Re: 485 No Trace - Intensity Fixed!!

Robert Simpson

Got a trace!! Yeah, and duh.
Went to check some voltages on the CRT and found the connector loose. Didn't get it back on fully when I previously checked the filament. Ah well, it works now! I guess I can take off the reject sticker.

Bad diode in HV section
Bad Op amp HV regulator
Bad transistor in intensity control section
Loose connector

Thanks to all. I used both archive info and your direct help.

See photos in "485 no trace" album

I think I will just enjoy the view for a few days of two 485s. Actually the other one needs some work as well.

Thanks again,

--- In, "Robert" <go_boating_fast@...> wrote:

Back on the 485 after fixing some promised PC/LCDs ( yup, bad ecaps)

Intensity adjustment measured at test point 1781, Z out, now works!

After reading the discussion about transistors that can check OK on a simple transistor checker (low base current), but not work at higher base currents, I decided to recheck the transistors starting with Q1742. I had previously tested them with that Ebay tester discussed elsewhere and they all checked out with HFE in the low hundreds or better.

In circuit working 485, non working 485 Voltages:
Q1742 Good/Bad, intensity turned fully down
Base 14.07/14.08
Emitter 14.76/14.78
Collector 5.39/7.12

The collector values lead me to believe Q1472 in the bad scope is more on then it should be, so I swapped in a new 2N3906.

Collector 5.39/6.31

Checked TP1781 and now it varies when changing intensity at the front panel!

No trace yet. Plan to test voltages at the CRT pins to see if the beam is pinched off.


--- In, "Robert" <go_boating_fast@> wrote:

Based on your comments, I swapped in a good U1624 from the working scope. HV is now about 3012 varying by a couple of volts. Pin 2,3 went to -65 and -66 mv. Looks like I need a new U1624.

Thanks for the guidance. Found this a lot quicker with your help.

--- In, David <davidwhess@> wrote:

Hmm. The magnitude of the voltage at pin 2 is very low considering
that the bias current is only supplied by that 470 kilohm resistor. It
should be nominally -37.6 millivolts but could be as much as -235

Unfortunately there is no indication that Tektronix was grading U1624
for low input bias current but looking at pin 6 makes me think the
operational amplifier has no power or more likely has just failed and
there is a short from pin 2 to pin 3.

Whatever else is wrong I think the operational amplifier is bad. Do
you have any likely replacements you can use?

I was going to suggest pulling CR1626 and CR1627 next but if they were
leaking, then the symptoms would be different.

It is probably worth checking the voltage at the base of Q1618, the
voltage at TP1614, and Q1618, Q1614, CR1618, and CR1621 for shorts.

Does adjusting R1625 have any effect on the high voltage?

I have got to get myself a 465 or similar. You guys are having all of
the fun.

On Thu, 07 Mar 2013 19:11:36 -0000, "Robert"
<go_boating_fast@> wrote:

U1624: (measured with a Fluke 8800A)
Pin Bad Scope Good Scope
2 -8.5mv -62.6
3 -9.8mv -63.0
4 -15.0V -15.0V
6 +690mv +706mv
7 +14.95 +14.93

It looks like pins 2,3 are not negative enough. What do you suggest I check next?


--- In, "Robert" <go_boating_fast@> wrote:

Thanks, I will check.

--- In, David <davidwhess@> wrote:

I assume you are talking about R1642 which has 4 parts. I think it
more likely there is a problem in the regulation circuit than the
voltage divider.

Before fooling with the high voltage divider, I would measure the
voltage at pin 6 of operational amplifier U1624. If it is stuck
within a couple volts of the +15 or -15 rails then the regulator is
not functioning correctly.

I would also measure pins 2 and 3 of U1624. They should be within
about 10 millivolts of each other if the regulator is working
correctly but could be off as far as 100 millivolts because of input
offset current.

Both pins should also be about 40 millivolts below ground but could be
up to 250 millivolts below ground.

If you have a high input impedance voltmeter or electrometer, this
would be a good place to use it but a normal 10 megohm meter should
work well enough. A Tektronix DM502 has a high input impedance mode
for instance and so do many bench multimeters.

A 741 is not really the right type of operational amplifier to use in
that application. A low bias current bipolar type like a 308 would be
much better but would need external compensation. An LT1097, LT1012,
or LT1001 would be ideal drop in replacement.

On Thu, 07 Mar 2013 05:06:17 -0000, "Robert"
<go_boating_fast@> wrote:

Is there a good way to test a thin film HV control resistor set while in circuit?

Finally getting back to my 485 repair after fixing a major leak in the primary water line to our house under the concrete driveway. Working on the 485 is much more fun, and also some progress today.

I decided to check all the diodes in the restorer circuit area. (I used a simple single trace set up, 12V AC through a resistor, with scope in XY mode). They were all good until I got to the last diode to test. It was bad. (see photos in "485 no trace" folder). After replacement, the HV is now stable although too high. Originally the HV jumped around a 300-400V range. Now its stays steady at 3350V. Should be 3KV +-50.

The manual indicates the thin film resistor set controls the HV, (similar to 7603's I have worked on). Is there a way to test these resistors in circuit? I do not want to unsolder it unless I am sure it is bad.


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