Faulty Voltcraft VSP 2653

[BobbyK]

Hi,

I got a Voltcraft VSP 2653 on eBay, pretty cheap - except one channel is erratic. The supply looks very similar to the 2410 model, taken apart here: https://www.eevblog.com/forum/reviews/voltcraft-vsp-2410/
The two main channels, switch-mode as far as I can tell, are driven from an input switch mode supply, that gets the 220V input down to about 96 volts. The problem is that one channel gives an erratic output voltage.
Here are the symptoms:

when in dual tracking mode, both channels behave as they should, I can change voltages from 0 to 60 volts and current limiting works too. When switched to individual channels, ch1 behaves as it should, but channel two sometimes starts climbing all the way to 96 volts. It stays there, but keeps moving up and down about +- 2 volts randomly (is not stable). This goes on for a while, and the voltage control does nothing, but sometimes, changing the voltage, will cause the voltage to go back down to the properly set value (it is still not stable, and moves up and down by a volt or two).

Any ideas what may be causing the problem, and what I should check first?

[dundee]

I've found schematics for the VSP 2405.
http://www.produktinfo.conrad.com/datenblaetter/500000-524999/511142-sp-01-en-Labornetzgeraet_VSP2405HE.pdf

It should be very similar to your device.

I'm surprised about the 96 Volt. Shouldn't it be 320V DC?

[desy2820]

I would carefully check the connections for the channel 2 voltage adjustment.  Make sure that everything is well-connected.  Next, I would measure the resistance of the variable resistor/potentiometer.  I think yours is either open or has a bad spot in it.

While measuring resistance between the wiper to either end, turn it back and forth.  You should see a smooth change as you turn it.  there should be no sudden jumps or open (infinite resistance) spots.  Compare your channel two measurements with channel one.  They should be about the same, if the controls are ok.

If you can easily swap parts between channels one and two, you can also try swapping the potentiometers.  If the problem moves to channel one, then it's bad.  Otherwise, there's something else going on with channel two.

Note: when in tracking mode, channel one's controls sets the voltage for both channels, which is why it works properly. 

I hope this helps.

[BobbyK]

I've found schematics for the VSP 2405.
http://www.produktinfo.conrad.com/datenblaetter/500000-524999/511142-sp-01-en-Labornetzgeraet_VSP2405HE.pdf

It should be very similar to your device.

I'm surprised about the 96 Volt. Shouldn't it be 320V DC?

Great find dundee. The schematic will be a great help - the circuit boards are indeed similar - looks like they designed the entire line of supplies to be extremely similar, with minor changes to resistor values, and in some cases wire thickness, etc. Well, my gues is that the first stage (a switching supply) takes AC and turns it into 100V DC and feeds that to both channels. Then another switching supply, separate for each channel takes that and controls the final output voltage.

[BobbyK]

I would carefully check the connections for the channel 2 voltage adjustment.  Make sure that everything is well-connected.  Next, I would measure the resistance of the variable resistor/potentiometer.  I think yours is either open or has a bad spot in it.

While measuring resistance between the wiper to either end, turn it back and forth.  You should see a smooth change as you turn it.  there should be no sudden jumps or open (infinite resistance) spots.  Compare your channel two measurements with channel one.  They should be about the same, if the controls are ok.

If you can easily swap parts between channels one and two, you can also try swapping the potentiometers.  If the problem moves to channel one, then it's bad.  Otherwise, there's something else going on with channel two.

Note: when in tracking mode, channel one's controls sets the voltage for both channels, which is why it works properly. 

I hope this helps.

Thanks desy,

   I removed the pot, and checked - it is a single turn 100K, in parallel with a fixed 15K resistor. I replaced the pot with a new one just because it wasn't perfectly smooth, but that didn't help. I also traced all the connections on the voltage adjustment side, and there seems to be no issue. here is also a simple test I did, just to see if it's an intermittent connection:

1. I switched to individual mode. Then turned on the supply. the tapped and wiggled wires- the voltage stayed correct and stable.
2. I cranked up the voltage to 60V on channel two, again in individual mode. I waited 3 minutes without touching anything. The voltage started to get unstable, moving between 58 and 63, then after about 5 minutes it jumped up to 98 volts.
3. I switched to tracking. set both voltages to mid-way. Then I switched to individual - the voltage on channel two INSTANTLY (over 2 seconds) shot up to 98.

Each output channel is controlled by a TL494. What could make it the voltage on ch2 rise like that? or become unstable?