Technical > Electronics > Tektronix 485 - no power up

reply to drjim

Re: Tektronix 485 - no power up

Get your service manual from here:
»manoman.sqhill.com/tek.php
...and read starting at page 4-6 onwards on troubleshooting techniques. That page'll show a table of power supply voltages with max ripples. Check them off. Then look at "troubleshooting the power supply" page 4-7.

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

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

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.

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

Make a note of that and continue checking voltages.

+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

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).

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....

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.

DS1824 does light with the power on.

Keep you posted....

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

I can't find TP1951...

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

reply to 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....

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

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.)

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