tek 545 scope 'too bright'

if you go download:

ftp://bama.sbc.edu/downloads/tek/545a(2)/Tek%20545a(2).pdf

you'll have a service manual.

You probably should replace any of those black tubular bakelite capacitors.. They tend to get anywhere from slightly to horribly leaky. Any leakage over a microamp will mess up the grid biasing.

see the next to last page for the CRT circuit.

The brightness circuit has many high-megohm resistors-- thesetend to drift in value over the decades.

Also around the intensity control there are several capacitors in the C82x range -- replace them. They might measure okay under 1.5 volts from your meter, but leak badly when under high voltage.

"Ancient_Hacker" wrote in news:1169569646.335974.73670 @d71g2000cwa.googlegroups.com:

:( That would explain a lot.

Thanks, will give those a try.

The russian 12ax7s arrived today but did not fix the problem. Looks like it is going to be a long process to fix up this beauty.

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

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There's your starting point - if the power supplies are way off that can cause a lot of problems. Get your 220 supply back within specs and see how the scope works. From what I remember, the -150 is the reference supply that the rest are set to.

Granted, there could be something elsewhere in the scope loading down the

220V power supply (leaky caps, etc.) but it's the place to start troubleshooting.

Bob

"Bob" wrote in news: snipped-for-privacy@corp.supernews.com:

Smurfies law.... I pulled tubes and tested them. When I put them back in, I couldn't even get the -150 to adjust at all. Maybe I should not have 'straightened the pins' on the tubes. Maybe I have some sockets that are not making good contact with straight pins.

The russian 12ax7 doesn't help either.

Now, the -150 is about -87 volts and only a very small adjustment range. The 100 volts is about 130

Looking at resistances, I am seeing a 100 ohm 'load' to ground on the 100 volt line. That CAN NOT be right. That would require a 100 ohm 100 watt resistor and there is no such animal.

I was afraid of the bright beam damaging the CRT, even when 'off screen' so I tried removing the CRT's socket. BAD idea. No load on the HV supply allows the voltage to go very high and the scope protests with pops and crackles.

Current plans are to track down the 100 ohm 'short'(or find something that justifies it),

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

Well, you get brownie points by using your ohmmeter to find problems, rather than just plugging it in.

*But* in many Tek scopes they put eight 12-volt tube heaters in series and drive them from the 100 volt line. Mainly for the tubes in the low-signal sections, to avoid AC heater hum getting into the circuits.

And tube heaters have very low resistaance when cold-- when hot they'll draw 150 milliamps from the 100 volt line -- much closer to reality.

I'd try scoping the plates of the voltage regulator tubes-- often the electrolytics dry up and you end up with low and hummy regulated voltages.

You might try switching the 6080 tubes-- you might have the weaker one now on the 100 volt line.

"Ancient_Hacker" wrote in news: snipped-for-privacy@v45g2000cwv.googlegroups.com:

Brownie points don't fix things. :(

AHHHHHHHHHH. And I kept skipping over the filament wiring schematics, thinking to myself 'these can't have anything to do with the problem'. :(

Hm.... I see TWO 12 volt tubes, V180 and V130(I think), a 12AL5 and a 12AU6 with filaments in series, off of the +100 volt line, going to a point labled

+75 to pin 15 interconnecting socket vertical amplifier... now to check the schematic for the vertical amp...

What-do-you-know... 7 more tube filaments in the same string, hiding in the plug in module. Well, I guess I can pull the module and see if my 100 ohms goes up to 2k or so and then I will know that you were 'spot on'.

Waiting for the Capacitor ESR meter kit I ordered to arrive. It looks like a cool instrument to have around the shack!

And I might have mixed them up while testing them. :(

Also considering replacing the high voltage rectifiers with solid state diodes. Wonder if removing the filament load from the supply will mess things up. Probably!

THANKS again. Experience! It makes a difference.

-bz-

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bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

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The saga continues....

Removing the plug in module DID clear the 100 ohm 'short' to ground on the

100 volt line.

Swapping tubes did NOT fix the -150 volt supply however. It only measures -131 volts (after several minutes of 'post hv turn on' warm up). It drifts much more during the warm up than the -150 volt adjustment pot will swing to voltage. In fact I see very little change as I adjust the pot.

The 100 volt supply is putting out about 130 volts and the rest of the supplies look about normal (have not checked the HT supply to the CRT. I wonder if it being too high could cause all my symptoms).

Measured the current out of the -150 and 100 volt supplies (across the 5 and 10 ohm series resistors). ~.95 volts /5 ohms = 190 ma for the -150 supply ~1.4 volts/10 ohm = 140 ma for the 100 volt supply. I am assuming that those values are not unusual. I have not looked at ripple, etc., yet.

