DC on output Pioneer VSX-1021 - Page 1
Forum section: electronics / repair

author: synth-dude
author_rank: Contributor
timestamp: February 05, 2017, 12:27:54 pm
content: I have this amp in protection mode, its a Pioneer VSX-1021, the MCACC light is flashing. Service manual says it means DC on the output.I pulled out the amp board and tested the transistors, found they were all ok, except for FL. I found the main power transistors are both shorted, and also the pre-driver stage same channel. All other transistor tested OK in circuit. I pulled the same transistors off SBL (Surround Back Left, its a 7.1 amp), thinking that would get left and right working, until I buy the replacements for the surround later. Left Surround channel left with no transistors in it.Powered up and same problem again, protection mode. I dont know if the Left channel is working because its saying there are still DC issues. So I measured the DC on each output. Measured at the test points for biasing which are straight off the main transistor outputs, not the terminals on the back.Now it gets weird, I get +60v on each channel (all 7 channels). I'd say this is not normal. I'm looking for advice on how to proceed from here, I'm thinking replace all the transistors on the board, all channels, and check all passive components.Is this the best way forward or is there a better way?Also, because the NPN transistors are on the positive rail, does that mean the +60v means each NPN is now shorted? Sounds reasonable to me but maybe its not that simple?Thanks for any help anyone can give.Cheers.

author: Seekonk
author_rank: Super Contributor
timestamp: February 05, 2017, 02:07:34 pm
content: Quote from: synth-dude on February 05, 2017, 12:27:54 pmI caqn't think of anything worse to do. Electronics problems should be diagnosed, not taken a shotgun to. First find a 60W incandescent lamp and put that in series with the line power. This will keep everything from burning up when powered. Then look at the bias on each transistor to diagnose if it should be conducting or not. A great way to learn electronics. Or you can just keep blowing up fuses and replacing parts.

author: Armadillo
author_rank: Super Contributor
timestamp: February 05, 2017, 02:25:14 pm
content: Quote from: Seekonk on February 05, 2017, 02:07:34 pmLuckily its not a gryphon amp.The output transistors are supposed to be very well matched by the manufacturer. its like the "GEMs" identity of the amp. Only changed when it is actually damaged. And be careful of fake transistors in the market.

author: Armadillo
author_rank: Super Contributor
timestamp: February 05, 2017, 02:55:43 pm
content: Short the input and Measure each of the amp from the each of the output points shown in the attached pic [with 8 ohms load connected], with respect to the "speaker common" to identify which of the amp is turn-on 60V, first step. "First find a 60W incandescent lamp and put that in series with the line power."There are 7 amps, I think.

author: xavier60
author_rank: Super Contributor
timestamp: February 05, 2017, 11:28:49 pm
content: If the output transistors were short circuited, some of the emitter resistors are likely to be open by now.If there is damage in the idle current setting circuitry, the output transistors might conduct heavy rail to rail current.The bases can be connected together for initial testing as a precaution.