Toshiba sdp7000 no sound

[efilnikufecin]

I have a Toshiba SDP7000 TV/DVD combo that has no sound when used like one would normally use a television. When I hook it up to my high power stereo system via the headphone jack and turn the volume all the way up I can hear it through my stereo, but with a hum.

I downloaded a repair manual for this set, and began diagnosis:
1 Check input source: Pass
2 Check input/output IC1: I was unable to determine the input pin. The output pins gave the same hum when the volume was turned to a high level on my stereo.
3 Check input/output ic100,101: These are mps7720 audio amp chips. Same result as the output of IC1/the headphone jack.
4 Check the speaker: Pass

I suspect a problem in the amplifier circuits or the amps themselves. On page 40 of the service manual there is a schematic of the audio amplifier, and I verified I was getting the 15 volts required to the amps. I found no bulging caps.

I have a data sheet for a mp7720 audio amplifier chip made by MPS, but I am not certain I have the right one for the chips in question (mps7720 according to the schematic) and I cannot find a data sheet for mps7720.

I am considering removing some components from the board so as to disconnect the amp circuit, and jump the output of IC1 to the headphone jack, and give it to my son with some computer speakers to play video games on (if the whole idea works) but if I can fix it right, I would rather do that. Any suggestions on what I should try?

[t.j.]

Re: Toshiba sdp7000 no sound

dumb question! have ya made sure audio output set correctly in menu?

[budm]

Re: Toshiba sdp7000 no sound

Check the DC voltage of the IC 100/101 PIN4 which is the AUDIO MUTE Control line, It should toggle between 0 and 3~3.4V. I believe 0V = MUTE, >3V = SOUND ON.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

Pin 4 of IC 100 and 101 is at 1.82v both with mute on, and mute off.

In answer to the first question, I have checked, and I have had others go through the menus and check to see if they could find a setting that would disable sound.

Edit: btw that is not a stupid question, I have solved that very problem on a few RCA TVs with just a few button presses on a remote control.

[budm]

Re: Toshiba sdp7000 no sound

Ok It is not switching HI-LO on the Collector pin. It can be due to Q100 has leakage resistance. What you can do is to lift the Collector pin, this should allow the voltage on R127 (10K) to rise up. R126 (4.7K) and R127 (10k) is setup as a voltage divider, when Q100 is turn on, the voltage drop across R127 will be shorted out by Q100 Collector/Emitter which will cause the voltage drop on R127 to be around .3~.6V. when Q100 is turn off, the Collector voltage should rise up to about 3.4VDC.
The two ICs are Class D amplifiers.
By the way, I am not sure yet if HI = MUTE, or = unmute. Still looking for the spec sheet of the MPS7720.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

Q100 is a very tiny surface mount transistor. Would it be a major issue if I removed the whole thing? I don't think I can lift just the collector pin.

[budm]

Re: Toshiba sdp7000 no sound

You can. Just be careful not to damage the trace.

[budm]

Re: Toshiba sdp7000 no sound

OK, this is what I found, MP7720 has the same pinout as MPS7720. Pin 4 is an enable/mute pin.
http://pdf1.alldatasheet.com/datashe...PS/MP7720.html

Mute/Enable Function
The MP7720 EN input is an active high enable
control. To enable the MP7720, drive EN with a
2.0V or greater voltage. To disable the
amplifier, drive it below 0.4V. While the MP7720
is disabled, the VDD operating current is less
than 5µA and the output driver MOSFETs are
turned off. The MP7720 requires approximately
500ms from the time that EN is asserted (driven
high) to when the amplifier begins normal
operation.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

After looking at the parts list Page 31 in the service manual it shows:
IC100 AF500060 0IMCRMZ002A "IC,MICRO CONTROLLER" MP7720 MONOLITHIC POWER SYST
IC101 AF500060 0IMCRMZ002A "IC,MICRO CONTROLLER" MP7720 MONOLITHIC POWER SYST

The schematic diagram shows mps7720 and with a magnifying glass the chip shows mps7720. So the parts list says I have the right data sheet, but the diagram and chip differ. This is where my confusion comes in as to whether or not I have the right one.

[budm]

Re: Toshiba sdp7000 no sound

It is more likely to use the same voltage level for controlling even if the pin outs are not the same.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

Went to the link, same data sheet as I already had. Breaking out the soldering iron now to pull Q100.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

pulled q100, didn't damage the traces, but it came off in 2 pieces. Still no sound, and voltage at pin 4 of IC100/101 is still at 1.82v I cannot seem to find R106,107, or 126 on the board. I was wanting to check the +5v input to see if it was weak.

[budm]

Re: Toshiba sdp7000 no sound

Can you take the reading of pin 4 and ground? It may be one of the filter cap (MLC type) C115, 118 may be leaking.

Do you have any sound source that you can inject in to c112 negative side as shown in the diagram and see if you will hear any sound.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

Using the line level output of a cd player, R channel connected to an input jack for ground left channel connected to the negative pin of c112 and it's equivalent c111 for the opposite channel gave no output.

[budm]

Re: Toshiba sdp7000 no sound

So it is still muted. Resistance reading?

[efilnikufecin]

Re: Toshiba sdp7000 no sound

1854 ohms

[budm]

Re: Toshiba sdp7000 no sound

I expect around 10K since it has 10K resistor to ground. So are the caps C115, 118 surface mounted?

[efilnikufecin]

Re: Toshiba sdp7000 no sound

I checked knowing they would be, and sure enough they are.

[budm]

Re: Toshiba sdp7000 no sound

I would try removing them one at time and see if the resistance will go up.

[efilnikufecin]

Re: Toshiba sdp7000 no sound

1900 ohms at pin 4 with the 2 caps removed.