TEKTRONIX 2213 POWER SUPPLY ISSUES - Page 1
Forum section: electronics / beginners

author: beanhauler
author_rank: Contributor
timestamp: February 24, 2022, 06:39:28 pm
content: i am starting a new post as i am still having some issues with the old 2213 scope around the vertical controls which may be power supply related.https://www.eevblog.com/forum/repair/tektronix-2213-power-supply-dead/recently bought a complete board assembly of the vertical attenuator from those guys in greece.Looked like a good board and after plugging it in it seems that both channels are working but the CH1 has a horrible dc shift when range changing from 10V upwards.Biggest change happens when we go from 1V to 500mV until the dc shift is off the screen with no input.If i switch the inputs to ground then no shift occurs and i have adjusted the step change to be correct.Thought i would have a look around U120and the power supplies which is when the fun started.The DC supplies are not correct and the 8.5 plus and minus are about a volt over. Adjusting R946 appeared to do nothing.There is another post describing this issue and they recommend changing R945 TO 4K7, R946 TO 2K5 and strapping a 1 MEG resistor across R922 on the pre regulator board.The preregulator board outputs 43.4V but no tweaking will bring the 8.6v rail down.I have noticed that sometimes the power supply screams and sounds off frequency which is annoying.If i load the pre reg board up with 75R i cant get it to run off frequency but if i drop the resistance it starts to scream.I have recently repaired a 2235 which required a new transformer and the inverter was quite happy with 43.4v and produced 8.6V no problem.The circuitry for the inverter section of the 2235 is very similar to the 2213.i started to look around the main board inverter section which takes the 43v and drives the transformer for the output supplies.thought i would check the capacitance on C945/C947/C951. All of the electrolytics read Pf instead of ufarads ...could this be the reason i wonder that R946 refuses to vary the output voltage of 8.6V?Going to change R933 and R941 for the hell of it as well.If this works then back on the attenuator.Next plan is to change the electrolytics and try the mod to the power supply adjuster to see if that works.If i can get the supply to U120 down to 8.6V maybe the shift may not be as pronounced although i have grave doubts.Channel 2 has no vertical shift on range change and probing the output of U220 pin 6 there is no dc shift while range changing.Channel 1 U120 though has a definite DC shift on pin 6 when range changing .Has anyone had an issue with the attenuator switches putting resistance into the circuit causing a shift or is this again an issue with Q122 that someone else commented on in another thread.I had previously tried to fix the old attenuator board as the circuitry around the fets seemed troublesome so rather than continue i purchased a board and replaced the whole thing. Maybe i will have just the same amount of trouble trying to rid the dc shift from this one.Stop press. The mod did not work for me on the power supplyI did as suggested and the 1 MEG resistor across R922 does bring the pre reg down to a nadge under 40V but now the -8.6V does not make 6V.So i put it all back as was and changed c945 /c947/c951. Also changed R941 and 933. Bingo now adjustable to -8.6V.Tried the vertical attenuator on CH1...still horrible DC shift...back to drawing board but at least power supplies are all good and i have two traces and all controls work fine.Its just that DC shift which spoils ch 1. Onwards.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2022, 06:59:16 pm
content: Quote from: beanhauler on February 24, 2022, 06:39:28 pmIf you absolutely must start a new thread for some reason, could you at least link the old one so we don't have to go looking for it?

author: beanhauler
author_rank: Contributor
timestamp: February 26, 2022, 04:27:45 pm
content: Its nice to know somebody is interested.Added the link above.

author: bdunham7
author_rank: Super Contributor
timestamp: February 26, 2022, 05:34:13 pm
content: OK, I thought I remembered you had a longer thread on this, not just a 1-poster!A few thoughts about your issue.First, just looking at pin 6 of U120 doesn't tell you much about the cause. You'd need to look at pins 2, 3 and 5 as well, and you can compare that to CH2 if you like. The behavior you are describing can usually be adjusted out with the step balance pots, circled in the attached picture. However, since those are directly fed by the +/- 8.6V supplies, there's not much point in doing anything until you get the power supply fully under control.You still may have a damaged U120 or leaky C119 or C121 or?, but I wouldn't jump to any of those conclusions yet--even though CH2 seems OK. As for your PSU issues, if you have dried out capacitors than you need to do a bulk replacement. You're not getting anywhere trying to troubleshoot one of these scopes without solving all of the PSU issues first. These older 2000-series scopes can be a long slog to repair--they're pushing 40 years old.The manual I have lacks PSU schematics. Do you have a correct PSU schematic for this model?