Looks like I am going to have to measure voltages at several points and try to make sense of any patterns that emerge.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

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bz wrote in news:Xns98C44814D5A8FWQAHBGMXSZHVspammote@130.39.198.139:

in troubleshooting those old tube scopes,I used to keep a few electrolytic caps with clip leads to bridge the PS electrolytics,when I had a low supply.

--
Jim Yanik
jyanik
at
kua.net

Jim Yanik wrote in news:Xns98C4626E2AFF1jyanikkuanet@64.209.0.85:

....

Good idea. Thanks! (note to self: remember to discharge any such caps as they can 'build up' a charge again, once disconnected.)

--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+nanae@ch100-5.chem.lsu.edu

I don't remember whether you posted that your scope is the older 545 model, or the later 545A or 545B model. I still have a copy of the 545B scope handy. As I remember, the low voltage power supplies are pretty close from the basic 545 thru to the 545B models. According to my manual, the resistances to ground (power off and all supplies totally discharged, no plug-in installed) are:

-150V 2K

+100V 2K +225V 2K +350V 1K +500V 15K

Ripple specs are pretty tight, and must be to specs for a good trace and waveform display.

-150V 5mv

+100V 10mv +225V 5mv +350V 20mv +500V 20mv

Going back to your earlier posts, you started with a good -150V source, but all others were way out of tolerance ????!!!!! Kinda hard to do with this instrument, since all the other LV sources are referenced to the -150V source. Are you sure that your test instrument is good?? If you have another source of known voltage, check it and eliminate or identify that as a source of error.

Next, remove and reseat all the tubes in the power supply section. Old tubes and sockets get a bit of fungus or mold in the works, and can create all sorts of havoc. Work the tubes in and out of the sockets several times; you might even clean the tube pins with a bit of fine emery cloth.

Cheers!!!!!

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Some days you're the dog, some days the hydrant.

"DaveM" wrote in news:YcedncY5EOW5FyfYnZ2dnUVZ snipped-for-privacy@comcast.com:

It is a 545. No A or B.

That 'no plug-in installed' is more important than I knew!

Will double check these, but I think that seems like about what I was seeing.

Will check.

-150 was ok as were MOST of the others: "Checking voltages, the 220 volt test point on the upper right side is only about 135. Other voltages (500, 350, 100, -150) are all good."

Moderately sure but I have several meters that I can check it with.

Will do.

Oxidation, etc., can cause problems, for sure!

I may do that and run a small rat tail file in and out of each pin in the tub sockets.

I did pull the power supply tubes and check them in my tube checker. Straightened the pins while doing that (always do in order to protect the tub sockets in the checker). It was after I checked the tubes that the -150 volts went to hexxxx went crazy.

Thanks for the suggestions. Once I fix the scope, then I have another piece of equipment that I wanted to use the scope on. :)

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

bz wrote in news:Xns98C4761E72DC4WQAHBGMXSZHVspammote@130.39.198.139:

I used a 1Kohm 2W resistor glued to a plastic stick to discharge caps. Bent the leads to fit the cap's terminals. I also used this for switcher PS line caps.

--
Jim Yanik
jyanik
at
kua.net

and to think, i through out years ago, 2 perfectly working 545 boat anchors.

--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Jamie wrote in news:1Syuh.300$ snipped-for-privacy@newsfe03.lga:

I _had_ two(with carts!). My wife "disposed" of one.

If you look at the prices on 12AX7s on the web, you will realize that you may have thrown out thousands of dollars worth of tubes. Some vintages of

12ax7 go (to audio buffs) for unimaginable prices! Of course the russian 'knock offs' go for less than 10 bux.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

NO NO NO! The sockets contacts are plated (tin?). A file will remove the plating, opening up the base metal to further corrosion. Instead, spray some contact cleaner into the sockets, and then run something scrubby but non-abrasive through them, like a pipe cleaner.

Someone else suggested touching up the tube pins with fine emery cloth. That's OK - tubes are replaceable. But in practical terms, sockets are not.

Bill

Bill Jeffrey wrote in news:4SKuh.3530$ snipped-for-privacy@newsfe11.phx:

Sometimes tin, sometimes silver, gold on the best, IF I remember correctly.

Pipe cleaners (those I am familiar with) are usually too thick for miniature tube sockets (might work on octal but not loctal) and many are built on a wire base that could distort the shape of the spring. Perhaps a wooden toothpick, used with caution.

You have a good point. I wouldn't have taken that step unless things were really bad. I may need to use a sharply pointed tool to 'tighten' some of the sockets, but, again, that is a last resort.

I do think that many of the strange problems I have been seeing are due to bad connections on the tubes. I 'made the mistake' of seating them fully.

Looking at the tube pins closely, I can see that they have been a fraction of an inch ABOVE the 'fully seated' position for many years and the pins have oxidized on the exposed areas. Seating them fully was thus a mistake on my part. :(

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap