Gould 407x display help

[mole42uk]

I'm still involved in the restoration of the Gould 4074 which is making me work for success. The power supply is now working properly after a full rebuild. The logic is doing everything I'd expect but the display is not nice, as shewn in the accompanying photographs:

The first image is the display when the instrument is first switched on, the second is the display after pressing the 'menu' button. The text can just be read but it has an obvious timing error.

The other problem is that the intensity controls do not work, these images are taken with both the trace and alpha intensity fully anti-clockwise.

I can follow the fault-finding in the service manual, but the chart leads to the 'Bright-Up Amplifier' which is a mess of SMDs. If I could get some helpful pointers it might save my eyes and my hair!!

[kevinaston1]

I have not looked at the Gould service manual; but my immediate impression is, no CPU reset.


Kevin

[dave cox]

Is there a manual available for download ?

dc

[mole42uk]

I have the manual. It's just that my eyes aren't as good as they once were and the sections of circuit I need to fault-find are all SMDs!

[cmjones01]

To me that display looks OK but with either the CRT astigmatism misadjusted so the spot is a tall, thin line instead of a fine point, or there's a load of hum or noise on the Y axis which is smearing the image vertically. Check those things first.

Chris

[Noopy2014]

I agree with Chris.
You don´t have a problem with the brightness. You have a problem with a noise on the Y-axis. The lines consume the brightness which should power up only small dot.

But the interesting question is: Where does that come from?

I would suggest the following:

1. Measure the voltage between the plate Y1 and ground and Y2 and ground directly at the base of the CRT.
You should see whether both plate potentials are ok.
If it´s so you have aproblem with your CRT. I don´t know a way to missaligne the CRT in such a way so there could only be a broken Joint in the CRT. If it´s so I would have one in stock.

2. If there is a noise on the Y-deflection it gets more interesting.
The supply for the deflection amplifier supplys first the Y-amp than the X-amp. Noise on the supply should be visible on both deflections which we don´t see.
There could be a defect in the Y-amp. But I would first try to test whether there is a problem with the small signal coming to the Y-amp. That´s a bit tricky because it´s a differential current interface. Best way is to activate the external plotter an measure the signal on the plotter interface at the backside of the 4070. This signal is picked up between the Y-DAC and the Y-amp. Strange signals at the plotter interface would lead to the display card. I would suggest to display one grounded channel so you should see a single value for a longer period. If it´s a mess and not a value...
In the end you need either a new DAC08 or you have to do a logic search or you have to do a more ugly search in the SMD-Y-Amp but let´s see what you get. Edit: There could also be a connector problem...

Of course I have some spare parts here...

Best regards,

Richard

[mole42uk]

Thanks guys, I'll be in the workshop again tomorrow so I'll check. I have a spare tube, so I might try that as well.

I am interested that the brighness controls are both at minimum in the photos are above. That seems wrong.

[RogerEvans]

Do you know if the brightness controls directly control the CRT voltages or are they just read by the CPU which then sets control voltages via a DAC and analogue multiplexer to set the CRT voltages? Several of the Tek digital scopes use the 'fly by wire' approach and if the DAC references drift then all the analogue settings are affected eg position, trigger levels, brightness ... The advantage of the extra level of complexity is that you can set everything via the GPIB for automated testing.

You may well find a CRT 'grid bias' preset to adjust the present brightness levels to something more acceptable and then readjust when things are fixed.

Roger

[Noopy2014]

Oh, didn´t realize that the brightness controls are at minimum. That´s strange.

The brightness isn´t controled by the CPU, it goes directly to the tube driver amps.
CRT-Settings are the only things that you can´t control over GPIB.

I think you have more than one problem.
Regarding the brightness there could be a problem in the brightness-control-circuit in the CRT-amp. Unfornunately it´s also possible that there is a failure in the power supply which generates the voltage for the voltage control grid. There is a small transistor taking the control voltage from the tube amp and adjusting the control grid voltage around 100V on the Basic high potential (~3kV).
There is also a protection Diode on the tube base board (D1) which in case of failure could lead to a potential on the brigthness-control-grid so that the reading is very bright.

[mole42uk]

Updates from inside the 4074 on the workbench:

Have you ever had one of those days when every step makes two more? I started with a display on the screen, replaced a few failed components and finished with a blank screen and no further forward.

