Toy arrived from China...

majoco

Member
Joined
Dec 25, 2008
Messages
3,652
Location
New Zealand
I didn't know which forum to post this in, but it has to do with an HF receiver, so I guessed this one....

I'm making a replacement for my JRC NDH 515 receiver memory unit. It was dodgy when I first bought it in 2007 suffering from leaky batteries - the acid has been slowly eating the plated-through holes and the tracks on the PC board until it finally gave up. Studying the schematic showed that it was quite a simple method of controlling each digit with binary coded decimal numbers 0 to 9 for each of the 6 digits of the frequency, 0.000,0 khz to 29.999,9 MHz. A row of 6 thumbwheel switches produces the BCD data and 6 LS TTL 74126 line drivers isolate the receiver from the memory unit when not required. Easy - well, no. The receiver reads the incoming data, but if the data is invalid, the display is blank. My display was blank - every one. I found that every output line was high all the time so the receiver locked up. Testing the IC's in circuit showed the same but I couldn't test the IC's out of circuit. I ordered a "TTL/CMOS IC tester" from China before the lockdown and it has finally arrived. I have now tested all the IC's and they are all good. Here's come pics of the tester - it was only NZ$36 including postage, say US$20.

At switch on....
DSCF1442 sml.jpg

Press the "UP" button to select the IC number...

DSCF1443 sml.jpg

Put the IC into the socket but forget to close the lever, press "TEST....

DSCF1445 sml.jpg

Close the lever and test again...

DSCF1444 sml.jpg

...and all is good! Here's a pic of the "work in progress". The "Channel" and the "Memory" buttons are now just dummies. The "Preset/Manual" button disconnects the receiver from the memory unit for 'normal' operation. Now lets see if I can make it work!

DSCF1446 sml.jpg
 

majoco

Member
Joined
Dec 25, 2008
Messages
3,652
Location
New Zealand
... and I believe I have found the problem with each line of the output BCD being high - when the input of the 74126 gate is not held high, then it is grounded via a 47k resistor - the input sources about 20uA when low, so 20uA x 47k = 0.9volts which enough to make the output go high! Now to replace 24 47k resistors with 3.3k !
 
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