Replacement caps for Tektronix 434 Power supply board.

[John McGivern]

Hi guys, I live in UK trying to find caps for above scope electros and some dog bone I believe to be ceramic.
If someone could advise type value and voltage as some values are not available. How critical value is?

Many thanks, John.

[stj]

Re: Replacement caps for Tektronix 434 Power supply board.

value matters.
maybe if you list the caps you need it would help

[budm]

Re: Replacement caps for Tektronix 434 Power supply board.

How about getting the service manual?

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Here you go......

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi guys thanks for replying.
I have all the service manuals being a novice I don't have the practicle experience.
Will these caps be Tantalum electrolytics?
What value uF and voltage?
Replacement type?
These are the caps on secondary board.
uF. Voltage.
15. 300
12. 150
510. 25
100. 20
15. 100

I have read conflicting opinions regarding values when replacing tantalum electros with aluminium electros.
That is if they are tantalum, how do I check? John.

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Off the top of my head I'd say they probably aren't tantalum, I can't ever recall seeing a 300V rated tant cap. Look up the part numbers in the service manual and order them that way.

[budm]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi guys thanks for replying.
I have all the service manuals being a novice I don't have the practicle experience.
Will these caps be Tantalum electrolytics?
What value uF and voltage?
Replacement type?
These are the caps on secondary board.
uF. Voltage.
15. 300
12. 150
510. 25
100. 20
15. 100

I have read conflicting opinions regarding values when replacing tantalum electros with aluminium electros.
That is if they are tantalum, how do I check? John.

Can we actually see those caps? And what are the cap designators as printed on the board and on exactly which board? Pictures of the boards will help also.

[PeteS in CA]

Re: Replacement caps for Tektronix 434 Power supply board.

Off the top of my head I'd say they probably aren't tantalum, I can't ever recall seeing a 300V rated tant cap. Look up the part numbers in the service manual and order them that way.

I think Tek stopped making that model over 30 years ago. Finding exact replacements - the same P/N, not just the same value and voltage in a compatible package - could be difficult or impossible. The value, voltage, same dielectric, compatible package approach might be more feasible. For some 'lytics, being too good is possible, of course.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I think Tek stopped making that model over 30 years ago. Finding exact replacements - the same P/N, not just the same value and voltage in a compatible package - could be difficult or impossible.

The finding of exact Tektronics parts is unnecessary except in a few cases. This is a fantastic analog scope, well worth repairing. There are precious few parts here that need exact replacements. The critical items are going to be the CRT and any of the specialized mechanical switch parts on the front panel.

John, I don't recommend repairing the scope as a first time electronics project. I would start out by stating the obvious...what are the symptoms? Is it totally dead? Second, I would make a list of capacitors and other parts that are obviously faulty by visual inspection. Perhaps you know someone who can guide you?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Hi Longbow. Thanks for the information. I retired last year I unfortunately have no one to bounce questions off so my progress is slow.
I realise this is not an easy project as I cannot understand how the circuit for the power supply works.
1st symptom turned on scope was clicking.
Measured all ref. voltages all measured approx 50% of spec.
Due to my inexperience I connected scope through I/T and dim bulb and possibly shorted Q1080.
I have replaced transistor and 5 caps.
The old caps where incorrect values according to circuit diagram.
I replaced with as close to original value.
Replaced 2 precision resistors 6.81kohm and 5.62kohm they where reading approx 4kohm each.
I am waiting for uA723 10 pin can to arrive then put back together and fingers crossed switch on not through dimbulb.
Many thanks, John.

[PeteS in CA]

Re: Replacement caps for Tektronix 434 Power supply board.

While I have seen a uA723 used as a switching regulator, it's a linear regulator IC. If you haven't found it already, finding a datasheet online should be fairly easy. It's been working in electronics longer than I have.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

John, why don't you explain your skill level so we can know what to tell you? This scope has an off-line switching power supply. Do not use the 100 watt light bulb trick on it. Find yourself an isolation transformer and save yourself lots of headaches.

Fortunately Tektronics manuals are outstanding. They explain how the circuits work and how to troubleshoot them. The clicking noise you hear is probably the power supply trying to start up, failing and re-trying. Obviously your first efforts will be to get the power supply fixed. To do that, you must understand how it works. Begin with page 3-18.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Just as a point of interest for those who read through a recent post : HP9845 power supply... Just a slice of the power supply was shown on any one sheet. This Tek supply shows the operation of the mystery secondary feedback transformer. Here it is T1848 which they call a current transformer. Its purpose is to insure adequate dead-time for the switching transistors. These days, a special controller IC usually takes care of that problem.

I wonder if the designers would have used IGBT's or power MOSFET's if they had been available?

