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How to troubleshoot a faulty 7B92A?
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Hello
I have a faulty 7B92A that I would like to repair I have a 7704A as a scope The jumper for mainframe selector is on P835 for "other 7000" I am not sure if the jumper for time/div variable selector should be on P760 (delayed) or P761 (delaying) for the 7704A ? There is nothing showing on the screen when pressing the beam finder button When the 7B92A module is inserted into the scope, the trace displayed by the other timebase is vibrating a bit and it can be observed that the position of the readout is different for the other timebase/input. Do you have any recommendations on the approach for troubleshooting a 7B92A? Thanks Thierry |
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Replies interleaved:
On 3/18/2021 7:03 PM, Thierry Delaitre via groups.io wrote: HelloOK, a little fast for the 7704A but that only shows up on the fastest sweeps. The jumper for mainframe selector is on P835 for "other 7000"I think that's right, the other setting may be for a 7854, which makes a difference.? I think it may end up changing where the illumination power comes from. Either is valid.? It depends on how you decided to use it.? You'll get different behaviors, of course. And I'll assume that the beam finder works properly with another horizontal plugin.? So it's not the switch. What I'd suspect is that the output of the plugin is out of scope (!), that is, is going between? extremes at the horizontal input stage.? In other words, the plugin is driving the horizontal input of the mainframe badly. Do you have any recommendations on the approach for troubleshooting a 7B92A?Check the power supplies first, in case there's too much loading from a bad capacitor. Look at the output to the scope, it ought to be wrong. Crosstalk from H to V and between the H channels can happen with the trace deflected off the screen.? A symptom is that you may see the displays (and other trace) wiggle when the 7B92A sweep triggers.? The rate of "wiggle" may vary with the sweep settings. I'd suspect all of the outputs from the plugin until I have verified that they work. Harvey
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On Thu, Mar 18, 2021 at 07:03 PM, Thierry Delaitre wrote:
The jumper for mainframe selector is on P835 for "other 7000"This should be correct for your scope. P834/P835 affect the hold-off timing at fast sweep rates: approximately 1.2 us when set to "7800/7900", and 2.2 us when set to "other". I am not sure if the jumper for time/div variable selector should be on P760The choice of delayed or delaying variable selector is unrelated to the mainframe. I think most users keep this in the "delayed" position so that the variable control is available in "normal" mode (i.e. when the delaying and delayed selectors are locked together). There is nothing showing on the screen when pressing the beam finder buttonWhen the time base plug-in is operated in a horizontal slot, the "sweep gate" signal, the differential positive-going ramp signal, and a host of other control signals all have to be ok in order to produce a useful display. If you are reasonably confident that the 7B92A isn't putting out signals that are so far out that they may damage whatever mainframe slot you plug it into, then I would suggest the following tests: - Install the 7B92A in the Left Vertical slot. - Set the 7B92A to NORM, AC, INT triggering. - Lock the delaying and delayed selectors on the 7B92A together and set them to 1 ms/div. - Install your other (working) time base in a horizontal slot, and set it to 1 ms/div, AUTO, AC, INT triggering. With this setup, you should be able to get a horizontal line on the screen, whose vertical position can be set by 7B92A's POSITION control. (You will need to move it towards the clock-wise end of the range in order to get the line on the screen.) If no setting of 7B92A's POSITION will give you a visible trace, then push the BEAM FINDER to see where it is. If you do see a trace that you can control with 7B92A's POSITION, then change the 7B92A triggering to AUTO. Now you should see diagonal lines on the screen. The steps described above will tell us whether the 7B92A is able to drive the differential output signal at all, and will side-step any potential problems with the Sweep Gate and other control signals. dan |
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The horizontal line is generated, of course, by the 7B53.? That the line is in the middle or can be adjusted so suggests that the position control and circuitry into the frame is working.? Now what doesn't work in the vertical compartment would be blanking, which would make no sense to have.
