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Marantz 1030: Extreme Bias Voltage and Distortion on Left Channel

A

AudioRestore

New Member
Hey all. I have this Marantz model 1030 amplifier with some problems.

Problem 1: Left channel has extreme distortion under any kind of load. If you crank the bass control, it distorts to hell and gone.

Problem 2: When attempting to set the bias voltage for the left channel, it's ENTIRELY too high. The service manual states 7.5mVDC at idle. It won't go below 130mV. The right channel has no distortion and I was able to successfully set it's bias current to 7.5mV.

I think what's going on here is that something is pulling the bias voltage way up on the left channel and when you load that channel down, the high bias voltage causes the transistor to be driven into distortion.

What might be pulling the bias voltage up? Will this kill the transistors in short order?

TIA.

Service Manual: https://data2.manualslib.com/pdf4/89...57cabed1622d74
 
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Measure the dc voltages to ground at J709. Repeat for J713, J715.
Black mm probe connected to chassis.

EDIT: Also at J711
 
Yeah, it was just a punt, make sure nothing strange going on. I would have liked to see J715 to have been closer
to half rails voltage (25V). Decimal points are ok, expect H001 Vbe (J711,J713) to be closer to 0.6-0.7V

Does the left channel bias respond to trimmer adjustment or is it always 130mV.

I assume you don't have an oscilloscope, would be nice to see whats going on/part of the wave form that's distorted.
Also to ballpark the symmetrical clipping as per sm.

On one hand I'd be tempted to nudge R723 (for symmetrical clipping) to see if it has any effect on voltage at J715 ie, closer
to 25Vdc? Also measure voltage between J707(RED) and J705(BLACK), expect about 1.2Vdc
 
mbz said:
Yeah, it was just a punt, make sure nothing strange going on. I would have liked to see J715 to have been closer
to half rails voltage (25V). Decimal points are ok, expect H001 Vbe (J711,J713) to be closer to 0.6-0.7V

Does the left channel bias respond to trimmer adjustment or is it always 130mV.

I assume you don't have an oscilloscope, would be nice to see whats going on/part of the wave form that's distorted.
Also to ballpark the symmetrical clipping as per sm.

On one hand I'd be tempted to nudge R723 (for symmetrical clipping) to see if it has any effect on voltage at J715 ie, closer
to 25Vdc? Also measure voltage between J707(RED) and J705(BLACK), expect about 1.2Vdc
Click to expand...

A) It responds, but it's always entirely too high. (>120mV+)
B) Nope, no oscilloscope.
 
mbz said:
Yeah, it was just a punt, make sure nothing strange going on. I would have liked to see J715 to have been closer
to half rails voltage (25V). Decimal points are ok, expect H001 Vbe (J711,J713) to be closer to 0.6-0.7V

Does the left channel bias respond to trimmer adjustment or is it always 130mV.

I assume you don't have an oscilloscope, would be nice to see whats going on/part of the wave form that's distorted.
Also to ballpark the symmetrical clipping as per sm.

On one hand I'd be tempted to nudge R723 (for symmetrical clipping) to see if it has any effect on voltage at J715 ie, closer
to 25Vdc? Also measure voltage between J707(RED) and J705(BLACK), expect about 1.2Vdc
Click to expand...

H001 Vbe is 0.54V.
J715 to ground is affected by R723, got it dialed in to 25VDC.
J707 to J705 measures 1.1VDC.

rjsalvi said:
What mbz said...and have you checked the integrity of the solder joints, or for broken traces in the general vicinity of H703? Also, H005 is the temp sensing (dual) diode for that channel...does it have the same voltage drop as the other channel's (H006)? Do the voltages of H701 match those of H702? It's entirely possible (probable?) that your high bias is clipping the signal, OR the bias issue is a separate one from the distortion issue. If you have PRE / MAIN jacks in the rear of the unit, send your phone output into the MAIN jacks and see if it still distorts. If so, you've really narrowed down the suspects.
Click to expand...

Collector of H701: 27VDC
Collector of H702: 26VDC
Both temp sensing diodes are dropping 1VDC.
No PRE/MAIN jacks, but I injected a signal directly into the amp board, and the left channel still distorts.
 
Wondering if you made any progress with this?
Need to sort out the bias issue on left, R729 set H709b/e voltage then onto H001b/e voltages.
You should check R729 is functioning ok, with amp powered off/unplugged measure the resistance between the wiper
(middle leg) and the output leg (one combo will probably give near short, the other probably close to 470 ohms).
Voltage at H709b is too high, cause low voltage drop at R729 cause, either lower current through R729 or lower
resistance at R729 (ie trimmer failure). Diode check H713, H715, maybe leaking some current?
 
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