Help needed with - Racal Dana 9081

[frsimen]

Hello Mark,

That's an interesting set of results. Peter's description of the operation of the ROM and the service manual's explanation of the half wave filter switching point to there being a problem with the ROM or possibly its addressing. What you have measured indicates that everything else is working as it should, given that the signals from the ROM are not what they should be.

The feed from pin 5 is being used with this step size switch position. Possibly you would have different results if you were to try a different position of the step size switch.

There are a few things to try:

Monitor the voltage on pin 7 of IC11, with the tuning set to 300MHz. See if there is any position where the voltage drops to LOW.

If there is, tune up in frequency and note the frequency where it changes to HIGH. Then tune down from 300MHz to see if the voltage on IC11 pin 7 changes as you tune down. Note that frequency.

You are now in the territory that Peter is familiar with. Hopefully he will be able to suggest some measurements that you can make, bearing in mind the test gear you have. Luckily, in this part of the circuit, there is no high frequency activity and if you don't touch a control, you can read off any value that you wish without fear of it changing.

Regarding the waveforms in the photo. The timebase is running too fast on your oscilloscope. Slow it down and you will see some sort of square or pulse waveform. The sloping vertical edges are down to the relatively slow rise and fall times of the CMOS ICs.

Paula

[PETERg0rsq]

I agree there is something wrong with PROM addressing.

If you give me the voltage on pin 13 to 21 on the PROM, and pins 4 to 9 on the PROM WHENEVER ANY PIN 4 TO 9 change state I will tell you if the addressing is wrong, or there is a fault inside the PROM.

For Paula to understand the program I enclose the switching points of the ROM.

To help understand, convert inputs A1 to A8 into HEX. Then see how they compare to the switching points and test the outputs Q0 to Q2 and Q3 to Q5.

Remember there are two sets of data in the PROM (Range 1 and range 2,3,4,5) selected by Q0.

For example Q0 should switch to high at a value lower than 0x4E, or higher than 0x62 (in Range 1) and Q2 should switch at 0x52 (in range 1). In Range 2 Q0 switches at 0x4a and 0x62, and Q3 at 0x70.

Hope that makes sense.....

If the ROM is faulty I do have convertor boards that replace The PROM, IC11, and IC17.

Note a 9082 has a different program in he PROM, which I have also reverse engineered.

[3pinplug]

Hi Paula, Peter
Many thanks will run the tests and report back.

Peter, just so i am clear on the pin 4-9 measurements, would i take a set of readings at say 300mhz then repeat this for 310,320,330mhz etc until a pin (4-9) changes state?

[frsimen]

Hello Mark,

I think the best approach is to start at the low frequency end of the band. Monitor pin 4, then tune up in frequency. When you see a change in state, stop and measure the voltage on each of the pins 13 to 21 on the ROM. Continue to the high frequency end of the band, checking the voltages on pins 13 to 21 again if the state on pin 4 changes.

Repeat but monitoring pins 5, 6 etc in turn. There will be a lot of measurements! It would do no harm to note down the frequency for each change.

Peter,

Thank you for the information, you've supplied enough now to be confident in confirming the health status of the ROM.

Paula

[3pinplug]

Hi Paula, Peter
All understood now, many thanks will start gathering the data but may take a few evenings to come back with results.

[PETERg0rsq]

Originally Posted by 3pinplug

Hi Paula, Peter
Many thanks will run the tests and report back.

Peter, just so i am clear on the pin 4-9 measurements, would i take a set of readings at say 300mhz then repeat this for 310,320,330mhz etc until a pin (4-9) changes state?

Yes, as Paula says, you are determining the displayed frequency for each change on pins 4 to 9.

Pin 4, 5 and 6 (Q0-Q2) will have two change points per range, pin 7, 8, 9 (Q3 to 5) will only have one change per range.

Also the frequencies will be different when on range 0 (5-36MHZ) than the other four ranges.

So only need data from range 1 and range 2.

Not interested in exact volts on pins 13 to 20, just 6v or 12v (better if just logic 0 or logic 1).

So you will have something like enclosed spreadsheet for every time an output changes

You will end up with 18 sets of data (9 for range 1 and 9 for range 2)

Start off with a displayed frequency below lower range limit and tune up while measuring a pin. Repeat for each pin, noting data when pin changes state.

(board Pin 12 and 13 DISCONNECTED)

This will reveal if addressing is incorrect, or the PROM has an error.

