They are two different adjustments. One nulls the DC to the speaker. The other sets the no-signal current so soft passages are not "hoarse" OTOH the amp does not burn-up at idle. Both should be done.
It is odd that bias adjust needs major disassembly, but I do not know this model.
Use a DMM. The manual is, I assume, from before great low-price DMMs. A VTVM is ultra-handy for some measurements, but not for setting bias. And will typically be "more accurate" than a needle VTVM, which typically had a 1.5V (1500mV) low scale. Not that high precision is needed: "50mV" bias can be 40mV or 60mV no-problem. 5mV would be hoarse, 500mV could be HOT.
The cross-over bias adjust the class-A portion between the positive and negative Class-B peaks of the waveform. D. Self points out that too much overlap can cause distortion because with both drivers conducting, the combined output impedance may be lower than when only one side is conducting. So there is an ideal bias, not too much and not too little. It is common to skimp in order to improve thermal stability with ~problem amplifiers, at the price of a small amount of cross-over distortion.
"Current dumping" amplifiers operate the main power transistors in Class-C mode ("dumping") and use a passive network to combine the distorted main dumping output with the (class-A) driver output that has the opposite distortion as a result of the feedback attempting to correct the "dumping" output.
The cross-over is frequency/speed dependent because transistors tend to float on unless they are actively driven off as well as on. This is the importance of driver cross coupling and the reason old amplifiers (1970-80) often failed when a high frequency signal causes both positive and negative output transistors to conduct at the same time creating a massive "shoot-through" current.
"Current dumping" amplifiers operate the main power transistors in Class-C mode ("dumping") and use a passive network to combine the distorted main dumping output with the (class-A) driver output that has the opposite distortion as a result of the feedback attempting to correct the "dumping" output.
The cross-over is frequency/speed dependent because transistors tend to float on unless they are actively driven off as well as on. This is the importance of driver cross coupling and the reason old amplifiers (1970-80) often failed when a high frequency signal causes both positive and negative output transistors to conduct at the same time creating a massive "shoot-through" current.
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steveu- I think what Doug Self is saying is that correct bias adjustment is vital to proper operation of these old amps. I've experienced how a massive shoot-through current can cause a smoking amp.
Many times, my wife and son have asked why I spend so much time working on my "piece of junk" Marantz 1200 integrated amp. There are very few things I like more than sitting in my little shop, listening to vinyl LPs played on my DIY amps while working on my "piece of junk amp." I think that's the spirit of DIY. I obviously don't do this for a living, I would promptly go broke if I did.
I appreciate the answers from all you guys and gals who know more than I do about audio electronics. Happy Saturday.
Many times, my wife and son have asked why I spend so much time working on my "piece of junk" Marantz 1200 integrated amp. There are very few things I like more than sitting in my little shop, listening to vinyl LPs played on my DIY amps while working on my "piece of junk amp." I think that's the spirit of DIY. I obviously don't do this for a living, I would promptly go broke if I did.
I appreciate the answers from all you guys and gals who know more than I do about audio electronics. Happy Saturday.
VTVM is an old way of saying "Multimeter" 🙂 As long as you have a meter that measures millivolts, that's fine.
Can not bias the left channel of my 2330B, the reading is almost 0mv, the output 1khz sine wave is very ugly at the scope, tried moving the 300ohms pot but nothing happens, changed to a new pot and nothing, try to measure directly at the base of the outputs and the reading is the same, almost no 0mv, what am I missing? The manual indicates 16mv.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Schematics here: Marantz 2330 - Manual - Stereophonic Receiver - HiFi Engine
What does the waveform look like? A few faults are possible - faulty output transistors, drivers, the bias device, the diode stack...
What does the waveform look like? A few faults are possible - faulty output transistors, drivers, the bias device, the diode stack...
If the earlier photo is anything like yours, you probably have bad pot(s) The pot is usually in the lower side of the VBE multiplier so that when the pot fails, you get cross-over distortion and not overheating and failure. The pots I see in the earlier photo are notoriously bad. If it was my amp, I would get a good pot and find what value it needs to be set and then replace the pot with fixed resistors. Fixed resistors won't drift and corrode.
Screenshot-20181012-154342 — imgbb.com
Screenshot-20181012-154337 — imgbb.com
No, that's not my amp photo, the trim pot was checked and changed. Above are the two sine waves, the good and the bad one.
Also checked the power supply, changed between channels the plugs and no mv yet to the left channel. Also changed all the electrolytics at the amp board and checked for bad solder and appears everything fine. Both channels are with 0dc at output what's it's good, but only the right channel gets 16mv, the left one is a bitch.
Screenshot-20181012-154337 — imgbb.com
No, that's not my amp photo, the trim pot was checked and changed. Above are the two sine waves, the good and the bad one.
Also checked the power supply, changed between channels the plugs and no mv yet to the left channel. Also changed all the electrolytics at the amp board and checked for bad solder and appears everything fine. Both channels are with 0dc at output what's it's good, but only the right channel gets 16mv, the left one is a bitch.
Most do. And if you're still using one with a meter..... well it's time to join the 21st century! 🙂 I have an old Simpson 260 in the museum of by gone gear.
There are jobs that are easier with a meter... Simpson 260 it is something that 21st century still allow.
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