Tektronix 2235 - power supply issues

[AaronB]

My 2235 scope has a problem with the power supply. It blew the fuse (1A slow blow) as soon as I turned it on. I have downloaded the service manual and it says the correct fuse should be stated on the rear sticker, that is no longer there, it's just a plain aluminium panel. Someone has pencilled in 2A FAST next to the fuse holder, but I'm pretty sure that is wrong for a UK machine at 240V, and 1A slow is correct.

Tracing back from the mains input, it looks like the A6 filter board is OK - measures 470K. The next section from C904, the 4 rectifier diodes, and C906 seems pretty suspect - C904 is a dead sort (obviously that's why the fuse blows). The diodes don't measure how I would expect, and C906 looks original and probably a good candidate for replacement. I will replace these components (suggestions welcome) but what more can I test to see if this is the only problem, and that there isn't something else further down the line that caused these failures?

I guessed on posting in the repair section, rather than testgear, I will repost if this incorrect.

Thanks in advance for any help.

[akimpowerscr]

Hello and welcome to the forum.

There have already been many topics on the power supply of the Tektronix 2235 oscilloscope,

Before opening a new topic, I advise you to use the search tool to check if the answer to your question is not found in another topic

A first tip: do not replace any part until you are certain that it is defective.

In your case, this is obviously a short-circuit and therefore easy to detect.

You need to check (in the order of the list) if the following components are not shorted (possibly unsolder one side of the component and unplug it from the PCB to check it)
- mains filter A6
- C904
- diodes CR901, CR902, CR903 and CR904
- -C906
- Q9070
- Q935

NB: fuse: 1A slow blow.

[AaronB]

Thanks for the help, I did try searching but as you say, many topics, difficult to sift through.

I removed C904, it was out of spec at 3800pF instead of 2200pF, but that wasn't the short. Unplugging Q9070, allowed me to measure C906 - not bad at 77uF instead of 75uF, ESR 0.5. Diodes CR902 and CR904 measure OK on the board, It's the other two diodes in the bridge that are both shorted. After replacing them I need to check if the Q9070 FET and Q935 SCR are still OK.

[akimpowerscr]

Well done !

For capacitors, there is no need to worry if the measured value is higher than the nominal value.
The tolerance must be taken into account, for example, for an electrolytic it is + 50% and -20%
As for the ESR, there are many applications where it has little or no influence on the operation of the circuit.

In general, too high an ESR of a capacitor goes together with a much too low capacitance.
Preferably replace all 4 diodes, not just shorted diodes and check Q9070 FET and Q935 SCR

[AaronB]

I have a stash of 1N4007 diodes that should do the trick for the rectifier circuit.

It's handy that it is so easy to unplug Q9070. My cheapo component tester will normally recognise a mosfet, but shows Q9070 as a couple of connected resistors - so I figure that is also bad as I believe they fail closed.

Urgh, Q935 is in the corner of the board, right underneath the edges of the chassis, but pretty sure it'll have to come out to be tested. I guess I could leave it and hope that it is OK after fixing the above.

Is there an average number of fuses that will get blown before people figure out all their PSU issues?

[akimpowerscr]

It's handy that it is so easy to unplug Q9070.

Later, after the fault is repaired, I advise you to eliminate this connector and directly solder the wires to the pins of the mosfet.
These crap molex connectors cause bad contacts and heating of the mosfet pins.

In the event of a fault, Q9070 short circuit or explode ..... easy to tell the difference between a faulty mosfet and a good one

Q935 will short circuit when it is defective, it is easy to check in circuit with ohmeter , no need to disassemble it.
 

[Rickenbackerman]

The bits under the edges of the chassis are a pain but if you're careful, have a nice skinny tip (that's what she said), have just the right angle and enough light you can get to them.

If you scroll down you'll see I have a similar thread going on my own 2235 PSU.  No luck yet.

I'm starting to think bd193's plan of action of sticking a 48V Meanwell unit in these and calling it a day is the way to go, which is what I'm planning on doing.  The factory supplies are sketchy as hell anyway.

[akimpowerscr]

The bits under the edges of the chassis are a pain but if you're careful, have a nice skinny tip (that's what she said), have just the right angle and enough light you can get to them.

If you scroll down you'll see I have a similar thread going on my own 2235 PSU.  No luck yet.

I'm starting to think bd193's plan of action of sticking a 48V Meanwell unit in these and calling it a day is the way to go, which is what I'm planning on doing.  The factory supplies are sketchy as hell anyway.

The failure of your oscilloscope is different from that of AaronB and therefore requires a different diagnostic procedure.

I think we must therefore leave these two very distinct topics.

In fact, you have already been told what to do and you were asked a question that you did not answer:

"With an external 43V dc power supply, does it work or not?"

the crowbar protection trips at 51V, then 48V is a little too high .... to lower between 40 and 45V.

Why not just buy a 2x30V 2A laboratory power supply?

With the 2 adjustable 30V outputs in series, you can easily supply 43V to the oscilloscope.

[AaronB]

Q935 will short circuit when it is defective, it is easy to check in circuit with ohmeter , no need to disassemble it.

It doesn't measure as a dead short, but the resistance values seem pretty low: 11, 120 & 131 Ohms - I think she'll have to come out. The parts reference says it is a C106B2, but not readily available here in the UK, C106BG looks like a good alternative. It's not expensive, so I'll add it to my parts order.

