How to track down flawed sine wave in Leader LAG-125 oscillator

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How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Wed Jun 12, 2013 7:08 pm

My first few pieces of test bench gear included a vintage Heathkit IO-4510 oscilloscope and a newish Leader LAG-125 low distortion audio generator. I've got some vintage audio gear that I want to troubleshoot and refresh, so I knew I'd need a scope and an oscillator.

The scope isn't calibrated yet, but it seems to work OK. I've downloaded a tiny, cheap signal generator app for my iPhone and that produces a reasonable, low voltage, fairly noisy sine wave. It's smooth enough and properly shaped and shows fine on the Heathkit scope.

The Leader oscillator (solid state), on the other hand, produces a nice, strong signal, but the sine wave is badly flawed nearing and at the peak. (see attached image) This flawed shape is consistent at various frequencies, various voltages, on either of my two probes, and in either of the two channels on the scope.

So, I've opened up the oscillator and started probing inside. That's where I need some help:

1. The power supply is super simple: dual secondaries off the transformer (0V-10V and +23V-0V-23V-) go each into their own full wave diode rectifier network and then a few caps before heading to various other boards. I measured the outputs, they're all clean DC, and then replaced all the diodes and caps anyway, just because, and now the outputs are the same and I'm confident the power supply is doing it's job. The silk-screened values on the power supply BOARD do not match the measured output levels, but they are in line with the schematics. For example the board has "20+", "20-", and "5+" marked at the outputs, but I read +30VDC, -30VDC and +12VDC from each, respectively. I'm chalking that up to old board stock and new, correct, schematics. Am I wrong to make that assumption? The unit works well enough otherwise that I'm doubt over voltages like this would still yield proper operation.

2. The unit has huge variable capacitors (like the tuner of a radio) and resistors on switches to chose the frequency of the signal output. All of this is in a shielded box. That seems right and unlikely to flaw just a part of the sine wave.

3. There are two remaining major components: the oscillator board and the mainboard. I'm guessing the mainboard just amplifies the sine or sqaure wave output as per the attenuator selector knob.

4. The oscillator board has a fair number of caps, resistors, transistors and a LM741 op amp. I've removed the shielded cover for the oscillator board compartment, attached my oscilloscope probe, and tried to read the signal coming out of the op amp, going into the opamp, leaving the board, etc.

The signal leaving the board is clearly flawed, just like the output on the front of the unit.

http://i79.servimg.com/u/f79/18/35/87/05/img_0112.jpg

The signal leaving the output (pin 6) of the opamp is not a proper sine wave. It's also not a sawtooth wave. It's like a cross of the two: a smoothed sawtooth with a quarter-wave trough at the top of the peak. So, I'm thinking the opamp isn't the sinewave generator/converter OR the opamp is part of the problem.

Backing up one component and looking at the signal going into the opamp (pin 2), it's kinda hard to get a reading. With my probe set to X1 and my 15MHz scope set to the smallest setting (1mv/cm), I can get a smooth-ish sine-sawtooth kinda wave like above, but it is delicate and weak and I can see that same wave even when I'm not dead on the #2 pin. It's almost like the oscillator is braodcasting that signal enough for my probe "antenna" to pick it up without touching. I *must* be doing something wrong to see it this way.



So, I've halted my back-tracking through the signal circuit, hoping someone intelligent and experienced can point out what I'm doing wrong. I'm happy to desolder and replace as much of the circuit as I need to, but I'd prefer to investigate, make determinations, and repair/replace based on findings rather than just brute force. I can always buy a better oscillator, but I want to learn how this one works and how to diagnose and repair it.

Any ideas? Need more info or photos?

Thanks,
Doug
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by Ron Pond on Sat Jun 15, 2013 6:27 am

Any chance of posting a schematic?

Ron.
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Sat Jun 15, 2013 10:31 am

Sure! Here is the manual with schematics at the end.

Leader LAG-125 manual

I'm focused on the audio oscillator board at the moment. I'd love to get some advice on how to trace the signal (seems too low to measure on 2mV scope) back through the circuit to find where it is becoming flawed.

Failing that I'm ready to replace all the electrolytic caps, all the diodes and the LM741 opamp. THose supplies didn't cost much, but desoldering these boards tends to unseat the traces, even when I'm careful....
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by Ron Pond on Sun Jun 16, 2013 2:36 am

If you have or can get hold an ESR meter, you can read the electrolytics in-circuit.....takes only a few minutes to find any dud caps.
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by Polaraligned on Thu Jun 20, 2013 8:42 am

The electrolytic caps are a primary suspect.  I would check them first.

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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Thu Jun 20, 2013 9:21 am

UPDATE: I changed out all the electrolytic caps, no change to the flawed sine wave. I replaced the diodes, no change. Replaced the opamp, no change. So, I've got the dozen or so transistors on order and I may as well replace the resistors while I'm at it. The only component left unchanged after all that will be the oddball LDR: a CdS photocell in a sealed tube with a lamp. If I have to replace that, too, I'll have to construct my own. The lamp gets 4.493 VDC, roughly, and the resistance varies from 300 R to 1200 R.

Finding equivalents for some of these old transistors and diodes ain't easy. the 2SK40D were especially hard, even just to find the original operating characteristics.
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Thu Jun 20, 2013 9:23 am

The ESR of the electrolytic caps ended up being well within spec; much lower than I would have expected, actually. The actual capacitance varied more than I would have expected, but nothing suspicious. I don't know how to test either of those in-circuit, though. Anyone here with pointers on that?
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by Polaraligned on Thu Jun 20, 2013 10:21 am

A pic of the sinewave may be helpful.  If it is just near the peaks, you may be getting clipping.

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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Thu Jun 20, 2013 10:32 am

When I first tried out the unit, turning the variable output level to its highest setting would flatten the bottom and (somewhat) the top of each wave. I've backed off that max setting and I'm testing a 0 dB attenuation (1V P-P) and 10 dB attenuation. I don't see any flatness (which would be clipping, right?

The weird drop out really looks to me like a failing component, as several of you have suggested.

The latest image of the signal:

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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by 75X11 on Thu Jun 20, 2013 10:47 pm

That is a truly odd waveform.  I would be looking at the IC components that could form an inverted square wave in that manner on your output signal. check their pins and see if there is any corresponding signal they would be generating to that notch on your output waveform.  Also have you tried a load on the output to see if there is any change?
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by dlucy67 on Fri Jun 21, 2013 12:13 am

There are two JFETs and a dozen BJT transistors in the circuit that I have not replaced yet. I've looked at the input and output of every one and I can see flaws. The flaws are different in the oscillator circuit from the actual overall output (the above image). Here's what I see inside the circuit itself:



That image is two channels on my scope, each channel connected to each side of... one of the FETs, I think.

I'm guessing the flaw is caused somewhere and it's bouncing around as feedback to everywhere prior in the circuit. Does that make any sense?

The only IC in the circuit, the LM741 opamp, has been replaced with no change in output.
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by 75X11 on Fri Jun 21, 2013 1:34 am

I wonder if providing a load for the output of the generator may make a difference in it's operation.  Paragraph 2.8 in the manual discusses it.
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Re: How to track down flawed sine wave in Leader LAG-125 oscillator

Post by Polaraligned on Fri Jun 21, 2013 7:32 am

Looks like you are stuck replacing the semiconductors in the oscillator portion until this straightens itself out.

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