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

First thanks for replying.
Longbow I am retired trying to learn electronics to keep the old brain cells working. I restored a Pilot U650 date 1936 with the help from different forum's and Andy Davies by the sea.
My knowledge is limited to say the least.
I cannot follow the circuit for the power supply on this scope.
Where does the regulator get its voltage from?
Capacitor c1081 I don't follow connection What is its purpose?
Turned on scope today.
324Vdc at P119 also at collector of Q1080
No voltage at regulator.
No ref voltages.
The neons are alternating on/off.
Nothing on display, pressed beam finder still nothing.
Hope this helps.
There is no clicking

[srhofmann]

Re: Replacement caps for Tektronix 434 Power supply board.

Just as a point of interest for those who read through a recent post : HP9845 power supply... Just a slice of the power supply was shown on any one sheet. This Tek supply shows the operation of the mystery secondary feedback transformer. Here it is T1848 which they call a current transformer. Its purpose is to insure adequate dead-time for the switching transistors. These days, a special controller IC usually takes care of that problem.

I wonder if the designers would have used IGBT's or power MOSFET's if they had been available?

Maybe, would have depended on how reliable a design they would have come up with. Tek scopes have been mainstay in electronics for decades and are known for their reliability. The company I used to work for bought 475As by the skid. Very rarely have they used bleeding edge technology in their scope products. Don't think any of the new stuff supports USB3 yet and they still sell stuff with GPIB interfaces.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

I cannot follow the circuit for the power supply on this scope.

After you read through the circuit explanation I'll bet you will.

Where does the regulator get its voltage from?

I hope you are talking about the main power supply and not the high voltage area. Regulator is a broad term, you probably mean where does the power come from to operate the switching transistors Q1834 and Q1844??
These transistors get their power directly from the mains which is shown at the lower left corner of the power supply diagram. The mains voltage is rectified by the bridge, CR1821. A d.c. voltage of about 160 volts should appear accross C1826.

I'm guessing, but it is likely that your main power filter capacitors, C1822, C1823 need to be replaced. Since this is capacitor forum, you should look into getting an ESR meter since you will probably be replacing a number of faulty electrolyic capacitors. Only the faulty ones, of course.

The circuitry around U1910 actually controls what the switching transistors do, so I would call this IC and associated parts the actual regulator. Looks like U1910 needs 7.5 volts d.c. at its Vcc terminal 6. However, you will not see the 7.5 volts unless the switchers have started up. So, there has to be a starter circuit to get them going. Manual page 3-19 "Start Network". The start network uses some mains current to produce a kick to start the switchers. The kick is delivered through VR1831 to transistor Q1844.

The priority would be to get your primary 160 volts d.c. fixed. Make a complete check for shorts/opens around the switching transistor area. Then move on to the switching regulator area doing the same kind of checking for obvious faults. Keep in mind that you are working with mains voltage on the left side of T1860, so use caution. Do NOT connect a piece of grounded test equipment to the scope unless the scope is isolated with an isolation transformer. Also, the high voltage for the CRT is about 4000 volts. That's all I have to say about this subject.

Capacitor c1081 I don't follow connection What is its purpose?
324Vdc at P119 also at collector of Q1080

Sorry you lost me. I don't see any 1000 series parts in this scope. What page are you looking at?

No voltage at regulator.

where at the regulator? Which regulator are you referring to?

Nothing on display, pressed beam finder still nothing.

You are getting way ahead of yourself. Start at the plug. Then make sure your power supply is working. I think we will be there for a while. The part series in power supply is 1800 and 1900 so call out what part you are asking about.

Just a final word. These circuits are not easy to understand even with the excellent manual. Generally speaking it is important to learn electronics a bit at a time. Have a relaxing and satisfying adventure. My suggestion is to keep a journal and take lots of notes.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

Make sure your multi-voltage switch on the back is set to 240 VAC.

[Longbow]

Re: Replacement caps for Tektronix 434 Power supply board.

My way of looking at electrolytics - at least in an amusing way - is that all of them lose the same number of microfarads over time regardless of initial value. Obviously this is not a fully qualified scientific statement. However, it seems that capacitors with values of 1 uF, and 2.2 uF and so on drop out dependably, while the loss of a few uF in larger caps isn't that big a deal. And this Tek scope has lots of those little value caps in critical spots. Just an opinion.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Longbow you must be looking at the wrong S/number this scope is the earlier version it's in the subject title Trektronix 434 s/n below 500000.

[John McGivern]

Re: Replacement caps for Tektronix 434 Power supply board.

Here you go......

Hi this is the earlier PDF my scope is the older s/n below 500000.

See post 21.