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Old 19th Nov 2020, 8:29 pm   #1
g0hze snowy
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Default Racal Dana 9916 Frequency Counter.

good evening gents, have just purchased the above frequency counter as faulty spares or repair.
so far i have checked the voltages according to the manual and all are ok.
Q6 (ztx550 pnp) has 4.84v on both base and emitter but just 0.384v on the collector, replaced with new trannie, just the same, something beyond it dragging it down i suspect or is it (Q6) just a switch that should have the same volts on each leg if everything is ok?
pin 14 ic31 has just 0.384 rather than the 5v or so on it fed from Q6!.
lots of tants ( some blue types) mainly buff coloured, are these as prone to failure like the blues. no magic smoke or burnt areas visible.
Display shows a couple of dots and thats it.
I do have the manual so i can follow the circuit.
any and all help would be much appreciated, snowy
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Old 20th Nov 2020, 12:31 am   #2
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Default Re: racal dana 9916

I would check those tants.
I have had red and green ones as well as brown go short circuit.
If they are on a biasing circuit they just take the rail down.
If there is enough current available they just let rip.
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Old 20th Nov 2020, 8:39 am   #3
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Default Re: racal dana 9916

OK - so you have the manual so you can follow the circuit. Not sure the rest of us do though! If you would like helpful comments, then please post at least the schematic (and parts list if values aren't on the schematic).


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Old 20th Nov 2020, 12:14 pm   #4
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Default Re: Racal Dana 9916 Frequency Counter.

The manual is on the KO4BB archive, they seem to be limiting downloads so I couldn't view it last night.
Scroll down to Racal Dana 9916 Frequency Counter Service Manual; http://www.ko4bb.com/getsimple/index...uals&dir=Racal

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Old 20th Nov 2020, 12:27 pm   #5
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Default Re: Racal Dana 9916 Frequency Counter.

Q6 seems to be part of the channel select circuit, if I'm following it correctly it switches the +5V supply for channel B when B is selected, similarly Q13 does the same for channel A.
Check C15, C25, C26 for shorts? The +5V also goes to the L.F multiplier option board (if fitted).

David

Last edited by factory; 20th Nov 2020 at 12:34 pm.
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Old 20th Nov 2020, 3:50 pm   #6
g0hze snowy
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Default Re: Racal Dana 9916 Frequency Counter.

David, Rich and Refugee, many thanks for your advice, i have changed a number of tants with 'normal' electrolytic's as the are just for decoupling, i injected a 10 mHz signal into it and followed it as far as i could, nothing found wrong, so i went to the display, voltages are fine, signal on my scope from the LED drivers, thru' the resistors up to the board connector, voltages appear ok as well. I cant for the life of me think that all the 7 seg's have died in one go. had to get away from it for a while, i have another go probably tomorrow, thanks for all the advice / suggestions so far, keep them coming please.
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Old 20th Nov 2020, 6:42 pm   #7
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Default Re: Racal Dana 9916 Frequency Counter.

As you say above, it's unlikely that all the 7-segment displays have failed at once. More likely it is something common to all of them. It could as simple as a power supply issue, or possibly either the 7-seg driver (that drives each digit in parallel), or the multiplexing strobe signals, that select each digit in turn. Or, worst case, it could be the large LSI chip.
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Old 20th Nov 2020, 7:16 pm   #8
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Default Re: Racal Dana 9916 Frequency Counter.

Having finally accessed the schematic, here's my immediate thoughts on your initial post.

1. Q6 is a power supply switch to the B channel amplifier and AGC system. Channel selection is described on page 4-3 of the manual.

2. Q6 is a simple switch. It is controlled by IC13d, a NAND gate. When that gate output is high, then Q6 will be off. When it is low it will be on. The purpose of Q6 is to increase the amount of current that gate IC13d can control.

3. You measure 4.84V on Q6 emitter. The schematic says it should "+5V". According to the test procedure on page 5-4, the +5V rail should measure 4.95V +-0.15V, so it can fall as low as 4.80V and still be OK. This looks fine.

4. You say the base also measures 4.84V, which means IC13d is high. That means the Q6 is off, and so it will not be conducting.

5. So why is the collector measuring 0.384V? It should - theoretically - measure 0.00V because Q6 is off. My guess is that there is some leakage back through an IC somewhere, which is receiving voltage from elsewhere in the system. Its probably OK - and I suggest looking elsewhere before chasing this one.


My conclusion is that this bit of the circuit - the B-channel amplifier - is nothing to do with the display being blank. Not sure why you homed in on this at all. Your measurements do not suggest a fault around Q6.

