I found a very old low cost function generator (Sabtronics 5020A) NOT working:


The power supply voltage comes to the pcb because with the tester I measured 33V AC (it says on the pcb itself that it uses AC) when it was powered, but any wave and any output (TTL OUT, LOW OUT, HIGH OUT ..) does not show up on my oscilloscope (a classic tektronix) regardless of amplitude, frequency, waveform, etc.. set on the generator.

Here is a photos of the traces:



I tried cleaning only a couple of traces using isopropyl alcohol and the result was this: (so much dirt came off)

Why only a couple and not the whole pcb?
Because I am asking you if it can be the solution to clean them with alcohol or if you think the problem is something else.
Or if, seeing the state of these traces, you tell me that they are "unrecoverable" and it is not a matter of dirt tracks, but worn tracks.
Should I even desolder them all redo the connections?
Any else suggestions?
(I have never repaired anything so I am unable to tell whether these traces are worn or not)

Thanks!

 

Without a circuit diagram, repair is likely to be difficult
That looks like a dry joint below the resistor in the second PCB photo – re-touch it with solder.

On such an old piece of kit, it not working could be due to oxidised contacts in the many push switches and/or electrolytic capacitors having reached the end of their life.

 

I tried cleaning only a couple of traces using isopropyl alcohol and the result was this: (so much dirt came off)

If you cleaned near what appears to be a transistor that was replaced, you're removing flux residue that the repairer neglected to remove.

I am asking you if it can be the solution to clean them with alcohol or if you think the problem is something else

I suspect the problem is something else.

Should I even desolder them all redo the connections?

This would be pointless.

Any else suggestions?

Look for a schematic and verify that the power supplies are functioning properly.

 

A lot of those things used the ICL8038 IC function generator. If yours does, poke around the chip and go from there. Datasheet is attached. See if you can find some old Appnotes too. It's possible that your circuit is derived directly from a 'typical application' which may be a good guide.

As mentioned above, the switches and pots may need a shot of contact cleaner. My favorite is MG Chemicals 401B Nu-Trol Control Cleaner (Amazon). Wear gloves and goggles.

Other than that the standard stuff applies, electrolytic caps are suspect always. The repaired board and transistor is too.

Keep in mind that for about the cost of the contact cleaner and shipping you probably can buy a cheap DDS function gen (Direct Digital Syntheses) from Amazon but it is fun to try to fix things.

Good luck!

 

Can you take pictures of the component side , especially the chips.

 

The arrow is pointing at what appears to be a not-very-good attempt at repairing a lifted pad. The copper that should be inside that circle is gone, and a small wire runs from the component lead to the remaining trace.

​

I would not be surprised if that connection was flaky or even open. It should be checked for continuity and possibly reflowed with a bit more solder. Be careful, though—it would be very easy to dislodge the wire. The difficulty of doing this sort of thing without the right tools may be the reason it is so poorly done.

As far as systematic troubleshooting, it was good to check for the presence of the power at the PCB, but you shouldn't have stopped there. Since it is an AC supply, there will have to be a rectifier to convert it to DC for the circuit. That could be a monolithic 4-pin package, or four discrete diodes. The AC will connect to the bridge and there should be a DC output from it.

If you find that, move on to where the DC is routed and check the other side of those components. The power supply signal will either end up on all the things that need it, or it will disappear. Whatever came before that is the culprit, at least the immediate one. This general idea of signal tracing is one of the most powerful techniques for troubleshooting circuits.

It can be done with power, control, audio, RF, and logic signals and is fast. In fact you don't have to know how the circuit operates to do it, just how the components the signal is connected to should behave relative to the components directly connected to it. You are looking for dead (nothing), weak (diminished), distorted (the wrong waveform or overdrive) results if there is a problem.

For some of it, knowing the expected voltages makes life much easier. These may be printed on the PCB, or you might have to use a schematic to find them. Always try to find a Service Manual if you can. They contain schematics, troubleshooting information, and testing & alignment/calibration procedures.

 

There's a very poor diagram on the Antiquesradio.com website if you search for it. Unfortunately it doesn't have any values for I.C's etc, but at least it's a start.

 

There's a very poor diagram on the Antiquesradio.com website if you search for it. Unfortunately it doesn't have any values for I.C's etc, but at least it's a start.

https://antiqueradios.com/forums/viewtopic.php?f=8&t=221342
About halfway down the page. There’s a post from Sabtronics as well. Good catch!

 

Without a circuit diagram, repair is likely to be difficult

Look for a schematic

@Dodgydave
The only pattern I have found online is the one on this forum:


The integrated circuits are these:

 

My favorite is MG Chemicals 401B Nu-Trol Control Cleaner (Amazon

I have this at home, will it fit?

 

The only pattern I have found online is the one on this forum:

There are two primary methods for troubleshooting: start from the power supplies and work forward until you find something that isn't working or start from the outputs and work backwards until you find something working.

