Fixing Rocktile GB-15 bass guitar combo

I bought a damaged super-cheap bass combo for practicing at home. Description provided by the previous owner mentioned that signal overdrives and that it sounds unpleasant.

After turning this on for the first time I noticed that:

• there is unwanted overdrive/distortion
• distortion changes over time
• after couple of second there is no sound

As always I checked supply voltages first - they were fine. Power stage built around TDA2030 worked too so I concentrated on the preamplifier. Replacing opamp didn't help so I had to understand how the circuit works. There was no circuit diagram available so I reverse-engineered it from the PCB:

After connecting a signal generator to the amplifier I noticed that it operates normally for a while after touching pin number 2 or 3 of the 4558 operational amplifier. Using oscilloscope I observed that voltage on the pin number 2 of the 4558 is slowly rising and when it saturates the circuit doesn't amplify anymore:

I knew that this has got to do with an input bias current of the operational amplifier. In one of my favorites books "The Art of Electronics" I read that input bias current problems become visible some time after turning the circuit on, when impacted capacitors charge, for low input bias current amplifier it can be even minutes after turning a circuit on.

According to circuit diagram every input pin of the operational amplifier has got a DC path designed for input bias current. I measured R7 resistance on its pins and it appeared to be fine but by measuring resistance between pin 3 of the 4558 operational amplifier socket I noticed that the R7 is not there. I inspected the PCB and discovered that there is a crack on the PCB between the R7 and GND. On the movie below you will see (sound is important for that movie).

After fixing that PCB crack with a drop of soldering tin the amplifier started working

One more note here is the fact, that the input jack socket shorts input to GND when there is no plug inserted. It is so to avoid noise when there is no guitar connected. But to connect a signal generator to the PCB you have to be aware about that and to plug something in to open the input socket.

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.

Voltcraft VC840 RS232 interface circuit

Q0 is a photo-transistor which receives IR light from a multimeter. If Q0 is illuminated then current flows through R1 and turns Q1 and Q2 on and the output R will be pulled down to positive voltage, this means mark will be transmitted. If Q0 is not illuminated then there will be negative voltage at the output R so space will be transmitted.