"Upgrading" an old Analog Oscilloscope ?

[CochainComplex]

Hi,

one question came through my mind, recently. What would happen if I would replace all passive Parts in my Tektronix 2225. It is working fine but it will turn 30 years and especially the electrolytic caps will not getting better with years. The same happens with Carbon Composit Resistors, but not so obvious. So ...will the Oscilloscope profit of such a overall replacement with newer even better parts (thigther tolerances, lower tcr, metallfilm instead of carbon, maybe low esr Caps etc)? (except for prolonging the Scopes lifetime) Such as SNR, accuracy may small gain in bandwith.

(replacing semiconductors will be more difficult - ....maybe diodes with better specs) 

Thanks for your suggestions in adnvance

Best regards
Alex 

[ebastler]

Sure -- and it will also give you a more solid bass, crisp well-defined mids and transparent treble, and much better localization of all instruments especially when listening to classical music...

Seriously, don't mess with the scope. Preventive replacement of electrolytic caps may be worth a thought (although I would not do even that). But I don't think you should touch anything else unless you observe a specific problem with it. I would not expect any significant performance change, and would rather leave the scope in its orignal state.

[tggzzz]

Key question: is this the best use you could make of your time? The definition of "best" is for you to decide, nobody else.

Any replacement in any equipment carries risks:

• damaging the board or other parts of the equipment
• replacements are subtly different, e.g. faster transistor or lower ESR => oscillation, or higher ESR => higher ripple
• recalibration required

If something is broken to the extent that a replacement will significantly improve it, then replace it. If something is likely to break soon then replace it at the same time as other components are being replaced. Otherwise, leave well alone.

The main components that are likely to fail are PSU electrolytic capactors and tantalum capacitors. If one of those fails then replace all the Al electrolytics with a "switching" capacitor (e.g. Nichicon PW with double the voltage rating), or tantalum beads while ensuring that they aren't operating near their voltage rating.

[Fungus]

If it ain't broke, don't fix it.

[CochainComplex]

Thanks to both of you!

@ebastler...Oh yes I hope the highs at 30Mhz will be more crisp for my supersonic ears - I allways clean them with snakeoil
In general I meant preventive replacement of electrolytic caps, also tantalums. But if I open the device I could also replace some other parts (metalfilms are cheap). The tektronix scope has an easy to maintain layout - desoldering some throughhole resistors is not an issue. Metalfilms will be more stable over the next few years so less recalibration.
I obiously don't know much about Oscilloscope Topology but lets say we have some easy low order filters made of carbon composite resistors together with some electrolytics - wouldn't the cutoff frequency be shifted by aging parts? This will also happen if you use an active filter with the passive feedback parts. This leads to a broad missaligntment of the transferfunctions of the whole system. (but yes, probably tektronics used film resistors and ceramic/foil caps).

@tggzzz  How distructive will it be having some failing tantalums? I ve heard they might fail suddenly under several circumstances (I don't use them for my designs for several reasons - this is one of it). I guess the last 30 years those circumstances were good. It will be tough if they would take down some "not longer in production" semiconducters - so I want to prevent this.  Cap replacement in the PSU is also an option - could also be better for Ripplerejection.

Behind my thought was also this document (among others) http://www.st.com/content/ccc/resource/technical/document/application_note/9d/56/66/74/4e/97/48/93/CD00004444.pdf/files/CD00004444.pdf/jcr:content/translations/en.CD00004444.pdf page16 Better powersupply. I know this is a total different topic but isnt a stable powersupply crucial as "baseline" for any measuring device - especially if you have old opamps with not so good CCMR and Powersupply Rejection ? ADC might be an extrem and not suitable here.  But this ideas are going beyond simple replacements.

regards Alex

p.s.: (ESR can be crucial to stability (e.g. LM317 Iadj case)  thats why I added "maybe" in front of it.) 

[tggzzz]

@tggzzz  How distructive will it be having some failing tantalums? I ve heard they might fail suddenly under several circumstances (I don't use them for my designs for several reasons - this is one of it). I guess the last 30 years those circumstances were good. It will be tough if they would take down some "not longer in production" semiconducters - so I want to prevent this.  Cap replacement in the PSU is also an option - could also be better for Ripplerejection.

The consequences depend on the circuit. Tant beads can fail open circuit or short circuit.

My 485's switching PSU Al electrolytics had become troublesome. At the beginning of a day it took ~60s for some of them to reform the oxide and for the PSW to start. After replacing them, the 485 suddenly stopped working, and I traced that to a short-circuit 15V tant bead on a 13V rail. When it shorted it also fried a resistor.

If you want specific information about your scope, I srongly recommend

• reading the service manual and schematics to see what might need replacing and what you have to do to replace it (465 poor, 485 OK, HP1740 better)
• searching the Yahoo TekScopes group (invaluable very practical information, helpful denizens)

[oldway]

You intend to make some "preventive" repair:

- choose the right analog oscilloscope: 2235 instead of 2225 or Hameg HM605 (in my opinion, the best Hameg analog scope)

- as Fungus wrote: "If it ain't broke, don't fix it." excepted if there is some well known issue in this oscilloscope.

- buy a second one for parts.

- greatest secret of working a very long time: to work cooler. That's a problem of 2235, power supply work to hot...add a fan as in 2236

- check power supply for ripple.

[CochainComplex]

Thx to all your ideas and thoughts on this topic