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Marantz 140 amp DBT dead short question

So, just a hypothetical question, this unit was in service for several months before I boxed it and put it away. Obviously, the filter cap was questionable and was failed by the time I took it out of storage. What damage could have been caused if I kept running this unit and it failed while in service?
 
Educated guess - Cap probably failed on power up. Initial surge after being powered down for so long. I guess it wouldn't have failed if continuous use. (this conjecture really only applies to big filter caps. If a unit has been off for long periods of time, I bring them up the first time on a variac…)
Dr. Audio brought up the caps in his first two posts.... (just sayin')
 
jd3rdcoast said:
I’m curious how a large value Leaky vs. Shorted capacitor behave when testing with DMM capacitance check when value is higher than available scale? I have struggled in past trying to determine if Filter caps need replacing. I typically just check voltage and look for high ripple with a scope. I’m guessing that does not tell everything.
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I did not say to use capacitance range, just use resistance range. this test only shows up a shorted or very leaky cap that would cause the dbt to light up. I have a Heathkit capacitor tester that checks for leakage under various supply voltages.
 
Quick question, not at this point yet, but figured I ask now so I am ready. In the manual it says to adjust the DC balance with an oscilloscope, can I adjust it with just a DMM and if so how? And for the bias, it states to turn the pot CCW and bring the unit up on a Variac and then adjust the bias to 10mv above the initial reading. It doesn't really say where the bias should be, is there a value associated with this process that reflects the "normal" bias setting without turning the pot all the way down?
 
For the bias, they want you to measure the voltage across TP01 and TP02. This gives the voltage across the 2 emitter resistors. By Ohm's Law E (voltage) = I (current) x R (Resistance). So, to calculate the current, you divide the voltage by the resistance. The emitter resistors are 0.2 Ohms. 2 of them in series is 0.4 Ohms.
A good value for bias current is 20 - 40 mA. Let's adjust it for 30mA. So to calculate the voltage you want, it's current x resistance: 0.03 x 0.4 = .012V or 12mV. So turn the pot all the way down, like they said. Raise the variac up and watch the current. If it goes above 40mV, back it down and check things out. If you can connect only one amp at a time, do that. After you can set the bias and offset on one, leave it there and do the other channel. Then turn it on with both channels connected and no dim bulb tester and no variac, and reset the bias because it will now be higher than you want it. Allow it to sit for 20 minutes, checking and re-adjusting the bias as you go. When it stops changing, check the offset and re-adjust. Then you're done.
Do not connect speakers while you are adjusting, only when you're done, and always turn the amp off before connecting anything!
 
dr*audio said:
measure the voltage across TP01 and TP02
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Looking at the SM and the board are these test points J701 and J702? Also, after doing the right channel, should I remove it to do the left channel and then have both channels installed after adjusting each with only one channel installed?
 
I think what dr*audio is doing is answering your question about what the bias should be set to. The SM doesn’t spell It out, but if a typical bias current should be 30 mA, as he says, the math says you get that by setting the bias voltage to 12 mV across both emitter resistors. You measure that between J 701 and J702 (TP01, TP02) test points, and adjust with R742. DC Offset is measured at the speaker terminals, and adjust with R741 as close to 0 mV as possible.
 
Steven Tate said:
The SM doesn’t spell It out, but if a typical bias current should be 30 mA, as he says, the math says you get that by setting the bias voltage to 12 mV across both emitter resistors. You measure that between J 701 and J702 (TP01, TP02) test points, and adjust with R742. DC Offset is measured at the speaker terminals, and adjust with R741 as close to 0 mV as possible.
Click to expand...
Thank you very much Steven.
 
Same as what Steven Tate said. You don't have to remove the first amp once you get it adjusted, just install the other amp and adjust it.
 
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