[3pinplug]

Readings from band 2
Pin 4 changes from 12.29v to 6.83v at 39.0mhz

Rom pin readings
pin 12v 6v
13 0.00v 0.00v
14 12.31 12.31
15 0.00 0.00
16 0.00 0.00
17 12.31 12.31
18 12.30 12.31
19 0.00 12.31
20 12.30 0.00
21starts at 5.5v and contantly drops towards zero for both readings
22 0.00 0.00
23 0.00 0.00

Pin 4 then changes (same band) at 49.0mhz from 6.83v to 12.28v
13 0.00
14 12.30
15 12.30
16 0.00
17 0.00
18 0.00
19 12.30
20 0.00
21 starts at 7.7v and contantly drops towards zero
22 0.00
23 0.00

I will add the remaining sets of data for rest of pins this eveing, ahead of this I just wanted to let you know i can't take a set of readings on band 1 as when i disconnect pins 12 & 13 band 1 just displays zero's, this has been constantly doing this throughout. Do you think this could be a factor as sounds like you are expecting band 1 to work with these two pins disconnected?

[frsimen]

Hello Mark,

I think you have found a problem already. Check pin 21 of the ROM with your oscilloscope. It should be either HIGH or LOW, not drifting nor with any sort of pulses on it. Move back to IC18 pin 10. That should be HIGH or LOW, again without pulses. If it isn't, move to IC18 pin 8. Check it is connected to pin 9 and check that it is LOW or HIGH . If it is, but pin 10 is wrong, replace IC18. If pins 8 and 9 are connected, but the level there isn't LOW, check that R73 is connected. If that is right, check D13 and Q22. The important point is that the voltages should be a distinct high or low, without pulsing.

Band 1 should work if you turn the spin wheel down to reach a lower frequency.

Paula

[Cruisin Marine]

Just a thought as I don't know these sig gens very well. If the ROM is fitted in a socket, would it not be worth reseating it?

[3pinplug]

Next set of pin data Reading from "Pin 5" on band 2 - at 31.0mhz it changes from 12.28v to 6.63v
Pin
13 0.00
14 0.00
15 12.30
16 12.30
17 12.30
18 12.30
19 12.30
20 0.00
21 0.6 initially, immediately starts scrolling down to zero
22 0.00
23 0.00

at 39.0mhz it changes from 6.83v to 12.28v
13 0.00
14 12.30
15 0.00
16 0.00
17 12.30
18 12.30
19 12.30
20 0.00
21 0.6 initially, immediately starts scrolling down to zero
22 0.00
23 0.00

[3pinplug]

Confrim that the next few pins only have 1 change on them not 2 - Pins 6 to 9
Pin 6 changes form 6.83v to 12.28 v at 32.5 mhz
Pin 6
13 0.00
14 12.30
15 0.00
16 0.00
17 0.00
18 0.00
19 0.00
20 12.30
21 0.7 initially, immediately starts scrolling down to zero
22 0.00
23 0.00

[3pinplug]

Pin7

Changes from 6.83v to 12.28v at 41.5mhz

13 0.00
14 12.30
15 0.00
16 12.30
17 0.00
18 0.00
19 12.31
20 12.31
21 0.6v initially, immediately starts scrolling down to zero
22 0.00
23 0.00

[3pinplug]

Pin 8 changes from 6.83 to 12.28v at 33.0mhz band 2

13 0.00
14 12.31
15 0.00
16 0.00
17 0.00
18 0.00
19 12.31
20 0.00
21 0.8v initially, immediately starts scrolling down to zero
22 0.00
23 0.00

[3pinplug]

Pin 9 - changes from 6.83v to 12.28v at 27.4mhz

13 0.00
14 0.00
15 12.31
16 12.31
17 0.00
18 12.31
19 12.31
20 12.31
21 0.6v initially, immediately starts scrolling down to zero
22 0.00
23 0.00

Hi Paula
Will conduct the further tests as suggested and yes pin 21 looks odd throughout all the tests.

Hi Peter
Hope these are ok for you, I will look now at band 1 and repeat if i can get a frequency to show.

[3pinplug]

Just taken the measurements Paula, results as follows:-
Pin 21 = scope trace stays at zero no movement
IC18 Pin 10 no movement stays at zero no pulses
ICPin 8 and 9 12v
D13 + end changes state from 12v to 8v when wheel spun.
When i say scope trace at zero its the same position when probe not touching anything as when i do.
Should i change IC18?

