Fixing Bose Quietcomfort 35

Short version:
Bose Quietcomfort won't turn on - open it and check if a plastic piece sitting on the switch is intact.
Bose Quietcomfort won't enter pairing mode - it's possible that the switch on the PCB  can't reach  position responsible for entering pairing mode. Open the headphone and check if moving the switch directly helps, if so, continue reading.


Headphone won't turn on when the switch is moved.
The problem was this small plastic part which is visible from outside as a switch and which moves the actual switch soldered on the PCB. To fix it you will have to open the right headphone. Start with gently removing the ear cushion and the fabric inside, then undo three screws:

Without screws aluminium cover can be removed. The actual switch is highlighted:

From the other side of the aluminium cover you see a plastic part which should sit firmly on the switch. In this case it is broken:

Find two pieces of the broken part and glue it together, I used two compound epoxy glue.

It's not pretty but it holds well.

Bose Quietcomfort won't enter pairing mode.

The mechanical tolerances of the switch, this plastic piece and aluminium cover are so tight, that it can happen that the headphone will not enter pairing mode. You move the switch but it neither enters pairing mode nor removes paired devices. The reason is, that the oval hole in the aluminium cover is to small and the switch will not contact in the "bluetooth" position. I extended this hole. It was around 8,9mm long but I extended it by approximately 0,75mm towards the bluetooth logo. I used a round file for this:

It is barely visible that the hole is longer:

But this bigger hole allows reaching the final position of the switch.

Fixing Dell 2400MP projector with power supply PA-4331-1D-LF

Power, temp and lamp control lights are continuously on when projector is turned supplied. Nothing changes after pressing power button. In the user manual there is no description of such a combination of control lights.

Opening the projector:

There are silk screen descriptions on the PCB so it was easy to identify and measure voltages generated by the power supply PCB, the 5V net looked like this:

Which is mean that the power supply is not able to regulate voltage. It looks like a single cycle of the flyback is causing overvoltage and it leads to power supply shutdown and another try in couple of milliseconds when voltage drops to 4V. Visual inspection of the C206 5V output filtering capacitor reveals that the capacitor is swollen.

This capacitor has been replaced with a new low ESR capacitor but the power supply didn't start either. This particular power supply turns out to be very sensitive to correct filtering capacitors selection. After replacing the old capacitor with 3 electrolytic hybrid ultra low ESR capacitors the projector started working normally:

The C602 capacitor value is not critical (as long as it is at least 3300uF) but low ESR is.

I suppose that high temperature in the projector accelerated electrolytic capacitor's wear-out. Most of devices I repair is supplied by a flyback power supply and most of flyback power supplies suffer from issues with output filtering capacitor which usually works in high temperature, high current and high frequency.

Seeing problems with electrolytic capacitors another time I decided to buy a meter UT612 RLC meter:

I like price-quality ratio of UNI-T devices but looking at specs reveals, that the higher frequency you use for a measurement the lower capacitor values can be tested. In case of the projector with a capacitor of 3300uF I was able to measure ESR with highest  frequency of 120Hz. For a flyback power supply working with tens of kHz this measurement should be performed with at least 10kHz to reflect real conditions capacitor has to deal with.

Fixing tube amplifier Suprem Concort 80

The amplifier came with a note that it doesn't turn on. Control lamps remain dark after flipping the switch. The reason for that was damaged switch. It made a click noise but there was no connection. Establishing reliable connection requires force to applied on the lever in side direction, then the control lamps lit but there was no signal coming through the amplifier. After flipping the standby (Vollbetrieb) switch, one could hear that the power stage was working because it wasn't complete silence, it was typical quiet noise of a working tube amplifier.
For this amplifier there is no circuit diagram available, but there is one very similar (Superem Luxor 40 https://elektrotanya.com/suprem_luxor_40_sch.pdf/download.html ) which except for number of inputs and tubes reference designation is the same. I checked heater resistance of all tubes but they all appeared fine.
Keeping in mind that tube amplifiers require connected speaker I measured all supply voltages including bias and they were correct.
I observed that one of the ECC83 tubes look different, see the middle tube on the picture:

See milky deposit in the top part of the tube which lost vacuum, it should be uniform, shiny surface.

In a normal tube there is shiny surface on the inner part of the glass, above a getter. (see https://en.wikipedia.org/wiki/Getter ), the middle tube on the picture has no shiny surface but a milky deposit. This is a sign that the tube has lost vacuum, and that particular tube drives the end stage of the amplifier. This explains why the amplifier was mute. I have no vacuum tubes in my workshop so I shuffled other tubes in the way that there was no signal for vibrato but all input channels should work. And it works. It sounds pleasant with a bass guitar. I ordered replacement tube and will build it back together soon.

Fixing Allen & Heath Zed 420 mixer

The mixer was turned on 24 hours a day for around two years. After that time once its LEDs were not as bright as in the past and the signal was not getting through the analog channels. The mixer was returned from the service with a note, that power supply was damaged and that a price tag for the power supply makes it unprofitable to repair. After that, the owner brought the mixer to me. I observed that output filtering capacitors, right behind rectifying diodes are swollen. It was clear indicator that their lifespan was exceeded. See swollen capacitors, especially the one marked as (2):

 I replaced those with identical capacity and voltage, low ESR capacitors and it worked again.

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 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.

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

Fixing Ravensburger Tiptoi

Thanks to my college from work I had a chance to look at apparently popular toy called tiptoy. Basically it is a RFID infrared camera combined with a mp3 player. There are books with embedded RFID tags small points printed over the regular pictures. Those points are almost invisible to a human but tiptoy recognizes them and plays back some records when its tip touches tags in a book so a child gets interactive description of what is in the book. Different images or areas of images have different points pattern.
A typical problem of the tiptoy is that its speaker is not completely child proof and after some time the toy becomes mute. This particular toy worked with a headphones normal so my friend decided to try changing a speaker what solved his problem. Here come some pictures he took:

 Take batteries out, unscrew four screws.

 Take speaker out and desolder it.

Both speakers - the original one and the new not installed yet one.

This is close-up how the speaker is mounted.

You can ensure that the speaker caused a problem by measuring its resistance.

This is measured resistance of the new speaker, close to 8 Ohms is correct.

This is resistance of the damaged speaker. 2.3MOhms is almost open circuit - a speaker is damaged.