I think it may be the same point being made here but using the lightbulb limiter on a class A amp. is not likely to be helpful. Since you have established that one channel is working OK, disconnect +/- DC power to that channel and focus on the problem channel. Measure the current in each supply rail to the bad channel and let us understand something specific to it, without the influence of the good channel loading the power supply as well.
Last edited:
Tr1 and tr2 here is pnp transistor, the current should flow out of base, like this.
But here I don’t understand where the base current of tr1 and tr2 go，seems everywhere they connected to has higher voltage than the base.
what is actually happening here?
the working channel as comparison looks fine and logical
"What sets the quiescent current? You need to look to R30/31 for the answer. There is a small voltage across these resistors, and assuming that R30/R31 are accurate in value, you can determine that the standing current is 700mA approximately.
These small voltages are in fact DC offsets within each of the amplifier halves. Normally, one would "design out" DC offsets, but in this amplifier they are deliberately introduced by the 3M7 resistors (R6/11). Note that the exact values of these resistors might vary - and if your example uses 0.47Ω output resistors, expect to find something like 1M8."
so R6 and R11 sets the DC offsets, which goes wrong badly in bottom half the channel in my case
I highly suspect the old R6 went way out of spec causing this.
I cant desolder it right now because my soldering station is down for some reason, so cant measure it right now.
will see when my new soldering station come.
These small voltages are in fact DC offsets within each of the amplifier halves. Normally, one would "design out" DC offsets, but in this amplifier they are deliberately introduced by the 3M7 resistors (R6/11). Note that the exact values of these resistors might vary - and if your example uses 0.47Ω output resistors, expect to find something like 1M8."
so R6 and R11 sets the DC offsets, which goes wrong badly in bottom half the channel in my case
I highly suspect the old R6 went way out of spec causing this.
I cant desolder it right now because my soldering station is down for some reason, so cant measure it right now.
will see when my new soldering station come.
The high currents flowing are the subject interest. When they are throttled by the bulb limiter, you have no simple way of observing the circuit operation or identifying what's wrong, other than to examine and test all components individually, under realistic conditions. In other words, the circuit won't be working normally if power supply voltages and currents are less than necessary for normal operation. Consequently, you hit a brick wall when the normally high supply current becomes severely limited and the supply voltage must also drop in concert. Simply, you can't use that approach to prove whether a class A amplifier is working correctly.
Class AB amplifiers for example, can have a couple of orders less idle current than class A types so this type of problem seldom occurs in testing them. It's possible to disable the output stage of class A types and just check previous sections up to the drivers but feedback then has to be reconfigured and this becomes a lot more difficult and demanding than just tinkering with a DVM as your only tool and reference. You may need to check your funds for replacement semis when the smoke settles, as this is not simple and requires some serious test equipment.
Class AB amplifiers for example, can have a couple of orders less idle current than class A types so this type of problem seldom occurs in testing them. It's possible to disable the output stage of class A types and just check previous sections up to the drivers but feedback then has to be reconfigured and this becomes a lot more difficult and demanding than just tinkering with a DVM as your only tool and reference. You may need to check your funds for replacement semis when the smoke settles, as this is not simple and requires some serious test equipment.
I think you've made a succinct statement of the mystery. Where are these strange currents originating? Earlier posters suggested carbon tracks as a possible culprit (posts 20 and 38). I think that may have merit.
In post #24 you mention " --- r19 and zener diodes runs very hot." I suggest you raise the value of R13 and its opposite on the positive rail (R19? I can't read the Ref Des). These Zener regulators supply small currents to the amp and there's no reason for the diodes to be so heavily biased. (Original choice may have been to support preamp circuits.)
Because it's tied to a positive supply, I doubt R6 is the problem. I can't see how it could lead to the negative base voltages that are so problematic and baffling.
I agree that the value of R6 and R11 is an important issue. They help establish the claimed 700mA "standing" bias current. Since you've raised the supply voltage to +/- 30V, that implies 42 Watts per channel, more than 84W for stereo when accounting for earlier stage dissipation. Hence, the earlier suggestion to make R6 and R11 = 22M.
