Monday, September 12, 2016

Voltrcraft LSP-1403 / Maplin N27GG / Manson SSP-7080 / Extech 382260

I bought a Voltcraft LSP-1403 power supply on ebay. I intentionally choose the broken one so I could challenge myself and try fixing it. This power supply is a product of Manson but it's sold under at least three other brands which I named in the title of this note.
Voltcraft LSP-1403, Maplin N27GG, Manson SSP-7080 and Extech 382260


The power supply was turning on, display lit and showed reasonable numbers which could be altered by turning knobs, but it didn't provide any output voltage. 
Block diagram of the LSP-1403 power supply
One look at the PCB shown that Q2, U4 R26, R27 and D9 were burned component. I replaced those damaged components and added a light bulb in series, to prevent a small explosion in case I overlook any other damaged/shorted component. Additionally, all the time the power supply was connected to mains over an isolating transformer, this way I was able to connect an oscilloscope to whatever point in the circuit I wanted.

Part of the circuit diagram of the main power supply
After putting this all together power supply provided correct output voltage but after loading it a light bulb lit up which was a sign that something is going wrong with the circuit.
I measured voltage on a shunt resistor R28 ant it turned out to look like this:
Voltage on R28 before fixing the power supply (low output current)

Voltage on R28 before fixing the power supply (higher output current)

Which is wrong because of too high current value and too steep rise. The UC3843 controller was detecting this high current and was turning the MOSFET off to prevent damage, this was the reason the power supply wasn't able to provide demanded output current. Such a steep current rise in the primary winding of a transformer is not possible because of primary winding inductance. My first guess was a damaged transformer (shorted primary winding) but with another transformer, from a similar power supply symptoms were exactly the same.

My second guess was that the snubber circuit is damaged. I measured all snubber components for an obvious damage but they all seemed to be fine. I added two shunts just to see if the current observed on the low side of the primary winding is flowing through the snubber circuit. or through the primary winding. I measured that there is no such a high current in any of the upper shunts. I started scrutinizing the (single sided) PCB for a short but there was none.

The solution, the MOSFET's radiator is connected to GND over a 10nF capacitor (a green capacitor on the circuit diagram above). The isolating layer between MOSFET case and its radiator was shorted and this 10nF was connected to the drain of the MOSFET. The capacitor charged to almost 400V over primary winding and when FET turned on it had to discharge this capacitor first what caused heavy current spikes.

After isolating the MOSFET from the radiator power supply works correctly, voltage on a shunt during normal operation looks like this:

Voltage on R28 after fixing the power supply


Lesson learned: mica is not always isolating, before starting with wild theories check very basics. I did some research on google and it seems that there are often problems at the primary side of the main power supply.

I do have circuit diagram of this power supply but I am not supposed to publish it. However, I will answer all questions to components.

Monday, May 16, 2016

Fixing РКСБ-104 - Voltcraft HS-036 Geiger counter (replacement for ИЖЦ-3 LCD display)

Geiger counter RKSB-104 was produced in Belarus and exported under Voltcraft HS-036 brand to Germany. I bought one damaged counter on ebay. The defect is typical for this model: LCD display showing a huge black stain, everything else worked fine but the previous owner has removed the display.
I tried to purchase a new ИЖЦ-3 display but I failed to find a supplier, I decided then to reverse engineer the PCB and figure out if any modern LCD can be fitted into this device. I used this circuit diagram: http://c4r0.elektroda.eu/_hv/xrays/count2_sch.jpg

50494847464544434241403938373635343332313029282726
R1BPR1G1F1A1B1G2F2A2B2G3F3A3B3G4F4A4B4BPBPBPBPBPBP
Original RKSB-104 display pinout
BPBPE1D1C1BPE2D2C2BPE3D3C3BPE4D4C4BPBPBPR2BPR3BPBP
12345678910111213141516171819202122232425

RKSB-104 LCD pinout, BP is a backplane, R1, R2 and R3 are some symbols which I couldn't identify based on a circuit diagram.


I suppose that DE119 LCD can be used to fix other similar devices for example for example РКС-107 or Bella (DBG-01N)

I prepared a small PCB which is an adapter between a original PCB and a new display

Adapter PCB circuit diagram

Adapter PCB layout
New LCD mounted on the adapter PCB

Display before fixing

The new display and adapter mounted on the counter PCB

The counter with a new display