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Old 5th Jan 2018, 10:56 pm   #1
sm6sven
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Default Racal 9081 repair questions

Hello, My name is Sven and im new on this Forum.

I hawe to repair a RACAL 9081 Signal generator and i have some questions.

The Generator have all Voltage OK and 120Mhz ist OK Counter shows only
zero . There is pulse from Logic board.

1. How to check if Divider and Phase detector board is working ?
I find it 22Volt out from Varactor line to VCO !!

2. Where can i find which value for the Test points on Divider and Phase detector board ?

3. If the Phase detector is damaged is there any eqvival Transistors for Q15 ( Dual FET E412) and Q16 ( FET U898E ) ?

4. How to check VCO ? Perhaps is it also damaged with the 22Volt from Varactor line....!

Excuse for My broken English...
Kindly Regards Sven SM6RZB
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Old 6th Jan 2018, 6:39 pm   #2
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Default Re: Racal 9081 repair questions

Hi Sven,

Welcome to the forum!

I take you have the service manual for the 9081, it is "out there on the net", I got it from there.
Unfortunately the block diagram is missing from my versions o the manual...

DISCLAIMER: I do not have the generator, just the manual.

1. Page 52 briefly describes the operation of the phase detector circuit.
First thing to check if both clock signals are reaching the detector. The reference signal is at IC3a/3, the divided down signal from the VCO is at IC3b/11.
Those ICs are CMOS circuits, running from 12V, so you should see 12Vpp clock signals there.
The manual does not mention in the text what the frequency is, you have to go through the signal sources and divider chains to find that frequency. (That frequency could be marked on the block diagram though.)
When the generator is running correctly, both frequencies are the same.
The logic circuits on that board work with 300kHz clock signal, so those frequencies to the phase detector are probably in the kHz range.

2. As mentioned, you have to work through the signal chains to figure out the exact frequencies.

3. Just what makes you suspect those FETs in the circuit?
The E412 is a dual JFET, Siliconix has an equivalent the U412.
That is a 40V device, Idss about 0.5mA - 5mA, Vgs 1V - 3.5V.
Nothing special about JFETs, except that it is a single chip, good thermal coupling and matched parameters. If push comes to shove, I imagine you could get a handful of BF244A or similar and select them for best Idss and Vgs match.
Could not find any data on the U898E FET.

4. Chances are that 22V would not damage the VCO.
To test the VCO you could disconnect the tuning voltage input, drive it from a regulated PS, and measure the output frequency if it varies as you change the tuning voltage.

Keep reading the service manual, lots of information in there from where you can find the data points you want to measure.

PS: Your English is just fine!

Good luck, Peter
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Old 6th Jan 2018, 7:52 pm   #3
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Default Re: Racal 9081 repair questions

I don't know the equipment but it might be worth checking the actual physical transistor in the Q16 position is marked U898E (as printed in the parts list) and not the common U1898E FET. Just in case of a typo?
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Old 7th Jan 2018, 5:51 pm   #4
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Default Re: Racal 9081 repair questions

Thanks for your answer orbanp1 and buggies.

I think Ive been out of track with my fault traces. I have resolder all 40 ICs on N board and tested It with a Simply IC tester but i have not find some defective 40 IC, offcourse the IC tester probably not test with any High Frequency and only sort out IC with shortcircuit. wherefor my conclusion is it any fault in Phase Detector circuit.

Offcourse the right way to do an Logical and effective fault tracing is to Measuring the signal chain. Im not so familiar with Measuring in Digital circuits who works with higher Frequencys . I think a Frequency counter with High input impedance can be to some help but I cant see any Pulse shape with it . An Logicprobe works only to measuring in "slow operating" Digital circuits.

I think in this case I need a Logic analyser. hawe you any suggestions for an good an not so expensive measuring equipment for Fault tracing in just this case ?

Regards / 73

Sven SM6RZB
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Old 7th Jan 2018, 6:33 pm   #5
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Default Re: Racal 9081 repair questions

I have found that a cheap frequency counter is perfectly good enough to track signals through signal generators using synthesisers.
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Old 7th Jan 2018, 6:44 pm   #6
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Default Re: Racal 9081 repair questions

As you might guess from my main interest (classic computers) I have done a lot of digital troubelshooting.

You can do a lot with a simple logic probe, used intellegently, but it is limited. You can certainly find ICs with outputs stuck at '1' or '0', missing clock lines, etc.

The instrument I use most (and recomend if you can find one) is the HP LogicDart. It was called an 'advanced logic probe'. It's a handheld instrument that acts as a logic probe, voltmeter, frequency counter (not particularly good ones but enough to see if a supply
rail is way off or a clock is running at half frequency) and a 3channel logic analyser.

After that you need a logic analyser. All mine are old but that is not a problem if you are working on old stuff anyway, they are fast enough. If you are buying one second-hand, make sure it has the leads/pods with it. The instrument is useless without them and finding them as 'spares' is almost impossible.