At present, carbon tracks are the only mechanism I can think of to explain the weird symptoms. The only method of exploring this notion that occurs to me is to purse my proposal in post #78. If the carbon suspicion is correct, the strange bias voltages will remain at the end of step 2. Assuming this is the case, next step would be to remove TR1 and TR2, and look for voltage drops around the bias paths that don't make sense. Carbon adjoining nearby tracks with negative voltages would be chief suspects. Carve on suspect areas with a hobby knife to see if that reduces the errant voltages.
I agree with BSST in #85 that tracks should be thouroughly inspected and measured.
The large amount of current causing a massive amount of voltage across R18 (2k2) cannot be easely explained.
Suppose the right or maximum current is running there (voltage across R5 / R5), say 0.5mA, 1V over R18 is also a maximum.
But 1.5V over R18 implies more current, or a increased value of R18, or a bad contact (bad track, bad soldering) there.
Given the various blackened spots, a discontinuity is apparent.
The large amount of current causing a massive amount of voltage across R18 (2k2) cannot be easely explained.
Suppose the right or maximum current is running there (voltage across R5 / R5), say 0.5mA, 1V over R18 is also a maximum.
But 1.5V over R18 implies more current, or a increased value of R18, or a bad contact (bad track, bad soldering) there.
Given the various blackened spots, a discontinuity is apparent.
Hi Citizen,
I appreciate your critique of my thinking.
I've been imagining a spurious carbon resistance that forces TR1 to conduct, eg. resistance between base and collector of TR1--- wild speculation. snowjx has cautioned that some of his measurement may be in error because the light bulb changes conditions during the course of his measurements. I suspect this happening, as I spot several readings that seem improbable: 2v base/emitter at TR9; 0.9V across R26 in the wrong direction if TR9 is biased on; -11.5V at base of TR5 should yield -10.9V at emitter instead of reported -12V. Point being all reported voltages should be regarded skeptically as long the cursed light bulb is in use.
BTW, I believe your posts have been offering great advice.
Thanks again.
Best regards,
Steve
I appreciate your critique of my thinking.
I've been imagining a spurious carbon resistance that forces TR1 to conduct, eg. resistance between base and collector of TR1--- wild speculation. snowjx has cautioned that some of his measurement may be in error because the light bulb changes conditions during the course of his measurements. I suspect this happening, as I spot several readings that seem improbable: 2v base/emitter at TR9; 0.9V across R26 in the wrong direction if TR9 is biased on; -11.5V at base of TR5 should yield -10.9V at emitter instead of reported -12V. Point being all reported voltages should be regarded skeptically as long the cursed light bulb is in use.
BTW, I believe your posts have been offering great advice.
Thanks again.
Best regards,
Steve
Not ment as critique, but as a 'counter-thought'.
Your thinking 'spurious carbon resistance... base and collector TR1, mine 'leaking from c to b by an almost broken TR1. Same result, apart from what it actually is. as I do not use the light bulb help, but it's it is a pointer indeed.
"...2v base/emitter at TR9; 0.9V across R26 in the wrong direction if TR9 is biased on..." I missed that one, very well observed!
"-11.5V at base of TR5 should yield -10.9V at emitter instead of reported -12V" TR5 cannot conduct and not conduct at the same time; what's happening there?
I was also thinking of a nasty (partial) shorting down under somewhere, carbon or otherwise.
Rails drop by serious increased current through... TR10/TR9 are the usual suspects. Hence the bulb condition.
I've already shown my concerns about various caps (C6, C7, C13, C14, C10), which are wrongly reversed and should be bipolars or poly's. But that would not explain the negative rail side to go out of its normal setpoint enough.
So again, ... spark! (while typing)
TR5 does not conduct but is forced as much as possible by TR1/TR2 and the feedback loop to open. So, there must be a nasty shorting near or just after TR5 preventing this! TR9 can do such a thing, notice the voltages across R26 and the Vbe of TR9. Your analysis!
Hi Citizen,
I'm retired and one of the things I miss most is technical discourse when tackling a problem. I think I'm also living vicariously through all you guys doing actual work on the bench. 😉
I'm retired and one of the things I miss most is technical discourse when tackling a problem. I think I'm also living vicariously through all you guys doing actual work on the bench. 😉
I didnt change any parts value, all replaced like for like. R13 have two values because, in original a1, one channel supply power to preamp.
in this ma50, it’s a power amp, r13 was 680r both channels. And the rail power was 32v initially it’s a beefed up a1, with higher rail voltage
BSST / Steve,
This lovely discourse, that's what's all about.
