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465b blew when GND was connected to High Voltage - little help!

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Dasmooler
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465b blew when GND was connected to High Voltage - little help!

Post by Dasmooler » August 26th, 2018, 9:52 am

Hi,
New to this forum, I'm hoping someone can point me in the right direction. An engineer gave me his old 465b that he broke by connecting the ground lead to HV without floating the scope. Now it blows the input use at turn on. I have opened it up and visually can not find any melted traces, burned components, or any signs of damage at all. Given the mechanical assembly of the scope, access is very difficult Can anyone point me in the right direction? Any help would be appreciated.
Thanks,
John

strick
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by strick » August 26th, 2018, 1:42 pm

John,
You can download the 465B service manual - - I just got it from Tek. It has procedures on troubleshooting the power supply toward the rear of the manual. The directions are not simple and you have to have a lot of equipment.
You will certainly need a good multimeter and perhaps another oscilloscope. Lacking either of these is bound to be disappointing.

The good news is that the power supply is not a switching power supply. If it was, I would tell you to stop unless you have worked with and are comfortable with 500-600 volts.

The bad news is that it has a single large power transformer T14500. If it got cooked by the high voltage (a real probability), then you are done. Good luck finding another one.

Were it me, I would disconect everything I could from the transformer, hook a variac to it and check for voltages. You do have another scope?

- good luck!

:) Strick

Dasmooler
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by Dasmooler » August 26th, 2018, 2:40 pm

Thanks, Strick,

I have been reluctant to take it apart for fear of not getting it back together! I have the manual, but even with that it seems a daunting task. It seems this was not made to be taken apart! I am comfortable with HV - I designed switchers back in the 80's (dating myself) and HV neon sign xfmrs. My equipment is an old Fluke 8050 DVM, 0-30V split supply, and a variac. My scope bit the dust years ago.

If it is the xfmr that would make sense because the fuse doesn't blow until the variac is partially up - how high I don't recall). All of the internal fuses feeding the circuits are good, and the ones that aren't fused didn't draw excessive current when powered with my power supply - although I don't have a way to supply 150V.

I am doing this mainly for fun - I don't get to play with electronics much at work anymore, it's just when I opened it up and saw how it's constructed - well, my confidence level dropped like a rock!

Thanks for your help - I didn't even consider the xfmr being bad.
John

strick
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by strick » August 26th, 2018, 6:34 pm

John,
Well, if you really need a scope, I sell used Tek scopes - primarily TDS500-700 series that I've recapped, replaced the NVram and calibrated at cost. Again, like you, I do it for fun, not my regular job.

Anyhow, If you open all the fuses past the main and disconnect everything you can find on the schematic on the transformer, you should be able to check the voltage levels on all the transformer leads, even with an old meter. However, much past that, you really need a scope to dig into any very far in a Tek scope, even an old one...

I'm just finishing up recapping a 2465 300MHz scope that I've had for a couple of years. If it doesn't blow up when I put it together tomorrow, I'll be happy to send it to you if you will pay the shipping. I really don't want or need it (I have everything from 784C to 540As) and my workhorse is a 540C with 2M and a hard drive...

I live in Florida and the scope weighs about 20 lbs...

Strick

Dasmooler
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by Dasmooler » August 27th, 2018, 5:34 pm

Thanks Stick, but the thing is, I don't ally need a scope these days! My interests have turned to hobby machinist where I enjoy making stuff with a mini-lathe and mini-mill. Truthfully, I was expecting this to be a blown trace somewhere and be an easy fix. But, being an engineer at heart, I am going to continue to plug away at it bit by bit.

Thank you for your very generous offer. I could bring it to work where I have Tek scopes costing >$10k at my disposal, but I don't have the space or the time.

I'm pretty sure I won't get it fixed, but I'm gonna have fun trying!

strick
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by strick » August 27th, 2018, 6:00 pm

Ok,
No problem.
Strick

hexley ball
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Re: 465b blew when GND was connected to High Voltage - little help!

Post by hexley ball » August 30th, 2018, 8:51 am

Some random comments about the 465B power supply:

1. The most common power-related problem on these scopes is for one (or more) of the dipped tantalum bypass capacitors to short circuit. They are prone to do this if exposed to a voltage over their rated maximum. I suppose it is just possible that when the scope's ground was connected to HV, the resulting current surge down the ground path could have coupled into the voltage regulation circuits, caused a momentary spike on one or more supplies, and thus caused one or more tantalum capacitors to short circuit. Those would be the first things I would check.

2. The regulated outputs for +55, +15, and +/- 5 volts have clamp diodes to prevent those supplies from going towards the wrong polarity with respect to ground. See CR4426, CR4405, CR4425, and CR4515, respectively. Might be worth checking those also for damage.

3. The 465B uses the +55 supply as its main reference for the low voltage supplies. So you might want to start troubleshooting the +55 supply first; if it is down, then the +15 and +/-5 rails won't work properly.

4. It is telling that the internal fuses are intact even though the line fuse blows. That suggests that the fault is in some unfused part of the power supply circuits. The transformer is one possibility, as has been mentioned. Another place to look could be the +110V and +55V regulator circuits, as those are unfused. Also, the fuse in the +15 supply comes **after** the internal regulator circuitry, so if something failed there the supply rail fuse would not be affected. You might want to look for shorted pass transistors and things of that nature in these areas.

5. As Strick mentioned, the service manual describes how to isolate the supplies from their various loads. Sometimes it just requires removing connectors; other times it requires unsoldering Service Jumpers. This is essential to isolating the fault, of course. Please refer to the manual sections, "+55V Supply Troubleshooting" and "+15V, +5V and -5V Supplies Troubleshooting" for details.

Good luck,
hexley

Dasmooler
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Country: United States

Re: 465b blew when GND was connected to High Voltage - little help!

Post by Dasmooler » September 2nd, 2018, 10:03 am

Think I found my culprit. After removing the pcb that was blocking access to the bottom of the board with the bridge rectifiers I found CR4516 was shorted from pins 4-3 and 4-2. I'm pretty sure I can replace the bridge without removing the board (tight quarters) but, of course, I'm out of solder wick so the repair will have to wait.

One thing that has me absolutely puzzled is how the switch/pots for the triggers are assembled. Both the inner switch and the outer pot are held with set screws. The outer part completely blocks access to the set screw for the inner switch. I had to pry the knobs off (believing the switch was a press fit) and then discovered the inner screw. Not even a ground down allen wrench could fit in there. Totally stumped. I hope I can press fit it back on (tappy tappy). :D

strick
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Country: United States

Re: 465b blew when GND was connected to High Voltage - little help!

Post by strick » September 2nd, 2018, 5:15 pm

The 465B service manual has explicit disassemble directions. Always best to look in the manual to keep from busting stuff. If you break some plastic part, good luck finding a replacement (unless, of course you have several lying about..).

:) Strick

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