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Old 22nd Jun 2017, 10:02 pm   #1
high_vacuum_house
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Question Philips PM5190 waveform shape fault

Good evening,
I was successfully going through some fault finding on this early digital LF signal generator and had replaced one O/C reed relay which stopped the square waveform getting to the amplifier stage.

At this point the sine triangle and square waveforms were very good with no distortion.

I still had issues with the output attenuator stage with nearly no output to the BNC when all of a sudden the sinewave shape changed and now has a sudden DC shift part way through the waveform which corrects itself. I switched to triangle wave and the waveform has the same sudden DC shift. This fault shows itself exactly at the same point in the waveform at any frequency that is dialled up.

I think the fault may be with the DAC but this fault is a little beyond me. Could someone point me in the right direction to look.

Such a shame as I thought I was getting close to finally clearing up all of the faults in it.

Many thanks,
Christopher Capener
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Old 22nd Jun 2017, 10:43 pm   #2
ronbryan
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Default Re: Philips PM5190 waveform shape fault

It looks as if one of the fairly significant bits feeding the 9 bit DAC is missing. Can you get to the input pins of the DAC to see if one of the more significant bits is stuck?

Ron
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Old 24th Jun 2017, 9:31 pm   #3
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Smile Re: Philips PM5190 waveform shape fault

Good evening,
Not much progress on the waveform fault though I have replaced a defective reed relay in the output attenuator so the output P/P voltage is much closer to being correct.

I found a very poorly seated IC on the digital processor board which I thought might cure it but no!!. I also re seated all of the socketed IC's on both boards.

I have gone through the DAC which is made up of discrete components. All of the resistors in this section read correctly.

Going along the buffered inputs to the DAC shows that all 9 data lines seem to be doing something roughly correct. Going down the signal combiner resistor chain (R660-R667) shows the signal being built up stage by stage though the waveform is rather all over the place until the output stage.

The final DAC input (which I think is the MSB on the DAC) has an unusual burst which is shown on the oscilloscope trace. The probe is on the junction of R629 and the emitter of Transistor 310 (on channel 2) Channel 1 is triggering on a 1KHz signal on the DAC output.
This burst when magnified is approx. 8.5MHz which is very close to the main generator clock frequency. I don't know if I am going down the wrong route looking at this.

Any more help would be much appreciated.

Many thanks,
Christopher Capener
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Old 25th Jun 2017, 7:52 am   #4
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Default Re: Philips PM5190 waveform shape fault

What does the triangle wave look like? Does it have the step in it?
If not, then from the block diagram you've given, I think the DAC is OK and the problem is in the sine lookup table ROM (or associated circuits). If there is a step in the triangle wave then I would debug using that one, since you should, I think, just have simple regular square waves on the inputs to the DAC (those 74LS08's wired as buffers), each one half the frequency of the previous one.

Is the service manual for this instrument on the web? If so I can take a look (this is the sort of thing I work on for pleasure...)
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Old 25th Jun 2017, 8:38 am   #5
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Default Re: Philips PM5190 waveform shape fault

Yes the triangle waveform also has the step in it at exactly the same point.

Christopher Capener
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Old 25th Jun 2017, 11:42 am   #6
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Default Re: Philips PM5190 waveform shape fault

Hello,
here is a repairing thread with ccts of analog & digital boards, as premium, Mathima`s reengineered "SineProm.bin files", or an SM too
Regards, Karl

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Old 25th Jun 2017, 1:50 pm   #7
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Smile Re: Philips PM5190 waveform shape fault

Good afternoon,
Many thanks karesz, looks interesting I shall defiantly check all the skeleton pots on the analogue board!!

I have been poking around the circuitry for the MSB to the DAC and I am thinking that IC 370 is not working correctly.

The output to the DAC is pin 15 and when it is active it has a solid burst which I have measured at 8.5 MHz which is very close to the clock frequency. I am thinking that this looks very wrong to me. I think it should go high when active and I think the clock signal is getting to the output pin.

