Fixing acoustic research Phantom Sub 12D

In the subwoofer there is one externally available fuse and two built in fuses which protect symmetrical low voltage supply for the analog path and the amplifier.
The problem was that internal fuses blew immediately after turning power supply on.

This is a D-Class amplifier and there are some descriptions on the web that in this case output filtering capacitors C16 and C17 should be replaced what the owner already did but fuses kept on blowing. I received the Amplifier without speaker housing and took the amplifier PCB out:

Type code of key components on the PCB are removed but it turns out that this PCB is exactly the same as the one used in JBL SCS150SI, SCS160SI and SCS180.6S subwoofers.
The circuit diagram and service manual are available online.

With the circuit diagram available I noticed, that the owner replaced C16 and C17 as they were damaged (you can see corrosion on the amplifier PCB resulting from leaking capacitor's electrolyte) but there were more components requiring replacement, in this case it was Q10, Q11 and Q5. I assume that output filtering capacitor C16, failed to short circuit because of electrolyte leakage what caused Q10 and Q11 to work with a short circuit as a load, this damaged that MOSFETs. Q11 was failed open and D10 failed as a short circuit between gate and drain what caused Q5 to fail. After replacing all three mentioned transistors, the amplifier started to work.

Fixing Ebike battery, resetting OZ890

My Ebike's range dropped significantly. I took the battery apart and and found 7 pouch Lithium-Polymer cells and a BMS printed circuit board based on the OZ890. Every cell consists actually of two smaller cells connected in parallel so the battery architecture can be described as 7S2P.

Firstly I noticed that there is one cell with unusually fast voltage drop, when the cell voltage dropped to 3,2V the BMS turned the complete battery of to prevent damage. Removing kapton foil revealed a crack on the battery pouch:

Replacing complete battery is not an option, 6 of 7 cells are still fine. I wasn't able to find exactly the same replacement cell so I decided to cut the corners and built a cell combination which will fit into the place of the damaged one. 6 pieces of 18605 cell fit perfectly and if they are connected 2S3P then their voltage matches original li-po cells and their capacity is even higher than rest of the cells. I ordered 6 new 18605 cells already welded in the desired shape and put it instead of the damaged cell.

My strategy was to fix the battery by replacing damaged component and I believe that as long the new cell has got the right voltage and at least the same capacity (ideally a bit higher) then it has to work.

OZ890 BMS had to be reset to accept the battery. In this case one of the middle cells were replaced, it means, during the complete operation OZ890 was powered up continuously and it detected 0V on one of the cell which was replaced. I wasn't able to reset the OZ890 with original o2micro USB I2C interface, but disconnecting the BMS from the batterypack completely did the trick. After reset the BMS allowed normal charge and discharge procedure. This means, that there is no Coulomb counting in the BMS and that there is voltage supervision only. I don't expect this trick to work with more sophisticated BMSes. Fixing this battery wasn't all that difficult. It has been couple of month since I did it and the battery still serves its purpose. 

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.

Fixing DENON DCD-510AE CD player

While visiting friends I heard their complaints that they have a CD player which keeps on resetting. It wasn't able to eject the CD (there was one inside). On the display it was continuously saying "READING". From time to time this player was able to start playback but it ceased when pressed "next". I was told that the CD module was already replaced twice with no result. My first thought was that it has got to do something with supply voltage. Circuit diagram is easy to find on https://www.manualslib.com/manual/1115299/Denon-Dcd-510ae.html#manual .
I started by checking 5V and 8V net, see sample trace below. I also observed (you have to believe me because I have no waveforms for that) that 8V was dropping to lowest values when a skip track or eject button was pressed. Spinning the CD loaded the 8V and 5V net significantly too.

Blue trace i 8V and yellow is 5V.

5V and 8V are both generated by linear regulators and it is obvious that 8V is overloaded so much that 5V falls below reset threshold. I inspected C914, IC93 and C916, they appear fine. I measured current consumption on 8V (approx. 1A max) and supply transformer voltage. According to my crude calculations C914 value is way too low, and I believe that the power supply is calculated without margin. I don't have 22000uF capacitor and I didn't want to order one but I realized that I have a 230V to 5V 1A switching mode power supply and keeping in mind that the main load on the 8V net is the 5V regulator I decided to add an additional power supply to provide 5V from another supply.
I removed IC94 and connected additional power supply (see yellow rectangle) parallel to C917. This new power supply is connected in parallel to the primary winding of the main transformer of the CD player T901

It's not ideal, I could try experimenting with the original power supply and low ESR high capacity alternatives for the C914 but I had an extra switching mode power supply lying around so I used it. It works just fine.

Ni-Cd battery deep discharge protection circuit

I have a LED lamp in my workshop which I supply from a recycled 12V 3Ah Ni-Cd battery. To protect the battery from a deep-discharge I built a simple circuit from what I found around. The circuit is built into the lamp and starts to beep when the supply voltage drops to 10V. The beep means that the battery has to be replaced.

The potentiometer PR has to be adjusted: Just set the desired voltage at which the circuit has to beep and then find a position of the PR at which the circuit starts to beep. The TL431 is a shunt voltage regulator and will pull down the pin 1 of the CD4093 when the voltage on its pin 1 is higher than 2.5V. One gate of the CD4093 forms an oscillator running at audible frequency. 3 unused gates have their inputs tied to GND. Piezzo is a of a passive type, it doesn't have built in generator.

The circuit has been built a piece of prototype PCB.

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.

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

50	49	48	47	46	45	44	43	42	41	40	39	38	37	36	35	34	33	32	31	30	29	28	27	26
R1	BP	R1	G1	F1	A1	B1	G2	F2	A2	B2	G3	F3	A3	B3	G4	F4	A4	B4	BP	BP	BP	BP	BP	BP
Original RKSB-104 display pinout
BP	BP	E1	D1	C1	BP	E2	D2	C2	BP	E3	D3	C3	BP	E4	D4	C4	BP	BP	BP	R2	BP	R3	BP	BP
1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25

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