I'm having difficulty adjusting the channel positive and negative output current limiters. It seems the pots R541 and R542 have no effect.
All AC voltages are peak measurements not RMS.
If I follow the service manual (Maximum Power Output) and adjust the test input to get a 25.5V output into a 4R load then the output current should be 25.5 / 4 = 6.375A (actual measured is 6.08A). Probably due to losses in the load test leads.
The 0.1R sense resistors R531 and R532 should drop 0.608V with 6.08A current. But instead they drop 0.317V. Which suggests the resistance is actually 0.055R and not 0.1R.
With the pots (R542 and R541) set to max, the Vbe supplied to the sense transistors is only 0.307V.
I don't have any SPS2325 nor SPS2324 sensing transistors (Q516 and Q517) so I am using KSP42B and KSP92B transistors. The tiny current on Vbe of these transistors is tested at about 0.748V. This means the these transistors are always in cutoff independent of pots R542 and R541 settings. Hence, no current limiting.
Causes might be:
Has anyone sorted this out, short of bumping up the current sense resistor values? To say 0.15R?
Any and all help is appreciated.
All AC voltages are peak measurements not RMS.
If I follow the service manual (Maximum Power Output) and adjust the test input to get a 25.5V output into a 4R load then the output current should be 25.5 / 4 = 6.375A (actual measured is 6.08A). Probably due to losses in the load test leads.
The 0.1R sense resistors R531 and R532 should drop 0.608V with 6.08A current. But instead they drop 0.317V. Which suggests the resistance is actually 0.055R and not 0.1R.
With the pots (R542 and R541) set to max, the Vbe supplied to the sense transistors is only 0.307V.
I don't have any SPS2325 nor SPS2324 sensing transistors (Q516 and Q517) so I am using KSP42B and KSP92B transistors. The tiny current on Vbe of these transistors is tested at about 0.748V. This means the these transistors are always in cutoff independent of pots R542 and R541 settings. Hence, no current limiting.
Causes might be:
- the sense resistors (coils) are not dropping enough voltage.
- the sense transistors I used for Q516 and Q517 have higher Vbe than the originals
- this circuit has AC reactive behaviors I don't understand.
Has anyone sorted this out, short of bumping up the current sense resistor values? To say 0.15R?
Any and all help is appreciated.
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Hi f350bronco,
I've attached a service manual for common reference and for other members wishing to contribute.
25.5 VRMS corresponds to 25.5* sqrt(2) or about 36.06V, peak voltage.
The test procedure calls for use of a scope to detect threshold of clipping. Are you using a scope?
It's good that you're looking at drop across R531 and R532 in an attempt to understand the problem. But in addition to driving with lower than specified current, you are missing that the currents flowing in these two resistors are only half-cycle sine wave pulses; R531 delivers the positive polarity, and R532 the negative polarity; a voltmeter across either of these resistors measures only half the voltages you noted in your post--- and there's the issue accurately sensing the RMS voltage of the half-wave waveforms.
Your really need the scope. I suggest using a scope to confirm current limiting behavior and setting the prescribed thresholds. I wouldn't actually short the outputs--- just trust the design and don't court trouble.
Good luck!
I've attached a service manual for common reference and for other members wishing to contribute.
I believe this is the fundamental problem. Marantz is specifying RMS voltage, not peak. So you are driving way below rated power, about half the specified test power.
25.5 VRMS corresponds to 25.5* sqrt(2) or about 36.06V, peak voltage.
The test procedure calls for use of a scope to detect threshold of clipping. Are you using a scope?
It's good that you're looking at drop across R531 and R532 in an attempt to understand the problem. But in addition to driving with lower than specified current, you are missing that the currents flowing in these two resistors are only half-cycle sine wave pulses; R531 delivers the positive polarity, and R532 the negative polarity; a voltmeter across either of these resistors measures only half the voltages you noted in your post--- and there's the issue accurately sensing the RMS voltage of the half-wave waveforms.
Your really need the scope. I suggest using a scope to confirm current limiting behavior and setting the prescribed thresholds. I wouldn't actually short the outputs--- just trust the design and don't court trouble.
Good luck!
You nailed it. Setting up for 25.5V RMS allowed me to dial in the four current limits on my scope. Thank you!