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This is a sub-selection from Tektronix 485 - no power up


neonhomer
KK4BFN
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join:2004-01-27
Edgewater, FL
reply to Mdoc

Re: Tektronix 485 - no power up

I think that is the same manual I have.....

Yep.... I picked up my copy from »bama.edebris.com/manuals/tek/485/



Mdoc
Ehh... munch munch... what's up, Doc?
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Sterling, VA
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1 edit

Mine is 242MB in size. Yours is 30MB.
Although, having checked yours, it seems to have the same content. If that's true, shows mine has a lot of dead space.



neonhomer
KK4BFN
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join:2004-01-27
Edgewater, FL

So far, checking voltages, on the +50 pin, I barely get 10v out of it, and it pulses w/ the two neon lamps.....



Mdoc
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Make a note of that and continue checking voltages.



neonhomer
KK4BFN
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join:2004-01-27
Edgewater, FL

+15 : Maybe 1-2v
+5 : 0.2
-5 : -0.2
-15 : -0.2 to -2.4
+59.4 : 0 to 11
-5.5 : -0.5
+120 : 3 to 35
+9 : 0

The HV lamp that is discussed in the tek forums post is lit, but dim. Silvered on the inside, so you really cant see it.

The two caps mentioned also are okay, not shorted.
--
"F is for Fire that burns down the whole town...
U is for Uranium...... Bombs...
N is for NO SURVIVORS!!!!!" Sheldon Plankton



Mdoc
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Yep... check out T1960 on "A15 Transformer board." Fig. 8-18. Circuit on diagram 15, part of which is shown above.


Mdoc
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reply to neonhomer

Wait, I thought the input to the T1960 transformer is 115V AC.

Go back prior to T1960, diagram 14 and look at DS1824. Is it lit? Also check out the line voltage selector (230v/115v).



neonhomer
KK4BFN
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join:2004-01-27
Edgewater, FL
reply to Mdoc

Why do I have the feeling the T1960 is similar to a Horizontal Output Transistor?

If it's bad, I don't think there is a replacement...... regardless now I have to strip this thing down to get to it....



Mdoc
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said by neonhomer:

Why do I have the feeling the T1960 is similar to a Horizontal Output Transistor?

If it's bad, I don't think there is a replacement...... regardless now I have to strip this thing down to get to it....

I think the culprit is before T1960 on diagram 14.


Mdoc
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reply to neonhomer

If DS1824 is blinking, the 115v power is ok up to this point before the inverter and protection circuit. And this means either the startup circuit (VR1831, C1829) is not kickstarting the inverter (Q1834, Q1844) or the protection circuit is preventing the inverter start up. My guess is the latter, but you can isolate that.

To do that, see the first 4 entries in table 4-4, starting on pg 4-7. Make some of the checks noted.



neonhomer
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join:2004-01-27
Edgewater, FL

DS1824 does light with the power on.

Keep you posted....



neonhomer
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join:2004-01-27
Edgewater, FL

Removed comb "U", bent pin 3 out of the way, and plugged it back in. Same thing.

I can't find TP1951...



Mdoc
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diagram 14, U1910 pin 2, that's the balance node input. U1910 is part of the protection circuit on A13 power board.



Mdoc
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reply to neonhomer

Click for full size
said by neonhomer:

I can't find TP1951...

Found it.


neonhomer
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join:2004-01-27
Edgewater, FL

Once you said U1910, I found it.... must be blind... LOL

From ground to the BAL point, I get 60-350mv, and it bounces around....

I get 160V at the shell of Q1834, and between -70 and -140v at the shell of Q1844

So it sounds like T1960 might be bad.....
--
"F is for Fire that burns down the whole town...
U is for Uranium...... Bombs...
N is for NO SURVIVORS!!!!!" Sheldon Plankton



Mdoc
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said by neonhomer:

Once you said U1910, I found it.... must be blind... LOL

From ground to the BAL point, I get 60-350mv, and it bounces around....

It's supposed to "bounce around" if you're using a DMM. The proper way to measure is not with a DMM but with a working scope with external trigger on TP1926. For the first 4 entries in table 4-4, do what you can with a DMM. See what you can do to borrow a working scope.

