Tektronix 2235 repair thread - Page 1
Forum section: electronics / testgear

author: bd139
author_rank: Super Contributor
timestamp: November 23, 2018, 02:26:10 pm
content: 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 issues1. Pre-regulator explodedThe 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 unitI 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 lineThis 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 flickeringNot 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 ownOnly shows both or channel 2. Likely switch issue. To debug...Reference - power supply schematic:

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 03:12:43 pm
content: 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.

author: bd139
author_rank: Super Contributor
timestamp: November 23, 2018, 03:38:30 pm
content: 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 scopeI 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.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 04:54:22 pm
content: Quote from: bd139 on November 23, 2018, 03:38:30 pmThat would explain it. Many other but not all reported failures may be due to the same problem.QuoteI 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.QuoteI 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.QuoteI am not aware of more problems being reported when running on 240VAC instead of 120VAC except for the Rifa input filter capacitors.

author: bd139
author_rank: Super Contributor
timestamp: November 23, 2018, 05:52:53 pm
content: Quote from: David Hess on November 23, 2018, 04:54:22 pmThat's rather interesting actually. You can see roughly when that MOSFET gave up here:Quote from: David Hess on November 23, 2018, 04:54:22 pmDo 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.

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2018, 07:47:34 pm
content: Quote from: bd139 on November 23, 2018, 05:52:53 pmAbsolutely they work without the DSO board installed and that is one of the standard diagnostic procedures. Without the DSO board, they function just like a 2235 as that is basically what they are.The 2230 was a continuation of the 468 analog and digital storage design which use an analog channel switch and single digitizer but despite the limitations this imposes, it works really well. The user interface is not what you would expect of a more modern DSO but it is usable and has a certain charm. The two most annoying things about it are that the DSO configuration settings are not saved and the 20 MHz bandwidth limit does not apply in DSO mode.The 2232 uses separate digitizers and has a recognizably modern DSO interface.The 2230 is much more difficult to work on because removing the storage board requires mechanically disconnecting it from the front panel. On the 2232, the storage board is trivial to remove for access.

author: floobydust
author_rank: Super Contributor
timestamp: November 24, 2018, 09:38:51 pm
content: Recapping these, I also replace C944 1uF tant. and I count four Rifa X and Y-caps C904 (bastard behind on/off switch) and C900, C902, C903 (EMI filter PCB).I upsize the primary capacitor to 100-120uF as modern parts are -15% actual and 75uF is the absolute minimum to begin with.My 2235 V/DIV knob (clear) dials are fragile and broke, I couldn't find any so I made some new ones. I have to try perfect the text and laser kerf.https://www.eevblog.com/forum/repair/tektronix-2215-scope-repairsome discussion on the MOSFET Q9070 must be low capacitance, instead of low RDS on as the Holy Grail.

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2018, 12:04:41 am
content: One of the reasons I have been compiling the changes Tektronix made in the power supply is to eventually refurbish them to the last latest configuration and get some idea of what might have been changed to make the power supplies more reliable. Mostly they increased the size of the output capacitors to handle higher current.Tektronix had added the 39 ohm gate resistor and source ferrite bead by the time of the 2230 and they remained through the 2235A/2236A/2232 which were the last versions. (1) Note that the 2235A is just a 2235 with all of the various improvements Tektronix made over the 6 years between the 2235 and 2235A.The gate drive in this case is important because we know the power MOSFET has a reliability problem for whatever reason. The gate resistor and source ferrite bead would be exactly the things needed to control the gate drive or suppress a parasitic oscillation but I would want some way to monitor the gate drive waveform at the MOSFET before doing more than adding a similar gate drive resistor and tuning it as described in that discussion thread. Like I said before, I have not done this because none of my 22xx oscilloscopes have failed.(1) There may have been an early and late version of the 2232; there was an updated service manual released but I have not found any differences between them or between the 2232 and 2235A/2236A.

author: bd139
author_rank: Super Contributor
timestamp: November 25, 2018, 10:31:12 am
content: Yes I’ve seen the oscillations in my attempts to build RF amps with switching MOSFETs. The main thing that improved what I was working on was keeping the source short and on a ground plane and adding 10 ohms or so in series with gate. Neither of these are happing with 10cm of wire between the board and the MOSFET. Not exactly a nice design.If I can’t repair this preregulator I will probably buy a reasonable quality 48V SMPS module and bypass it. Should be able to turn it down to around 43V. Can be mounted on back of chassis. Cheapest solution!

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2018, 02:10:31 pm
content: Quote from: bd139 on November 25, 2018, 10:31:12 amIt is usually not a problem in switching regulators because the transistor spends so little time in its linear range. But the long lead lengths make this more of a possibility.My suspicion is that the long lead lengths and lead dressing result in either poor switching or oscillation leading to excessive power dissipation or exceeding the maximum gate voltage.QuoteSomeone on the TekScopes list did that and reported good results.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 26, 2018, 02:16:49 am
content: I recently had both a 2235 and a 2235A open on the bench and one significant physical difference is the switching transistors heat sink on the non 'A' version is only around a third the size of the one in the 2235A. And when running these heat sinks get very hot indeed. Also noted while looking up info on these old timers that there is provision for a fan and some people have retrofitted one successfully. Although they did not indicate which direction the air flow would be. I guess that would be inwards?.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2018, 04:05:48 am
content: Tektronix used a couple of different configurations to power the fan when it was included. It is 13.6 volts on the 2236 and 8.6 volts on the 2230 and 2232. The older fans used the custom brushless DC motor shared with other older Tektronix instruments with a really impressive looking and performing impeller. At some point they changed to using a standard tubeaxial fan.The fans on my 2230s and 2232s blow outwards but if I installed one from scratch, I would probably configure it to blow inward with a dust filter on the outside.I think what happened is that Tektronix did a "final" design of the 2 channel 22xx series with the 2235A, 2236A, and early 2232 released in 1990 where as many assemblies were shared as possible. The 2235 and 2236 were the oldest models of the 22xx series so they brought them up to the standard of the "new" 2232. The 2213 and 2215 do not count as the oldest because they were more like prototypes and quickly replaced by the 2213A and 2215A which came out *after* the 2235.

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 26, 2018, 06:10:27 am
content: I made my fan blow inwards, easy to filter and it blows air directly onto the hot bits.Tek specify a 20ohm resistor, but I used a 10ohm IIRC to adjust the speed of the fan I used.I made a thread about mine, and a video, I managed to cross reference the parts in the Tek service bulletin to modern parts and mine is working well.I didn't swap the line filter module yet, but I found a shop selling TDK-Lambda brand filters for about $10, so I might grab one and stick it in.Links below for reference:https://www.eevblog.com/forum/repair/tektronix-2235-upgrades/msg1862394/#msg1862394I'll be following this thread for further upgrades, I can't help tinkering where I shouldn't...

author: floobydust
author_rank: Super Contributor
timestamp: November 26, 2018, 10:50:54 pm
content: I don't think the Tek pre-reg PSU is properly understood. It looks like a flyback converter but has a resonant winding/cap C908, I think for low EMI.The TL594 oscillator might have to be bang on. But the components differ in models:2215: R920 20k + 1800pF C920 27.8kHz?2235: R919 11k + 1800pF C919 50.5kHz?2235A: R918 was hand-picked for frequency, not sure of the target.I would rather have the supply working properly instead of adding a noisy fan. I like a silent scope.I did make a mistake and replace C908 with a part 10X bigger 2,200pF, the PSU MOSFET ran much hotter.

author: bd139
author_rank: Super Contributor
timestamp: November 26, 2018, 11:09:42 pm
content: Yeah I don’t want a fan in this. My main digital scope is a TDS210. Another silent item. The TDS210 actually replaced my DS1054Z because it was quieter and did the job fine! I was hoping to have the TDS210 and 2235 on the bench and leave it at that. Get rid of the other four scopes!2235 targets 60Khz apparently. I was wondering if the frequency was related to loading. Higher loading, higher switch frequency.I have all the parts in now to fix the power supply however this might not happen until the weekend due to other things I need to do.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 12:24:22 pm
content: Had a few minutes. I think we have a smoking gun!Capacitance was fine but I noticed some browning on the bottom of the tape seal on the outside of C907. So I peeled it back about 20 turns and found some heat damage. Not sure if this was the cause or an outcome yet.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 12:49:29 pm
content: And another smoking gun. Someone has been at this. I noticed this, perhaps later than I should have. Some numpty has changed two of the 510K HV resistors for what looks like some 470K ones they had lying around. In the process of doing this, they left shit all over the board and managed to burn the power switch shaft.The human race disappoints me sometimes. This wasn't hard to not fuck up.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 02:29:01 pm
content: Ok quick update. I have replaced capacitor, the driver transistor, MOSFET and the switching diode from the list above. I added the 39R resistor in series with the gate as per the service note.Turned it on, blew the fuse straight away. Hmm what did I cock up?Turns out I put CR907 in the wrong way.Idiot! Swapped it round, brought it up on 46V DC this time across the preregulator filter cap and bingo, it powered up fine. Thank goodness that didn't break anything. Or did it?Plugged mains in, boom fuse went again. Checked voltage at schaffner filter and all is good there, thus there is something wrong along the line. I reckon it's one of the diodes gone short. I will debug that later.Good progress at least.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 02:45:22 pm
content: Ok fuse is my fault. I'm using fast blow where I should have slow blow in thereInrush current to the main filter cap is blowing it before it gets to do anything.I'd be dangerous if I knew what I was doing