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What I suspect is that the sweep is never triggering, remains in a holdoff state.? That would blank the display, which I think you see. I'd be looking at the sweep. Harvey On 3/19/2021 4:56 PM, Thierry Delaitre via groups.io wrote:
thanks both |
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Thanks. Here are some observations from the measurements around the U820 sweep control IC on the horizontal logic board 7
U820 - Sweep control IC - Pin 6 (auto delay) is 0.5V ; strange ?? other side of the R828 resistor (180K) is 14.7V - Pin 7 (trig D) is 0.5V - Pin 8 (hold off timing) is 0V ; strange? according to the schematic there is a pull-up resistor to the 15V line - Pin 10 (hold off) shows a 4V DC voltage (no sweep) - Pin 17 (trig disable) is 0V - Pin 18 (lockout) is low - Pin 19 (auto mode) is low as it should for auto triggered Thanks Thierry |
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On Sat, Mar 20, 2021 at 07:16 AM, Thierry Delaitre wrote:
U820 - Sweep control ICU820 contains comparator and discharge circuits on pins 6 and 8. U820 allows C828 to start charging when it stops receiving trigger signals. That is how it eventually reaches free-run in AUTO mode. Your pin 7 being low suggests that your TRIG'D light is on. Is that correct? It would explain why pin 6 is low, and why you never reach free-run mode. If your 7B92A is still in the vertical slot, set for AUTO, AC, INT, then there should not be any trigger signals, and the fact that U820 is in triggered mode is suspect. - Pin 8 (hold off timing) is 0V ; strange? according to the schematic there isPin 6 and pin 8 cannot be reliably measured with a DMM, as they both normally have positive-going ramps, and should never get much higher than +5V. You will need to look at them with a 10X high-Z probe. (Remember to use an IC probe tip adapter on your probe, or else be extremely careful not to let the probe tip slip and short adjacent IC pins. I find that it is usually safer to probe at some other location: R828 for pin 6, R871 or R872 for pin 10, R834 for pin 8.) U820 allows hold-off capacitors to start charging at the end of the sweep, so if pin 8 is stuck at 0V DC, then U820 may think the sweep is still in progress, or it may have never received a HOLDOFF START pulse on pin 16. - Pin 10 (hold off) shows a 4V DC voltage (no sweep)Are pins 10 and 8 both constant DC, or is there a waveform visible with a 10X probe? dan |
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thank you.
The 7B92A is still in the vertical slot. Regarding pin 7 and the TRIG'D light, the light is off when the scope gets switched-on (with AUTO, AC, INT). The TRIG'D light switches on if i change the mode to NORM. The light stays on if I revert the mode back to INT. Pin 10 remains at a constant DC high voltage and Pin 8 remains at a constant DC low voltage. both pin 10 and 8 do not show any visible waveform I use a scope probe on the same scope to troubleshoot. Thanks Thierry |
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Thierry,
Q410, Q412, etc are 0367. Check all that have this on them from C to E. Likely leaky. Dan is right about U820 possibly being defective. I had one of these bad before. It is possible dirty contacts, if this one is in a socket. If you are not getting any sweep on standard or delayed, I have seen Q812 open B to C. This transistor will make U820 look bad if the transistor is bad. A KSP10BU will replace this and the 0367 types. I have had that happen at least once. The Cherry switch on the right side of the plug-in at the front not physically mounted right. That will be open or grounded to P911. See if it is closed and open by a multimeter when you pull and push the time base selector. This is done with no power applied. The screws that mount to the readout board (A12) are oblong and allow movement of the switch. Make sure you have the pin that activates the microswitch there. You will see it with the right side panel removed. R479 and R939 should be 1W. These are stressed and could be high to open. I have heard of someone saying U856 can go bad. This is a 7474 flip-flop Pulling the time base knob engages the delayed sweep. I do not know if you did or did not test this. See if you get sweep this way. The small intensity knob to the left of the time/div knob. That could be too low. Turn that CW to see of you get sweep. When working right, it will get very bright at full CW. The contrast to the right of this control may need adjusting. The trace sep. knob my be off. If your S/N is below 60000, it is possible the tunnel diodes are bad on one or both triggering boards. It is possible the internal dc bal. and/or trg. sens. pots were turned. They may need a slight adjustment. It may bring the triggering back. Cleaning the switches might help if you have already done so. I hope this helps. Mark |
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On Sat, Mar 20, 2021 at 04:19 PM, Thierry Delaitre wrote:
Regarding pin 7 and the TRIG'D light, the light is off when the scope getsThis is definitely abnormal. When switching from AUTO to NORM, the TRIG'D light should flash briefly, but then go out. This suggests that the sweep timing capacitors did not charge, or that sweep end detection circuits are not working. It also explains why the hold-off and auto ramps are stuck low. As Mark suggested, if you have not already done so, make sure all mechanical switches, especially S490, operate correctly. I would also look at pins AV (Aux Gate signal) and AR (Main Gate signal) to see whether the plug-in thinks it has started a sweep or not. These are normally square pulses, but yours are probably stuck. Whether they are stuck high or low may provide a clue as to what is wrong. Pin 10 remains at a constant DC high voltage and Pin 8 remains at a constantThis now makes sense given your TRIG'D explanation above. While this is not what these signals should look like, I think it is the result of problems earlier in the sequence of events. They may be ok once the plug-in is able to complete the sweep ramp. I use a scope probe on the same scope to troubleshoot.Sounds good. dan |
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thank you Dan/Mark. nearly there...