The voltages at Q1834/Q1844 may be from attempts to start the inverter, so it's meaningless at this point. The voltages are supposed to be double the 115v AC input voltage, or the p-p value of the line voltage. Read "Line Input Circuit" pg 3-6 2nd paragraph.

And I doubt T1960 is bad; we can't say it's bad if the problem points to circuits before T1960. It is working (in a manner of speaking) because the secondary voltages are showing the continuous inverter startup attempts made by the startup circuit. Something is preventing this, and the best method to find this out is by monitoring TP1951 with a working scope.


Mdoc
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1 edit

And if you can't get a working scope, the best way then is to look at all the inputs to the balance node indicated in the block diagram on pg 3-11 and look for shorts or opens in circuits leading up to those inputs. Look for scorch marks, too. (Cold solder joints is a possibility, too--push/pull on points on the board with a pencil or plastic probe.)



neonhomer
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Edgewater, FL

All I have is a Fluke 789 Process Calibrator, and a Fluke 11 DMM.

I don't know anyone with a working scope. That was the reason I bought one, so I could have one.

I will hae to fight with it more tomorrow. I wish I knew someone locally (Daytona Beach, FL area) who could assist me in fixing this thing. Its either that or I keep searching for a working scope....
--
"F is for Fire that burns down the whole town...
U is for Uranium...... Bombs...
N is for NO SURVIVORS!!!!!" Sheldon Plankton



Mdoc
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Yes, try that.



neonhomer
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Edgewater, FL

You mean unplug the high voltage lead, or all the wiring to the CRT?



Mdoc
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all wiring to the CRT (leave the HV in)



neonhomer
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Edgewater, FL

Oi vey.. that's going to be a bitch.......

I had the power inverter board and the sweep board out last night, and it was a bitch to get apart.

It says I can pull certain combs to isolate certain areas. I do know if I pull the "R" comb, the "clicking" goes from slow to fast.
--
"F is for Fire that burns down the whole town...
U is for Uranium...... Bombs...
N is for NO SURVIVORS!!!!!" Sheldon Plankton



Mdoc
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I'll look it up. How fast is fast?



Mdoc
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reply to neonhomer

said by neonhomer:

Oi vey.. that's going to be a bitch.......

I had the power inverter board and the sweep board out last night, and it was a bitch to get apart.

It says I can pull certain combs to isolate certain areas. I do know if I pull the "R" comb, the "clicking" goes from slow to fast.

The faster clicking is caused by pulling the "R" comb, Table 4-4, pg 4-8, top entry. Nothing new there.

If I'm not mistaken there should be a plug with wires attached to it on the rear of the CRT; you just pull that off.



neonhomer
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Edgewater, FL

You pull the rear cover, and there are three connectors..... one has 3 wires, and two that have one wire each....



Mdoc
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Pull them all off. The big HV lead goes to the tube anode, you can leave that in.



neonhomer
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Edgewater, FL

Haven't forgotten. Just don't feel like opening this thing back up right now. Will probably open back up after the kids go to bed in about 45 min or so.



neonhomer
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join:2004-01-27
Edgewater, FL

Oooookay... Pulled the three connectors that go from the xfmr board to the crt. Nothing. Same thing.

Walked through some of the tests on 4-7. Nothing to note.

I think I have reached the limit of my equipment. I hate to have to send this to a shop, as I know it will be a few hundred $$$ to get it fixed.
--
"F is for Fire that burns down the whole town...
U is for Uranium...... Bombs...
N is for NO SURVIVORS!!!!!" Sheldon Plankton



drjim
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join:2000-06-13
Long Beach, CA
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Try and find a local ham radio club. Most of the old guys there (like me!) probably have an oscilloscope, and might be willing to help you track down the problem.
--
One man's Magic is another man's Engineering.



cowboyro
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reply to neonhomer

said by neonhomer:

I think I have reached the limit of my equipment. I hate to have to send this to a shop, as I know it will be a few hundred $$$ to get it fixed.

Yeah you can hunt a working unit if you are patient. I have a 2232 that I got for $100 *shipped*.