high pitch noise from a tek 422 oscilloscope

[crazyhog]

The disassembly was quite complicated. To put things back into a functioning state will be similarly complex.  I don't want to just put the unmodified transformer back in.

Anyway, $4 worth of silicon caulk is in.  I'll give it a few days to cure and will know.

[Toasty]

How are you going to reattach the rest of the housing with that much caulk?  It is shielding for the transformer...

T

[crazyhog]

Here is a pic with the lid and clamp.  Excess caulk are squeezed out for a perfect fill.  Hopefully the caulk doesn't shrink too much after it's cured.

[Armadillo]

The sound is definitely not the 60 hz AC.  I have a isolation transformer on my bench and I know the 60hz hum well.  Here is a 7khz tone on youtube and the noise from the power supply sounded like that.

https://youtu.be/-vAqiFrG9cg

Good thing, I have a natural 7K filter installed, can't hear a thing.     

[voltz]

Not wanting to cause any alarm here, but some silicon sealants react with copper badly. Silicon sealant with high acetoxy will rot copper and turn it green. Check your silicon and see if it contains acetoxy. It will have a strong smell of vinegar if it does.

Its still possibly ok, because the copper wire has enamel covering of course. But years ago i made some traps for a dipole and filled the trap tubes with silicon sealant. Nice job i thought... What i did not know at that time was the silicon had a high acetoxy content. Its an acid released during the curing process. Once its cured it stops reacting. Anyway, my traps stopped working after just 1 week. I took them apart and they were totally green inside. And if i remember correctly, that was even with enameled copper wire.

So just check what you have. Neutral sealants without acetoxy or low content (no vinegar smell) would be fine. Just thought this was worth mentioning possibly for others reading this post and about to silicon any copper wires or parts.

[crazyhog]

 

The silicon caulk label doesn't list its ingredients.  It has a distinct smell that I can't identify.  The label says low odor.  Most of the transformer is coated at the factory with a white rubbery coat. But the lead wires are only covered with enamel.

I got that pot committed feeling. 

[crazyhog]

Everything is back together and working(pic 1).  The high pitch noise is much quieter but not completely silent.   From 3 feet away, I cannot hear it.  However when I am right up against the scope I can still hear a faint  high pitch noise.  The noise frequency has dampened to around 4khz.  Whereas the AC only Tek 422 is completely silent.

So far no problem with the silicon caulk; fingers crossed.  While waiting for the silicon chaulk to cure, I desolder and check every electrolytic caps in the power supply.  I replaced 4 more with high ESR.  Surprisingly they all still had good capacitance.  They were part of other capacitor banks.  Their higher ESR fractionally increased the capacitor bank's overall ESR.  The power supply is definitely happier now.  The AC current draw has been reduced by about 20%.

[crazyhog]

Recently, I spotted an AC only Tek 422 at a local electronic recycler selling for a song.  Of course I had to rescue it!  Amazingly this machine, older than me, fires right up.  The main problem, as typical, was dirty switches.  Although I still need to check all the capacitors

Here are some pics of the AC only 422 and AC/DC 422 scope and a couple of my favorite old timey multimeters. 

Both meters are quite special in that they're hybrids.  They're the bastards of VTVM and VOM!  They are battery powered even though they're part VTVM!  It's crazy!  The world is a more colorful place because they existed!  Hahah!

[bd139]

Never seen those meters before. Rather interesting

[voltz]

Nice. Looking good.

[crazyhog]

The left meter is a Simpson 260 with a Model 651 DC VTVM adapter attached at the base (like Voltron, the defender of the Universe!).  The right meter is a Triplett Model 631 VTVM-VOM.  Both meter can operate in plain VOM mode or in VTVM mode for measuring DC voltage. 

Both came to me in really sad state.  There were short circuits, burnt resistors, and wrong wiring.  It was quite obvious the previous owners tried to repair their meters.  However, the sloppiness of their effort were very clearly evident and that sloppiness pretty much doomed them from the start. 

That sloppiness also sometimes lead to a happy outcome!  For example, the previous owner wired the Triplett 631 meter movement terminals wrong!  As a result no current can ever get into the meter movement and therefore protected it!  The meter movements, if fried, are the only component that can't be repaired.

I'd like to also give a shoutout to Alan Wolke on youtube.  I've learned a lot from his videos.  Here for example is a video on analog meter movement that I studied before I repaired these hybrid bastards.  Allan is excellent at explaining idea simply and clearly.

[crazyhog]

Here is a picture of the Tek 422 with the front cover.  The shell, including the cover is made of aluminum with the thickness of a coin!  The whole Tek 422 exudes quality.  No mucking about!

Here is a pic of my favorite VOM: the Simpson 269.  This meter used to cost more than 2x the much more popular Simpson 260. The area of the meter face is 2x that of the Simpson 260.  The meter movement has 100,000 ohm/volt vs the 260's 20,000 ohm/volt.  At the 8 volt range, the impedance is already 800,000 ohm.  Its impedance is close enough to the 1M ohm of a oscilloscope with a 1x probe!  All this from a passive VOM!

There are 3 Simpson 269 series.  The best and rarest is series 3.  It comes with Taut Band meter movement that eliminates needle movement friction.  No need to tap on the meter face to unstick the needle to get an repeatable reading.  The other 2 series doesn't have taut band.  However, series 3 is very rare; I hardly ever see it on ebay.

Anyway, the Simpson 260 is hugely more popular and the Simpson company probably sold 100 to 1 the 260 vs the 269.  You can get a rough estimate by counting how many 260s are for sale on ebay vs the 269.  However, the Simpson 269 easily trounce the Simpson 260 in almost every way.  The Simpson 260 and Triplett 630 fanboys are missing out!

[The Guy]

I have one of them scopes... Beautiful machine. I agree.

Good luck gettin it back to life.

[crazyhog]

Hah, that guy with the belated well wishes!     I'll take it anyway . As I reported in previous comments above, both of my Tek 422 scopes are back in service doing my bidding.

[lordvader88]

Just popping in to say thanks for the decent photos. Lovely bit of engineering that scope. I hope you win the battle

Oh I never realized it runs off batteries too. I have a working Tek422 on the way soon, can't wait.