So the fault is that the display is blank?


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Old 20th Nov 2020, 7:37 pm   #9
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Default Re: Racal Dana 9916 Frequency Counter.

Take a step back from it for a short while, and do'nt dive into bits of circuit randomly.

The heart of the 9916 is IC4, which is the counter, gate, and display multiplexor. This is a custom chip, so be careful around it.

It also handles most frequencies in the counter.

So I would start there and find out exactly what is not working.

The schematic gives a good description as to what each pin does, so follow that.

1 Do you have the 10MHz reference on pin 5?
2 Can you see the input frequency from front panel on Pin 22?
3 Are you seeing the gate pulses on pin 19?
4 Is there a 1MHz output on pin 6?
5 Is Vref OK?
6 Is there 10kHz on pin 7?
7 Is there any display data being clocked out of pins15, 16, 17 and 18?
8 do inputs 8, 9, 10 change state with frequency switch?

After making these initial tests you can then decide where the fault(s) lie, and start a logical diagnosis.

Also what happens when you select lamp test? All digits should show 8. Does just one show 8, or still just dots. Decimal points are displayed seperate to the digits.

After checking report back what you found, and should be able to guide you to next stage.
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Old 20th Nov 2020, 7:39 pm   #10
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Default Re: Racal Dana 9916 Frequency Counter.

If the display is blank, apart from a couple of dots, then the place to home in on, is the display itself.

To make sure this isn't just "finger trouble" from not setting controls right, try going through the "Segment, decimal point and self check" test, section 5.15 on page 5.6 of the manual. And then tell us what you get?

If that test fails, then if you have not checked the +5V rail that is on the emitters of Q18 to Q25, then please do so now. Its probably OK, because you say there are these couple of dots that are lighting up, suggesting some voltage is getting to the 7-segment displays.

After that, I think you are going to need a scope to get into the nitty-gritty of the multiplexed display system.


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Old 20th Nov 2020, 7:44 pm   #11
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Default Re: Racal Dana 9916 Frequency Counter.

Tantalum caps. We have the late Les Lawry-Johns to thank for the "Blue" tants concerns. Reds, blues, buffs, all the same. I have had very few failed ones over the years (plenty in the Grundig TVs I worked on for years, NEVER a failure).
However, they seem to fail quite quickly if used in decoupling voltage rails, but ONLY if their voltage rating is too close to the voltage in the circuit.
Usually if one has gone short in such circumstances, there is a good chance its head is visibly blown off, or a black burn mark somewhere.
Les.
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Old 20th Nov 2020, 9:18 pm   #12
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Default Re: Racal Dana 9916 Frequency Counter.

The tantalum caps might be suspect, but my view is that they are unlikely to be the problem. First reason is that many of them as just signal couplers - if they fail, they will just mess up the bias in that bit of circuit, and at the moment I can't see any affecting the display.

Then the few tants that are actually across supply rails are unlikely to be a problem, because a) there are no burn marks according to the OP, and b) the OP says the voltages according to the manual are OK. So it would appear no tants are currently taking out supply rails.

One possible clue about the display being blank is in clause 4.43, which says "The display is also blanked out in the event of the failure of the 10kHz synchronizing signal." Its failure ultimately puts 1 IC1 pin 4 to zero. You might start there, and see if that pin is in fact a zero - then work backwards if it is.


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Old 20th Nov 2020, 10:05 pm   #13
g0hze snowy
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Default Re: Racal Dana 9916 Frequency Counter.

Well gents, that's a lot of info for me to assimilate, i will make notes and make a start in the morning, have done some of the suggestions but will do them all again, this time making notes, my grey cell is not what it was ha ha. Thanks ponce again to all.
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Old 21st Nov 2020, 1:52 pm   #14
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Default Re: Racal Dana 9916 Frequency Counter.

Good afternoon Gents, ok this is what i have found after printing off all your advice, i will address it to all rather than individuals, hope you dont mind.
IC1 pin 4 has 3.124v on it, so that i think is ok.
IC4 pin 5 has 10mhz, pin 22 has front panel input on it, gate pulses on pin19, 1mHz output on pin 6, 10kHz on pin 7, pins 15 - 18 showing data pulses, inputs on pins 8 - 10 change with position of front panel switch, nothing happens when i select lamp test, only LED,s lit are the decimal point ( alters in conjunction with front switch), top led showing mHz. so thats what i have found today. over to you gents,TQ
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Old 21st Nov 2020, 2:08 pm   #15
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Default Re: Racal Dana 9916 Frequency Counter.