author: m3vuv
author_rank: Super Contributor
timestamp: February 27, 2022, 08:07:21 am
content: is it the same psu as the 2215 with the triac pre regulator?

author: beanhauler
author_rank: Contributor
timestamp: March 01, 2022, 10:45:44 pm
content: The power supply is not using the triac circuitry. It uses the preregulator board which is mounted above the main board and then a metal cover goes over it to keep your mitts out of the way.if you need the pre-reg circuit i have it here somewhere.I think i am getting an old hand with these now. The prereg should output 43.4v on the two pins feeding the inverter which is on the main board.As said i have replaced the c's around this stage. They measured pf in some cases which means they were proper dried out.Anyway i fired up the unit and adjusted perfectly the -8.6V and all looks good. ....APART FROM the DC shift on CH1 when range changing.So i first set both switches to ground on the channel inputs and did the adjustment per the manual. Both channels have no offset when range changing with inputs at ground. Now put the inputs to DC and centre the line and start with X1 and 10V which is hard cranked left. Start range changing up and as you slowly crank up the switch the dc line shifts in increments until it disappears off the grid in the northerly direction until its gone.CH2 remains spot on and on max right setting is a minimal shift with some very small noise.So start poking around u120 . On U220 ch 2 both pin 2 and pin 3 are firmly rooted at 0V regardless of switch setting.U120 pins start at 0v and as you increase the switch the dc changes by a few millivolts and provides an offset after R119 to the gate of the fet Q122.Of course if i hang a DVM on the gate then it injects a considerable amount of noise which is seen on the output.Generally the whole chain is dc shifted right up until the plug leaving the board and connected to main board.One of the things i did was to plug the Ch2 connector onto the CH1 attenuator board and the DC shift is replicated now on CH2 which proves the problem is in the CH1 chain. It also makes sense with the millivolt changes around U120 and on the entry to the resistor array R189.So it looks like the issue is somewhere around U120 the fet and the feedback boot strap circuitry.I need to replace C121 ,C119,R119 Q122 and CR119 i think as a starter.I did think that maybe the range switches were an issue and that they had greater resistance and upset the dc balance but this is more about the DC restorer not functioning.It could be that the c's around CR119 have changed and upset the dc balance or that the CR119 Diode capacitance has changed or leakage has increased.C121 the 1n in the loop is very suspicious although these ceramics tend to hang on for years...could be that someone sent an input that the cap objected to?Incidentally CR 119 is listed as 152-0246 and is extremely difficult to match. It has a very low 100pA reverse leakage to support the high gate impedance of the FET. Trying to find an equivalent is tough. 100pA leakage and 6pf capacitance...really tough.The fet is not much better to match. A J310 may work i just dont know...J300 is impossible to find.It may be that i have to accept the offset on ch1 but it does annoy me and it is not completely fixed per se.Still i have a perfectly functioning power supply pre reg now and everything else functions as it should.Anyone who has experienced similar issues to this then do let me know because the front end of these scopes tend to be very similar.I attach the pre reg circuit but encourage you to use the services of Artek manuals who do an excellent service of providing tech manuals that have been rescanned to not be annoying to read from page to page.Plus they are nice folks with it to boot.I bought this attenuator board from Qservice and i am sure this had the issue for a long while.Its frustrating as the original board i gave up on after replacing many items on it including the output amps and resistor networks.I did replace U120 and U220 on that board but could get no signal past the fets. I could not find a fet that worked in place of the J300's.Thought i would opt for the easy way out...yeah right.Still it tries my patience which cant be bad and i do get a buzz when something finally works....maybe.Here is a link to the video showing the issueChecking the part 151-1124-00 which seems universal across a lot of the tektronix scopes and looking in the tek parts list seems to list it as a j300This does not make sense with the pin out.Trying to change Q122 with a J 310 but the pinout for a j 300 does not seem to match what is on the boardwith the flat side of the fet towards you it should be drain source gate but it looks like source drain gateThe j300 is listed as TO92  what is this that is on the board?Case marking is very poor but looks like could be SI234? does this make sense?Ok i have tried a J310 in its place and its worse than the original.I am going to try a 2n5486 to see if that works.Need to do some research first though.