TC Electronic Classic 450 Bass Head - no sound

[GregS]

Hello,

I'm presently trying to troubleshoot a TC Electronic Classic 450 amp in which there is no sound. The amp's owner reports that nothing happened when it was last used - just that one day he powered it up and it was in this state. The green LED on the front panel is lit indicating power on but there is no sound at the speaker at all. No hiss, hum, buzz, nothing. Also it appears that no signal is even getting through the pre-amp. With a signal at the input the compression LED's do not illuminate and there is no signal at the effects loop send. The obvious such as gain, volume not being turned up, mute button on etc. has also been eliminated. Also, there are no burnt parts or other obvious signs of damage inside.


The power supply seems to be good as the rails are measuring plus and minus 65VDC which are the correct voltages. However there is a red LED illuminated that is beside the speaker out jack on the same circuit board as the AC power receptacle. It is labelled "Protect" and "LED1". So it seems that the amp is in some sort of protection mode for some reason. The problem is that I don't know what that LED is supposed to indicate and have had no luck searching the internet so far. I'm also not able to find a schematic for the amp so can't get an idea from that either.


As a last resort before telling the owner that he'll have to send it to TC Electronics I was wondering if anyone on this forum would have any experience or insight into this.


Thanks,

Greg

[nosaj]

Hello,

I'm presently trying to troubleshoot a TC Electronic Classic 450 amp in which there is no sound. The amp's owner reports that nothing happened when it was last used - just that one day he powered it up and it was in this state. The green LED on the front panel is lit indicating power on but there is no sound at the speaker at all. No hiss, hum, buzz, nothing. Also it appears that no signal is even getting through the pre-amp. With a signal at the input the compression LED's do not illuminate and there is no signal at the effects loop send. The obvious such as gain, volume not being turned up, mute button on etc. has also been eliminated. Also, there are no burnt parts or other obvious signs of damage inside.


The power supply seems to be good as the rails are measuring plus and minus 65VDC which are the correct voltages. However there is a red LED illuminated that is beside the speaker out jack on the same circuit board as the AC power receptacle. It is labelled "Protect" and "LED1". So it seems that the amp is in some sort of protection mode for some reason. The problem is that I don't know what that LED is supposed to indicate and have had no luck searching the internet so far. I'm also not able to find a schematic for the amp so can't get an idea from that either.


As a last resort before telling the owner that he'll have to send it to TC Electronics I was wondering if anyone on this forum would have any experience or insight into this.


Thanks,

Greg

Test the speaker. There are a lot of surface mount components in there, so if it's not the speaker and I cannot find a schematics it'd probably be best to send it in to TC?
Nosaj

[GregS]

Test the speaker. There are a lot of surface mount components in there, so if it's not the speaker and I cannot find a schematics it'd probably be best to send it in to TC?
Nosa

Thanks for responding. It's definitely not the speaker as I'm using one of my own that I know works. Also, there's no signal at the output of the pre-amp either which I've confirmed. Unless someone here has already experienced this, knows what exactly it is, and it's fixable I'm going to simply give it back to the owner who can decide whether he wants to send it to TC Electronics to be fixed or not.


Thanks,

Greg

[g1]

You said the 65V were the 'correct voltages'. I guess they are marked on the board? Otherwise you would not know without the schematic.
Are there any lower voltages marked, like +/-15V ? A loss of one of those might disable the preamp and turn on the protect light. Otherwise, 'protect' mode usually disables the power amp, so not really sure what is going on there.
TC is a pain. No service support, just send it back for expensive repairs.

[GregS]

You said the 65V were the 'correct voltages'. I guess they are marked on the board? Otherwise you would not know without the schematic.
Are there any lower voltages marked, like +/-15V ? A loss of one of those might disable the preamp and turn on the protect light. Otherwise, 'protect' mode usually disables the power amp, so not really sure what is going on there.
TC is a pain. No service support, just send it back for expensive repairs.

Yes, the +/- 65V is marked on the board. There are no lower voltages marked. I suspect that most likely the problem is a malfunction of the diagnostic circuit and not an actual problem with the amp.


Greg

[J M Fahey]

I suspect that most likely the problem is a malfunction of the diagnostic circuit and not an actual problem with the amp.

Just thinking aloud, it´s statistically WAY more possible a diagnostic circuit detecting a real fault (that´s their job and faults are an everyday reality) than a confused diagnostic circuit detecting a fault that is not there and muting a perfectly good amplifier.
Just sayin´

[Jazz P Bass]

There is a block diagram in the OM that shows a common mute circuit on the input and the power amp.

I would expect that if the amp is in Protect that these would be triggered.

It is an awfully small unit (8lbs.)
So It most assuredly uses an SMPS and a Class D output.

[J M Fahey]

Yes, the +/- 65V is marked on the board. There are no lower voltages marked.

In any case the preamp will work with +/-15v if Op Amps are present and if onboard DSP it will work with +5 or +3.3V (not my area here) .
They may be derived from 65V rails (possible but quite inefficient) or most probably from own low voltage windings at the ferrite transformer.
So look at it and adjacent power supply area, I bet you will find 15V regulators, maybe also a 5V or 3.3V one.
Test for voltage both raw going in and regulated going out.
Also if Op Amps present, although SMT type, they should still have +/-15V on pins 8 and 4 .
All this generic stuff can be done without schematics.
Post a couple gut pictures and board closeups, we might suggest test here or there.

[GregS]

Thanks for the suggestions. However, I don't want to spend any more of my time on this or waste anymore of anyone else's time either. Even if I were able to find a voltage missing somewhere it's not likely I could do anything about it anyway. It's almost all modern miniaturized surface mount components that I'm not able to work with. It seems that this is a very common issue people have had with these amps so I was just wondering if someone on here might already know what the problem is and if it is fixable. Otherwise I'm just going to give it back to the owner and tell him that his only option is to send it to TC Electronics for repair.


Thanks again for the responses.

Greg

[nosaj]

In any case the preamp will work with +/-15v if Op Amps are present and if onboard DSP it will work with +5 or +3.3V (not my area here) .
They may be derived from 65V rails (possible but quite inefficient) or most probably from own low voltage windings at the ferrite transformer.
So look at it and adjacent power supply area, I bet you will find 15V regulators, maybe also a 5V or 3.3V one.
Test for voltage both raw going in and regulated going out.
Also if Op Amps present, although SMT type, they should still have +/-15V on pins 8 and 4 .
All this generic stuff can be done without schematics.
Post a couple gut pictures and board closeups, we might suggest test here or there.

Thanks for making suggestions on various ways to troubleshoot in leiu of a schematic. I really need to loose my crutch for them.
nosaj

[J M Fahey]

Oh, had to get used to repair >50% of stuff without access to schematics or service manuals.
As the Japanese say: what does not kill you makes you stronger