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Tektronix 2235 repair thread

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bd139:
Another Tektronix repair thread. Thought I better break this off the TEA stuff so it's easier to find if anyone else comes across it.

Bought a 2235 from a hamfest type event for a whopping £25. Described as working, which it mostly was for a bit so no complaints there. Powered it up, seemed ok bar a couple of issues. Decided to do a full refurb on it as these are known to be occasionally dodgy as hell.

Pretty good condition:



First step I did was empty it of the ageing electrolytic capacitors and mains filter:



New high quality Panasonic and Rubycon parts were substituted in. New schaffner filter installed. I replaced the RIFAs with NEW Kemet RIFAs. A sticker will go on the back of the unit saying these need to be replaced in 10 years. They are consumable, not dangerous.

Powered it up, all good. For a bit. About 30 minutes after power up, the IRF710 in the preregulator blew it's arse out.



Took the driver transistor out and a diode.



After this explosion, I tested the pre-regulator mostly worked by applying 22V across C925 to see if the SMPS controller was still working. It was. I applied 43V across TP940/950 to bring the scope up on DC and it came up fine.


Outstanding issues

1. Pre-regulator exploded

The pre-regulator MOSFET Q9070, an IRF710 exploded. This took out Q908 and CR907 too.

Q9070 is being replaced with an IRF820. Crap by modern standards but cheap and with a higher voltage headroom and similar characteristics to the original device without being too daring. Also known compatible as some units shipped with them in.
Q908 is being replaced with a PN2907A.
CR907 is being replaced with a STTH2R06 ultra-fast rectifier.
As a precautionary measure C907 is being replaced with a Vishay MKT1813 1uF 630V unit

I will lap solder the connections onto Q9070 and shrink it as the original connector was damaged by the explosion.

RESOLVED. Substituted above parts in and it worked.

2. Noisy 100V line

This is a cock up on my part. There is a lot of switching noise there, above min specification. I used a standard cheap ass 33uF cap on that instead of a low ESR type. I will replace that once I get the preregulator working.

RESOLVED. Replaced capacitor with low ESR one and it's fine now.

3. Bit of intensity flickering

Not sure what this is yet. Intensity flicks in and out a little bit. I have done some googling and looks like it may be the 510K resistors in the HT chain (known problematic) or the z-axis or the multiplier. If it's the latter I'm going to cry. This was occurring before I did any refurb work.

RS don't have any suitable replacements until mid December so I will shelve that problem until then.

If anyone has some advice or ideas here I'd appreciate it.

UPDATE: 510K resistors replaced. Unfortunately this made no difference. Have since isolated to the multiplier and have ordered a replacement one.

UPDATE: multiplier replaced. Improved but is still happening. Am going to inspect all remaining resistors in the HT section.

4. Won't show channel 1 on its own

Only shows both or channel 2. Likely switch issue. To debug...


Reference - power supply schematic:

David Hess:
You are lucky that the failure of Q9070 did not propagate through the drive transistor and diode into the switching regulator controller.  I would have socketed and replaced the TL594 as well.  I probably would have replaced C907 and CR907 also.

It would be nice to know why Q9070 failed.  If you have the ability, taking a close look at the current and voltage waveforms may reveal something.

I think Tektronix stopped using those sockets for the TO-220 switching transistor and started soldering them to the wire extensions after they identified the socket as a reliability problem.

Something else I have wondered about in that design is the wire extension on the TO-220 power transistor.  Sufficient inductance could cause poor switching or high peak gate-to-source voltage.  Lead dressing would have a significant effect on this.  I would consider braiding the wires to minimize loop area but measurements should be made to find the best configuration.

Excluding the 2213/2215 models with the preregulator modification, the 2235 was the first model of this series of oscilloscopes.  One of my ongoing projects is to document the component changes between it and the last version of this design which was used in the 2235A, 2236A, and 2232.  Some of the changes were to support higher power, some were to replace obsolete parts, and I assume some were to improve reliability.

I do not have significant diagnostic data on this power supply design yet because none of my 22xx power supplies have failed or misbehaved and I am leery of messing with a working instrument.

bd139:
Very lucky indeed it didn't take anything else out. Replacement controllers are available for cheap next day free delivery here so I figured it wasn't worth replacing until I know I've killed it. If it does go I will replace it and add a socket. It was outputting ramp and PWM outputs at around the correct frequency once the turn on voltage was exceeded so I figured it probably works.

