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 VOX AD60 VT guitar combo

The VOX AD60 VT blown the main fuse when turned on. I received it with a comment that someone has already tried repairing it.
After opening the housing it was obvious that someone has already played with transistors in amplifier stage.

For test purpose I used two power supplies with current limitation to supply the power stage. I connected them to points marked with yellow boxes on the circuit diagram below. After turning power supplies on, I observed that without input signal both power transistor (Q13, Q16 - blue ovals) are turned on completely. My power supplies entered constant current mode. I removed both power transistors (Q13, Q16 - blue ovals) and turned supply again, this time I observed that R87 (green box) overheated. I measured voltages across R84 and R79 (blue boxes) and realized, that Q12 and Q15 (green ovals) are fully turned on, while they shouldn't. While inspecting pairs of transistors Q5-Q6 and Q7-Q8 I noticed that in each pair there are different types of transistor, while they should be actually the same. I swapped Q6 and Q8 as they should be according to the circuit diagram and after that the amplifier (still without power transistors) draw approx. 20mA. I soldered power transistors back and tested the amplifier. It works well. 

The problem was, someone was trying to fix this amplifier or to test transistors but mixed them up while putting them back in the circuit. This caused a shoot through current, the power transistors survived that but the fuse tripped. 

On the very beginning, when I observed that there is high current flowing from both positive and negative supply rail, I thought it is something with phase inverter, and not with upper or lower power stage. I did once mixed up transistor while fixing a power stage and after that I can say, I learned my lesson and this time was able to find the problem quite fast.

EMC Troubleshooting speaker relay matrix

There is relay a matrix designed to switch between active and passive speakers in a music shop. There are actually two matrices, each in a different showroom. It has been observed that one matrix switches speakers almost inaudible while the other makes a quiet but annoying click. Here is a screenshot of a signal recorded at the output of the speaker relay matrix. This is matrix switching line signal for active speakers so it was possible to record it directly:

Recorded distortion on the output of the matrix, at the beginning and ant the end there are disturbances which I believe are 4 bytes digital transmissions coupled on the audio signal.

Because of the location and surrounding equipment only one rack with a matrix can be opened, namely the one which generates audible disturbances, it looks like this:

Wiring arrangement in the matrix

The matrix consists of daisy-chained modules, each module is switching one input signal to 1-of-8 output jack sockets. A single module can be simplified to the following:

Single module of the relay matrix

In reality a single module is more complicated (I assume GND is switched too) but it describes the idea. A complete matrix is built in two rows and wired like this:

Current wiring of 10 modules in the matrix (outputs not shown because they are shielded on the other side of the panel)

See how many times digital signal cable is crossing and running parallel to analog signal wiring. I believe there is magnetic coupling between digital and analog lines. To make it worse, both are wired without shield, such arrangement is actually prone to cross-talks and requires longer cables. I suggested changing arrangement to something like this:

Improved wiring of 10 modules in the matrix

In the improved wiring concept there are shorter cables, and there is no unnecessary zigzag. I believe this will remove unwanted disturbances on the switched analog signal. Rearranging is possible because every module has its own configurable address, I am not even sure what exact type of the digital transmission is used. Maybe daisy-chain order can be changed even without readdressing modules. I will update this post when it's done.

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.