 pm by GoodCrossing »
 

Offline bob91343

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #18 on: June 19, 2019, 05:02:09 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #19 on: June 19, 2019, 08:56:45 pm »
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)
 

Online jdragoset

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #20 on: June 20, 2019, 12:26:31 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #21 on: July 08, 2019, 02:14:33 pm »
Alright, parts arrived and were replaced, and I'm still facing the same issue. Any other troubleshooting I can do?
 

Offline bob91343

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #22 on: July 08, 2019, 04:01:33 pm »
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.
 

Offline GoodCrossing

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Re: Tektronix 2215, pulsing/chirping noise, no trace.
« Reply #23 on: July 08, 2019, 04:36:10 pm »
Are there any techniques to isolate the problem to a part of the circuit or something? I'm honestly not sure where to continue.
 

Offline pbarton

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« Last Edit: July 08, 2019, 06:53:36 pm by pbarton »
 


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