There was no reference voltage . I've therefore replaced U752 (LM741). This has restored the sweep on the interface board, AV, AU (holdoff) and can now see the sweep on TP450 I still have the 7B92A on the vertical slot and cannot see any sweep I found that Q902 and Q912 were leaky and have replaced both on the output board. I can now see the sweep for the delayed time base but not for the main timebase (ie horizontal line). I can see the sweep at TP450 so not sure where things go wrong on the output board It is not that easy to troubleshoot the output board with the horizontal logic board on top of the interface board Thanks Thierry |
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Thierry,
See if the Cherry microswitch is is opening and closing. That will do what you describe if it is not mounted right. This is pushed by the pin on the front knobs. The white spool is between the pin and switch. This plastic piece looks like a spool or bobbin used in the underside of the area where the needle goes down into is on a sewing machine. The main time knob that pulls out activates this switch. I have seen this problem before. The two screws that are on the readout board attached to the switch on the small metal plate adjust the switch physically. Check AR like Dan G. said. His advice is the best place to start signal tracing. That AR point is T.P. waveform 8 which shows 6V peaks. This point is the extreme lower left of the board. A 51 ohm resistor is going to that test point. If no signal there, go back to test points in the hexagon numbers to see where you have it. See if TP550 waveform is there. That you can solder a short piece of wire at the base of the TP and bend it out towards the front to access it with a probe. At least the board can stay screwed back on for further testing. Remove the wire after the problem is found. At point AF above P801 should be about .8V peaks according to number 11 waveform on <4> which is also waveform 9 on <5> Are the pulses coming in on pin 1 of U820? It should be about 450mV peaks. I am assuming the contacts on things have been cleaned and known working. If not, clean them. The three switches below the time knobs are frequently dirty being open on the end allowing dirt in. The voltages on the triggering boards can be off. Depending upon the S/N of yours, check the voltages at points were internal adjustments are to see if any are off. Int. DC Bal and/or Sens pots could have been turned by the previous owner. If your S/N is below 70000 then the tunnel diodes are the most likely cause of no triggering. Checking waveforms at the test points 7 on <2> or at pin 1 of U820 (waveform 8) will be a good indication of the tunnel diodes operation. Another thing I have seen is the solder connections of these stiff gold pins not be adequate. It is possible one or more of these need additional solder. The ones that I have seen bad this way are usually the ones where the trace is on one side of the board and the solder for the pin is on the other side of the board. The ones that have this problem, I added a bit of solder at the base connecting it to the trace. This plug-in is one I have seen this on where the logic board is connected to the main board. Check the other transistors that are 3571TP. You will find the Q numbers in the parts list. You may have more leaky ones giving you trouble. R82 and R86 will run rather warm. These are the 510 ohm 1/2 carbons. See if you get the 5.7V on the load side (pins 8 and 9 of U74). These may be high. If your are bad, I suggest using 1W. These are used on S/N 70000 and higher. They are located on the main trigger board between U74 and U122. I hope this has been of some help. If Dan or anyone else has suggestions, help him out. Mark |
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Thanks Mark
an update I have cleaned the time/div contacts. I also verified S800 and S490 offline as well as using the scope. AR only works when the time div knob is pulled. When the time div knob is pushed - AP remains low and BM remains high - J & K of the 1st flip/flop are high and the flip/flop is meant to operate in toggle mode according to the 74LS112 data sheet - the clock continues to operate at the input of the flip/flop - is it expected that AP remains low and BM remains high ? There is a nice sweep at TP450 and no sweep at TP550 I verified all transistors for the DLY'D sweep gen (5) There does not appear to be a signal at AD (voltage is 0.2V). I therefore checked TP355 on the delayed triggered board (3) and can see a signal at TP355. Should I be searching why there is no signal at AD which may explain why there is no sweep at TP550? Thanks Thierry |
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Thierry,
I am glad you are making progress. The schematic shows that when the div. knob is pushed (usual operation), that grounds the 74112 through the 68 ohm resistor (R861) J & K inputs making the Q outputs static as you see. When it is pulled, the 74112 oscillates. The function table shows the states of the inputs and outputs. From the table, this IC appears to be working correct. See if you have the 400mV pulses at the collector of Q358. This is waveform 7 on <3>. You can pull and reinsert the short coax between J260 and J270. The inner contact could be dirty. The trig. sens. pot may need to be adjusted. The TD bias pots may also need adjusting a bit. There should be a pulse at DA at ,5V d-c. Now back to the other side of the plug-in on the interface board. Check the waveforms 1, 2, 3 and 5-11 on page <5>. See if you have them there. Also check the transistors there with the number mentioned earlier if you have not already done so. The leakage in these will be from collector to emitter as a diode check. An analogue meter will have the needle vertical on the Rx1 scale. Usually the same number that has a leakage of the 0367 in something, the others likely will be leaky as well. The manual will tell you the Q numbers. Mark |
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