Creative DDT2500 Decoder keeps resetting/muting? (Solved by workaround)

[ITguy]

Hello,

I've recently aquired an old Desktop Threatre DDT2500 from Creative. It worked fine the first hours, then it started turning off and on the sound periodically around every second. In the off-intervalls it's completely muted, even the slight hiss noticable with no input signal is gone. As the system in this phase also doesn't react to keypresses so I would assume an IC resetting itself - probably caused by power issues.

First I checked the power supply and also tried with my bench power supply - but the problem stayed.

Researching I discovered that this problem seems to happen quite often to this system. Henrik Haftmann claims a bad cap on his page, which I replaced with a 22µF 50V (Panansonic 105°C Low ESR 5000h). The problem stayed and the old cap seems to be ok, my capacitence meter read 23µF.

Then I found out that the bigger model, the DDT3500 also seems to have problems with 3 capacitors near the decoder chip, so I also replaced the 47µF and 100µF caps. Again the problem stayed and the old caps seem to have correct capacitance.

The regulator south from the decoder outputs 3,3V as expected, alltough its input voltage seems a bit low with 4,8V. The replaced 100µF cap seems to be its buffer.


At this point I'm lost.  Anyone has an idea how I could fix this device?

With Kind Regards,

ITguy

[Rasz]

tried with my bench power supply

was it drawing more just before shutting itself off?
you could try looking at power rails with a scope

[ITguy]

When playing the current is about 600mA and drops to 450mA in the silent phases (When muted manually, it's also 450mA). I found three voltage regulators, one 5V and two 3,3V -> Voltage levels are stable.

I then found a B2B interconnect which has three pins labeled with MUTE1 to 3. I probed them and they are 5V when playing and 0V when silent. So I tried feeding them with 5V from the regulator and when doing this with MUTE3 then the audio stopped dropping

Next I will try to trace what is causing that MUTE3 pin to go low.

With Kind Regards,

ITguy

[Rasz]

interesting, so its a logical fault, not the classic bad supply
why are there 3 mute signals?
hmmm there is a mechanism to detect if speakers are connected, that would probably send mute signal when triggered
are all the speakers ok? maybe one of them intermittently goes open (when warm?) and triggers it

[ITguy]

I traced the MUTE3 Pin, it connects the Standby-PIN of the TDA7372A via a 10k Resistor. MUTE2 and MUTE1 seem to be responsible for Center and Sub respectively (each a TDA7360) but I only tested, not traced them yet.

On the other board, it ends with a another 10k resistor (103 SMD this time) in a Creative CT8941-NBT (seems to be proprietary with no data sheets) chip. Measuring at this pin I get 5V DC switching on and off perodically - looking as expected on the scope. But there also seems to be a AC signal I can't really interpret.


I don't think it has to do with the speakers, as the effect is the same even if no speaker is connected.

With Kind Regards,

ITguy

[Rasz]

this does sound like short/open protection build into the amplifiers, they have internal logic that restarts after a while
have you checked amplifier caps? starting with svr and standby ones of TDA7360
old cap could be pulling standby low

[ITguy]

I don't think its the Amps or their caps, because they stay on when supplied with 5V on the STBY pin.

It looks like something is causing the Creative CT8941-NBT to only briefly set the output pin to high and then drop to low again. Unfortuantelly I can't find any datasheet on that chip

With Kind Regards,

ITguy

[ITguy]

As much I would have liked to fully repair that decoder, I gave up now after endless hours of probing that undocumented IC. So I decided to add bodge wires from 5V to the Amps' standby pins, which allows me to use the device again.

While most digital functions are nonfunctional and I have to use analog input now, surprisingly the stereo signal upmix still works. This is great, as this was one of the main reasons why I bought this device.

Maybe I will add a poweron delay to that Standby-lines to avoid the pop at device startup.

With Kind Regards,

ITguy

[ITguy]

Small update, maybe anybody is still reading this. I've now managed to replace the loud pop at power-on by the clicking of a relay. For this I orderd a dirt-cheap delay module (NE555 based) from China:

https://www.aliexpress.com/item/DC-12V-Delay-relay-shield-NE555-Timer-Switch-Adjustable-Module-0-10S/32819633423.html

After removing the screw terminals and trimming the PCB to minimum size, it fitted nicely into the front of the decoder housing. The module is powered by 12,5V tapped from the board to board connector from which I also got the 5V.

I've tried the 5V version of the module first, but the relay didn't trigger with the 4,95V provided by the decoder

With Kind Regards,

ITguy

[depatto]

Hi ITguy, thanks!! I registered just for this!