Thanks to Noopy2014 I checked the protection diodes on the tube base, three of the four were short circuit (well, ~170R in both directions). I tried the other tube base PCB that I have but that showed no trace at all, so I robbed two of the diodes which tested okay and now the previously working tube base doesn't let the tube display anything.,

I checked the Y amplifier outputs at the tube base, Y1 is 87.5V, Y2 is 87.3V so that seems good. I didn't try my other tube because I got into testing other parts. I did try out my other driver board which displays on the screen in a similar way to the other but it keeps the instrument in self-test because of corrosion from a battery leak some time ago.

I am still concerned that the brightness controls do nothing although I can measure that the trace brilliance control moves from -6V to 1.5V and the alpha control moves from 1V to 2.5V. The trace/alpha switch seems to work, believable voltages on the switch pin and the outputs and the display changes when I press the 'menu' button. The 245V supply to the Bright-Up Amplifier is present so I suspect that there's a failed transistor or diode in the Bright-Up circuit. That's where things start to get interesting, partly because it's all SMD and crammed in under the CRT and partly because the component layout in the service manual that I have is different to the PCB that I have on the bench.

I think the simplest way out might be to replace all the active parts of the Bright-Up amplifier, three transistors and four diodes. My reasoning is that the supply protection resistor (R922, 100R) was o/c and, although I replaced it, the circuit still isn't working. It was probably a failure in the driver transistor Q801, the emitter of which should move between 20V and 60V over the screen intensity range and it doesn't, that popped the resistor R922.

That's all for today, folks!

[mole42uk]

Well, I decided that I’d make an extender cable to outrig the pre-amp PCB as instructed in the service manual.

I am glad I did because it made testing much easier and soon a 120k resistor dropping 260V because of a s/c zener was discovered. I can’t work out why the 1.3W 120V zener had failed when the supply resistor is only ⅛W and was undamaged. There wasn’t a suitable zener in the drawer so that’s it for a day or so.

[mole42uk]

Here's the pre-amp PCB proudly rotated to allow work on the underside and an annotated pic:

[Noopy2014]

I´m really pleased that you make progress!

[dave cox]

You could probably use a couple of lower voltage / lower power ones in series.

0.125W series resistor sounds a bit tiny at that voltage ?

dc

[mole42uk]

I'd agree except that it survived the 260V - the ⅛W resistor usually drops half that but the shorted zener that drops the other half didn't give it an easy time. I think the PCB layout engineer knew it might be a little critical since the SMD resistor has about 5mm all the way around it on an otherwise pretty densely populated board.

Pondering on some of the design issues in this equipment, one of the things that seems to be evident is that small resistors are used in the supply rail as resistor/fuses so that if a failure occurs that might damage the supply or other parts the resistor becomes sacrificial. There have been two or three that I've found o/c with a down-stream transistor that has shorted c-e to potentially draw a higher current.

[mole42uk]

Another update: having replaced the 120V zener I now have that supply rail working, and having found the pictured dead transistor and replaced it and the 100R resistor in the supply rail, I now have the 270V woprking again.

There's still no display on the screen, the brightness controls do change the voltage to the bright-up circuit but there's still something wrong in there. It's not made any easier because the layout diagram I have is only a little similar to the PCB I'm working on. This means that every time I need to check a component I have to find out which one it is by tracing the circuit layout on the board. Tedious, yes it is.

[Noopy2014]

I would suggest to do the following:

1. Is the output of the brightness control ok?
Do you see changes around 1kHz at PLTB2 on the tube socket board? (or on the same potential on the brightness control whatever is better for you)

If not D2 on the tube socket board is dead or you still have something to do in the brightness control itself.

2. If the brightness controll is 100% ok the next possible explanation is a problem in the power supply.
I would measure the high voltage on the tube socket board between R2 and C2 and at PLTA9. At PLTA9 there should be something a little bit above -3kV, between R2 and C2 there should be a changing voltage around -3kV. If the voltage is constant lower than at PLTA9 the electron beam can´t leave the heater.

A problem here could point to a defect in the power supply. If Q16 in the power supply on the EHT board is shorted the voltage on the brightness control grid is constant lower than on the cathode.

3. Last explanation is a defective CRT.

4. If nothing helps you should check your own work till now.

[mole42uk]

Thanks Noopy, I will try those tests soon.

Meantime, I wrote to Advance to ask if they had an archived service manual for the Gould 4074 since my copy is issue 3 and the ‘scope is issue 7, but they wrote to say that although they have lot of archived documents, that one they don’t have.

[karesz*]

Richard,
as I see, it can exist as Gould or Schneider 4074 too, and as WME_Bill wrote in an older thread: doc`s for 4072 can be helpend too...
Rgds, Karl

[Noopy2014]

I also tried to concact Advance Electronic Solutions for more Information.
If you ever get more information let me know!