For the power rail voltages to drop to only +/-10V with respect to ground, there must also be quite a bit of extra heat dissipated somewhere there - check for hot spots. You may have shorts, wrong resistor codes, reversed cap polarities or dead semis there. So yes, this means something is very wrong and you need to review any assumptions made about your work thus far .
it wouldn't be the first time i've pulled a "new" capacitor out of my parts stock only to find it it bad.
so please tell me what sort of testing was used to make your determination that it's "good"
and as i've already mentioned the carbon contamination of the PCB could well trump/fault a new part.
so please tell me what sort of testing was used to make your determination that it's "good"
and as i've already mentioned the carbon contamination of the PCB could well trump/fault a new part.
I think I know what caused the rail voltage drop.
it’s because I have a series testing bulb connected in the main, to avoid damage in case of any shorts.
So I noticed when power rail voltage drop, the bulb is a lot brighter. In which case, the bulb took more voltage drop. Indicating there are some kind of shorts in the amp ( or at least current is much larger).
I tried to remove the short circuit protection bulb, the power rail voltage goes much higher. but the amp got hot very quickly. So I powered it off.
In all these tests I have speaker attached, and have music played. The music plays fine. Ever since I replaced c10, c13. Music sounds fine, without distortion.
despite it sounds ok now. The original problem still haven’t gone away completely. This channel still suffer from wrong voltage:
r8 voltage still negative, hence tr1 base voltage is negative.
I also found that ever since the circuit protection bulb goes brighter, and power voltage drop, the voltage on R15 is a lot larger, around -4.7v, while normal voltage should be around -2.5v.
When I remove the circuit protection bulb, R15 goes up to -12v, make it very hot.
it’s because I have a series testing bulb connected in the main, to avoid damage in case of any shorts.
So I noticed when power rail voltage drop, the bulb is a lot brighter. In which case, the bulb took more voltage drop. Indicating there are some kind of shorts in the amp ( or at least current is much larger).
I tried to remove the short circuit protection bulb, the power rail voltage goes much higher. but the amp got hot very quickly. So I powered it off.
In all these tests I have speaker attached, and have music played. The music plays fine. Ever since I replaced c10, c13. Music sounds fine, without distortion.
despite it sounds ok now. The original problem still haven’t gone away completely. This channel still suffer from wrong voltage:
r8 voltage still negative, hence tr1 base voltage is negative.
I also found that ever since the circuit protection bulb goes brighter, and power voltage drop, the voltage on R15 is a lot larger, around -4.7v, while normal voltage should be around -2.5v.
When I remove the circuit protection bulb, R15 goes up to -12v, make it very hot.
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I tested with multimeter, the capacitance is fine, and it is not shorted. It’s newly replaced cap, if it does goes wrong again, it won’t be the source of the problem, it more sound like the result of the problem being charged negatively. So I can’t just keep swapping it, that make no sense. I probably better order some dual polarity caps to replace these as Citizen suggested
please describe your test method... because as for one, i ,after 30 years of doing service work on electronics cannot conceive of a test with nothing more then a multimeter and say that a cap is ok...
The strange thing is the sound is good now, as I said, since the first c10, c13 replace. Despite the voltage still incorrect.
how could this possible?
how could this possible?
do tell,is this from the same batch of caps that was used for the previous replacement?
if component substitution isn't changing operating conditions then the only thing left is the PCB...
and thanks for the clear description of your test procedure!!
if component substitution isn't changing operating conditions then the only thing left is the PCB...
and thanks for the clear description of your test procedure!!
See pm.
Replace C13, C14 and C10 for bypolars (or reversed-series polar caps), even if it's the wrong value. They're dc blockers only.
Replace C13, C14 and C10 for bypolars (or reversed-series polar caps), even if it's the wrong value. They're dc blockers only.
Here are what I measured based on current situation (protection bulb bright, hence dropped rail voltage, amp sound fine ). Please not the voltage drift a bit during measuring because of the series bulb used, it makes power rail voltage not stable. so the voltage I measured across the circuit, will be some inconsistency depends on time its been measured. but it should still be able to provide some kind of knowledge about what is going on.
Please only look at the voltages in Bold, not in circles which comes from original schematic.
Please only look at the voltages in Bold, not in circles which comes from original schematic.
and here are the measurements from the other (good) channel. Please note, the rail voltage goes up to 17v in this measurement, its just the variation between two power on, and time.
Do you have bench power supplies--- i.e. adjustable voltage and adjustable current limiting?
Do you have a drawing similar to #53 of the working amp with bias voltages shown but operating at +- 30V, your original goal?
Do you have a drawing similar to #53 of the working amp with bias voltages shown but operating at +- 30V, your original goal?
No I dont have bench power supply.
If I remove the bulb, power will go up to 30v, but its risky to burn something, it gets very hot quickly
#52 is the problem channel, initially sound is distorted as described in #1, then I replaced c13 and c10, sound is normal, I cant hear obvious distortion. But voltage still measured problematic, I think it will fail again if I leave it like this and run for a long time.
#53 is the good channel measurement
Working channel is also hot, its the nature of this amp design — class A bias. It will problematic if run say 10 minuets or more with out the cover (which is basically heat sink) on
The problem is the broken channel. I remembered initially, the sound was fine on this channel, but after I run it for a while, its broken, with a serious of element burned, including c15 (short), d9 or d10, can’t remember which one, zd1, and also r26 (become infinity), I really dont want go through that again. So not quite willing to remove the protection bulb