The IC (370) is a 74LS175 and I have drawn out the circuitry showing the individual logic gates in the IC. The data comes in on pin 5 and goes through 3 flip flops before exiting to the DAC on pin 15.

The service manual for the Philips PM5190 is available as a PDF online

Many thanks for your help
Christopher Capener
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Old 25th Jun 2017, 2:19 pm   #8
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Default Re: Philips PM5190 waveform shape fault

That little circuit of 3 d-type flip-flops (3/4 of the 74LS175) will delay the incoming signal by 3 clock cycles. If working correctly there can never be a feature of the output narrower than 1 clock cycle. So the maximum frequency is half the clock. If you are getting something at the clock frequency, then that IC has failed.
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Old 25th Jun 2017, 2:50 pm   #9
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Smile Re: Philips PM5190 waveform shape fault

I think that has nailed it. I save just scoped pin 14 which is Qbar on that gate which is not connected and I get a nice square wave at 5V logic whereas pin 15 which is Q, and MSB out has a clock burst and is only getting to around 4.5V peak. I shall order the 74LS175N and see what happens.

Many thanks for your help,
Christopher Capener
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Old 25th Jun 2017, 5:45 pm   #10
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Default Re: Philips PM5190 waveform shape fault

The kludger in me would think of clipping the 3 pins on that last section, lifting them away from the PCB, then wiring the D input to Q/ of the section that drove it (so feeding it with an inverted signal) and then taking the output from the good Q/ of the faulty section (inverting again, so no overall change).

I woild not leave it like that (the IC is likely to fail further as people who bought from a certain mail order company in the 1970 know well!) but it might prove the point.
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Old 25th Jun 2017, 6:52 pm   #11
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Default Re: Philips PM5190 waveform shape fault

The old IC is now out and I shall fit an IC socket in its place. I didn't want to leave it too long in case it takes something else out with it!.

I wander why it was working then failed?. I have 2 IC's on order and will see what happens next!!. If all ok I shall refit the screening tins then redo the calibration.

Christopher Capener
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Old 25th Jun 2017, 7:09 pm   #12
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Default Re: Philips PM5190 waveform shape fault

I would use a turned-pin socket, they are a lot more reliable than the normal type.

I've had ICs fail from old age when doing repairs. Very annoying as I first of all think it is something I've done, and look in the area I'm working on rather than actually think about the symptoms. In general there doesn't seem to be an external cause for the failed IC, it just died.
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Old 25th Jun 2017, 7:46 pm   #13
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Default Re: Philips PM5190 waveform shape fault

Christopher,

I have some PM5190s one of which has a dodgy keyboard but I'm pretty sure the electronics are ok. I could break it down and let you have a replacement board if your current efforts fail.

Ian
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Old 1st Jul 2017, 6:26 pm   #14
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Smile Re: Philips PM5190 waveform shape fault

Good afternoon,
Good news in a fashion, The MSB 74LS175 IC was replaced and we now have correct sinewave and triangle waveforms present. The replaced reed relay in the attenuator has given me significantly greater output.

I have still got some more problems to sort out though. There is a significant DC bias when it should be zero and the output P-P voltages are still too small and don't correlate with the dialled reading.
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Old 1st Jul 2017, 9:40 pm   #15
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Smile Re: Philips PM5190 waveform shape fault

Good evening,

Looks like I can see the light at the end of the tunnel !! The horrible DC offset and wrong P-P voltages turned out to be another of these horribly unreliable reed relays in the selector circuitry to the buffer amplifier, its contacts O/C again. 3 so far have been dud.

Luckily I had bought all 3 DIL reed relays that the Rugby Maplin had in stock at the time and soon fitted a new one which cured all of the issues and now the DC offset is in the millivolts and the P-P voltages correspond to the dialled in voltage.

Just need 3 key caps for the lower keys as my youngest son spotted it and broke them off! Repairing them is virtually impossible!

Many thanks for all of your help.
Christopher Capener
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