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 03:04:35 pm
content: Ok so after 20 minutes on 46V DC straight into the filter board it's running nicely. I have noticed another issue though which I will add to the list at the top.IRF820 runs entirely cold at 46V. Lets see how it does on 350V or so later!

author: TERRA Operative
author_rank: Super Contributor
timestamp: November 27, 2018, 03:20:00 pm
content: I found a little Deoxit went a long way on my unit.A few of my switches were crunchy, but they came up like new after a bit of a clean.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 03:25:11 pm
content: Good to hear. I’ll have a go with that (I have a can). The flickering is the only thing that is worrying me. If it’s the multiplier or transformer then I’m going to need another unit.

author: bd139
author_rank: Super Contributor
timestamp: November 27, 2018, 05:24:30 pm
content: Ok did some analysis on the flickering while it's open and safely connected to DC and I found that voltage at TP842 drops whenever it flickers. This is not on the driver side of things after hooking a scope to the collector of Q840, setting persistence to infinite and leaving it there for a few minutes so I assume the flickering is an HT issue. I will replace the known hooky HT resistors first and see if that solves it. If not, I will do all the capacitors. If not that I will cry into it and find a donor unit for multipler and transformer.I will win if it kills me

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2018, 04:40:07 am
content: Quote from: floobydust on November 26, 2018, 10:50:54 pmWhat I have been told is that C908 and the extra winding inject a counter current into the chassis cancelling radiated emissions and this is what the schematic shows with the separate power supply and chassis grounds.Quote from: bd139 on November 26, 2018, 11:09:42 pmBut the 2235 and 2235A have the lowest power requirements which is why they have no fan.I would take a very close look at the operating frequency. The TL594 clock is 60kHz but since only one of the outputs is used, why isn't the output frequency 30kHz?

author: floobydust
author_rank: Super Contributor
timestamp: November 28, 2018, 06:24:34 pm
content: Quote from: David Hess on November 28, 2018, 04:40:07 amThanks David for pointing out the two different grounds on C908. I'm not sure how T906 is wound but this extra winding surely is to lower EMI.When I see care and attention like this, I wonder how using a 48VDC replacement SMPS can work without wrecking the scope's noise floor.

author: floobydust
author_rank: Super Contributor
timestamp: November 28, 2018, 07:06:29 pm
content: Quote from: bd139 on November 27, 2018, 05:24:30 pmNever give upThe HV 0.01uF caps are pretty loud when they arc over inside. Looking at the pcb in the complete dark you can see any arcing. But the multiplier parts are encapsulated and hard to see/hear anything. It's possible to test the HV multiplier with a car ignition coil and HV probe, and CCFL tubes as a load.Good thread on remaking a Tek HV multiplier:https://www.eevblog.com/forum/repair/tektronix-sc504-hv-multiplier/I think the 2235 is well worth the trouble to repair. The scope is a great drive. So easy to see subtle signal qualities in the analog world, 2mV/DIV sensitivity you can actually use instead of SMPS hash all over the place like you see with modern DSO's.The 2235A I have is a total dog in comparison. I hate that scope, it never triggers properly and seeing the plethora of Tektronix ECO's, it looks like the scope was a rough change when Tek went to CADD. I have to spend time and see if it needs repairs or ECO's done.

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2018, 07:34:02 pm
content: Yeah I’m really liking this scope. When it worksThe multiplier doesn’t worry me too much. I built and played with lots of multipliers (usually resulting in swearing and zaps) as a kid. The transformer does worry me though!My TDS210 is amazingly quiet. Another keeper that one

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2018, 09:17:46 pm
content: Quote from: floobydust on November 28, 2018, 06:24:34 pmIf the TL594 preregulator is not being used, then there is no interference from it to cancel.

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2018, 09:33:09 pm
content: Quote from: bd139 on November 27, 2018, 05:24:30 pmThose points are the output of the z-axis amplifier which controls the trace brightness.Are you saying that TP842 glitches but the collector of Q840 does not?An intermittent failure on the high voltage side of the z-axis amplifier is possible but I would expect it to have the same effect on both side of R842 which is just there it isolate the capacitance from the amplifier output.Before getting too involved with replacing parts, I would fix the problem with the focus resistor chain that you identified. The original Allen-Bradley carbon composition resistors R888 to R892 were replaced with Vishay VR37 resistors and R886 was replaced with a Vishay VR25 resistor. These high voltage film resistors are inexpensively available.

