A bit of a strange problem - my Dual-Mono Buff's been working fine for many months but this morning, following a powering-down then power up, the RH channel no longer works. The mute light's on for that channel, and checking the voltages to Buffalo, connections etc. all seems well. BUT the BP HD Placid supplying the Legato on that channel showed only 2V+, 4.5V - . I disconnected the load (both BP HD Placids supply the relevant Legatos and Ballsie -Lites for unbalanced output) & readjusted the shunt and voltage (14.5V in this case). However, on connecting up & under load the voltages drop way down. The shunt voltage, however, remains at 0.06.

I'm really puzzled by this. Surely the power to the Legato wouldn't influence the lock to the DAC, and what would cause the voltage of the BP HD Placid to drop so much under load?

All help and advice gratefully received. I'm very much a newby to DIY electronics so please be gentle...

Edited by user Wednesday, December 28, 2011 8:38:48 AM(UTC)  | Reason: Not specified

Thanks for your help Leon, I shall try that tomorrow. I still can't understand why a possible fault with the Legato should affect the lock on the Buffalo II though?

OK, today I've had another try at isolating this problem, but it's still confusing! I tried swapping BP HD Placids round & this resulted in the voltages of the original LH Placid dropping, but only on the +V side. Disconnecting the Ballsie Lite made no difference. I then swapped the Buffalos round & reconnected up to the Sp/DIF input but the non-locking problem migrated with the Buffalo, so now the LH one doesn't lock. Previously, although the RH channel wasn't working, the LH channel sounded fine but with them swapped all I get is a very distorted peep from the RH channel.

When powering down, I notice the Placid BP on the RH side takes a while before the LEDs fade, whereas the LH's fade instantly. Without load, the RH Placid BP reads fine & everything adjusts OK, it's just under load when it drops, but the readings are inconsistent - today, without adjusting anything, it now reads (under load) 9.77V+, 2.87V-, with the shunt current high on the -tive side, low on the +tive side.

It does seem to me that both the Legato and the Buffalo II on the RH channel have developed faults simultaneously. Is this a likely scenario?

Next thing I guess is to pull it all apart & reset up as a single Buffalo DAC so I can more easily swap units around. There are no obvious signs that anything is amiss, and all LEDs work.

Unfortunately I'll not have the chance to do anything else for a week or so but I'll keep you posted!

Saw your posting earlier on today.

I agree with you, those pots are unreliable. I just had an incidence, been away for a couple of hours. When I returned and fired up the DAC - only very faint sound coming to my ears.

Measured the shunted current on the Placid BP. = 0mA on the positive rail and only 3v output. Readjusted to 80mA, and the voltage returned to 12.5v - and it's running again.

Remind you - the DAC was running perfectly a couple of hours ago.

Fixed resistors! At least for current set. That must be the solution.

Lucky you.

In the past this usually indicates a problem with R4 soldering. Double check it.

I would always suggest using fixed resistors on Placid HD once you know your desired operating point.

I would suggest just running it at about 100ma when hot. There will be some thermal coefficient because LEDs as a voltage ref will always have some.

Looking through the posts on the Trident thread, I notice that likely culprits are the op-amp & the two transistors. The former is a Nat. Semiconductor LMP7731MA and the latter, Fairchild 860CWs? Is this correct please?

Edited by user Tuesday, January 10, 2012 4:57:58 AM(UTC)  | Reason: Not specified

I finally got round to attempting a repair on the Trident but no joy. I renewed both transistors and the op-amp. Twice (just in case), The Trident only outputs 0.8 volts. Any other suggestions please?