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Re: SC502 Trace Issues - Beginning and ending of trace connected.
Bob,
Looking forward to the video! Thanks! Those capacitors are actually on the A5 Aux board. I had that board out for cleaning and inspection. I did not check the capacitor values. I've been delayed... lost 2 transistors on the -20V supply. Thanks! Barry |
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
The DC Restorer caps are C881, 2, 3, 5 and 7. They go between the main (A1) board and the HV auxiliary (A5) board. In my experience, they are more likely the problem than the diodes on the A5 board. But replacing the caps is a bear. By coincidence, I am doing the final edits of a video on replacing two tantalums on the main board which are under the A5 board and require removing the A5 board. I will add this intensity problem to the video, which I hope to get out today.
-- Bob Haas |
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
CR884 and CR885, they are on A5 board.
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Regards, Ignacio El 05/03/2024 a las 17:33, Barry Breaux escribió:
Mark, --
Este correo electrónico ha sido analizado en busca de virus por el software antivirus de Avast. www.avast.com |
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Re: 7704A - Readout Stretches Vertically
When I get the vertical amplifier's damaged IC replaced (and the problem is still there), I'll add a picture in the group page.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Ozan" <ozan_g@...> Hi Barry, |
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Re: 7704A - Readout Stretches Vertically
Hi Barry,
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Is there a picture of how the distorted characters look like? Ozan On Tue, Mar 5, 2024 at 06:36 AM, n4buq wrote:
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
Mark,
Thanks for the help! Greatly appreciated! I am thinking the Bias pot is R873, its labeled Beam. Last week I found that pot had come apart. No trace. If I pressed down on that pot the trace came alive. I ran the scope with a 20K pot and it functioned. Then I wired the circuit with fixed resistors. 25k, 25K with a center tap until the replacement pots arrive. I checked the 1uf capacitors with my Sencore meter and do not suspect them to be the issue. I will order replacements. The ones in there are axial and the ULD you recommended is radial. Is that what you had in mind? I have ordered the 1N4937 diodes. It was previously recommended to replace the diodes in the DC Restorer circuit. I am not sure what the DC Restorer circuit is. Can you point that out. Thanks! Barry |
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Re: 7704A - Readout Stretches Vertically
At issue is that the stretching occurs even when the trace is not extended past the top or bottom graticule. It isn't horrible but quite noticeable. I have another 7704A, two 7603s, and a 7514 and none of those show this problem quite like I see it on this 7704A. Yes, with an AC signal that approaches or passes the CRT's vertical limits, I see rippling on virtually all these scopes and I understand that but this "stretchy, multiply-scanned image" thing is only seen this way on this 7704A.
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Again, it isn't like I can't read the characters, etc., it just seems it is abnormal and would like to find out why. Thanks, Barry - N4BUQ I looked back through this thread, and I don't know what the deal is here, so |
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Re: 7704A - Readout Stretches Vertically
Disable the high voltage, unplug the CRT, certainly defocus the CRT, IfHarvey, I think we are talking about different things. Your suggestion was to view the V en H signals of the faulty 7704A on another scope, with the other scope in XY-mode. The bright spot is on the CRT of that other scope. Albert |
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Re: 7704A - Readout Stretches Vertically
I looked back through this thread, and I don't know what the deal is here, so please forgive if I've missed something. In the OP, I think it was an issue about the readout distorting severely when the vertical signal is over-driven and off-screen. This is entirely normal behavior. The Tek verticals tend to have great dynamic range, but even they have their limits. I think that Albert et al have touched on this in previous messages about what it looks like at normal levels.