Since you have nothing from all 3 outputs, I'd be inclined to start from the power supplies. There isn't much circuitry, so one method has no meaningful advantage or disadvantage.

 

As you are now, go and identify the common.

 

Can you take pictures of the component side , especially the chips.

The arrow is pointing at what appears to be a not-very-good attempt at repairing a lifted pad. The copper that should be inside that circle is gone, and a small wire runs from the component lead to the remaining trace.

View attachment 302613
​

I would not be surprised if that connection was flaky or even open. It should be checked for continuity and possibly reflowed with a bit more solder. Be careful, though—it would be very easy to dislodge the wire. The difficulty of doing this sort of thing without the right tools may be the reason it is so poorly done.

As far as systematic troubleshooting, it was good to check for the presence of the power at the PCB, but you shouldn't have stopped there. Since it is an AC supply, there will have to be a rectifier to convert it to DC for the circuit. That could be a monolithic 4-pin package, or four discrete diodes. The AC will connect to the bridge and there should be a DC output from it.

If you find that, move on to where the DC is routed and check the other side of those components. The power supply signal will either end up on all the things that need it, or it will disappear. Whatever came before that is the culprit, at least the immediate one. This general idea of signal tracing is one of the most powerful techniques for troubleshooting circuits.

It can be done with power, control, audio, RF, and logic signals and is fast. In fact you don't have to know how the circuit operates to do it, just how the components the signal is connected to should behave relative to the components directly connected to it. You are looking for dead (nothing), weak (diminished), distorted (the wrong waveform or overdrive) results if there is a problem.

For some of it, knowing the expected voltages makes life much easier. These may be printed on the PCB, or you might have to use a schematic to find them. Always try to find a Service Manual if you can. They contain schematics, troubleshooting information, and testing & alignment/calibration procedures.

So, from the schematic (you can find it here or I have attached it somewhere in this thread) I realized that there is a mosfet connected to C1 and C2 (about in the middle, bottom) that produce +-15V out supply.. am I wrong?

I powered up the circuit and measured the potential difference between the + of C1 and the - of C2 hoping to find 30V ... but I measured 0.15V
Just so I don't get confused, I tried all combination of connections between the 4 terminals of the two capacitors .. in any case less than 1V
I should have measured 30V, right?

@atferrari

I think that the one circled is an LED that notifies when the generator is powered, right?

On the generator there is a red LED, but it does not light up when I power the generator or even when I first the Power button for waveform output.
If I follow the tracks of said red LED though, they don't seem to match the one highlighted in red ... they don't lead to any R49 (it doesn't exist in the pcb! I couldn't find it and have still been looking for it for half an hour) and no ground ... rather it is connected to the + pins of C1 and C2

 

Hello,

What did you use to power it up?
A DC adapter will not work.
The power comes from an external transformer (as I interpetate the schematic).

Combining the schematic and your picture I see the following chips:
Z1 LM318
Z2 8038
Z3 LM324
Z4 LM318

Bertus

 

Hello,

What did you use to power it up?
A DC adapter will not work.
The power comes from an external transformer (as I interpetate the schematic).

Combining the schematic and your picture I see the following chips:
Z1 LM318
Z2 8038
Z3 LM324
Z4 LM318

Bertus

Yes, the chips are clearly visible in the photos I took. I use an external transformer (it is the one that came with the generator, so it is from manufacturer)

 

Hello,

What are the voltages on each of the capacitors ?

Bertus

 

I powered up the circuit and measured the potential difference between the + of C1 and the - of C2 hoping to find 30V ... but I measured 0.15V

The +/-15V supply is a little questionable. You're going to get slightly more than 16V on the filter caps, so there's little margin for a cap rated at 16V. They'll probably tolerate that slight overvoltage, but I wouldn't have done that. Maybe the transformer is always loaded enough to pull the voltage down.

Does the "Power On" LED turn on? What is the voltage at the power jack?

What is the device generating +12V?

 

View attachment 302641
The +/-15V supply is a little questionable. You're going to get slightly more than 16V on the filter caps, so there's little margin for a cap rated at 16V. They'll probably tolerate that slight overvoltage, but I wouldn't have done that. Maybe the transformer is always loaded enough to pull the voltage down.

Does the "Power On" LED turn on? What is the voltage at the power jack?

What is the device generating +12V?

LED does not light up and if I follow the tracks of said red LED though, they don't seem to match the one highlighted in red ... they don't lead to any R49 (it doesn't exist in the pcb! I couldn't find it and have still been looking for it for half an hour) and no ground ... rather it is connected to the + pins of C1 and C2

 

Hello,

What is the device generating +12V?

Above the transformer in the schematic, there is a 12 Volts regulator.

Bertus

 

Above the transformer in the schematic, there is a 12 Volts regulator.

Thanks. That's a weird way to draw a 3 terminal regulator.