Author Topic: Best practices for repair of old equipment  (Read 2102 times)

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Offline Sparky

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Best practices for repair of old equipment
« on: August 02, 2018, 06:26:31 am »
Having recently repaired my HP 6227B (see thread here) I'm curious of "best practices" for repair/restoration of older equipment.

For example I've often read here of people replacing the electrolytic caps.  It seems that's a "good practice" to follow, especially as the caps might otherwise appear perfectly fine (which was the case in my HP 6227B).

If one is replacing electrolytic caps, what about mylar or ceramic?  Do they suffer the same performance degradation over time?  Or just electrolytics because of the particular dielectric they use?

Some capacitors in the vintage equipment are quite massive compared to current parts of similar capacitance and voltage rating.  Are the original (often customer) parts somehow "special", or it's okay to replace with current generic equivalents?

I asked a few questions like these in my repair thread.  I'm hoping someone can drop a few comments so I can more completely restore the HP 6227B power supply.

Thanks!
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Offline Zucca

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Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #2 on: August 02, 2018, 07:05:37 am »
Having recently repaired my HP 6227B (see thread here) I'm curious of "best practices" for repair/restoration of older equipment.

For example I've often read here of people replacing the electrolytic caps.  It seems that's a "good practice" to follow, especially as the caps might otherwise appear perfectly fine (which was the case in my HP 6227B).

If one is replacing electrolytic caps, what about mylar or ceramic?  Do they suffer the same performance degradation over time?  Or just electrolytics because of the particular dielectric they use?

Electrolytic caps may need replacement, but not necessarily so. It depends on how they were made, how they were used, and their operating environment. High ripple currents, hot nearby components, poor ventilation, bad seals, etc. lead to the caps drying out. However, some of them can be just fine after 50 years.

Unless the electrolytic caps were produced during the era of the capacitor plague (1990's to early 2000's), I wouldn't shotgun replace them. First, visually check for signs of leakage, bulging or other deformation. For filter caps, check for excessive ripple while it's running. Of course, for final diagnosis of a suspect one, check out of circuit for capacitance, ESR, leakage, etc.

Tantalum capacitors can be problematic because they usually fail short circuit, explode, release magic smoke, or all of the above.

Old Rifa capacitors used across the mains inputs are notorious for burning up. If their plastic casing shows any signs of cracking, replace them with properly rated X or Y capacitors, depending on where they're being used.

Old Schaffner power line filters that are often used on the mains input of old equipment are also notorious for becoming incendiary (they have Rifa caps in them). Change those things, too.

On really old stuff (e.g., tube gear), wax and paper capacitors should be changed straight away. They're a disaster waiting to happen.

Other types of capacitors are generally OK and don't need to be replaced from a preventive point of view.

Quote
Some capacitors in the vintage equipment are quite massive compared to current parts of similar capacitance and voltage rating.  Are the original (often customer) parts somehow "special", or it's okay to replace with current generic equivalents?

If they're electrolytic, it's fine to replace them with modern equivalents. Be sure to meet or exceed their voltage rating. The capacitance can be slightly higher if an equivalent isn't available. Going too high (e.g., 2x or more) might cause higher inrush current that could be problematic, depending on what they're used for in the circuit.

Some old caps look like electrolytics, but may instead be wet tantalum or something else. If in doubt, find out.
I TEA.
 
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Offline Bashstreet

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Re: Best practices for repair of old equipment
« Reply #3 on: August 02, 2018, 09:49:52 am »
There is not much point randomly replacing electrolytic caps.
If the caps are good quality ones just leave them be.

If you deal with real old equipment that has wax/paper etc caps get rid them as they all will leak.
Ceramic, polyester, polypropylene etc caps let them be.

There are times you might want to change all types of caps if they are suspect or play crucial role and you want to keep them on speck.

There is a lot of hype about changing all electrolytic caps what is nonsense. Electrolytic capacitor tolerances are often large and it wont make much difference if there is some age degeneration.

If you deal with 1990's microcomputers it is good to note many manufacturers used absolute rubbish electrolytics and my commodore 64 is fully recapped.


 
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Online shakalnokturn

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Re: Best practices for repair of old equipment
« Reply #4 on: August 02, 2018, 12:23:39 pm »
I've seen MKP and MKT caps loose capacitance.
Usually on capacitive voltage droppers on mains,  half bridge SMPS or induction cookers.
 
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Online bd139

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Re: Best practices for repair of old equipment
« Reply #5 on: August 02, 2018, 12:34:32 pm »
My own principles:

1. Only fix what is wrong or is going to explode.

That is it :)

Edit: things likely to explode.  X2 caps by RIFA, wax/paper caps, anything which is damaged in a visual inspection. As a rule I don’t swap the electrolytic caps out unless they are actually measurably dying, leaking or more than about 30 years old.
« Last Edit: August 02, 2018, 12:36:31 pm by bd139 »
 
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Online Kosmic

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Re: Best practices for repair of old equipment
« Reply #6 on: August 02, 2018, 01:43:10 pm »
1. Only fix what is wrong or is going to explode.

Totally agree with that  :-+

It's also important to note that in a lot of high-end test equipment the quality of the components used was really high. So if you replace a old working cap (with really low ESR) with a brand new one but with lesser spec, you might just make things worst.
 
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Online TimFox

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Re: Best practices for repair of old equipment
« Reply #7 on: August 02, 2018, 01:44:38 pm »
Paper capacitors, unless in hermetic packages (like military), in tube-era gear, including plastic cased, have probably absorbed moisture into the dielectric.  Typical screen bypass caps will leak enough to depress the screen voltage.  I replaced all the plastic-cased paper caps in my Tektronix L,C meter with polypropylene; every one showed a lousy Q value after removal.
 
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Offline Sparky

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Re: Best practices for repair of old equipment
« Reply #8 on: August 02, 2018, 04:35:45 pm »
a few inch above...
https://www.eevblog.com/forum/repair/vintageclassic-renovation-techniques/

Thanks zucca!  Can't believe I missed that!  A treasure trove of repair info!

Electrolytic caps may need replacement, but not necessarily so. It depends on how they were made, how they were used, and their operating environment. High ripple currents, hot nearby components, poor ventilation, bad seals, etc. lead to the caps drying out. However, some of them can be just fine after 50 years.

Unless the electrolytic caps were produced during the era of the capacitor plague (1990's to early 2000's), I wouldn't shotgun replace them. First, visually check for signs of leakage, bulging or other deformation. For filter caps, check for excessive ripple while it's running. Of course, for final diagnosis of a suspect one, check out of circuit for capacitance, ESR, leakage, etc.

<snip>

If they're electrolytic, it's fine to replace them with modern equivalents. Be sure to meet or exceed their voltage rating. The capacitance can be slightly higher if an equivalent isn't available. Going too high (e.g., 2x or more) might cause higher inrush current that could be problematic, depending on what they're used for in the circuit.

Some old caps look like electrolytics, but may instead be wet tantalum or something else. If in doubt, find out.

Awesome tips, bitseeker!  Really great info that "shotgun replacement" is not necessarily the way to go, and which brand/type of capacitor need to be on the look out for.  I did not see any RIFA cap in my HP 6227B -- most of the caps seem seem custom HP parts...possibly manufactured by another company.

There is not much point randomly replacing electrolytic caps.
If the caps are good quality ones just leave them be.

<snip>

There is a lot of hype about changing all electrolytic caps what is nonsense. Electrolytic capacitor tolerances are often large and it wont make much difference if there is some age degeneration.

Thanks, Bashstreet! It's good to hear similar advice from few sources :)

1. Only fix what is wrong or is going to explode.

:-+  bd139

It's also important to note that in a lot of high-end test equipment the quality of the components used was really high. So if you replace a old working cap (with really low ESR) with a brand new one but with lesser spec, you might just make things worst.

Excellent point, Kosmic.  Many cap might be custom and difficult to know it's exact spec, so replacing it might could be a complete guess...

Paper capacitors, unless in hermetic packages (like military), in tube-era gear, including plastic cased, have probably absorbed moisture into the dielectric.  Typical screen bypass caps will leak enough to depress the screen voltage.  I replaced all the plastic-cased paper caps in my Tektronix L,C meter with polypropylene; every one showed a lousy Q value after removal.

Great advice, TimFox that paper caps can be replaced by current polypropylene type.



Lots of great comments!  That helps a lot to get a better understanding of the approach to repair/restoration from the pro's.   :-+

Happy restoration!  :-/O
 

Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #9 on: August 02, 2018, 06:29:45 pm »
Happy restoration!  :-/O

You, too! It's quite fun to get old stuff up and running again.
I TEA.
 
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Offline Sparky

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Re: Best practices for repair of old equipment
« Reply #10 on: August 02, 2018, 06:48:33 pm »
Happy restoration!  :-/O
You, too! It's quite fun to get old stuff up and running again.

Indeed, and very satisfying, too! 

I have a Power Designs TP343A to work on next :)  I just got some aluminium knobs that were missing from the voltage controls.  The "pull to release" tracking switch is broken so I need to find a replacement for it.  The voltage adjustment pots grind when adjusting...so they need replacement, too.  I will start a new thread when I get to it.  It will be a fun project!
 

Online bd139

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Re: Best practices for repair of old equipment
« Reply #11 on: August 02, 2018, 07:35:59 pm »
Happy restoration!  :-/O

You, too! It's quite fun to get old stuff up and running again.

Unless EVERYTHING is broken. Heap of D83’s I’m looking at you.
 

Online tautech

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Re: Best practices for repair of old equipment
« Reply #12 on: August 02, 2018, 08:32:32 pm »
Best practice should include safe practice as we cannot know the skill level of those we advise online.
This alone is the #1 reason why a 'Repair Guidance' thread has never become a sticky for fear that someone's gunna get themselves in over their head with disastrous consequences.  :scared:


A few links to old threads that have some little gems that can help those setting up with the 'needed' knowledge and gear.
No particular order.
https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/
https://www.eevblog.com/forum/beginners/life-hacks-for-the-beginning-ee/
https://www.eevblog.com/forum/beginners/definitive-capacitor-identification-chart-(w-pictures)/
Avid Rabid Hobbyist
 
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Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #13 on: August 03, 2018, 03:09:46 am »
I have a Power Designs TP343A to work on next :)  I just got some aluminium knobs that were missing from the voltage controls.