author: bd139
author_rank: Super Contributor
timestamp: November 28, 2018, 09:44:19 pm
content: Yes that is correct. TP842 glitches but Q840 collector doesn’t. Looks like whatever is happening is only pulling the voltage down there by about 4 volts. It is weird as I’d expect Q840 to drop a bit there too but it doesn’t. I stuck the scope across both sides of it and did a differential measurement which matched. I have my suspicions the resistor isn’t healthy. It will be replaced with a nice fresh Vishay MRS25 when I do the dividers.I have already ordered some fresh VR37 510k resistors. 50 of the things as it was cheaper than buying 6 from RS... unfortunately back ordered due on 6th December.Fuse is now sorted. I got some slow blow ones (arrived in a massive cardboard box big enough to get 2500 sheets of paper in covered in fragile tape. More). Power up doesn’t pop the fuse now so the power supply is repaired and working. Runs pretty cool. After 30 minutes the bracket was only warm to the touch and nothing to worry about.Can’t progress further yet so I am shelving this and fixing my 465 and 475A until the resistors arrive. Got a collection going now

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2018, 10:12:33 pm
content: Quote from: bd139 on November 28, 2018, 09:44:19 pmR842 might be bad then but there is nothing special about it and any film resistor can be used.I am not sure about the output impedance at the collector of Q840. That is the feedback point so up to a certain load, the impedance is effectively zero but the maximum load is pretty low. The quiescent current through Q845 is about 4 milliamps.4 milliamps through R842 can only produce 1 volt but if TP842 is pulled low enough, the collector-base junction of Q840 will be forward biased and unlimited current will be provided through it and CR840 so that could explain the voltage drop across R842.

author: bd139
author_rank: Super Contributor
timestamp: December 05, 2018, 09:34:20 pm
content: Ok have replaced the resistors in this and no change.Did some more debugging and removed the anode supply entirely and the display is (as expected) dull and entirely stable.That means it’s either the multiplier which is shot or the tube is. I can’t find any evidence of flash over on or around the anode cap so my current thought is the multiplier.Fuck!

author: tautech
author_rank: Super Contributor
timestamp: December 05, 2018, 10:11:11 pm
content: Good PDA multiplier rebuild guidance here:https://www.eevblog.com/forum/repair/tektronix-465/

author: tautech
author_rank: Super Contributor
timestamp: December 05, 2018, 10:14:25 pm
content: Quote from: bd139 on December 05, 2018, 09:34:20 pmYou mentioned a crack when discharging the anode connection. Discharge then reconnect and repower. Shut down and check if the anode is recharged.

author: David Hess
author_rank: Super Contributor
timestamp: December 09, 2018, 07:55:21 pm
content: If the multiplier was causing a problem then I would expect more than just an intensity change; beam deflection also depends on the PDA (post deflection acceleration) voltage.The CRT can be operated without the PDA attached. The deflection should be about half and the trace will be dim and fuzzy but it should still be visible.

author: bd139
author_rank: Super Contributor
timestamp: December 09, 2018, 08:09:40 pm
content: The deflection changes as well. If you stick it on X-Y mode it loses focus and shifts slightly. In sweep mode, the sweep length drops. I made a very hooky looking high voltage AC probe with some left over VR37 resistors and 5x 1n4007's in series and stuffed it across the ground / T948 p23 junction and there was no significant amplitude change going into the multiplier when this event was occurring. I even disconnected the multiplier and checked the transformer unloaded and that was fine. The -2KV is rock solid out of the multiplier as well so I think this is the 6x chain in there. I have run it without the PDA and it's perfectly stable there but as you say half deflection and brightness.Ergo I concluded the multiplier. It could be the CRT anode cap leaking but that usually leaves deposits around the cap and it's totally clean.Good news however. I managed to get my hands on an entirely dead 2235. This one's mains filter exploded and the attenuators are shot but the thing comes up on DC apparently fine. Also the channel switch is good if I have to hook that out. The cost? One £12 crate of strongbow and waiting until mid Jan.Seeing if I can get a multiplier before then as well. I really want this to work as for an analogue scope it weighs a lot less than a 465!

author: mnementh
author_rank: Super Contributor
timestamp: December 10, 2018, 04:11:01 am
content: You bastard... you and your stick-to-it-iveness had me up at 2AM calculating the charge level I'd need to put to 3 LiPo batteries to get a stable 42-43V.mnemWhere is the "Kick that guy in the junk!!!" button on my keyboard?!?

author: bd139
author_rank: Super Contributor
timestamp: December 10, 2018, 07:52:27 am
content: Trick is you don’t need exactly 43V. Anything around that but less than the trip voltage works nicelyIn fact it seems to run around 35V fine.

