Yes, nothing at TP9 or 10. I get 65v at both other legs of Q4, and nothing at Q6. At R20 and R25, and R21 and R24, I have 65v and -65v respectively. I checked the 1N3070s at D27 and D27 earlier, and they're ok. The ceramic resistors are all giving me 1ohm rather than the rated 0.47. Also, rather than the called for 25mV +- 5, I'm getting a solid 0.958V on each one. Adjusting the trim does change the resistance of the pot itself properly, but it doesn't change the voltage across the ceramic resistors a single bit.
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When you measure a 100k resistor, the ohm or so your probes add doesn't matter. WHen you try to measure 0.47 ohms, it does. For low resistance measures, always short the probes together and see what reading you get. You will then subtract that amount from the resistor reading. SO if your probes have half an ohm resistance, and the resistor also has half an ohm, that adds up to a 1 ohm reading.
It is like weighing your cat. You weigh yourself, then you pick up the cat and weigh yourself again. The difference in the readings is the cat.
Those resistors are either going to be open or they will be OK.
OK OK, 99% of the time.Education is what you're left with after you have forgotten what you have learned.
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What do you mean you are getting 0.958v ON each resistor? You mean to ground? If you are getting a whole volt ACROSS each half ohm resistor, then they are sitting there conducting TWO WHOLE AMPERES. If you are just measuring to ground that just means you have a DC offset on the output. Is that the case? The bias doesn't adjust that. The bias note says 25mv ACROSS each resistor. It says more or less, average them.Education is what you're left with after you have forgotten what you have learned.
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Originally posted by stevenrb718 View PostI'm measuring 3.1mV regardless of pot position. I assume this is not saturating the transistors properly? Would that point to my MPSA06s at Q5 and Q6 as actually being faulty?
THEN, while monitoring Source resistors, carefully adjust the bias to where you start seeing millivolts across all six of your MosFET's Source resistors. We have no idea how well matched they are at this point, so be wary of any that are twice the 25mV nominal voltage needed across each one. If you get to where you finally have near-equal Source Voltage thru those 0.47 ohm resistors, you've achieved proper bias. AND, you should see proper voltages at TP9 & TP10.
Logic is an organized way of going wrong with confidence
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Originally posted by stevenrb718 View PostWell I took a nice, long break away from the amp as it was bothering me. I replaced both Q5 and Q6 with new transistors. The old ones both measured lower hFE than the new ones, but nothing changed. I'm still not getting voltages at TP9 and TP10. What am I missing?
As I had suggested in my last post, lifting up the gate and source resistors on the MosFET output stages is recommended, so you're JUST working with this Driver Stage, and not suddenly have the output stage turn on hard enough to kill any of the MosFET's. You still have a fault in this bias circuit that needs to be found. It won't work until you solve that.
Logic is an organized way of going wrong with confidence
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Originally posted by nevetslab View Post
Maybe a bad (open) bias pot? Or any of the resistors in the bias adj string R27 thru R30, including the bias pot. If that string is open, I believe you'd have NO current flow thru Q4 & Q13, hence NO voltage at TP9 & TP10. Man, this amp is one mean bugger to keep taxing your/our ability to find where the fault is.
As I had suggested in my last post, lifting up the gate and source resistors on the MosFET output stages is recommended, so you're JUST working with this Driver Stage, and not suddenly have the output stage turn on hard enough to kill any of the MosFET's. You still have a fault in this bias circuit that needs to be found. It won't work until you solve that.
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R67/68 & R69/70 are 10W resistors, dropping the supply voltages that run the power amp down to +/- 16V Zener regulated, and are continuous duty, so they will ALWAYS be running hot, but no doubt within their rating in circuit.
I assume your test leads are measuring ok. When you short them together on your low ohms scale, what do you read? Typically around 0.1 ohm or less for proper probes.
At this point, I'd measure from component lead to component lead in the Voltage Gain stage and bias circuit. Essentially the series current strings from V+ thru all components to V- for those stages. This is now looking for any open traces between components, since thus far, nothing is found to be defective, but....you're NOT getting voltage readings at TP9 & TP10. If there IS a current path it should measure a voltage drop, or there's open circuit(s) someplace yet to be found.Logic is an organized way of going wrong with confidence
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