Trouble shoot Leader 9 Hz to 1 MHz Audio Signal Generator Model LAG-120A

[oneplustwo]

Hi folks,

I picked up a Leader LAG-120A Signal Generator on ebay and it seems to be more or less kaput. When I hook it up to my scope, I get a noisy looking sine wave that doesn't change amplitude, frequency, or waveform upon pressing any button or turning any knob.

I've already reached out to the seller, but thought I would also see if anyone here has any thoughts on whether this is something that you've seen before that might have an easy fix or if this is indicative of a big problem.

I'm tempted to negotiate the price down to practically nothing and then take a crack at fixing it. (This is diyaudio after all.) But if chances are that it's totally kaput, I'd go for a complete refund.

Looking forward to your thoughts.

Thanks!

 

[EssB]

I've no experience of that unit but the schematic looks simple enough to fix up.
Manual in this thread http://www.diyaudio.com/forums/equipment-tools/222306-leader-lag-120b-teardown-testing.html

 

[EssB]

.. that thread is for the 120B, here's the 120A schematic, doesn't look all that different

 

[oneplustwo]

Thanks for the link. I'll see if I can get the guy to give it for me at "parts only" cost and then add this to my project list!

 

[oneplustwo]

Well, I opened it up and didn't see anything obviously wrong. No burned resistors or bulging caps. All looked as it should. Build quality seemed quite good actually. Any guidance on where to start for trouble shooting?

 

[Elvee]

Any guidance on where to start for trouble shooting?

Power supply in general, then Elytics elsewhere.

 

[RACB]

Similar situation here:

I have a LAG-120A which is giving me some trouble.

As I received it from a frined, it was producing a distorted waveform wich stayed always around 4-8 Hz no matter the settings of the front panel.

I could not find the 120A service manual, but the 120B service manual mentions that if the output signal is distorter, the TH101 Thermistor should be checked for continuity.

I did exactly that and I had to bend it slightly. As soon as I moved the TH101 my LAG-120A started to work. But on the higher frequency settings the amplitude of the signal is not always stable....

I have tried to find a supplier or datasheet of the TH101 thermistor without any success. All it says on the schematic is " TH101 STT-300H"

Google is no help. The best I found is a chinese forum where someone asks the exact same question:

Where is it possible to get a STT-300H or a modern substitute. Or a least its specifications....

Thanks for reading and Best Regards,

Rafael

 

[BSST]

Hi Rafael,

I wasn't able to find a service manual--- only schematic, nor anything about the thermistor. If you've found any actual service info, would you point me in the right direction?

But the circuit is a classic Wein bridge oscillator. At stable oscillation, the amplifier stage must generate a gain of +3, implying that at equilibrium the thermistor's resistance must be 2x R119, that is 2x 680 ohms ~1360 ohms. The thermistor must have a negative temperature coefficient so that its resistance falls with rising temperature as the thermistor heats with increasing sine wave amplitude. I've pasted a search link at Digikey. The list is large.

I've not sorted out the nominal output voltage at TP102, but I surmise it might be 6 V rms, open circuit. It seems the nominal amplitude is determined solely by the thermistor selection and R119.

Temperature Sensors - NTC Thermistors | Sensors, Transducers | DigiKey

If you want to experiment, you could try an incandescent lamp instead of the thermistor. See figure 2 of this link: Application Report SNOA665C–October 1999–Revised April 2

http://www.ti.com/lit/an/snoa665c/s... WAVE GENERATOR&tisearch=Search-EN-everything

The lamp replaces R119 and the 430 ohm resistor relates the thermistor. Trim the 430 ohm to tweak amplitude.

Good luck!

 

[BSST]

Hi again Rafael,

I did a bit more exploration and found this link that was informative.

https://www.electronics-tutorials.ws/io/thermistors.html

I suggest as experimental parts the following Cantherm NTC themistors available from Digiikey: MF52A2103J3470 MF52A202J3470 MF52A682J3470

e.g. MF52A2103J3470 Cantherm | Sensors, Transducers | DigiKey and etcetera.

Instinct tells me to keep the leads long to maximize thermal resistance to the circuit board.

Good luck.

 

[PRR]

...I've not sorted out the nominal output voltage at TP102

The output stage is unity-gain (0.9 or so). What is the box rated at?

The Heath also ran 40V supply and would do rated 10V. THD was a bit lower at lower level. Depending on era and market, Leader may have used 6V, though it is odd (not decimal, not a multiple of 0.775V).

Agree with your suggested AGC parts. Me, here, I would go find the last 120V 7Watt incandescent nite-light in the store. H-P's 200 boxes used 230V 7W xmas lamps. Lamps get pretty screwy below 20Hz, though I used to run the Heath sub-Hz and accepted the mostly-round wave for deep response testing. (Cap-tuned oscillators won't go that low without gallons of variable cap.)

 

[1audio]

General Radio used a similar thermistor in their microphone calibrators. Its quite obsolete today. I found a vintage Fenwall catalog reference that may help find suitable ones if they are still made. These may be close: NTC Type BR Glass Encapsulated Bead Thermistors - Amphenol Advanced Sensors | Mouser However quite expensive.

 

[RACB]

LAG-120A service manual

Thanks everyone for their answers and help.
I'll try to get back to this project in Christmas Holidays.

However, I managed to get a copy of the service manual from Leader.

I leave it in my server for anyone interested.

http://ftp.cesinel.com/web_data/manuals/LAG-120A SERVICE MANUAL.pdf

Best Regards,

Rafael