Advice needed repairing my Tek 213 DMM & battery replacement - Page 1
Forum section: electronics / repair

author: gareth
author_rank: Contributor
timestamp: October 13, 2014, 12:20:45 pm
content: Overview - I was given a faulty Tektronix 213 DMM/Scope for my birthday, and firstly I have fitted an option 1 capacitor to operate it from 240V mains in the UK. Thanks to the forum members who clarified the capacitor I needed! I'm back for some more advice please.I plugged the boards back together and prayed to the gods of old technology and turned it on - no visible indications, power light etc but I immediately started to smell the bad smell, and switched it off again. I was hesitant to touch parts to try to establish if anything was hot in case of high voltage charge remaining, and the bad smell wasn't obviously coming from anywhere in particular.So I know I need to start troubleshooting with the power supply board, but I want to be sure that I can run it disconnected from the rest of the scope without damage.That is, if I leave the various loads disconnected. will this cause any damage to the power supply boards? Schematic here:http://w140.com/tekwiki/images/7/7b/Tektronix_213DMM_Battery_Charger_and_Power_Supply.jpgI plan to disconnect the secondary power board at P420, connect 240V ac supply and measure the voltage at TP12. Assuming it is good at 330V dc, I'll then plug it back in and see if I get the expected voltages at TP4 and TP13, then work my way across the board seeing if I can find any problems.I know that I'll need to replace the batteries, and I expect that without them fitted the low volt shut down circuit will operate. I can fake the battery voltage temporarily to ensure the circuit is operational in all modes.The batteries in the case were 2x Panasonic NiCd D cells - rated at 4000 mAh (picture attached). I have found possible replacements in both NiCd and NiMH - see herehttp://cpc.farnell.com/yuasa/1dh4-0t/battery-ni-cad-d-4ah-tagged/dp/BT05361and herehttp://cpc.farnell.com/gp-batteries/gp700dh1a1p/battery-ni-mh-d-7ah-1-2v/dp/BT01866.The NiCd cost a bit less (isn't really the main consideration here) and have the original rated capacity - but NiMH come in up to 11Ah - so that's nearly 3x the original running time. Can I fit these as a drop in replacement? Should I look over the charging circuit and make some adjustments there - maybe to increase the charging current slightly otherwise I might be looking at a charge time of 48H!!! (the original charge time stated in the manual is 16h).Any advice would be very gratefully received.

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2014, 05:05:45 pm
content: That power supply design scares even me. I cannot say that I understand the design well enough to answer your questions accurately without studying the service manual which I lack.Quote from: gareth on October 13, 2014, 12:20:45 pmUsually no damage will occur but correct operation without a proper load is not guaranteed.QuoteCheck TP12 but that is probably not a problem. I would expect more than a burning smell if the problem was there. Better would be to check C417 and C418 before trying this. And C436. And C446.The other thing I would look for is shorted solid tantalum capacitors in the rest of the oscilloscope.An easier and safer diagnostic test would be to use an external DC power supply to replace (or in addition to) the battery with the 213 left disconnected from the AC power line.QuoteI have made this kind of replacement before and it has worked out well so far however keep in mind that NiMH cells are not as tolerant of overcharge and trickle charging as NiCd cells. I would plan on keeping the original NiCd charging current and just living with a longer charging time at least until the oscilloscope is working.

author: gareth
author_rank: Contributor
timestamp: October 13, 2014, 07:45:47 pm
content: The service manual seems wonderful to me, compared to modern manuals that don't really qualify as user guides even.It has a section on the operation of the power supply circuit, but I confess I don't understand quite a bit of it. It does seem 'sub-optimal' in that the manual cautions against touching the ac plug when battery powering the scope as dangerous voltages could be present on it !?! Also there is a mod to this device which removes the constant 6mA power draw. If this draw is really constant then it kinda undermines the point of the low volt shut down!I have some power resistors I could use to present loads if I could work out what sort of current they are designed to supply.... Any ideas how I could do that?I appreciate there aren't any guarantees that damage won't occur - but why might it? What sort of situations are power supplies damaged without loads? I've heard that rf amplifiers need to be connected to a load or damage can occur - is this the same phenomena?I'll go over the board looking at the in-circuit resistance of tantalum caps - my research suggests that a 'normal' value for ESR is 1-2.5 Ohms - would you expect older caps (date-codes on the rest of the scope say 1976) to be in this range or did they have low esr types back then?The service manual has this to say about the theory of operation of the battery charger:QuoteLooking again at the NiMH option - even at 350mA that seems to be about 0.03C for a 11000mAh battery - practically a maintainance charge, never mind a slow overnight charge! I think I'll try a pair of NiCd and see what the actual charging profile is with them.Many thanks for your adviceGareth

