frequency counter repair help

[duldog]

i recently acquired an intermittently working LEADER LDC 824 frequency counter in a deal that is hard to resist (reads from 10hz to 520mhz)

sometimes it works (reads frequency) after 20mins warmup, sometimes after an hour other times it wouldn't work even after 2hours warmup time. Also at times it works even when the set is cold then suddenly stops working as it warms. it simply displays 0.00000, all the buttons seem to respond except that it doesn't read frequency

but when it happens to work, it works perfectly and reads as accurately as my other counter (a radioshack portable counter, signal source is a vhf handheld radio)

i can't find a service manual or even a user manual, the closest i can get is a user manual for LDC-823 (250 mhz) but it seems to be different

first course of action is to look for cold solders, and discovered that the unit has been resoldered recently and i can't find any loose/cold solders and no obvious physical defects

next, checked for supply voltages, found them to be spot on (5v, -12v, -19v, -38v, etc)

then i checked for hi-esr caps using a homebrew esr meter, found none except for a 33uf 16v tantalum cap which read 2ohms while the other same value cap read .5 ohm

internal oscillator reads exactly 10.0000 mhz on my other counter, probed it on the bnc post at the back of the unit

thinking that this might be a thermal problem, i used a hot air desoldering station to heat up one IC/transistor at a time to provoke the unit but none seems to react with heat

i am not willing to give up yet, even with lacking diagrams and limited test equipment and not so strong foundation on digital circuits and hard to find parts, i humbly seek for assistance, help me out what and where to look for, will do the hardwork and learn at the same time. a service manual would be greatly appreciated. if it can't be repaired i just want to know what went wrong, thanks!

 

[betwixt]

You really need an oscilloscope to find this kind of fault. The fact that the display is working is a big bonus, it points to a break in the signal path that is preventing the input reaching the counter stages although it isn't conclusive that the display simply isn't being updated with valid counter data.

Have you checked the non-electrical parts like the input socket. If it's a BNC type, quite often you find that someone has forced the wrong impedance plug into the socket and splayed the center hole too wide so it no longer makes contact with the plug.

Brian.

 

[mister_rf]

I have found this:

 

[duldog]

thanks for the replies,
@betwixt, already checked that my friend the socket is good so with its connecting wires to the board, and sadly i don't have access to an oscilloscope which makes it difficult for me to repair
@mister_rf, i've seen that too my friend, only the display section is the same with my unit, input section is different

 

[mister_rf]

Can you clarify if the LDC824 use also TC5032 six digit decade counter?

 

[duldog]

yes my friend, my unit uses TC5032

found on the net, exactly the same with my unit
**broken link removed**

- - - Updated - - -

yes my friend, my unit uses TC5032

found on the net, exactly the same with my unit
http://bbs.38hot.net/read.php?tid=15924

 

[BradtheRad]

The fact that it was resoldered recently suggests it was giving problems that were hard to track down, and hard to troubleshoot. Likely the same problem you're having.

Look for cracked traces on the board. You'll need a magnifying glass. Bend the board a little to see if you can make the problem recur (if it's working), or go away (if it isn't).

Look for cracks in solder joints around switches, connectors etc.

Check for continuity through switches, etc.

Can you inject maybe a 1Hz signal at various points? While watching a voltmeter.

then i checked for hi-esr caps using a homebrew esr meter, found none except for a 33uf 16v tantalum cap which read 2ohms while the other same value cap read .5 ohm

You probably know that 2 ohms may be high enough to prevent proper operation in certain situations. I don't know specifically what it would do in your circuitry, but that's what I've seen technicians report. However if it were high ESR causing your problem, then it would likely occur at all times, whereas it is intermittent right now.

 

[betwixt]

I doubt the TC5032 is available now but if it turns out to be faulty, it should be possible to make a substitute using a PIC microcontroller that does the same job then wire it into the original chip holes on the board. It's only a decade counter with multiplexed outputs.

Brian,

 

[mister_rf]

You may use the schematic to check the counter/display section. First you need to check the ‘Master Reset’ pin 27. Normal value= LOW level. (The reset input active for ‘High level’).
Be prepared to cut some PCB traces and to inject a test signal (1-10Hz). For a very simple signal detector use the spare buffer + LED.
Also try to do some tests by disconnecting the counter input pin and apply a 10-1000Hz signal to the counter input.

 

[duldog]

@bradtherad, yes mate since i was not sure about the esr of the cap, i just replaced with another one but sad to say it has not improved the unit, tnx!
@betwixt, thanks for the tip!
@mister_rf, i greatly appreciate your efforts my friend! i will do as advised and will report my findings, tnx!
btw, do you have the service manual? if so, can you pls share with me and send to jrea79@yahoo.com, tnx!

 

[mister_rf]

I don’t have the LDC824 schematics, but in my opinion only the input amplifier/prescaler are different from one model to the other. Everything else are similar to, so now you have all the needed information to start troubleshooting that unit. ;-)

 

[duldog]

@mister_rf, thanks mate! i really appreciate the effort, i will tear down my unit again tomorrow and report back,,, man you're a genius! how i wish i had that skill in understanding circuits, tnx!

 

[duldog]

FINALLY FIXED IT!
I initially suspected the input/prescaler board but even after almost scorching the board with my hot air station I cannot provoke it to work, I turned my attention to the display board
It didn’t take long before I had a specific area to concentrate, its around an HD74S112P ic. It had not been recently resoldered, no suspicious connections but its full of flux residue, had it resoldered all connections around the area just to be sure and cleaned off the excess flux residue. Much to my delight, this seems to have cured the troubles of this counter. I had the unit warmup/cooled/then warmed up again but it just didn’t quit working
Lesson learned? Persistence pays off, don’t be afraid to ask
THANKS for all the help!

 

[BradtheRad]

Very positive.

I have not heard of flux residue causing broken connections. I went for years figuring I didn't need to clean it off my boards. I usually left them looking dirtied up (but I did clean off my latest few projects).

Or it might be possible for a solder bridge to be camouflaged by flux residue. It could make and break contact with temperature fluctuations.

 

[duldog]

me too, i don't think the residue caused the trouble, it may have masked something amiss which i did not noticed at first, i just resoldered everything in that area and cleaned off , it just that they are too much residue around the area