Racal 9082

[Pattymor]

Dear all,

I went and register to yahoo groups in hope for some help but not replies.

Racal 9082 Series 302x Frequency 0000 on AM Band 1.5Mhz - Fixed top Freq on all bands


I had come across with this Signal Generator 9802 which was originally on dead screen. After a day of working on insulating the 12 volts DC lines which was in short circuit; I got in to the FM loop Board 19-0844/4 and found short circuit on C78 =47uF/16V, C46=4.7uF/35V open Resistor R81= 47 Ohm.

When powered the 9802 ON the AM band showed 00000 and all other bands showed over scale values with not tuning control at all (red lights on spinning wheel). Steady values on other bands: 68.2 Mhz, 137.2 Mhz, 274.2Mhz, 549.2Mhz. It does not matter the other control settings frequencies are steady.

Applied the 1Mhz to External standard with same results on Frequency counter and checked for LK1 good 2.0V on 19-0849 120 Mhz Loop board, good 300khz, 5Mhz, 10 Mhz. I suspected something could be wrong on same FM Loop board cause the LK1 was supposed to have 2.5V and it only had 0.7 Volts ,Not signal of around 370Khz found on TP2 or TP3. Thanks to comments from Peter on the FM Loop common problems

I found a burnt resistor R47 = 100 Ohm and a tantalum C32 = 4.7uf /35V on Short circuit. After replaced bad components, TP2 is around 110Khz and TP3 a Saw signal 220Khz (is still wrong far for 370Khz) and LK1 is on rail 24Volts DC. I Guess 24V on LK1 instead of 2.5 Volts is not going to be beneficial to other circuits.

Source of the 24 Vdc at LK1 is an OpAmp IC3 uA301 (IC5 on Paul's Schematic), Replaced the suspected IC3 with a new LM301 and LK1 is still on 24VDC. When opening the LK1 jumper, there is 2.1 Volt on the Varicap Diodes side.

It seems like nothing I had fix to this point on the FM Loop board changed the AM band to be on 0000 and the other bands to be on over frequency.

Please let me know your thoughts,

Sincerely,

Patty

[theoldtrout]

I use a 9081 in my workshop and have repaired a couple of these including the 9082. All the problems I had with these were due to shorted/nearly shorted tantalum capacitors pulling down supply rails, burning out resistors and damaging some of the semis; many tant caps are used to de-couple the supply rails. It is rather tedious but removing the various pcbs and doing a quick resistance test across these capacitors is a good start. If you do a forum search, these sig gens have been discussed before. Hope this helps, Angela.

[PETERg0rsq]

I am away from home in the Philippines at the moment, so do not have access to schematics to help.

I will return in about 10 days and can give some more specific guidance.

As a starter some comments

The frequency display is an actual counter of the output frequency (using the same reference) so if it displays 0 Hz then there is no rf output.

If all the bands show max or min frequency then most likely fault is oscillation in the digital board, or a fault with band edge detection.

Eliminate band edge detection by disconnecting the "up" and "down" connections to the digital board from the divide by n/counter board from the rom chip logic.

Then check the pulse output from the digital board with an oscilloscope to make sure only get output when spin wheel rotated.

Another useful check is to disconnect the output from the phase detector and apply a separate 0-10v dc source to the varactors and check the main oscillator and divider chain.

As said I can not offer any specific help until I get the schematics in front of me.

If you are still stuck I will contact you on my return.

[Pattymor]

Thanks for replies

After chasing everywhere for more tantalum capacitors on short I had found nothing.

Any how all the boards in this thing belong to TIPH169xx series being Logic Board 19-0948-1 TIPH1695-1, Divider 19-0950-1 TIPH1698/1, Output 19-0949-1 H1696/1 , Counter 19-0845/3 H1697/1.

By following Peter's advice, I had open the Oscillator Voltage which was on 21 Vdc (hope nothing else has burnt on the pathway) , applied 3.3 Volt to Varicaps Diodes and then AM band now shows 1.57Mhz, as soon as I got closer to 10 Volts the AM band collapsed and counter shows 0000 again. So the result of the counter showing 0000 on the AM-HF band was caused by the over voltage on Oscillator diodes causing a Frequency around 100Mhz and up on the Output Board.

Tracing back the 37.5Khz Reference Signal I found a LOGIC high on Pin18 at the Divider board, means not oscillation at all; so, I went to check Logic Board and found the 300Khz is solid at the IC1 pin 1 NOR Gate CD4001, and the High Logic level is steady on pin 3. Pin 2 is at Logic level Zero which is enable for the NOR gate to follow the Pin 1 at the output. I suspect The cd4001 is not performing so this will be the first CMOS IC I will be replacing (Hope is not many of them).

