There's a lot of possible causes, but I'm speaking about 'no' measurable bias current, or current that fluctuates from a normal value to something lower, and back again (or not).
The root of the problem is Pioneer's protection design, which is effective but horribly complex. The Spec-2 contains no less than eight (8!) relays, performing various functions ranging from a soft-start, to switching in an alternate set of resistors to change the current limiting curve when the amp is switched from 8 ohm to 4 ohm operation.
The relay I am speaking about at the moment interrupts the flow through the current mirror in the bias section of the main amp. The protection circuit of the amp does not allow bias current to flow until the last moment of operation. I'm not particularly crazy about the design, but it is what it is, and if you bypass the relay, you'll get nasty thumps as the protection circuit relinquishes control on power up.
What I have been finding with increasing frequency is that, once the protection has finished its power-up sequence, bias is unsteady, low, or non-existent. This will result in various levels of distortion in the affected channel - ranging from gritty highs and unbelievably flabby bass (think 'nearly unlistenable') to near total loss of signal.
The culprit is RL1 (as designated on the individual amp schematic and the foil pattern), or RL5 'a' and 'b', as designated on the large fold-out schematic. There is one on each driver board, and you can quickly see on the schematic that this relay is designed to interrupt the bias current. The stock Pioneer relay is, frankly, crap, and is likely the cause of a lot of crappy sounding Spec 2 amps.
Here is what the part looks like (circled in yellow):
The root of the problem is Pioneer's protection design, which is effective but horribly complex. The Spec-2 contains no less than eight (8!) relays, performing various functions ranging from a soft-start, to switching in an alternate set of resistors to change the current limiting curve when the amp is switched from 8 ohm to 4 ohm operation.
The relay I am speaking about at the moment interrupts the flow through the current mirror in the bias section of the main amp. The protection circuit of the amp does not allow bias current to flow until the last moment of operation. I'm not particularly crazy about the design, but it is what it is, and if you bypass the relay, you'll get nasty thumps as the protection circuit relinquishes control on power up.
What I have been finding with increasing frequency is that, once the protection has finished its power-up sequence, bias is unsteady, low, or non-existent. This will result in various levels of distortion in the affected channel - ranging from gritty highs and unbelievably flabby bass (think 'nearly unlistenable') to near total loss of signal.
The culprit is RL1 (as designated on the individual amp schematic and the foil pattern), or RL5 'a' and 'b', as designated on the large fold-out schematic. There is one on each driver board, and you can quickly see on the schematic that this relay is designed to interrupt the bias current. The stock Pioneer relay is, frankly, crap, and is likely the cause of a lot of crappy sounding Spec 2 amps.
Here is what the part looks like (circled in yellow):