author: bd139
author_rank: Super Contributor
timestamp: December 10, 2018, 05:25:12 pm
content: Got another multiplier here for ref as I'm too impatient to wait until Jan:https://www.ebay.co.uk/itm/283298267147... I found the guy looking through Google and asked if he had any more so has fished them out of some cannibal scopes he has on demandAlso has transformers:https://www.ebay.co.uk/itm/283292726524

author: David Hess
author_rank: Super Contributor
timestamp: December 10, 2018, 11:21:52 pm
content: Quote from: bd139 on December 10, 2018, 07:52:27 amThe inverter output transistors operate in their linear region when turned on to regulate the output voltage so the voltage from the switching preregulator needs to be high enough to prevent them from saturating or the voltages on the secondary will drop.Later versions of this circuit in the 4-channel 22xx series and 24xx series did away with linear regulation in the inverter which may have been a reliability issue and relied instead on the preregulator to produce the exact voltage needed.

author: mnementh
author_rank: Super Contributor
timestamp: December 12, 2018, 12:05:29 am
content: Yeah, well... my problem is not go/no-go, but one of noise, which I blame for a RAM error in the POST (Tek 2230, not 2235). I want to bypass the prereg entirely so I can determine if it is the source or just a victim of oscillation further down the line. So yeah, I wanna get pretty close to right on.mnem*poit!*

author: mnementh
author_rank: Super Contributor
timestamp: December 20, 2018, 05:32:29 am
content: Quote from: bd139 on December 10, 2018, 07:52:27 amI see what you guys are saying... at first I thought David's post was contradicting yours, now I realize he wasn't talking about the prereg and you're both coming to the same point by different routes. LiPo Storage charge is 3.7-3.8V/cell; With the three 4S packs I have handy that's 44-45.6V. Or I could do similar with 2 of my 24V drill packs. Should be good through a dim bulb tester I'd think. Now I need to drag it out and actually DO it instead of just thinking about it.Yeah, yeah... I know. I should treat myself to a nice old Lambda CC/CV supply that can do it just by twisting a knob; what kind of TEA addict am I.mnem*ZZZzzzzZZZzzz...*

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2018, 03:45:49 pm
content: They are contradictory. The inverter will run on 35 volts but the outputs will not be regulated.

author: mnementh
author_rank: Super Contributor
timestamp: December 22, 2018, 07:37:16 pm
content: Well fine then... guess I need to read up on the "Principles of Operation" part of the service manual I bought so many years ago.mnem"Electrons may be very, very tiny; but when they gang up on you they ALWAYS win." ~me

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2018, 09:59:20 pm
content: On the 2213/2215 which have an earlier version of this power supply, operation is more explicit. There are two adjustments; one controls the regulated output voltage and the other controls the headroom which is the voltage across the inverter transistors which keeps them operating in their linear region. On the 2235, the headroom voltage is fixed by the switching preregulator.The 4-channel 22xx oscilloscopes extended this design with a separate transistor used as the linear pass element and the inverter transistors run in saturation. I suspect matching of the two transistors operating in linear mode in the 2235 design caused problems with transformer balance although that does not seem possible with a two transformer saturable inverter; the parts list does seem to indicate that they were used in matched pairs.

author: bd139
author_rank: Super Contributor
timestamp: March 06, 2019, 08:51:55 pm
content: Ok finally an update on this one. My projects take too long plus xmas went crazy wrong for me. Firstly the guy who was giving me the 2235 parts mule let me down. I don't have it. I managed to get the replacement HT transformer that arrived ages ago in this evening and it's working somewhat better than it did before. Off DC anyway. Can't hear any more ticking now which is good.However it's still not totally healthy. It has still got somewhat jumpy focus (zips in and out) and shows some deflection changes when this happens. I'm going to measure, inspect and replace the remaining resistors in the HT section as I've done all the 510k ones now.

author: TERRA Operative
author_rank: Super Contributor
timestamp: March 07, 2019, 02:22:53 am
content: How's the focus pot looking?My 2235 was a little jumpy when focusing and a NOS pot from ebay has it all nice and smooth again.

author: bd139
author_rank: Super Contributor
timestamp: March 07, 2019, 07:42:36 am
content: Seems ok. I can’t see any voltage jumps anywhere around it which I’d expect if focus was changing.I’m wondering if this is leakage around the anode cap or in the tube base.

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2019, 10:03:02 am
content: Quote from: bd139 on March 07, 2019, 07:42:36 amAnode cap you'd hear it and CRT base is only at EHT voltages.Either Focus, Intensity or EHT components breaking down.

author: bd139
author_rank: Super Contributor
timestamp: March 07, 2019, 11:04:06 am
content: Yeah can't hear a thing. Anode cap is pretty noisy - been thereMay replace all the HV capacitors in it and see if that helps as I have some of the right value. If that doesn't work I'll see if I can find a suitable focus pot. Seem to be unobtainum. It's a 5M AB modpot which has to withstand 2KV.

