EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: electr_peter on April 04, 2021, 08:20:12 pm
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Tektronix 465 analog oscilloscope stopped working abruptly :-BROKE. During power up loud crackling started and display did not show anything, it was powered down as soon as possible.
Scope is set to 230V input. When powered from variac, crackling sound starts from ~90 VAC level. It is not clear where crackling sound is coming from - almost sounds as if something is arcing or popping.
Fuses are not blown, visually all caps are intact. Few main PSU caps were pulled out, but they tested fine.
I fear it has something to do with HV circuit (due to high voltage and very limited access).
What could be the issue? Have you heard such sounds in faulty analog scopes?
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I've recently been working on a Hameg scope, during initial power up there was a buzzing/crackling, I found 2 resistors with burn marks on, one measured open the other was out of tolerance. Might be worth doing a visual check to see if there are any components which show discoloration or signs of damage which might be causing the sound?
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I've recently been working on a Hameg scope, during initial power up there was a buzzing/crackling, I found 2 resistors with burn marks on, one measured open the other was out of tolerance.
Interesting, in which part of the circuit did resistors fail?
Might be worth doing a visual check to see if there are any components which show discoloration or signs of damage which might be causing the sound?
Good idea, although it will be difficult to see everything.
I will add recording of the sound as description of it may not be sufficient to identify source of the problem.
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Main issue was found in HV PSU section. Arc has jumped across 6kV rated capacitor and lit up neon lights.
(https://www.eevblog.com/forum/repair/tektronix-465-does-not-power-up-crackling-sound/?action=dlattach;attach=1211766;image)
Where can I find a replacement part?
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There is a lot of deposit in that area. Carbon deposits will start conducting at this high voltages and will arc out. It is quite normal for a scope of this age. Before you do anything else, clean up all the soot and let it dry for a few hours. Make sure you don't knock anything in the process. If there are any bad solder joints, run some new solder on them. Be careful of those ceramic bars as they are not easily available and they have lead free solder if I am not mistaken. Do not use leaded solder on them.
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Thanks for the tips.
I cleaned HV area slightly before taking photo, but it is still dusty. It is under metal cover and attracts dust particles. Other areas without cover do not have dust. Tek 465 has a fan BTW.
On the label it was noted to use only silver bearing solder (I presume this means lead is not allowed) - otherwise solder may come loose from ceramic pieces. I have some silver solder to use when I track down the source of capacitors.
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Main issue was found in HV PSU section. Arc has jumped across 6kV rated capacitor and lit up neon lights.
Where can I find a replacement part?
https://www.qservice.tv/vpasp/shopexd.asp?id=9339 (https://www.qservice.tv/vpasp/shopexd.asp?id=9339)
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"On the label it was noted to use only silver bearing solder (I presume this means lead is not allowed) - otherwise solder may come loose from ceramic pieces. I have some silver solder to use when I track down the source of capacitors."
Try to source one like this closer to you.
https://www.ebay.com/itm/015-Silver-Solder-3-Silver-100-inches-Length-KESTER-Flux-Core-Low-Melt/391907299741 (https://www.ebay.com/itm/015-Silver-Solder-3-Silver-100-inches-Length-KESTER-Flux-Core-Low-Melt/391907299741)
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Fixed quite a few of these. Should be fine if you clean it. Dry brush the soot off then wipe with 100% isopropyl and let it dry for a few minutes.
Best place if it’s still no good is hvstuff.com
While you’re under that panel, if you put the scope on the feet and look at the bottom left there’s a 47uF tantalum capacitor which sits on the unregulated line for the HT oscillator. Replace that while you’re in there with a 100uF electrolytic. The tant is notorious for going short. I bought about 5 with dead HT and that was all that was wrong with them!
The tant always goes short the moment you’ve fixed everything else and put the case back on! :-DD
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Capacitors are underway and the hunting season for tantalums is open :D
Pictures in the service manual show a filter element near the fan. Filter was missing in a scope, only metal screen remains. Part number for filter is 378-0044-01, "FILM ELEM, AIR CO:2.5 SQ * 0.25 INCH THK".
I wonder if filter would help with reducing dust build-up in HV area.
(https://www.eevblog.com/forum/repair/tektronix-465-does-not-power-up-crackling-sound/?action=dlattach;attach=1212007;image)
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A filter should be used.
Get the 1/4" thick material sold at the home supply for window AC units and cut to fit.
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I never bothered with a filter. The main use is to stop the grille hitting the fan :-DD
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A filter should be used.
Get the 1/4" thick material sold at the home supply for window AC units and cut to fit.
That's exactly what I used in mine, it's worked great for about 20 years now. The filter does help, I've had to wash mine a couple times over the years.
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On the label it was noted to use only silver bearing solder (I presume this means lead is not allowed) - otherwise solder may come loose from ceramic pieces. I have some silver solder to use when I track down the source of capacitors.
Classic Tektronix oscilloscopes, with the ceramic terminal strips, require a silver-bearing solder to avoid leaching the silver metallization from the ceramic surface. I believe they used Sn62, Pb36, Ag2 ternary eutectic, which has a similar melting point to the non-silvered Sn63, Pb 37 binary eutectic that is more popular (except for this application).
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Update - I changed failed capacitor and the scope works now.
I have one new high voltage 6kV capacitor leftover. If anyone needs it for a repair, you can contact me via message.
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as regards the ceramic strips and solder,if i remember solder with a high silver content was used,my 453 had a small spool of it fitted behind the rear cover,maybe the same on yours.
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There was no solder inside. I used solder with silver though.
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heres a couple of pics what was in the back om my scope,was fitted to the rear under the rear cover,was ages before i spotted it tho.
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another
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Including that bit of silver solder with the older Tek gear just spoke of their commitment to quality and long service life!! I had several older pieces of Tek gear and remember well the schematics with voltages listed and a functional description of every part in them. Many of my Tek units had that silver solder included, I know my anniversary edition 453 had it. Once Tek got sold into 'Megagihugic' corporate anonymity where profit before anything else was key the quality slipped, prices went stratospheric, parts became obsolete before the warranty ran out, they incorporated Microshit operating systems which when crashed handed you a $100,000 brick that the factory would repair for $15,000 and active probes began to cost as much as a used Lexus. Loved the old Tek, curse their new stuff.
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The Sn62Pb36Ag2 solder was necessary with the ceramic terminal strips used for point-to-point wiring on the vacuum-tube era Teks. If you used normal Sn63Pb37 or Sn60Pb40 solder, the silver metallization under the solder would dissolve in the molten solder, and the terminal would lift from the ceramic groove.
In my MRI days, when we were paranoid about ferromagnetic material, we bought SMT capacitors without the nickel barrier layer (special order) and used Sn62Pb36Ag2 solder paste to be compatible with the PdAg metallization.
Whence my paranoia? It started when I was contacting capacitor companies about non-ferromagnetic parts and one factory engineer assured me that theirs were OK, since the leads were "pure nickel".