I was standing at my in-house bar-counter after my previous post, sipping a port wine, pondering the benefits of a good in-depth discussion with a chess mate. Same game, different colours. It's strategy, not the last move. Retirement is due for me in less then 10y.
Cheerio!
This lovely discourse, that's what's all about.
I was standing at my in-house bar-counter after my previous post, sipping a port wine, pondering the benefits of a good in-depth discussion with a chess mate. Same game, different colours. It's strategy, not the last move. Retirement is due for me in less then 10y.
Cheerio!
Last edited:
I also note voltage drop on r18 is too big, but I suspect that was the problem of measurements. Because the voltage keep changing up and down.
they are chances that if I measure two points in longer interval, say more than 1 minute, it will cause inconsistency.
as I measured the voltage drop on r18 individually again, it seems always around 0.8v.
I can’t guarantee consistency across the whole circuit, because the voltage vibration. Please do keep this in mind. If any thing looks suspicious and need remeasure individuall, let me know I will do it
see the burn marks on cob? These we’re original, and they are on every zener diodes. Also if you Google images for musical fidelity ma50. They all looks like this. I replaced the zener diodes with higher power rating ones.
Got my new soldering iron. Lifted r6 and r11.
r6 measures 15.8 Mohm
r11 measures 16Mohm
can I just lift them and do the test? I don’t have 22mohm resistors
r6 measures 15.8 Mohm
r11 measures 16Mohm
can I just lift them and do the test? I don’t have 22mohm resistors
step 1, lifted r6 and r11 when powered on with bulb in circuit, rail voltage initially keep rising in tens of seconds, up to about +-22v, with bulb become dimmer. Measured tr1 emitter voltage become positive, about 250mv. While base still negative.
and then the rail voltage started fluctuating, jump up and down with bulb dimmer - brighter -dimmer, back and fort.
tried to remove the bulb, transformer stared making noise, r30 and r31 become hot very quickly, so I powered it off soon.
I think this doesnt help much, right?
Interestingly, when I solder r6 and r11 back on, and power on the amp, this time everything looks very promising actually.
with bulb still in circuit, I got about +-30v rail voltage. Bulb is very dim (as expected it the amp is normal with no unusual bias current)
when I measured, every voltage is good as it should be,
r8, tr1 base small positive value
tr1 emitter 550mv
tr2 base small positive value
r15 -2.5v
tr9 collector around 200mv
all values becomes perfect as they should be
only problem is bulb occasionally becomes bright for 1 or two seconds and then back to dim. So it’s no very steady yet.
all I did was
1. desolder r6, and r11 then test.
2. remove bulb from main tested again, r30, r31 became hot.
3. Solder r6, r11 back in
does that indicate the problem is some kind of parts pcb contact problem? But when I push pull parts, or twist pub little bit, here is no effect, can’t reproduce the problem.
or does that mean it’s kind of carbon tracks contamination? (I don’t really know what that means)
That could also explain when last time I replaced tr1, voltage dramatically changed, it looks like every time I replace, solder/desolder something, if can cause this issue.
with bulb still in circuit, I got about +-30v rail voltage. Bulb is very dim (as expected it the amp is normal with no unusual bias current)
when I measured, every voltage is good as it should be,
r8, tr1 base small positive value
tr1 emitter 550mv
tr2 base small positive value
r15 -2.5v
tr9 collector around 200mv
all values becomes perfect as they should be
only problem is bulb occasionally becomes bright for 1 or two seconds and then back to dim. So it’s no very steady yet.
all I did was
1. desolder r6, and r11 then test.
2. remove bulb from main tested again, r30, r31 became hot.
3. Solder r6, r11 back in
does that indicate the problem is some kind of parts pcb contact problem? But when I push pull parts, or twist pub little bit, here is no effect, can’t reproduce the problem.
or does that mean it’s kind of carbon tracks contamination? (I don’t really know what that means)
That could also explain when last time I replaced tr1, voltage dramatically changed, it looks like every time I replace, solder/desolder something, if can cause this issue.