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2019, 11:16:12 am
content: Quote from: bd139 on March 07, 2019, 11:04:06 amNot at all easy these days so maybe a strip and clean might be an option. 5M will most likely be a carbon track so could just be real dirty and wiper needs a tweak.In the D83 I had a HV zener in the CRT socket went leaky and needed replacement, check for that. IIRC it was 150 or 175V rated and set up the grid bias or something. Might be on the PCB in your Tek.

author: bd139
author_rank: Super Contributor
timestamp: March 07, 2019, 12:09:20 pm
content: Nothing in tube base at all. Straight wired. Have checked bias everywhere for jumps - nothingThe tube bias and blanking is totally stable.I just disconnected the deflection plates and anode and powered it up again and it's 100% stable as well, just dull obviously. No jumping or anything. I have already replaced the multiplier with another one and the issue persists exactly the same.I just found my HV probe after digging around in the cupboard for nearly an hour as well. The PDA voltage is 7.1KV (about right) and not jumping.Totally stumped!

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2019, 12:52:50 pm
content: Time to pull out the isolated push stick and give things a poke ?Dry joint perhaps ?

author: bd139
author_rank: Super Contributor
timestamp: March 07, 2019, 12:53:53 pm
content: Possible. I might take the tube out and inspect it as well. Look for flashover. Wonder if it got gassy

author: med6753
author_rank: Super Contributor
timestamp: March 07, 2019, 12:55:18 pm
content: Quote from: bd139 on March 07, 2019, 12:09:20 pmGee...almost sounds like it's the CRT itself. Perhaps a loose internal element? Try carefully tapping the neck of the CRT while powered up and see what happens.

author: tautech
author_rank: Super Contributor
timestamp: March 07, 2019, 12:58:41 pm
content: Quote from: bd139 on March 07, 2019, 12:53:53 pmThis looks mint to me:Gotta be control circuitry but maybe CRT socket pins could be cleaned, if for nought than to eliminate them.

author: bd139
author_rank: Super Contributor
timestamp: March 07, 2019, 01:09:07 pm
content: I did a quick video earlier of it.This is in X-Y mode, deflection plates disconnected (to eliminate amps).Brightness unstable, deflection is vertical (scope was on its edge)https://youtu.be/Lo-WzwbXh1Q

author: David Hess
author_rank: Super Contributor
timestamp: March 07, 2019, 07:47:38 pm
content: A change in the acceleration voltage causes a change in deflection.A dirty insulator or high voltage breakdown might cause it but would be difficult to track down. Still, try examining the high voltage sections in a dark room and maybe it will be visible. Or just scrub down and clean the high voltage parts of the board with distilled water.Easier to test are the acceleration voltages and especially the primary side of the switching regulator since this design does not rely on secondary side regulation of the acceleration voltages. I would make a careful measurement between the collector of Q939 and ground to see if it is jumping. Maybe a part in the regulator circuit has developed flicker noise.

author: bd139
author_rank: Super Contributor
timestamp: May 28, 2019, 07:54:43 am
content: Just a final update on this one. I managed to get hold of a later fully working 2235A so this 2235 is now relegated to parts mule status I'm afraid.The 2235A does however have a terribly whiny power supply. This goes away after it has warmed up but it's a bit irritating for the first 5 minutes.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: May 28, 2019, 11:27:45 am
content: Sounds like it needs new psi caps

author: bd139
author_rank: Super Contributor
timestamp: May 28, 2019, 11:32:20 am
content: Have tested them all and they are good on both ESR and capacitance.

author: wasyoungonce
author_rank: Frequent Contributor
timestamp: May 29, 2019, 03:06:42 am
content: I have a 2236 and its a little whiny as well

author: bd139
author_rank: Super Contributor
timestamp: May 29, 2019, 07:22:36 am
content: Thanks for the reply. I'm probably just going to leave it TBH. If anything goes wrong I'll fix it. I don't fancy a fight after that original 2235 that started this thread

author: paronaram
author_rank: Regular Contributor
timestamp: December 19, 2019, 05:07:52 am
content: Hello Everybody, Just joined this forum. I am learning electronics, reading here there, watching videos.. Trying to fix faulty audio equipment; troubleshooting. I start purchasing some test equipment for this task and that included Tek 2235 oscilloscope.Scope was cheap because it it has ONLY one problem (as per guy who sold me). It was listed no horizontal sweep on left side. I was hopping maybe something with PS, but no.. And after troubleshooting more I find out that I have no Ch1 as well ...Can someone help me where is this problems maybe coming from?I checked all voltages on TP they are fine, was trying to do reverse by adding signal on the Ch1 and see where it gets lost, but fist stage U10 is buried under 3 PCBs, I don't even know how to check TP there ...

