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Hi Albert- With my DMM neg probe on the -3kV TP and the pos probe on the bottom of R1719 (goes to pin 3 of CRT) I get about -142 volts. This is with the Readout off.

If I am reading the manual and the troubleshooting guide correctly, the control grid is normally 40-50 volts more negative than the cathode supply, and may cutoff if 65 volts more negative. Correct?

So maybe the bias is so far off that it is blanking the CRT?

Adjusting R1674 moves it only about +- 2 volts.

Hmm... If this is true, does that point to the HV board A18? That's a very difficult board for access to components. Just randomly testing a few components on the HV board I haven't found anything obviously bad, yet.

Your thoughts?

Thanks,
Keith Ostertag

--- In TekScopes@..., "mattko87" <matt8@> wrote:

That ist right.
585, 581 are a epic technologiy.
high bandwith, distributed deflection and tunneldiode -base triggering (only with A-version)

IMO, Tunnel diode trigging in the A models is a VERY IMPORTANT difference. The non-A's only triggered at a tiny fraction of their BW rating.

Not much of a fan of 4 division vertical as well. The 545 and 545A shared this same trait. Lots of BW for the time, but at an expense.

The design is interesting for its day, but I don't find them too useful as working scopes.

- Steve

Hello Mickey,

Try unplugging the ribbon cable that goes to the CRT assembly and re-try powering it on.

Jay

--- In TekScopes@..., "mickeyaslam" <mickeyaslam@> wrote:

I checked the no load voltages according to repair manual and most of the power rails are not powering up. For example 25v power rails show only 4.8v or so. Only good power rails was 5V and yes pin one and three on J6 were shorted as suggested in the manual. I also hear the hissing sound from the main transformer for low voltage power rails. It is located on the side of the PCB and it is the biggest transformer of all the other ones.

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

My first thought was capacitors.

How did you narrow down the cause to the LV power supply module?

On Wed, 11 Jul 2012 13:44:50 -0000, "mickeyaslam"
<mickeyaslam@> wrote:

I have a TDS684A scope which refuses to turn ON, instead when power button is pressed it tries to power ON but shuts off right away and then it get stuck in half second ON half second OFF loop. I have narrowed down the cause to be LV power supply module. I have replaced all the electrolytic caps on the power supply but no change. All the power diodes and MOSFET seems to be working fine.

Any help in the right direction will be greatly appreciated. Even better if someone has schematic for power supply module.

thanks'

mickey

My R7603 has developed a problem where the readouts jump around a lot
based on the input signal frequency and amplitude, signal trace is
fine. With no input, readouts are steady. This readout problem occurs
with several different vertical and horizontal plugins, problem
appears to be in the 7603 frame.

Power supplies measure well and look good on scope, dont think they
are involved. I have been unable to determine if the readout board or
the vertical or horizontal amp board is involved as I do not have
spares.

Has anyone seen this problem and resolved it?

Larry



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Yahoo! Groups Links

7704A



and my scope



Thanks.

Bill,

I'm wondering if that's noise from the D-to-A converter circuit, maybe because a low-pass filter cap is open. I'd suggest to check on the Storage Display Logic Board (A16), which is the top-most board. Check around op-amps U571A, U352 and U558. See if you've got that noise on the signals on pin 6 of those last 2 op-amps, and pin 1 of U571A. Oh, In between, there's a 250 kHz switching pulse on U523D. Hhhhuuummmm????

Jimmy

--- In TekScopes@..., "maninashed1" <bill.valvetechnology@> wrote:

Ive added some photos of the scope displaying the fault. These are in my photo folder under bill.valvetechnology.
Ive checked the waveforms and ripple from both power supplies against the manual and both the analogue and digital power supplies look fine.
Can anyone identify this fault? It has been suggested that it could be the clock signal being imposed on the waveform and the cursors in digital mode.
Thanks
Bill

You messed up the second link, but anyway, it's because the 7633 has a
storage tube.

On Thu, Jul 12, 2012 at 4:28 PM, Chris Wilson <chris@...>

12/07/2012 23:23

My new old stock 7633 has on screen graphics that look like they are
hand drawn, and have never been incredibly sharp. I have seen an ad
on a well known internet auction site for a 7704A showing some
screen shots. the on scren graphics are far sharper and in a totally
different font. Why are the two models of scope displaying such
different on screen graphics please? I have put the shot taken from
the 7704A ad and from my own 7633 on my web site as:

7704A



and my scope

------------------------------------

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Hi Keith,

Vgk = grid to cathode voltage.
Use your DMM at 200V DC range. Be careful, both leads will carry about -3kV w.r.t. chassis! So connect the DMM before you switch on the 'scope, and don't touch it. Then vary Intensity (while the time base is free running) and check that the reading changes accordingly. Vgk (negative) is ideally equal to the difference of the voltage at TP1876 and the voltage at the slider of R1674, but will be less now because of DMM load. The internal load is R1684, 22M.
I guess cut-off (dot disappears) voltage is somewhere between -50 V and -100 V.

Albert

Oh, now I see it, R1674 is marked CRT Grid Bias.

--- In TekScopes@..., "keithostertag" <keitho@> wrote:

Hi Albert- I probably can measure across pins 3 and 5 of P1710, carefully, but would I need to use the HV probe? What does Vgk stand for?

Which pot controls Grid Bias?

Keith

You messed up the second link, but anyway, it's because the 7633 has a
storage tube.

On Thu, Jul 12, 2012 at 4:28 PM, Chris Wilson <chris@...> wrote:

12/07/2012 23:23

My new old stock 7633 has on screen graphics that look like they are
hand drawn, and have never been incredibly sharp. I have seen an ad on
a well known internet auction site for a 7704A showing some screen
shots. the on scren graphics are far sharper and in a totally
different font. Why are the two models of scope displaying such
different on screen graphics please? I have put the shot taken from
the 7704A ad and from my own 7633 on my web site as:

7704A



and my scope

------------------------------------

Yahoo! Groups Links

12/07/2012 23:23

My new old stock 7633 has on screen graphics that look like they are
hand drawn, and have never been incredibly sharp. I have seen an ad on
a well known internet auction site for a 7704A showing some screen
shots. the on scren graphics are far sharper and in a totally
different font. Why are the two models of scope displaying such
different on screen graphics please? I have put the shot taken from
the 7704A ad and from my own 7633 on my web site as:

7704A



and my scope