author: David Hess
author_rank: Super Contributor
timestamp: October 13, 2014, 08:51:30 pm
content: Quote from: gareth on October 13, 2014, 07:45:47 pmI am sure the service manual is great except that I do not have one.QuoteI would do the others tests I suggested first. Then I would ask the question on the TekScopes@yahoogroups.com email list because someone there undoubtedly knows.QuoteIt is unlikely in a Tektronix design because they tended to cover things like that. I would just be cautious until other safer diagnostic options have been tried including powering it with an external DC supply replacing the batteries.Just to give an example, if the power supply oscillates with no load which might be aided by a bad output capacitor, the high voltage might damage the output transistors.QuoteThe tantalum ESRs should be roughly inversely proportional to capacitance. A bad one will likely be a dead short unless it exploded. Close visual inspection might show cracks or metal eruptions which look like solder stuck to the epoxy coating.QuoteIt looks like challenging stuff. I have not heard of the power supply section in these failing which is probably a good sign.QuoteI replaced 2 D NiCd cells with NiMH cells in a test instrument (not a 213) many years ago and they are still going strong last time I checked. I just lived with the extended charge time. After 14 hours of charging, it still runs as long as the NiCd D cells did.I suspect low discharge NiCd would be more reliable but I have yet to see them in a D size.

author: gareth
author_rank: Contributor
timestamp: October 14, 2014, 10:49:02 am
content: Thanks for all the advice, I've requested to join the tekscopes group you suggested and I'll ask the same question there too.In the meantime I can start testing caps for shorts!G

author: gareth
author_rank: Contributor
timestamp: October 14, 2014, 05:01:18 pm
content: I've started testing the scope for bad caps, specifically bad tantalums that have failed short - and what do you know, I've found three!One is C446 on the schematic I posted earlier, smoothing the main +15V rail, the other two are C550 and C555 - both smoothing the +6.5V rail and shown on a seperate schematic here:http://w140.com/tekwiki/wiki/File:Tektronix_213DMM_HV_and_CRT.jpg.So I can expect that both the 15V and 6.5V rails have been shorted - I wonder if this could have caused any further damage?I've started to go through the rest of the scope to see if I can find any others. In the meantime, can I ask, should I seek to replace these with tantalums or are there better alternatives these days? I'm sure that capacitor density has experienced it's own version of Moore's Law and if there are better (i.e. longer-life, better temperature tolerance etc.) alternatives I'm happy to spend the extra few pennies on them.Thanks againGareth

author: David Hess
author_rank: Super Contributor
timestamp: October 14, 2014, 05:27:08 pm
content: Quote from: gareth on October 14, 2014, 10:49:02 amGood times!

author: gareth
author_rank: Contributor
timestamp: October 15, 2014, 01:33:49 pm
content: Update - I've taken the three suspect capacitors out of the board and wierdly, two of them are now showing high resistance! The 100uF C555 is showing as 0.1 Ohms but the other two seem fine.I've got a kit transistor tester and it reports the two others as having 31uF and 156 uF!I don't know if it's related but I spotted that a zener diode VR440 was broken - I had a compatible one so I replaced it.Looking at the circuit again, I can see how a failed cap at C555 would also make C550 seem shorted, D'oh!But I can't work out why C446 seemed bad and now, having replaced the zener, it seems fine.Oh well, I'll just wait another 24 hours to see if any tekscopes people can give any additional info, then I'll test on batteries.

author: Lcarra
author_rank: Newbie
timestamp: May 02, 2017, 10:09:21 am
content: Hello, sorry for restarting the subject again, but I have the same problem as fellow Gareth, I have a TK213DMM and I would like to power it with 220 Volt. To repair it, Gareth comments in his first intervention that he did the option 1: change the bridge for a capacitor, could you tell me the type and value thereof?Thank you and sorry for my English.a greetingHola, perdón por reiniciar otra vez el tema, pero tengo el mismo problema que el compañero Gareth, tengo un TK213DMM y quisiera alimentarlo con 220 Volt. para repararlo, Gareth comenta en su primera intervención que le hizo la opción 1: cambiar el puente por un condensador, ¿Me podrían indicar el tipo y valor del mismo?Gracias y perdón por mi Ingles.Un saludo

author: Lcarra
author_rank: Newbie
timestamp: May 02, 2017, 10:44:32 am
content: I already found the thread referenced by Gareth in his thread, (Tek 213 Repair / Fitting Option 1 for 240V) I put it in case it is worth any moreThank youYa encontré el hilo al que hace referencia Gareth en su hilo, (Tek 213 Repair / Fitting Option 1 for 240V) lo pongo por si le vale a alguno más https://www.eevblog.com/forum/repair/tek-213-repair-fitting-option-1-for-240v/msg527016/#msg527016] https: // www. Eevblog. Com / forum / repair / tek-213-repair-fitting-option-1-for-240v / msg527016 / # msg527016 [/ ftp]Thank youYa encontré el hilo al que hace referencia Gareth en su hilo, (Tek 213 Repair / Fitting Option 1 for 240V) lo pongo por si le vale a alguno más https://www.eevblog.com/forum/repair/tek-213-repair-fitting-option-1-for-240v/msg527016/#msg527016Gracias