author: bd139
author_rank: Super Contributor
timestamp: December 19, 2019, 08:07:08 am
content: If it’s not sweeping on the left side only check the horizontal wires attached to the tube. Sometimes these fall off. Might be as simple as that. If not it’s going to require a lot more effort to find.

author: paronaram
author_rank: Regular Contributor
timestamp: December 19, 2019, 09:23:58 pm
content: Quote from: bd139 on December 19, 2019, 08:07:08 amThanks will check that later today

author: paronaram
author_rank: Regular Contributor
timestamp: December 20, 2019, 05:24:13 am
content: Yes, it does.

author: paronaram
author_rank: Regular Contributor
timestamp: December 20, 2019, 05:50:11 am
content: bd139 - you are correct. H cable is cut, but it's not just cut, it's cut from glassI honestly not sure, because I can't see there, clearance is about 1/2"Now I got another 2235, and I think tube is working but case is bent badly (what the heck are they doing with this equipment???) on the sides, and can not remove cover, over the weekend I will cut it with DremelIs it allot involved in tube replacement?Any major calibration that I will need to do ? (aka buy another scope to calibrate this one)Thank you

author: bd139
author_rank: Super Contributor
timestamp: December 20, 2019, 08:55:57 am
content: Oh that’s not good. See if you can feel around in the hole. Sometimes the connection is just bent.I’ve had that problem getting the case off a 2235 as well. Looking at the front it’s usually jammed on the right hand side next to the power supply section. When some inevitable idiot drops it, it bends the rails slightly. I had to hammer a metal ruler down the sized to get it off.Tube replacement isn’t terribly difficult. I think there’s instructions in the manual. It will need recalibration afterwards though. Make sure you work on top of a towel or something and wear safety glasses. There is an implosion risk. I haven’t imploded one yet accidentally but I have disposed of one on purpose and it’s quite loud and glad goes everywhere

author: paronaram
author_rank: Regular Contributor
timestamp: December 21, 2019, 06:38:40 am
content: Removed scope tube (very easy BTW), and guess what, pin is there, it's just cable was not connected. Looks like someone was trying to troubleshoot already.So I reconnected both cable from PCB to Horizontal pins, start the scope, and same issue... Well something should start working, cable was completely disconnected from that pin.On the internet I saw a video were guy was troubleshooting unrelated issue on the same Tek, and he was talking about pots on the top PCB (see picture), I move this pots around about 20 times, and turn the scope back On. See last two pictures ... very unstable but I see A and B now!But Horizontal sweep still not thereWhen I press beam finder dot gets in the area when I don't see signal at all, so tube is capable to display something there

author: Shock
author_rank: Super Contributor
timestamp: December 21, 2019, 08:18:47 am
content: Regarding power supply noise, doesn't say specific to 2235a but there is this service note.http://www.hakanh.com/dl/docs/hardtofind/2200_ps_noise.pdf

author: bd139
author_rank: Super Contributor
timestamp: December 21, 2019, 08:28:27 am
content: Power supply noise is normal on these. Usually goes away after a few minutes. Or the power supply explodesQuote from: paronaram on December 21, 2019, 06:38:40 amThat’s good news then.Next steps are:1. Check power supply voltages2. Stick it in X-Y mode and inject a signal into X and see if that works.3. If it doesn’t then signal trace that through the amplifier stages until you find the bit that stops making sense.

author: H713
author_rank: Regular Contributor
timestamp: December 22, 2019, 07:17:17 pm
content: Hello, I just purchased a Tek 2235 off university surplus with what seems like a similar issue. No trace, however when I push the beam find button, I get some part of a trace that sweeps appropriately with the time base setting. I can also move it up/down and left/right with the X and Y position controls (though with limited movement). When I feed a signal (random sine wave in this case), there is still no trace, however I can see some part of the signal displayed when I push the beam find button. I measured the power supply rails, (or at least the test points on the board) and they all seem to be in spec, so I'm inclined to say that it is not a power supply issue.Both channels behave exactly the same, so I don't necessarily expect it to be the vertical amplifiers (they'd have to have both failed in the same way). In my case, I've got other scopes that I can use to troubleshoot this one. Some direction on where to start looking, however, would be welcome.

