Bose Wave Radio/CD buzzing

[rmk]

My Bose Wave Radio/CD (top loading) has developed a speaker buzz. This is what I have observed:

1) When I turn on the unit, there is no buzzing noise. The radio works fine.
2) After 3-4 minutes, an intermittent buzz starts to happen, in sync with the flashing display (which is flashing because I have not yet set the time). The time interval is about a second. The buzz is coming from both speakers.
3) Slowly the buzzing becomes more constant until after a few minutes (1-2), the buzzing is constant.
4) The buzzing occurs even when the volume is turned down to zero.
5) I have looked closely at the circuit board and can not see any obvious capacitors that have either leaked or have bulging tops.

I have done the following:

1) Replaced the amplifier IC (TDA7375A). This is not make a difference to the buzzing and its behaviour.
2) I changed two capacitors (C202, C205, both 470µF 35V 105°C), which was recommend in the Bose trouble shooting guide. This is not make a difference to the buzzing and its behaviour either.

I have read a lot online about Bose Wave Radio/CDs that hum, not buzz, however having followed the advice I found, the problem has not been solved.

I am not an electronics expert, however it seems as though the problem is related to a component getting warm, and perhaps somehow tied into the clock.

I have the circuit diagrams if anyone wants to see a copy of them.

Thanks,
Richard

[German_EE]

Is there a voltage regulator somewhere in the power circuit that is getting stressed? That three to four minutes could be the time it takes to heat up.

[rmk]

How can I easily check if a voltage regulator is failing? There are two that are part of the audio circuit - one for -15V and one for +10V.

Thanks.

[wraper]

In my experience it is electrolytic cap fail with 95% certainty. Could be in PSU or somewhere in circuit where it can impact low level signal. Like 10-100uf cap connected to some power pin of IC which are connected to the power rail through something like 47-470 ohm resistor. In such case ripple is going into the signal due to insufficient power filtering.

[wraper]

For example could be cap connected to pin 6 of the TDA7375A (supply voltage rejection).

[rmk]

I tested the output voltages for the voltage regulators, and found the following. When the buzzing starts, when I test the voltage regulator rated for output -15V (LM320LZ-X), the voltage is about 13.8V, and if I use the ground terminal from another part of the circuit (e.g. the +10V voltage regulator), the buzzing stops almost immediately. When I take the probes off the voltage regulator pins, the buzzing starts again. I can reproduce this every time.

Does this mean that the voltage regulator is faulty?

See the attached circuit diagram sheet for the unit.

Thanks.

[OldSchoolTechCorner]

Bad electrolytic capacitors, I restore quite a few of them and you have to look at pcb plated through holes. If they are black then they were eaten by electrolytic that leak out of capacitors and you have to remove the masking on affected area and neutralize the acid and clean up the area and run thin strand of wire though the PCB plated through holes to make reliable connection to other side of the board.

[rmk]

Given my observation on the voltage regulator, which capacitor should I focus on? I cannot see any obvious damage or blackening. This is the circuit diagram of the voltage regulator section.

[wraper]

Given my observation on the voltage regulator, which capacitor should I focus on? I cannot see any obvious damage or blackening. This is the circuit diagram of the voltage regulator section.

WTF, they use half wave rectifiers  . Cheapass bastards. What is the voltage before the regulator (pin 2 of U200)? I would say replace C202, but it is already replaced. If the voltage beofore U200 is ok (at least -19V), then I would think such possibilities. U200 is faulty or unstable because of dead C229, or this rail is overloaded for some reason which causes the voltage sag.

BTW LM320LZ-15 is LM79L15 differently marked if you need replacement and can be replaced with any flavor of 79L15.

[mikerj]

WTF, they use half wave rectifiers  . Cheapass bastards.

Welcome to the world of budget engineering in "premium branded" consumer devices.  To be fair they are very low current rails, and half-wave rectification means you get positive and negative voltages from a  single (non-centre tapped) transformer winding which must have saved a few cents.  They also saved a couple more by using a regulator/heatsink with inadequate power dissipation and then stuck a resistor across it.  Hopefully there is a well defined minimum current draw on the 10v rail.

I tested the output voltages for the voltage regulators, and found the following. When the buzzing starts, when I test the voltage regulator rated for output -15V (LM320LZ-X), the voltage is about 13.8V, and if I use the ground terminal from another part of the circuit (e.g. the +10V voltage regulator), the buzzing stops almost immediately. When I take the probes off the voltage regulator pins, the buzzing starts again. I can reproduce this every time.

Does this mean that the voltage regulator is faulty?

Do you actually mean -13.8v?  You certainly shouldn't be seeing a positive voltage on the input or output of this regulator, with respect to the common pin.

The buzzing could indicate that a regulator is oscillating.  3 terminal regulators can be quite sensitive to input and output supply decoupling, or lack of, so ensure C225,C229,C232 are present and correct.  You could try wedging a known good cap of similar value to C229 (e.g. 2.2uF-10uf) across the test points TP231/TP232.  Ideally you need to be getting a scope in there to look at the rail voltages.

[OldSchoolTechCorner]

The buzzing problem fix many time. It due to bad capacitors and more likely damage traces, more likely is C225,C229,C232. Do recommend to replace all of them, as they all go leaky and leaving them in will just cause more issues down the road.

[rmk]

Thanks for the comments. I will follow the suggestions and report back later in the week. Is it worth investing in an ESR and capacitance meter? You have all been a huge help.

[rmk]

I changed the voltage regulator U200 (-15V) and the buzzing has disappeared. The old voltage regulator was outputting -13.8V and it was not stable.

The unit is now working correctly, all for the sake of a £0.22 component, which I bought online from Farnell.

Many thanks for all your help.

[OldSchoolTechCorner]

I changed the voltage regulator U200 (-15V) and the buzzing has disappeared. The old voltage regulator was outputting -13.8V and it was not stable.

The unit is now working correctly, all for the sake of a £0.22 component, which I bought online from Farnell.

Many thanks for all your help.

Nice