Advice needed repairing my Tek 213 DMM & battery replacement

[gareth]

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.jpg

I 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 here http://cpc.farnell.com/yuasa/1dh4-0t/battery-ni-cad-d-4ah-tagged/dp/BT05361 and here http://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.

[David Hess]

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.

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.jpg

Usually no damage will occur but correct operation without a proper load is not guaranteed.

I 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.

Check 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.

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).

I 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.

[gareth]

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:

Battery Charger Converter
The battery charger converter is frequency controlled with the battery charge current regulated by the control transformer T423.
Transistors Q422 and Q424 form a power converter. Feedback through R427 and pins 4 and 5 of T423 maintains oscillation at a frequency near the LC resonant frequency established by C427 and the primary of T430. When operating near the resonant frequency, the output voltage of the secondary of T430 is about 3 to 4 volts peak. When this voltage is rectified and filtered by CR434, CR435, C436 and L435, it causes the batteries to charge.
The converter transistors Q422 and Q424 are not regenerative when first turned on; therefore, CR426, a bidirectional negative-resistance breakdown diode, is added to provide a start pulse. As C425 begins to charge through CR425, a point is reached where breakdown of CR426 occurs and pulses Q424 into operation.

Battery Charger Regulator
As the battery charge increases to about 350 milliamperes, the voltage developed across R447, which is sensed by Q444, is sufficient to cause an increase in current through Q442 and pins 6, 7, 8, and 9 of T423. This increased current flow changes the permeability of T423, which effectively reduces its inductance and raises the operating frequency of the converter. As the converter frequency shifts away from resonance, the output voltage from T430 decreases and thus reduces the charge rate.
CR427 and CR428 are fly-back diodes which catch stored energy in the LC resonant circuit to improve efficiency. CR421, CR422, CR423, and CR424 are protection devices for Q422 and Q424. Rectifiers CR421 and CR423 prevent a short circuit of the supply by insuring that one transistor is off before the other turns on. CR422 and CR424 prevent forward biasing of the collector-base junction during the fly-back period. R423 and C423 prevent feedback oscillation in the drive transformer. VR440 prevents charger runaway should the batteries be disconnected while the charger is in operation. When the charger reaches the breakdown voltage of VR440, current flows through the T423 control windings and drives the converter off resonance which reduces the output voltage. CR430 and CR432 provide a negative supply voltage to operate Q442 and Q444.

Looking 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 advice

Gareth

[David Hess]

The service manual seems wonderful to me, compared to modern manuals that don't really qualify as user guides even.

I am sure the service manual is great except that I do not have one.

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 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.
 

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?

It 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.

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 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.

The service manual has this to say about the theory of operation of the battery charger:

...

It looks like challenging stuff.  I have not heard of the power supply section in these failing which is probably a good sign.

Looking 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.

I 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.

[gareth]

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

[gareth]

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 again

Gareth

[David Hess]

In the meantime I can start testing caps for shorts!

Good times!

[gareth]

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.

[Lcarra]

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 greeting

Hola, 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

[Lcarra]

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 more
Thank you

Ya 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 [ftp]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 you

Ya 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 [ftp]https://www.eevblog.com/forum/repair/tek-213-repair-fitting-option-1-for-240v/msg527016/#msg527016
Gracias

[harrimansat]

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

[HRMartinez]

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

[rea5245]

Gareth, if you're still around: did you ever get the Tek 213 running?

- Bob

[harrimansat]

Try lithium

[harrimansat]

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.

Regards

https://www.eevblog.com/forum/repair/tektronix-221-main-transformer-rewind-and-lithium-instalation/

[rea5245]

Wouldn't lithiums have a problem with the trickle charge the scope provides for NiCds?

- Bob

[xxx2fan]

Try this sight for schematics for the 213 oscilloscope
Let me know if it helps you

http://w140.com/tekwiki/wiki/Main_Page
xxx2fan

[David Hess]

Wouldn't lithiums have a problem with the trickle charge the scope provides for NiCds?

- Bob

Yes, so a shunt voltage regulator would be added.

[rea5245]

Are batteries even necessary? The manual (http://w140.com/tek_212.pdf, PDF page 37) says:

When the instrument is connected to a power line the
AC power is capacitively coupled to the Power Rectifier.
The rectified DC is used to either run the instrument or
recharge the internal batteries. The batteries act as a
large filter capacitor for the Input Rectifier in the AC line
mode of TM 9-6625-646-14&P operation. When the
instrument is not connected to a power line, operating
power is provided by the batteries.

So 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

[David Hess]

So 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?

Yes, 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.

[rea5245]

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

[David Hess]

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.

[harrimansat]

213 Works with 2.4V, two NICD.

[harrimansat]

A protected 18650 will work without voltage regulator, when is charged mosfet opens