author: harrimansat
author_rank: Regular Contributor
timestamp: December 14, 2018, 11:34:13 pm
content: I can help you, I have two working 213, I´m who had modified the schematics showed in this topic. This scopes model drafts about 5mA batt when is power off. The mods are to reduce it. I´m not sure why tektronix do that, could be to preserve tantalium capcitors, I don´t know

author: HRMartinez
author_rank: Newbie
timestamp: March 09, 2019, 08:44:03 am
content: My Tea 213 is "working" but the batter life of the NiCds is poor.Was thinking about upgrade to NiMh.Are there any circuit modifications that need to be made for this?Should I just stay with higher capacity NiCd?Any advice is appreciated.-Henry

author: rea5245
author_rank: Frequent Contributor
timestamp: May 08, 2019, 12:33:25 am
content: Gareth, if you're still around: did you ever get the Tek 213 running?- Bob

author: harrimansat
author_rank: Regular Contributor
timestamp: May 21, 2019, 09:13:07 pm
content: Try lithium

author: harrimansat
author_rank: Regular Contributor
timestamp: May 23, 2019, 08:51:03 pm
content: HRMARTINEZ your inbox is full!I have seven 2xx series scopes, all of them with some degree of damage due NICD leak, some of them come original battery from 70´s! Luckily they were all saved, some of them now are working with litthium, more lighter and no leaks.Regardshttps://www.eevblog.com/forum/repair/tektronix-221-main-transformer-rewind-and-lithium-instalation/

author: rea5245
author_rank: Frequent Contributor
timestamp: May 23, 2019, 08:53:20 pm
content: Wouldn't lithiums have a problem with the trickle charge the scope provides for NiCds?- Bob

author: xxx2fan
author_rank: Contributor
timestamp: May 24, 2019, 12:18:00 am
content: Try this sight for schematics for the 213 oscilloscopeLet me know if it helps youhttp://w140.com/tekwiki/wiki/Main_Pagexxx2fan

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 02:09:28 am
content: Quote from: rea5245 on May 23, 2019, 08:53:20 pmYes, so a shunt voltage regulator would be added.

author: rea5245
author_rank: Frequent Contributor
timestamp: May 24, 2019, 04:43:08 pm
content: Are batteries even necessary? The manual (http://w140.com/tek_212.pdf, PDF page 37) says:QuoteSo let's say I was happy to always run it off AC. Could I just replace the batteries with a large cap? How would I determine the capacitance I need?- Bob

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2019, 09:29:58 pm
content: Quote from: rea5245 on May 24, 2019, 04:43:08 pmYes, I think so. Based on the battery size and run time, the DC input current seems to be about 350 milliamps. Operating voltage is regulated by the stack of 10 NiCd cells to between about 12 and 14 volts. So using the usual 8200 microfarad rule of thumb, 2 volts of ripple at 350 milliamps requires a filter capacitor of about 1500 microfarads which sets a lower limit on the size.But a shunt regulator to limit the voltage to about 14 volts (or a little higher) is still required. Power dissipation will be about 2 watts minimum so a couple of big zener diodes could be used for this.

author: rea5245
author_rank: Frequent Contributor
timestamp: May 25, 2019, 12:40:38 am
content: Thank you David. I was not familiar with shunt regulators, so you've prompted me to read up on them and learn something.Alas, your analysis of the capacitance needed is still a little over my head.The batteries are charged when the scope is ostensibly turned off. If I replace them with a capacitor and the shunt is going to consume 2W or more whenever the scope is turned off but still plugged in, I'll feel guilty. Maybe I'll stick with the batteries.- Bob

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2019, 04:02:39 am
content: I used a shortcut for the capacitance calculation. What it comes down to is that with an estimated 350 milliamp load, allowing 2 volts of voltage drop between 120 Hz line cycles yields a capacitance of about 1500 microfarads. There are enough unknowns in this case that 1500 microfarads just becomes a start point.dV/dT = I/C or the change in voltage over time is equals to the current divided by the capacitance.Zener diodes make fine shunt regulators but it is not common to use them at higher powers anymore. This is only a practical problem however because availability of high power zener diodes has decreased over time. But you can still use multiple diodes in series to distribute the power and 5 watt devices are still readily available.There are other ways to make a shunt regulator but a couple of zener diodes would be simple and adequate in this case. A pair of 7.5 volt 5 watt zener diodes in series would do it at less than the cost of one dollar.You are right about the waste of power. The power switch connection could be changed to make it on-off instead of charge-on.

author: harrimansat
author_rank: Regular Contributor
timestamp: May 26, 2019, 10:55:12 am
content: 213 Works with 2.4V, two NICD.

author: harrimansat
author_rank: Regular Contributor
timestamp: May 26, 2019, 10:56:16 am
content: A protected 18650 will work without voltage regulator, when is charged mosfet opens