One thing I have no direct experience of are the USB-interfaced logic analysers that use a PC for control and display. But I have been helping a chap repair a complicated digital device from the early 1970s, he has one of those and it seems to do the job. He has been able to look at signals and we can then discuss what the fault must be. So I will recomend those albeit as a third party user.
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Old 7th Jan 2018, 7:50 pm   #7
sm6sven
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Default Re: Racal 9081 repair questions

Is a TT LA160 a useful analyser in this case ? I have seen some of this in Ebay but without PODs and Leads.

I think i have to try a USB analyser, I haved found some with 100Mhz and + - 50 Volt input capability. But is it any risk of pull the signal level down with the instruments input resistans ?

HP had also an Logic Clip but i only works to 5Volt level. In the Signalgenerator Logik is it signal with 12Volt level.


Regards / 73

Sven L SM6RZB
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Old 7th Jan 2018, 9:29 pm   #8
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Default Re: Racal 9081 repair questions

Hi Sven, not a technical digital solution, but have you checked there are no tantalum capacitors that are short circuit? Old semiconductor type equipment is notorious for this type of component failure. I have often had this with old Racal test gear!
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Old 8th Jan 2018, 4:12 pm   #9
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Default Re: Racal 9081 repair questions

Hi Sven,

The simplest (and best) test equipment to debug a digital circuit would be a scope. This also implies that you should understand the workings of the circuit.
With a scope you can see all aspects of the signal.
For the CMOS circuits even a "slow" scope, e.g. 20MHz would be useful, but for the ECL circuits you would need a scope with a high bandwidth, and also understanding of the ECL circuit electrical characteristics.

BTW, do you have the block diagram in you manual?

Regards, Peter
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Old 9th Jan 2018, 10:18 pm   #10
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Default Re: Racal 9081 repair questions

Quote:
BTW, do you have the block diagram in you manual?
Peter

Do you need the block diagram?

I have full paper version of manual here and can scan any page you would like, including the block diagram.

Peter
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Old 10th Jan 2018, 10:00 am   #11
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Default Re: Racal 9081 repair questions

Hi Peter,
I missed the block diagramms in my manual too, thank you very much if you woould send me your scans...
Best regards, Karl
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Old 10th Jan 2018, 3:17 pm   #12
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Default Re: Racal 9081 repair questions

Quote:
Originally Posted by PETERg0rsq View Post
...Do you need the block diagram?
I have full paper version of manual here and can scan any page you would like, including the block diagram.
Hi Peter,

Yes, if it is not too much trouble for you, I would like to have a copy of the block diagram.

Thanks, Peter
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Old 10th Jan 2018, 7:43 pm   #13
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Default Re: Racal 9081 repair questions

Hi Sven,

I second Rob's comment about the tantalum capacitors.

I used to have a 9081 but I got rid of it because the red tantalum bead capacitors were going short circuit. As soon as I had found and replaced one faulty capacitor another one failed. The only real solution was to replace them all, but there are so many in that piece of equipment that I sold it for spares/repair some years ago.

If you have located the fault to one particular area check that supply voltage is getting to all devices. If there is a short circuit capacitor you should be able to locate the problem that way.

Good luck,

Peter
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Old 10th Jan 2018, 10:08 pm   #14
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Default Re: Racal 9081 repair questions

Here are the block diagrams
Attached Files
File Type: pdf Fig 3.1 Simplified Block Diagram 9081.pdf (188.6 KB, 86 views)
File Type: pdf fig 3.2 Theoretical Block diagram.pdf (1.02 MB, 78 views)
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Old 19th Jan 2018, 5:23 pm   #15
sm6sven
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Default Re: Racal 9081 repair questions

Thank for all Help and advices now I have find the fault, A Bad soldering at pin 6 result in no 12Volt supply to Output board ! I think the bad soldering is an result from an earlier fault tracing ! Now works the 9081 Perfect! Thanks to Peter , who help me to bring any "life" in instrument with the tip, from an external Power source feed the VCO so I have any signal to trace. My next projekt is learning how to
fault tracing with a Logic analysator.

73/ Sven SM6RZB
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Old 24th Jan 2018, 10:33 am   #16
George G4EUF
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Default Re: Racal 9081 repair questions

Hello Sven,
I had a problem with my sig gen a while back which turned out to be two "tant" capacitors, I am unsure why but "tants" in Racal gear appear to be a fairly regular problem.
Regards.
George
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Old 24th Jan 2018, 11:57 am   #17
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Default Re: Racal 9081 repair questions

Hi George,
Not only Racals but Tek and others have similar problems after30 years or so, further we see the Rifa (X2) condensers case too; I believe its a problem of the early age of the equiments. Otherwise, cca 25 years ago, we had only after 1-2 years lot of problems with shorted Alu SMD Alu-elkos-i.e. D=6,3mm...
rgds Karl

Last edited by karesz*; 24th Jan 2018 at 12:04 pm.
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