Pioneer VSX-709S Trouble - diyAudio
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Old 3rd February 2010, 01:20 AM   #1
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Default Pioneer VSX-709S Trouble

I am hoping this is the proper forum for this.

I have some issues with this pioneer receiver, it first came in with a blown front right channel, the resistors to the driver transistors were open, and a 2.2k resistor was open. someone else was in here and already replaced the outputs plus a couple gate resistors and the overload resistors.

I repaired the amp and fixed the +HV and -HV fuses that supply the driver transistors, and the amp now works with some trouble.

After a minute or more the main heatsink starts getting extremely hot on one side, before the whole heatsink gets hot. The sound isnt very loud either. and when you turn it up to a volume where it is decent, the amp cuts out completely into standby from overload protection.

I found test points to measure current flowing through the output emitter resistors, and the channel that was not blown, it idles perfectly at .000 same with the other side.

When audio is playing, the left channel moves upward to .005V DC or so and stays there. The right channel bounces up and down with the music, all the way up to .1v any higher, the amp will just cut off. I am assuming the extreme bias offset from the music playing is what is shutting down the amp.

Anyone have any of these issues with this amp or know what to do next? I am at a loss, its gotta be something stupid as the sound is clean. no distortion whatsoever.
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Old 3rd February 2010, 12:09 PM   #2
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anyone????
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Old 3rd February 2010, 01:41 PM   #3
SQLGuy is offline SQLGuy  United States
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Did you set the bias when you repaired it? If not, that certainly sounds like a place to start.

BTW, "gate resistors"? Does this amp use FETs mixed with bipolars?
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Old 3rd February 2010, 02:23 PM   #4
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Yes the amp has FET outputs. the entire driver/amplifier stage is all bipolar. I pulled each transistor and tested for leaks, and shorts. all appear fine. there are 68 ohm gate resistors that couple into the bipolar drivers. the blown channel had these open, along with several other resistors in the bipolar circuits open. but none of the bipolars were damaged, only the FETs.

Also setting the BIAS does no good. it bounces around like mad when music is playing through it. idle bias is fine on both channels. .000v drop across both emitter resistors. i guess its fine. heck i dunno. the bias pot makes this go up or down. on both channels.

but like i said the un-blown channel the bias hikes to .005v when music is playing, and it stays there, no bouncing... while the right channel which was blown, but repaired, it jumps all over the place with the music. I have a fluke, so i can catch it on the bargraph at the bottom. if i play music, like techno, it jumps up and down like a VU meter. while the left channel does NOT do this...


Edit:

both channels produce clean audio BTW. The channel that does not bounce around, left channel, is the side of the heatsink that gets the hottest. before it propogates down the heatsink to make it the same temp all the way around. The amp uses an IRF540/IRF9540 pair in each channel, and its also to note I am running 4 ohm speakers, as those are my bench test speakers.

Last edited by mbates14; 3rd February 2010 at 02:29 PM.
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Old 3rd February 2010, 02:35 PM   #5
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Oops im sorry, i refer the emitter resistors as the "output" resistors on the FETs. i guess Source resistors? I have the meter set on DCV and measuring between the sources of both outputs, across the two source resistors. each resistor is 0.05 ohm, so a 0.1ohm total resistence, with music it spikes to around .11t to .141v depending. bringing me to around 1.1 amps flowing through the resistors. ouch.. lol.

Last edited by mbates14; 3rd February 2010 at 02:39 PM.
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