LEADER LFG-1300S Function Generator - DEAD

[timetec]

Hi all

Just wondered whether anyone had come across or played with a LEADER LFG-1300S 0-2Mhz function generator before, or perhaps repaired one ?

I have a 1300S, reliable as you like and a pleasure to use, but switched it on the other day and there was absoloutely no output whatsoever above 100Hz......

Having said that, there is some 'intermittent' output below 100Hz, but it does not tally with the scale readings and is way out-of-range.

It powers up OK, the O/P fuse is fine and all the power supply voltages are spot-on.

A 'stab in the dark' I know, but I have read lots of posts relating to the use and maintenance of Signal Generators here

Cheers, Richard

[Enzo]

I don't know the unit, but it is electronic, so can;t hurt to go inside and explore its power supply.

[audiohead]

Hi there:

Clean the range and function switches contacts with pro gold.

Art

[timetec]

Thanks very much the tips, audiohead & Enzo

The contacts on the range / function switches are all OK, but I have cleaned them with Servisol, just to make absolutely sure.

There are 3 x LM741 op-amps which make up the 'master oscillator' - these were removed and tested (all OK) - but I've replaced them with new ones anyway.

The only other IC I can pin-point as being a likely candidate, is a NE521 comparator, which is now oblesete and no longer manufactured..

Cheers, Richard

[toprepairman]

Sadly we can't get Pro Gold here. Unfortunately the Servisol is next to useless but there is little option. Can you still get 'Blue Stuff' in the States?

[audiohead]

Hi Rich:

Do you have the service manual/schematics?

I have the stuff, Let me know if you need them.

Art

[timetec]

Hello Art,

Yes I do - not a brilliant copy, but readable and available here :

LEADER LFG-1300S Service Manual

The troubleshooting / faultfinding guide starts on page 14, with the block diagram on page 22 and master generator schematic on page 23.

Perhaps you might just be able to spot something obvious, I've overlooked...

Very many thanks, Richard

[cliffyk]

I have a high-quality scan of an original LFG-1300S manual. The hard-copy manual cost me almost $35 so I am not releasing it to the multitudes--however send me a PM with your email address and I will send it to you.

The main pages were scanned at 300 dpi in 8-bit gray scale, schematics and block diagrams at 600 dpi.

[ChrisA]

Originally Posted by timetec

There are 3 x LM741 op-amps which make up the 'master oscillator' - these were removed and tested (all OK) - but I've replaced them with new ones anyway.

More likely a capacitor has failed than a broken lm741. Check for open, shorts or leaks. If the LM741 are in sockets maybe thing about an upgrade, you can improve performance by an order of magnitude for like $2

The good news is that (1) a quality USB audio interface and some software can make a pretty decent audio function generator if the computer audio can run at 96K and (2) if that is not good enough, high end HP gear sells on eBay for <$100. My old generator used to make little spikes on a spectrum display. My "new to me" 40 year old HP generator shows a thin line right on the fundamental and not much else.

[timetec]

Hello everyone - good news...

I'm pleased to say that I have finally managed to repair the function generator

It turned out to be a leaky dual-fet (a IMF3898 ), actually partially short-circuit S-D, the upper of the 2 FET's in the circuit diagram, reading around 3200 ohms.

Although an IMF3898 is shown in the parts list for Q105 - a upa71A was originally fitted.

I initially replaced it with a couple of 2N3819's - the generator sprang to life, but would only operate up to approx 800 kHz max.

After routing through my spares, I found what looks like a mil-spec device, a WD179 device manufactured in 1972 - date code 7235. I can find no information on it whatsoever, but it tests as a dual N-channel FET using an ATLAS DCA55 semiconductor analyser.

The generator works fine with it fitted up to 2Mhz, albiet at a very high temperature - ouch....

I need a TO18 heatsink if anyone has one - I just can't seem to get one here in the UK and Digikey want £12.50 for shipping one from the USA, which I simply can't justify, for something that weighs no more than 10grams !!

Below are a few photos : I replaced 4 IC's in total : 3 x 741's & a NE521 (SN75107AN equivilent), not that any of them needed replacing - the 4 timing caps were fine, these and the IC's are marked with green dots in the photos. The dual-fet (the culprit) is outlined in red.

Many thanks for everyone's help in getting this fixed - much apreciated guys

Richard