Ooh, what do those knobs look like? I upgraded the plastic knobs on my TP340A. Pics at https://www.eevblog.com/forum/repair/power-designs-tp340a-repair-and-facelift/msg770494/#msg770494

Quote
The "pull to release" tracking switch is broken so I need to find a replacement for it.  The voltage adjustment pots grind when adjusting...so they need replacement, too.

I've used Gemini/Alco DPDT Locking Toggle Switch (125V, 5A), part #A201KZQ as a replacement for that. The pots are high-quality Bourns multi-turn.

Quote
I will start a new thread when I get to it.  It will be a fun project!

Let me know when you start that one. I also have a TP343.
I TEA.
 
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Offline Sparky

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Re: Best practices for repair of old equipment
« Reply #14 on: August 03, 2018, 06:17:02 am »
Ooh, what do those knobs look like? I upgraded the plastic knobs on my TP340A. Pics at https://www.eevblog.com/forum/repair/power-designs-tp340a-repair-and-facelift/msg770494/#msg770494

I saw your TP340A thread!  I have one too!  :D  Mine had the plastic knobs just as yours did, and I didn't like it either how they would stick out from the chassis.  Unfortunately I lost one plastic knob during my cleaning...I still have two of the plastic originals...but then came the urge to get metal knobs.  I ended up buying a "for parts" TP343 cheap locally and took the knobs off it for the TP340A.  I thought the TP343 would be in really bad shape but it isn't, hence the new item on the restoration list!  ;D 

Now I needed knobs for the TP343!  I like the ones with flange you used for your TP340A. They are a great match to the unit and being large diameter are good for fine control.  Unfortunately no more available from the seller on eBay.  Instead I got a few of these.  Luckily I did not pay $8/knob and the seller accepted my offer!

Quote
I've used Gemini/Alco DPDT Locking Toggle Switch (125V, 5A), part #A201KZQ as a replacement for that. The pots are high-quality Bourns multi-turn.

Thanks for the info!  Glad to know I can find a replacement locking toggle switch!

I have a question about the Bourns multi-turn pots --- I'll write in your thread because you mention the issue I want to ask about.

Quote
Let me know when you start that one. I also have a TP343.

Will do! :D
 

Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #15 on: August 03, 2018, 06:23:29 am »
Ah, yes. The old, "I'll get it for parts," only to find out it's too good to be just a parts mule. That's one of many ways you end up with GAS.
I TEA.
 

Offline Sparky

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Re: Best practices for repair of old equipment
« Reply #16 on: August 04, 2018, 04:59:11 am »
Let me know when you start that one. I also have a TP343.

I got it off the bottom shelf today -- I made a mistake, it's a TP325.  Power Designs sure made a lot of models with only slight differences!  It will be a fun project and as shown in the pic there's really not that much wrong with it from the outside.

Ah, yes. The old, "I'll get it for parts," only to find out it's too good to be just a parts mule. That's one of many ways you end up with GAS.
Exactly right!  But what is GAS?
 

Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #17 on: August 04, 2018, 05:38:17 am »
I got it off the bottom shelf today -- I made a mistake, it's a TP325.  Power Designs sure made a lot of models with only slight differences!

They made a lot of power supply models. I have eight different ones and I've seen probably a dozen or more in addition. I still see ones I've never seen before from time to time.

Your TP325 is in good condition. Just one split banana jack, but the faceplate is pretty good. I've seen some really banged up and scarred ones. :(

Ah, yes. The old, "I'll get it for parts," only to find out it's too good to be just a parts mule. That's one of many ways you end up with GAS.
Exactly right!  But what is GAS?

See the TEA glossary (scroll past the Points of Interest) for that acronym and more.
I TEA.
 
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Offline Sparky

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Re: Best practices for repair of old equipment
« Reply #18 on: August 05, 2018, 01:52:24 am »
They made a lot of power supply models. I have eight different ones and I've seen probably a dozen or more in addition. I still see ones I've never seen before from time to time.

Your TP325 is in good condition. Just one split banana jack, but the faceplate is pretty good. I've seen some really banged up and scarred ones. :(

My TP325 thread is up!  It has a little problem with Source A to fix :)

See the TEA glossary (scroll past the Points of Interest) for that acronym and more.

That thread is hilarious!  I'm already showing some symptoms...
 

Online bitseeker

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Re: Best practices for repair of old equipment
« Reply #19 on: August 05, 2018, 01:56:01 am »
My TP325 thread is up!  It has a little problem with Source A to fix :)

Thanks, I'll go check it out.

See the TEA glossary (scroll past the Points of Interest) for that acronym and more.

That thread is hilarious!  I'm already showing some symptoms...

Yeah, I detected some symptoms in this thread. Welcome to the TEA house.
I TEA.
 
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