IC1 pin 4 should be one of two logic levels - nominally 0V or nominally +5V. And I would expect from what the manual said it to be DC at one of those two voltages. So 3.124V looks decidedly suspect to me. Is there some waveform on that pin that your DVM is averaging to 3.124V?


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Old 21st Nov 2020, 2:21 pm   #16
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Default Re: Racal Dana 9916 Frequency Counter.

Btw.
I have seen many english people write mHz for Megahertz
and I think it is irritating.

Normaly mHz is Millihertz and MHz is Megahertz.

...my only 5c comment

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Old 21st Nov 2020, 4:02 pm   #17
g0hze snowy
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Default Re: Racal Dana 9916 Frequency Counter.

Richard, i was using my Fluke 73 for measurements. another check is in order i think.
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Old 21st Nov 2020, 4:03 pm   #18
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Default Re: Racal Dana 9916 Frequency Counter.

Quote:
Originally Posted by g0hze snowy View Post
IC4 pin 5 has 10mhz, pin 22 has front panel input on it, gate pulses on pin19, 1mHz output on pin 6, 10kHz on pin 7, pins 15 - 18 showing data pulses,.......... nothing happens when i select lamp test, only LED,s lit are the decimal point
As you have multiplexed signals on pins 15, 16, 17 and 18 then fault is with display de-multiplexor, or displays themselfs.

Probe IC1 on display section.

Can you see the multiplexed pulses on pins 1, 2, 6 and 7?

What is the status of the Blanking input pin 4?

Lamp test (pin 3) should be low but change when you select lamp test function.

Not shown in the schematic is the VCC and GND pins (16 and 8 respectively) so check for supply volts on them also.

Assuming these are all OK, then the next place to look is IC 23 and IC28.

IC23 should have pulses on pin 14, and A,B,C, and D (pins 1, 2, 8 and 9) binary counting when pin 2 and 3 are 0V (connected to the reset line).
IC 28 takes this binary count, and should output the corresponding line to select the digit to display.

It looks like the display supply is seperated from the dp supply, so make sure the 5V "A" supply is correct on the switching transistors. (a pair of Tants on it!).

Wouldnt concern myself with the input section untill the display section is working correctly. There may be nothing wrong with it.....

My philosophy is fix what you can know is wrong.
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Old 22nd Nov 2020, 9:44 am   #19
g0hze snowy
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Default Re: Racal Dana 9916 Frequency Counter.

Good Morning gents
Thanks for all your help Peter, all checked out ok.
using a magnifying glass and head piece i found a red spot on the PCB about 1" from the front panel connector, under red spot was a very thin track that was broken and the broken end was stuck on the next track, repaired that but that wasn't the main fault, its looking more like IC1 on the display board a 74247, so now today's work is to remove the display board and do some checks on and around the IC.

Stand by for an update.
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Old 25th Nov 2020, 2:10 pm   #20
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Default Re: Racal Dana 9916 Frequency Counter.

Good Afternoon Gents, right got my head in gear and had another go at the 9916 today. results as follows:
TRH01uk, i have 4.98v to each of the driver trannies, your suggestion was to check IC1 pin 4, its at 3.314v, your other suggestion checks out ok q1-Q5 appear ok.
MotorBikeLes, no sign of any burn marks or tants without hats.
Peterg0rsq. the checks you asked me to do have the following results:
Q1 to 4 and 6 yes
Q5 ? is that IC4 pin 11?
Q7 there is a 'sort' of waveform but it looks like mush compared to all the others i have checked out.
Q8 yes
The only front panel LED,s that are on are mHz, gate/charging and the decimal point which changes with the operation of the front panel switch.
now sorry but i forgot to include the author of the next set of observations but the results are as follows:
he suggests i probe IC1, multiplexed pulses on pins 1,2,6,7.
status of Blanking input pin 4, no signal but showing 12mv saw tooth type wave form
lamp test pin 3, changes state when test button pressed h/l.
Pin 8 shows 4.26v with respect to chassis, pin 16 show 4.18v with respect to 5v line.
now onto IC,s 23 and 28, i have pulses on pin 14 and a,b,c,d pins 1,2,8,9, pins 2 and 3 are at 0.106v
Ic28 is the same as IC23
Tants changed on switching trannies but no change, so they were probably ok.
finally just a list of the voltages on the test points:
TP2 23.45V ( SHOULD BE 24V)
TP3 4.91V
TP4 5.02V
TP5 GROUND
TP6 -4.2V
Here endeth todays observations
over to you gents..
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