Leader LTC 907 Tester Repair

[chriswood1900]

I’ve always had a soft spot for the Leader range of Japanese test equipment. However, quirky it may be, at the recent Royal Wootton Bassett sale in December, I spotted a leader LTC 907 Multifunction tester for sale on one of the stalls and bought it.
This unit is designed to function as a multipurpose tester and incorporates, a transistor tester an RF. Signal generator covering AM/FM in 4 bands, a 1Khz audio signal generator,. a signal tracer, and a function generator for television testing.
The LTC-907 is far more complicated than might be first indicated by its appearance. The transistor tester is operated by around 18 CD range TTL chips. The function generator uses a couple of TTL chips 4047 oscillator and 4066 frequency change. The RF generator uses independent Transistors one for each range switched by turning the power on and off to each of the oscillators and the tracer uses a 741 op amp.
The initial inspection indicated that it was dead, power was not reaching the circuit boards, and this was controlled by a multipole on off switch.
On checking this switch with the multi-meter whilst there was power to the switch. There was no output, switch cleaner made little difference so I decided to remove it and dismantle. Once it was disassembled. I could see that the silver plated contacts were so blackened. they were making a good insulator and stopping any power getting through Cleaning the contacts with silver polish on cotton bud followed by IPA, removed the corrosion and reassembly allowed it to work?
On an initial test, only the transistor tester seemed to function. There was no output from the RF stages, and the other low frequency stages, only gave out a single frequency around 2 kHz. The tracer was dead
I re-seated the cable to the RF generator it was now working, but the function generator. was outputting a single frequency around 2khz regardless of setting.
My initial suspect was the 4066 which was doing the frequency changing and I removed this and put a socket in and plugged a new chip in. This made no difference. Next I suspected the 4047 as faulty so I removed this and fitted another socket and, tried a new 4047 again no improvement. next suspects were the four capacitors which were switched in and out by the 4066. I remove these and tested them. they were also okay, this left with a bit of head scratching and I really struggled to understand where the fault might be with all the capacitors removed the 4066 removed again. The 4047 was oscillating had a wide range of frequencies, but none were controlled.
Time to leave things and put some thought in, I was starting to suspect, maybe a board fault or a resistor so with all the caps out and the chips out of the sockets I took to measuring the resistors then I discovered that R123 which is shown in the circuit diagram of 68K was in fact, 680 K has been in since the unit was manufactured and I can only conclude, it could never have worked properly. On replacing that resistor all the functions started to work properly and there was proper control over each frequency and adjustment. Finally the circuit tracer which turned out to be a dead 741 op amp.
With all the basic functions, now working properly, it was time to clean the unit up and check the calibration. Everything was now working properly and I’ve added another useful unit to my bench. I wait to see how useful it turns out to be. It will be a perfect unit to pass to my Grandson to get him started on the hobby.
The manual is available on the BAMA site here https://bama.edebris.com/manuals/leader/

I enclose two .pdfs to show the general functional layout and the RF and Function generator circuits for info.

[Julesomega]

Well done! How disappointing it must have been for the first owner when they couldn't understand whether they weren't using it correctly.

[Phil G4SPZ]

An excellent rescue and a nice write-up. It’s really satisfying when you find a faulty component that has been there since day one!

[newlite4]

What a useful instrument, wish I had spotted it, well done for fixing it. I shall have to keep an eye out for one of those at future events. Good write up, enjoy using it.
Neil

[Cruisin Marine]

Nice looker for sure.

[chriswood1900]

Thanks for all the kind words it is certainly very compact and neat. I was repairing an instrumentation amplifier a couple of days ago and not wanting to remove a hard to reach transistor I thought I would try the insitu test function of the LTC 907 and found it worked very well and correctly identified the transistor type and proved it was working. So if nothing else it may be good for that!

[toprepairman]

Big fan of Leader kit and have quite a few different items of theirs.

[chriswood1900]

Originally Posted by toprepairman

Big fan of Leader kit and have quite a few different items of theirs.

I agree I have many items of their test gear from the 80s though to the early 2000s, sadly they seem to have moved on from test gear for repairs to high end equipment or broadcast etc, I suppose that is where the market is now!