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Old 15th Jan 2018, 6:41 pm   #1
Markany
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Default Racal 9081 SSG No FM but he meter reads OK

Hi All

I have an early version 9081 the one with the LED on the from panel, I have a printed service manual but it unhelpfully says the FM mod PCB is for later versions only.

The problem is after the usual 12V PSU cap failures were sorted (for now anyway) upon testing there is no FM mod from the unit. Now the meter reads and the level set appears to work, the AM side is fine. So my conclusion is the AF osc works and the faults is more likely to be in the FM mod section.

Has anyone seen this problem before or has the circuit diagram for the earlier units, i can only find the more recent ones on line.

Regards

Mark
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Old 15th Jan 2018, 9:30 pm   #2
PETERg0rsq
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Default Re: Racal 9081 SSG No FM but he meter reads OK

Here are the two versions of the FM Board

The early one is a closed loop system, keeping the modulation depth constant over the tuning range. The newer version uses an open loop FM system, which must be constantly re-set, using the modulation meter, when changing frequency.

If it is the older version, I do have the complete scan of the manual, but it is typical Mauritron quality! (10MB)

The newer version is the file I uploaded to KO4BB some time ago.

Peter
Attached Files
File Type: pdf Fig 19 New Version.pdf (376.7 KB, 25 views)
File Type: pdf Fig 19 old version.pdf (53.7 KB, 19 views)
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Old 15th Jan 2018, 11:08 pm   #3
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Default Re: Racal 9081 SSG No FM but he meter reads OK

Rusty memory here! There is a DIL reed relay goes faulty. May be on FM board? Switches out the meter when tuning to stop it bashing about. Can't remember if it causes your exact problem or the opposite way round. ie FM OK, but no meter reading. I used to change as a matter of course. Remember, the FM meter indication is derived from an inbuilt mod meter.
Rob
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Old 18th Jan 2018, 11:36 pm   #4
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Default Re: Racal 9081 SSG No FM but he meter reads OK

Hi both

Many thanks for the files and information, I might have to do some more reading of the manual as the PDF marked as old is what i have in the book and assummed it was the newer one from what is said on the diagram.

Do you have the PCB layout as that will make it clear as the board has a different shape. The one with the cut out i thought was the newer one and and the one without the cut out is the older one.

Also the newer one has the relay on that side panel next to the FM loop board.

Started work on the second 9081 and so far working through short caps, one on the 24v, then one on the 12v rail, it powered up then after a short while the 12v rail went down again so another shorted cap to look for.

Many thanks for both of your input

Regards

Mark
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Old 1st Feb 2018, 4:10 pm   #5
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Default Re: Racal 9081 SSG No FM but he meter reads OK

Hi All

Just to let you know the unit is now fixed thanks to those diagrams, in the end i have three different circuits and finally matched up what was what. It would be nice to have the extra board layouts but the circuit is much more important.

The fault was three of the 24v rails down with SC caps and burned out resisters. Once i worked out how the unit functioned the red herrings of meter fine but no output and also the buffer amp having input but no output were ignored.

Basically the buffer amp has gain control and that was shut hard off due tot he loss of rails further upstream.

I now have two of these fine machines in working order, well until more caps go of course.

Regards

Mark
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Old 14th Feb 2018, 5:13 pm   #6
Markany
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Default Racal 9081 display only works on two ranges

Hi All

The repair continues on this 9081, after many faults I finally had the unit working then after a week of on off soak test the display started playing up, what I have found so far is :-

1: the output on all ranges is fine and follows the steps and dial.
2: the counter display is fine on two ranges the 256-520 and 35-64
3: on the other ranges I get random readings

I have traced the issue to the programmable divider area where the 1Mhz clock is divided into required time base, the only reliable selection appears to be the 62Khz one for th 256-520 range.

Before I go getting devices and replacing IC10 and 11 I thought I would ask if anyone has seen anything like this?

Many thanks.

Mark
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Old 14th Feb 2018, 5:20 pm   #7
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Default Re: Racal 9081 SSG No FM but he meter reads OK

Is the display actually a counter, ie it counts the frequency over a given period, or is it driven by the processor?
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Old 14th Feb 2018, 11:48 pm   #8
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Default Re: Racal 9081 SSG No FM but he meter reads OK

it counts the output frequency directly, but obtains its clocks from the generators clocks and dividers.

No uProcessor in the 9081

only "special chips" are the counter and the ROM (which i have reverse engineered and made an eprom replacement)
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