Best Regards,

Patty

[PETERg0rsq]

Originally Posted by Pattymor

Oscillator Voltage which was on 21 Vdc (hope nothing else has burnt on the pathway) , applied 3.3 Volt to Varicaps Diodes and then AM band now shows 1.57Mhz, as soon as I got closer to 10 Volts the AM band collapsed and counter shows 0000 again.

What frequency did you get before it failed?

(There is no "AM Band" as such, just five ranges). Measure the output of the VFO at pin 6 of the VFO board (19-0841). Should easily get 256MHz to 512MHz output.

Tracing back the 37.5Khz Reference Signal I found a LOGIC high on Pin18 at the Divider board, means not oscillation at all; so, I went to check Logic Board and found the 300Khz is solid at the IC1 pin 1 NOR Gate CD4001, and the High Logic level is steady on pin 3. Pin 2 is at Logic level Zero which is enable for the NOR gate to follow the Pin 1 at the output. I suspect The cd4001 is not performing so this will be the first CMOS IC I will be replacing (Hope is not many of them).

I agree, but IC2, IC9a and IC7a could also cause an issue as described, if their inputs are permanently high. CMOS have many modes of failure

You might find several CMOS destroyed if the 9082 has had a supply voltage fault!

However one step at a time is the best way to diagnose these signal generators.

[Pattymor]

Peter,

This is my readings
Screen Counter VCO 5 VCO 6
Before 0000 3.200.Vdc 240Mhz
Range 1 , 1.11Mhz 3.285VDC 242 Mhz
Range 1 , 1.50Mhz 3.322VDC 242.6Mhz
Range 1 , 32 Mhz 5.55 VDC 303.7 Mhz
Range 1 , 45Mhz 6.340VDC 330 Mhz
Range 1 , 00000 6.40VDC 334 Mhz

Screen Counter VCO 5 VCO 6
Zero Below 2.75VDC
Range 2 , 28.98Mhz 2.796VDC 231.5 Mhz
Range 2 , 32.0 Mhz 3.902VDC 256 Mhz
Range 2 , 64Mhz 15.540VDC 512 Mhz
Never Goes Zero 21VDC

Screen Counter VCO 5 VCO 6

Zero Below 2.55VDC
Range 3 , 57.10 Mhz 2.632Vdc 228.4Mhz
Range 3 , 64 Mhz 3.903VDC 256 Mhz
Range 3 , 128 Mhz 15.55VDC 510 Mhz
Never Goes Zero 21VDC

Screen Counter VCO 5 VCO 6
Zero Below 2.55VDC
Range 4 , 114.22Mhz 2.632Vdc 228.4Mhz
Range 4 , 128 Mhz 3.902VDC 256 Mhz
Range 4 , 256 Mhz 16.01 VDC 515 Mhz
Never Goes Zero 21VDC

Screen Counter VCO 5 VCO 6
Zero Below 2.55VDC
Range 5 , 228.5Mhz 2.632Vdc 228.4Mhz
Range 5 , 256 Mhz 3.902VDC 256 Mhz
Range 5 , 512 Mhz 16.12VDC 516 Mhz
Never Goes Zero 21VDC

I will order and replace IC1, IC2, IC9,IC7 and will let you know my progress.

Patty

[Pattymor]

Little progress after replacing the mentioned CMOS IC's. 37.5Khz is now present on Phase Divider Board. Furthermore not Pulses have been found on Pin 18 on logic board. Disconnected Pin 15 and 17 with same results. TP5 is hard wired to some sort of bulk connector/relay small board aside to FM loop board cavity not showing in any schematic, desoldered TP5 wire with same results. TP8 shows Logic direction on both switch and Wheel, High on Up and Zero for Down. Some sort of pulses are present as well on Ic3 and Ic4 when rotated Wheel UP or Down and that's as far as the pulses goes. Had exchanged and solder sockets on all CMOS from IC1 to IC15 on logic board ,same problem not visible pulses on Pin 18.

Patty

[PETERg0rsq]

Patty

Have you read the technical description in the service manual for this part of the circuit. It contains a lot of information?

When operating the spinwheel you should be getting good pulses at IC5 pin 3. These are the pulses that will eventually operate the counter/dividers.

As TP8 is responding correctly then everything up to IC5a must be good.

The pulse multiplier doubles the amount of pulses by generating a puls on the rising and also on the falling edge of pulses from the spin wheel. As there are two encoder tracks feeding into IC5a you will end up with four pulses for every step on the spinwheel.

These should be seen at TP9. The next place to check for pulses is at IC13 pin 7

IC13 divides the pulses when in slow tune mode by about 10 depending on the voltage on IC9b pin6

TP5 should then show the pulses if the 9082 is not in a locked mode, but should see pulses when ever the up/down switch is activated, even when locked (TP8 should change state depending on UP or Down)

Not sure what you have atached to TP5. Is it a Racal board?

Once you go past TP5 it gets complicated!