I have my suspicions with Q9070 that I actually damaged the heatsink bond when I was replacing a capacitor if I'm honest. This is based on the amount of time it took to explode. The SOA rapidly shrinks as case temperature rises on that part which it does pretty sharpish due to RDS(on). I wasn't overly happy with the tightness of the SOA on it either. The thing needs to deliver quite a bit of energy to C940 to keep it charged, possibly close to the limit. I didn't measure it. I will this time.

Could be right with Vds which is a very tight 400V. One reason I picked an IRF820 as a replacement was the Rds(on) is about the same, higher Vds (500V) and gate charge is low enough and Ids is higher. I hope that will give it a little more headroom.

I hope yours don't explode. While this is fun I rather wanted to use the scope :)

I didn't include this before but here is a high resolution reference shot of the power supply before refurb. May be of use to you:



Date codes point to around 1985 I think for this unit.

Edit: also wondered about mains voltages. They have risen considerably here over the last few years. Mine peaks at around 255V RMS whereas it used to be around 235V RMS. One reason I refurbished the entire front end.

David Hess:

--- Quote from: bd139 on November 23, 2018, 03:38:30 pm ---I have my suspicions with Q9070 that I actually damaged the heatsink bond when I was replacing a capacitor if I'm honest. This is based on the amount of time it took to explode. The SOA rapidly shrinks as case temperature rises on that part which it does pretty sharpish due to RDS(on). I wasn't overly happy with the tightness of the SOA on it either. The thing needs to deliver quite a bit of energy to C940 to keep it charged, possibly close to the limit. I didn't measure it. I will this time.
--- End quote ---

That would explain it.  Many other but not all reported failures may be due to the same problem.


--- Quote ---Could be right with Vds which is a very tight 400V. One reason I picked an IRF820 as a replacement was the Rds(on) is about the same, higher Vds (500V) and gate charge is low enough and Ids is higher. I hope that will give it a little more headroom.
--- End quote ---

I have some not quite reliable third party notes on what transistors were used as various times.  Eventually Tektronix used the MTP6N60E which is 600 volts, 1.2 ohms, 6 amps, and 125 watts for all models.  The transistor characteristics are not critical as far as I know.

The power supplies operate in continuous conduction mode so the peak switching current is only like 1.5 amps for the 40 watt 2235 and 3 amps for the 80 watt 2232.


--- Quote ---I didn't include this before but here is a high resolution reference shot of the power supply before refurb. May be of use to you:
--- End quote ---

I only have 2230s and 2232s which is one of the reasons I have not messed with the power supplies as they are much more difficult to access with the DSO board covering everything up.


--- Quote ---Edit: also wondered about mains voltages. They have risen considerably here over the last few years. Mine peaks at around 255V RMS whereas it used to be around 235V RMS. One reason I refurbished the entire front end.
--- End quote ---

I am not aware of more problems being reported when running on 240VAC instead of 120VAC except for the Rifa input filter capacitors.

bd139:

--- Quote from: David Hess on November 23, 2018, 04:54:22 pm ---
--- Quote ---Could be right with Vds which is a very tight 400V. One reason I picked an IRF820 as a replacement was the Rds(on) is about the same, higher Vds (500V) and gate charge is low enough and Ids is higher. I hope that will give it a little more headroom.
--- End quote ---

I have some not quite reliable third party notes on what transistors were used as various times.  Eventually Tektronix used the MTP6N60E which is 600 volts, 1.2 ohms, 6 amps, and 125 watts for all models.  The transistor characteristics are not critical as far as I know.

The power supplies operate in continuous conduction mode so the peak switching current is only like 1.5 amps for the 40 watt 2235 and 3 amps for the 80 watt 2232.

--- End quote ---

That's rather interesting actually. You can see roughly when that MOSFET gave up here:  :-DD




--- Quote from: David Hess on November 23, 2018, 04:54:22 pm ---I only have 2230s and 2232s which is one of the reasons I have not messed with the power supplies as they are much more difficult to access with the DSO board covering everything up.

--- End quote ---

Do you know if they run without the DSO boards installed? I have seen a number of those going cheap but they scared me off (and I have a TDS210 that works fine anyway). Might be a possible debugging route.

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