Would you confirm R5 is really 22K. Are D9 and D10 low barrier diodes ( i.e. Shottky)? What is negative rail voltage in post #13?
My suspicion is that TR1 is cutoff, and thus TR9. My best guess is that TR2 is defective--- I can't see any source for the T2's apparent base current.
I just discovered post #15. I think I'm echoing Citizen's comments.
My suspicion is that TR1 is cutoff, and thus TR9. My best guess is that TR2 is defective--- I can't see any source for the T2's apparent base current.
I just discovered post #15. I think I'm echoing Citizen's comments.
Sorry tr2 emitter was positive 110mv. The value varies between 100 — 300mv but never goes negative
That was the original burn mark, I bought these as parts only, and completely rebuild. ( the circuit board is also low quality, but thats not standard parts so I didn’t manage to replace it.
these burn marks are standard on this machine, if you search online every musical fidelity ma50 looks like this.
it’s due to r19 and zener diodes runs very hot. I changed them to higher rating parts, and checked circuit, fixed these disconnected places.
below are the original pictures
Emitters of TR1 and TR2 should be positive whatever, and you OP picture show this (overseen...) as {+} 200mV.
Too low, but why?
Dragged down and causing havoc in the amp's channel. Yet, this 10 seconds issue... charging - discharching... Replece the aformentioned cap... ahum... capacitors.
Too low, but why?
Dragged down and causing havoc in the amp's channel. Yet, this 10 seconds issue... charging - discharching... Replece the aformentioned cap... ahum... capacitors.
R5 and R12 are definitely 22k, as they are newly replaced, and value checked.
both channels used same parts and build at the same time, so they basically mirrored each other. if any parts used wrongly, I would have spotted right away. That also make this distortion problem more strang, as I really can’t figure out what I did wrong
d9 and d10, are all 1N4148 Standard Silicon Switching Signal Diode, 0.2a, 100v
Negative rail voltage is -31.4v.
I tested TR1, and TR2 with multimeter, they are both fine.
the parts that I measured failed including:
c15 failed becomes short circuited
r26 failed twice
zd1 failed twice
d9 failed once becomes short circuited
tr5 failed once
they all got replaced. The reason for failure varies. some of them are old parts maybe was failed before I got it.
some burned because of myself, because I accidentally grounded TR9 collector, or shorted parts with multimeter leads accidentally when trying to measure.
I certainly concur with Citizen's observation that TR1, TR2 emitters should be positive.
I loathe removing components when I'm uncertain about their failure, but in this case I'd remove both TR1 and TR2 as a diagnostic. I'd tie a 3.3K resistor from output to negative rail to act as a class A bias load. When powered, the output should should be about 0V. TR9 should be cutoff. With the input transistors absent, it will be easer to inspect feedback to TR2. When all looks as it should, replace TR2 with a new part. Base current should make sense. Then install a new TR1 and remove the 3.3K load.
I loathe removing components when I'm uncertain about their failure, but in this case I'd remove both TR1 and TR2 as a diagnostic. I'd tie a 3.3K resistor from output to negative rail to act as a class A bias load. When powered, the output should should be about 0V. TR9 should be cutoff. With the input transistors absent, it will be easer to inspect feedback to TR2. When all looks as it should, replace TR2 with a new part. Base current should make sense. Then install a new TR1 and remove the 3.3K load.
I also have A1, that was my last rebuild project, I completely removed the pot, selector and the tape selector switch. Make it a power amp eventually.
as I found theses parts caused so many problems, not only noise, but also severe left right channel cross talk.
this is my a1 after rebuild, basically used same parts as this ma50
Emitter tr1 and tr2 are positive. It was my typo. Sorry.
please see my correction comment in #23
replaced both c10 and c13 as suggest by citizen124032, they measured ok. But I still replaced them.
tested with speakers on, improved a lot !!
I am not sure If it fixed the problem completely. But I can not hear obvious distortion like previous any more.
also did new measurement, one thing I noticed that the voltage is very stable now, and the values are much closer to the good channel.
R8 -51.8mv
tr1 base - 52.5mv
tr1, tr2 emitter 482mv
tr2 base -73.9mv
r18 voltage drop 480mv ( vs 540mv working channel)
r15 -2.1v ( vs -2.5v working channel, I remembered I measured about -0.9v previously)
tr10 collector 112mv
tr9 collector 70mv
tr10 - tr9 collector voltage 38mv ( vs 120my working channel, although there are some voltage here now, but its much smaller compare to the other channel, does that means less biased current on this channel? )
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Just did. got very promising result.
see my post #33 previously when it powers on, I feel something is leaking, with sound turn bad with time, and measured voltage drops.
now measured voltages are quite stable, although the values are still a bit lower
If it was the negative tr2 base voltage caused c10, c13 capacitor failure (I am still not sure if they are failed completely, as their value still measured ok)
now tr2 base voltage is still negative which still put these capacitors in reversed polarity. This needs be solved, right?
Base of TR1 should not be negative, as its base current flows into R7 and R8, causing a positive small voltage.
If it is negative, current flows from ground of R8 into what?
Replace C6 & C7 also for bipolars.
Base of TR1 and TR2 should 'mirror' base of TR3 and TR4.
Different values can happen due to component tolerances, mainly ZD1 and ZD2.
If it is negative, current flows from ground of R8 into what?
Replace C6 & C7 also for bipolars.
Base of TR1 and TR2 should 'mirror' base of TR3 and TR4.
Different values can happen due to component tolerances, mainly ZD1 and ZD2.
I replaced R24, c15, TR1, and TR2 hope something magic will happen.
initially, where is no improvement. However, after some time, maybe a few power/on and off while doing measurement. Things gone even worse.
the power rail dropped to +-10v. I can’t understand why. Checked main filtering capacitor, and transformer, they are all fine. Tried to discount the negative voltage power rail in the bad channel. Power rail returned to normal voltage.
so I think something even worse is happening in this bad channel. Trying to figure out why
initially, where is no improvement. However, after some time, maybe a few power/on and off while doing measurement. Things gone even worse.
the power rail dropped to +-10v. I can’t understand why. Checked main filtering capacitor, and transformer, they are all fine. Tried to discount the negative voltage power rail in the bad channel. Power rail returned to normal voltage.
so I think something even worse is happening in this bad channel. Trying to figure out why
i'll put five bucks on C13 being bad...but it could come down to carbon contamination of the pcb...
c13 was newly replaced, and checked again it’s fine.
I used multimeter checked lots of component, didn’t find anything wrong, neither can I find any wrong connection. So frustrated.
how can my rail voltage drop to 10v? Even the main filtering capacitors are siting around 9-10v
despite now the main rail voltages are only +-10v, both channels are actually still playing music fine. even the broken channel I can’t hear obvious distortion.