If the RO works fine with signal displays on-screen, or a little beyond, then fine. If they don't do as well way off-screen, then you're tilting at windmills. The RO is irrelevant if you can't see a waveform anyway. If you see weird non-linearities or stepped conditions on overdrive, it's likely thermal delays, sweep rate, and RO framing rate getting into the picture too. What matters is how it looks when things are "normally" displayed. If OK, then case closed. At certain sweep rates and vertical excursion, it's quite normal to see the RO rippling up and down a little. The question is, what's acceptable. It should be not obnoxious, and preferably, but not invisible. Ed |
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Re: 7704A - Readout Stretches Vertically
Barry,
The resistors are: R3439, R3442, R3493 and R3494. The the 510 ohm resistor, R3494 should be 1/2W. These are on the readout board. From what you said happened to the resistor, transistor and else, the output IC is bad. See if the output transistors are bad by checking between pin 13 and 9/12. That will indicate the B/C junction is bad of the outputs. The internal 70 ohm resistors to transistors are likely dropping more voltage thus a larger current demand through the 33 ohm resistor making it run too warm. Mark |
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Re: 7704A - Readout Stretches Vertically
Mark,
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I'm not sure which are the 2400 and 1500 ohm resistors. I don't see those values on the readout or the vertical amplifier board. As for the character height, I reduced R3468 to what I believe to be the original value which reduces the vertical size of the characters. I tend to like that a bit better. I'm still convinced that the problem is in the vertical amplifier and, likely, it's the IC. I had a rather unfortunate incident over the weekend. R4418 was right on the high side and 12V was a bit shy (not remembering the values). I replaced it with a proper resistor but when I remounted the board, one of the output leads that protrude through the frame became stuck in the plastic insulating ring and pushed that lead up against the bottom of the board at a ground plane. That smoked R4419 and Q4420. I replace those but I think U4413 was damaged in the process. Focus is not as sharp as before, 12V is now 10V, and R4418 is getting warmer than it should. I'm looking for a replacement for U4413 and hoping that might cure the stretching character issue as well. Thanks, Barry - N4BUQ Barry, |
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Re: 7704A - Readout Stretches Vertically
Barry,
The 2400 ohm and 1500 ohm resistors should be 1/2W. The former and one of the latter are in the upper left of the schematic and the 1500 ohm resistors to the two transistors to the outputs. I wonder if the decoupling condeners are going bad. You can increase the capacitance if you want. The characters are larger in this model than others even when the height is adjusted to minimum for the readout. There could be something on the vertical board that is bad/out of tolerance. Mark |
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
Barry,
Look at the crt circuit in the schematic. The diodes can/should be replaced with 1N4937 and I strongly suggest replacing R888 with a Vishay VR37 series 22meg resistor. That carbon will be out of tolerance. I would also suggest replacing the 1mfd condensers for the 70V supplies with a Nichicon ULD type and can be higher in capacitance, e.g. 2.2mfd, and the 1mfd decoupling from the crt bias with a 250V film type. Check the crt bias pot to make sure it is still good. The pot could be open. I have found these and a few others open in the 7000 series then replace them with a 10 turn 3/4W type. Yours does not need to be this type, unless you want it and the pot is bad. Mark |
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Re: 7704A - Readout Stretches Vertically
Disable the high voltage, unplug the CRT, certainly defocus the CRT, If there's a fuse in the HV supply, you might try removing that.
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The problem is (on a bright spot), the density of the electron beam.? Diffused over a relatively large area, it won't hurt the phosphor.? Unplugging the CRT may be one of the easiest things. The above is for checking the character generator. if you have no sweep, then unplug the CRT and check the voltages everywhere.? Horizontal and vertical positioning controls should be able to move the (invisible) spot around. You can check the CRT voltages at the socket (be very careful, high voltage!), with the thought that the grid/cathode voltage never goes negative (grid volts = cathode volts => very very bright spot).? The CRT is not needed to check the voltages.? The grid ought to be able to be driven negative with respect to the cathode to turn off the tube. Some of the diodes coupling the blanking signal into the CRT can go bad, as has been mentioned. Ought to be a start. Apologies if you already know this. Harvey On 3/4/2024 3:58 PM, Albert Otten wrote:
On Sat, Mar 2, 2024 at 11:04 PM, Harvey White wrote:Another trick you can pull is to take the second scope, put it in XY mode, andHarvey, |
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Re: 7704A - Readout Stretches Vertically
On Sat, Mar 2, 2024 at 11:04 PM, Harvey White wrote:
Another trick you can pull is to take the second scope, put it in XY mode, andHarvey, Regrettably this is almost impossible without blanking the (otherwise extremely bright) central dot. It would require the use of one of the control signals (perhaps Z-axis Inhibit) in the 7704A to drive Aux Z of the test scope. Even locating the bright dot outside the visible CRT area is of no help here. |
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
If the intensity control doesn?t work the blanking problem is a consequence of it.
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With a high degree of probability one or more diodes of the intensity DC restorer are leaky. Since testing it is quite difficult, I would replace all. This problem happens in several Tek models and also in scopes of other makers (Who copied the circuits designed by Tek). Regards, Ignacio El 04/03/2024 a las 16:48, Barry Breaux escribió:
Interesting that you mention intensity. The intensity adjustment does nothing. I am troubleshooting that now. --
Este correo electrónico ha sido analizado en busca de virus por el software antivirus de Avast. www.avast.com |
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Re: SC502 Trace Issues - Beginning and ending of trace connected.
You might want to see if the unblanking pulses are doing what they're supposed to.? (or blanking pulses).? In some older scopes, there are diodes that have gone bad.? Do be careful, since they may run the cathode and grid at elevated voltages (like 2KV).
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Grid - cathode voltage *should* change with the intensity settings.?? Hmmm...... Harvey On 3/4/2024 10:48 AM, Barry Breaux wrote:
Interesting that you mention intensity. The intensity adjustment does nothing. I am troubleshooting that now. |
Bob Haas
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