[akimpowerscr]

If this thyristor is not shorted, do not replace it.
In 90% of cases, a defective thyristor has a dead short circuit between anode and cathode following the melting of the semiconductor chip.
The resistances you are measuring are no doubt due to the fact that you are measuring it in the circuit.
In general, thyristors are very rugged semiconductors.

With a new fuse in place and the switch on, you should no longer measure a short circuit at the 230V input.

[akimpowerscr]

If there is no longer a short circuit, it is now time to try powering your oscilloscope with an AC voltage source with current limiting.

For this, take a 230V 100W incandescent bulb and connect it in series with the oscilloscope.
Supply the oscilloscope + 100W bulb assembly in series by the 230Vac

If the oscilloscope tries to start and the lamp only lights up halfway, all is well, you can then supply your oscilloscope with 230V without the bulb.

[m k]

The parts reference says it is a C106B2

It's an internal over voltage protection and deliberately short circuiting thing.
Anything capable will do.

[TERRA Operative]

I did a video on restoring one of these scopes, you might find it useful.
I included the service upgrades too, so you can update your scope to improve performance and reliability.

[Rickenbackerman]

In fact, you have already been told what to do and you were asked a question that you did not answer:

"With an external 43V dc power supply, does it work or not?"

As I said in my thread, I do not yet have a suitable power supply.  I've got one on the way now and will report back when it gets here.

[AaronB]

The bits under the edges of the chassis are a pain but if you're careful, have a nice skinny tip (that's what she said), have just the right angle and enough light you can get to them.

And the CR903 diode is even more difficult to get to. But there is a solution.

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[AaronB]

Does anyone have a full size, single scan of the Power Supply, Z Axis + CRT fold out schematic from the Tektronix 2235 service manual? The pdf version I have has it split across two pages and there is a missing section between G/H.

[akimpowerscr]

https://www.eevblog.com/forum/testgear/tektronix-2235-repair-thread/

[TERRA Operative]

Hopefully this helps. It's from the 2236, but the missing parts should line up. It's almost identical to the 2235 if you include the greyed parts. Plus the addition of the fan (a worthwhile upgrade to ensure longevity).

I also added some more useful info, I can provide specific details on the parts needed for the service update if you decide to do it.

Does anyone have a full size, single scan of the Power Supply, Z Axis + CRT fold out schematic from the Tektronix 2235 service manual? The pdf version I have has it split across two pages and there is a missing section between G/H.

[AaronB]

I was thinking of adding a fan. I have a miniature 12V fan that runs acceptably, and only draws 35mA, from a 9V supply, so I was just going to hi-jack the +8.6 rail and mount it close to the heatsink for Q9070.  But it may be overkill for something that I would rarely ever use for more than 1 hour at a time.

[TERRA Operative]

There's a place to mount a fan on the rear panel, it just screws right into place and blows directly onto the PSU heat sinks.

[AaronB]

The fan I was considering is very small, maybe only 4cmx4cm. Kind of thing you would mount directly on a heatsink, like a CPU fan. I would stick some fins to the back of the existing, flat heatsink and then attach the fan to that. It may not be much, but better than nothing. Maybe even just the extra cooling fins on that heatsink would be enough. But that's in the future.

First off, I'll replace the broken bits I know of. If that doesn't fix it, then I'll consider the external 43V PSU option. If it is then working, then I'll think about the service updates and a fan. If I cannot get it working, then look out for an eBay listing for a broken 2235.

[AaronB]

Well that was disappointing    It has stopped blowing fuses but is otherwise still dead. I get nothing between TP940-TP950, maybe it's time to try the external 43V supply test. Is there any reason to isolate any other parts of the board or is it just a case of attaching +43V to TP940 and 0V to TP950?

[akimpowerscr]

There is no reason to be disappointed, repair is that, step by step.

If you don't have patience, repair is not for you.

You have taken a step, you no longer burn a fuse, that's already very good.

Let's temporarily abandon the repair of the pre-regulator to check if the rest of the oscilloscope is working correctly.

I usually solder two wires directly on the printed circuit, on the terminals of the capacitor C940 to apply 43v there - pay attention to the polarity!

Give us the results of your test with an external 43V power supply

[AaronB]

I tried a few more component extractions for testing that had been mentioned in other threads. The Q935 thyristor, C925, C943, and R898-R891 - no luck there. So I went for the addition of a 43V PSU across the two test points. The first PSU I tried didn't work so well and I switched it off as I thought things were getting hot. Second one I tried caused the classic tick, tick, tick and two flashing neons of a resetting power supply. Carefully touching the board for hot components, and visual inspection, showed a bit of shadow on the PCB around CR907. Sure enough the smoking diode - a dead short.

It is marked as UES1106, which is a fast recovery diode, not something I keep in stock. Not sure what part it has to play in the circuit when bypassing the original supply, but with it removed, and also Q935 and Q9070 out of circuit, I get the power LED on and no ticking  . I once briefly saw a fuzzy rectangle on the display when I pressed BEAM FIND  , but it has stopped doing that now  . The PSU is drawing about 700mA in this state. Do I need to replace that diode, or is it no longer part of the equation with the new 43V PSU in circuit?

Getting closer.

[bdunham7]

I'm pretty sure it will work without it (and I think you found your problem) but just to be sure just test all the power supply voltages to the right of the transformer except the -2kV high voltage, unless you can do that completely safely.  If the scope seems powered up but you have no beam, check R886, R888, R889, R890, R891, R892 and R894 to see if they are high or open.  If those are bad, let me know and I'll look up the p/n from Mouser, as there is a specific resistor that works well in that application.