author: paronaram
author_rank: Regular Contributor
timestamp: December 27, 2019, 04:53:53 am
content: That beam finder button maybe faulty ... ?If its affecting both channels

author: floobydust
author_rank: Super Contributor
timestamp: December 27, 2019, 05:18:54 am
content: I can't see your trigger section buttons. In Auto sweep, a trigger is not needed and the sweep generator does the work.It might be triggering/sweeping fine but the position is way off (beam finder reigns it in). Beam finder also cranks the intensity.Looking at the Tek service procedure, check first for a Z-axis unblanking signal at TP482, then sweep sawtooth at R707, then H-deflection plates having 40Vpp sawtooth.

author: ahmedseecs
author_rank: Newbie
timestamp: August 07, 2020, 10:24:03 pm
content: Hi, I have recently acquired a Tektronix 2235A that works fine for couple of minutes but as soon as the oscilloscope warms up the display get defocused. Can you please suggest the possible cause for it.I am quite new it I did try going through the service manual but was unable to understand the cause .Thanks for your help in advance.

author: bd139
author_rank: Super Contributor
timestamp: August 07, 2020, 10:54:28 pm
content: Possibly focus pot failing or the divider resistors. See further up this thread for 510k resistors. Very common problem. Replacement pot is fairly unobtainable. I managed to salvage a pot out of another failed scope (different brand) that worked.Try replacing the resistors first though. Think it’s 4x 510k ones. You need Vishay VR series which are properly voltage rated as there are quite a few bolts across them!

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 09, 2020, 06:31:14 am
content: I managed to find a new and unused pot on ebay a while back, so you might get lucky if you keep an eye out.Here's the resistors (in blue) I replaced in my 2235, I would replace all 6 if they are present in your unit, just to be safe (resistor are cheap anyway, and it means you don't have to open the scope up for this resistor issue again.)

author: Ze Pink Panther
author_rank: Contributor
timestamp: January 24, 2021, 04:30:24 am
content: Good evening everyone,I hope this thread is not completely dead.My problem concerns the Tektronix 2235 Service Manual. It is (now) widely available on the net (even fron the Tek web site), but all the versions and copies I've seen have all the same problem: all the folded pages (containing mostly schematics, which makes them even more important and critical) have a scan gap in the middle, which results in column H of the schematics having almost half of it missing.Can you point me to a source where the SM is _fully_ scanned?Thanks in advance and regards,ZPP

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 24, 2021, 08:58:54 am
content: I have a copy of all the 2235 foldouts, available here, along with a bunch of other stuff including high quality scans of the manuals:https://drive.google.com/drive/folders/1L2ptk_0OeEywzHXIV2jmTQYrtf5IzgIH?usp=sharingIt's the 'Tek 2235 Foldouts large.pdf' file

author: Ze Pink Panther
author_rank: Contributor
timestamp: January 24, 2021, 10:16:45 pm
content: Thank you, Sir!You've made my day...Best regards,ZPP

author: golashki
author_rank: Newbie
timestamp: November 23, 2022, 04:12:13 pm
content: Hi, I'm new here and I would like to ask you about repairing 2235I bought a Tektronix 2235 (a great, beautiful oscilloscope!). Unfortunately, the switch on the "Timing board" PCB is damaged. There is a completely cracked "knob insert" connecting the shaft to the Time Base "A" switch. Unfortunately, as a result of my inexperience, while taking the switch apart, I did not notice that the switches (those brown blocks) have no internal limiters and, additionally, they rotate very easily when the shaft is pulled out. In this way, I lost the "axial alignment" of the complex switch between its elements. The "knob insert" itself can be glued together, but it was problematic to assemble the switch to work as intended.So I have two questions:-if anyone can sell the whole "Timing board" for 2235/2236or the working switch itself (or broken, but not taken apart).The second beginner's question - is how to disassemble such complicated switches to avoid axial shifts of their position? Maybe someone has such experience and may want to share some proven ways how to fix alignment in multi-module switches, etc.

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2022, 02:35:16 pm
content: Quote from: golashki on November 23, 2022, 04:12:13 pmI have done that a couple times and I used the schematic diagram to realign the cams when I put the switch back together.

author: golashki
author_rank: Newbie
timestamp: November 24, 2022, 02:52:55 pm
content: Yes, unfortunately, the same path awaits me. I thought there is some way to facilitate this procedure. Thanks

author: Eduardo Millan
author_rank: Newbie
timestamp: July 28, 2023, 01:49:30 am
content: Hola, tengo este mismo modelo pero esta quemado el Flyback-Se puede conseguir uno nuevo? Gracias

author: porter
author_rank: Regular Contributor
timestamp: August 11, 2023, 03:57:40 pm
content: Thanks folks for the information.I replaced C902-C904 (RIFA), C900, and the Power Input Module on my 2235.One less thing to worry about.(My little-screw management skills are not so good. I guy I used to work with told methat the missing screw count indicates the number of people that worked on something.)