[PJL]

IC16 (ROM) Pin 21 is A9 and is driven by IC18 pin 10 which is the inverse of the range switch which is zero on range 1 and 1 on all others. Try measuring IC18 pin 10 on range 1 and then on range 2, it should change.

Everything points to a dry joint or a track fault between IC18 pin 10 and IC16 pin 21.

[frsimen]

Hello Mark,

Hold fire on IC18, but you have found the fault. The ROM is being switched to the coverage needed for Band 1 all the time. There should be 12V on pin 21 when you are on ranges 2-5, and 0V when you are on range 1. That explains why you can only tune the bottom part of the band. The voltage on pins 8 and 9 of IC18 is 12V, when it should be 0V. Try the range switch in all 5 positions in case one of them gives you 0V.

If all ranges give 12V, try lifting one end of D14 - D14 itself probably isn't faulty, but there could be a feed coming via D14. See if the voltage on pin 9 of IC18 drops to 0V. If it does check the voltage at the junction of D13 and R81. Expect it to be 12V. Lift one end of D13. If the voltage drops, change D13, if it stays the same, change Q22.

If the voltage on pin 9 of IC18 stays at 12V with D14 lifted, disconnect the wire from terminal 14. The voltage on pin 9 of IC18 should now drop to zero.

One or other of these changes should restore 0V to IC18 pins 8 and 9 If it doesn't, have a good look around for solder bridges on the PCB. If you don't find anything, replace IC18.

Paula

[frsimen]

Hello Mark,

The previous post had a mistake, so ignore the paragraph beginning "If all ranges give 12V". If you lift D14, you probably will measure 12V or near to it even if there is no fault in that area.

I forgot to say to check first that R73 is connected to pins 8 and 9 of IC18 and hasn't gone open circuit. The range 1 switching arrangement won't work if it isn't in circuit.

If all is well there, try lifting the D14 as suggested before. If the voltage on pins 8 and 9 drops to 0V, check for a short between the base and emitter of Q22. Replace it if there is a short. Replace D13 if Q22 measures fine.

I think you are very close to finding what is wrong now.

Paula

[Chris55000]

Pin 21 (Address Line A9) of the '3515 PROM IC16 has a very simple circuit controlling it, a direct feed from control line "RE" of the Range Switch Assembly S2, and is "LOW" on Range 1 and "HIGH" on all other ranges, so a simple ohmmeter/continuity test between the +12V Line and pin 14 (RE) should show open–circuit on Range 1 and low–resistance on Ranges 2, 3, 4 and 5, inverted by IC18c then directly connected to Pin 21 (A9) of the PROM.

Therefore Pin 21 of the PROM should be "HIGH" on Range 1 and "LOW" on Ranges 2 to 5.

If Pin 21 is "LOW" on Range 1, either IC18 is faulty or the '3515 has an internal fault in it between pin 21 and it's –Vss supply pin, +6V. If Pin 8/9 AND 10 of IC18c are all low and IC18c pin 10 doesn't change state when the Range Switch is moved between Ranges 1 and 2, lift pin 21 of the PROM and then change between Ranges 1 and 2 again – if IC18c pin 10 is still stuck in one state, either the range switch contacts/wiring on the front panel between Pin 14 and the +12V supply is defective, or there could possibly be an unwanted shunt/short–circuit path from IC18c pin 10 to earth, possibly via D13/D14 or IC11, as Paula suggests!

If the PROM pin 21 is "stuck low" but IC18 pin 10 changes normally when pin 21 is lifted, the PROM itself IC16 is defective and a replacement will need to be obtained!

The Range Switching Assembly S2 and Channel Spacing Switch Assembly S4 is wired in an electrically matrixed configuration that is quite difficult to follow from the front panel wiring diagram, but there is a small table giving the logic for line "RE" at the bottom of the left–hand corner Phase Detector/Divider Circuit Diagram Fig. 13 to clarify matters!

Chris Williams

[3pinplug]

Hi all
Will run through the checks suggested, one piece of feedback so far:-

Band (pin 21 of prom)
1 = 12.32v
2 = 0.6v scrolling down in voltage
3 = 0.6v scrolling down
4 = 0.6v scrolling down
5 = 0.6v scrolling down