Calculating Current Through a Transistor
- Thread starter LBPete
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ConradH
Lunatic Member
It's either a thermal problem, capacitor problem or bad/oxidized joint problem, since it clears up quickly under power. You've changed everything that's directly involved! I'd also change the 3V cap, as those are prone to all sorts of problems and issues with the current may not divide evenly through the pair. Then I'd explore with some freeze mist or a Q-tip lightly moistened with alcohol. If the problem is subtle, you'll probably need the freeze mist.
It definitely seems to be thermal related because it only occurs after a cold soak but I haven't been able to induce it with freeze spray. I'm not sure what you mean by a 3V cap. All the electrolytic caps are new and the two small uf value electrolytics were replaced with Wima film caps this includes the 2.2 uf coupling cap. Is that the one you are referring to?
That's extraordinarily high but does that amp even have a protection relay? If memory serves, it uses one of those crowbar circuits to ground the output.
The good driver board in the amp I'm working on, shows about 25mV DC offset on startup and drops a little during warm up and stabilizes around 15mV. That's how I would expect both boards to act.
- Pete
Am I right in thinking you've changed lots of transistors, caps etc., but not changed any resistors? The problem could be a bad resistor. Though resistors are usually pretty reliable you may just be unlucky here.
The other possibility is a micro-fracture in a track, maybe
BTW, changing the 3k resistors (R603, R604) wouldn't do any good because R604 isn't really doing anything anyway and certainly has nothing to do with DC offset. The main resistors regarding DC offset are R601, R602 and R612.
The other possibility is a micro-fracture in a track, maybe
BTW, changing the 3k resistors (R603, R604) wouldn't do any good because R604 isn't really doing anything anyway and certainly has nothing to do with DC offset. The main resistors regarding DC offset are R601, R602 and R612.
Still no resolution. Ghazzer, I understand what you are saying about solder whiskers but thought that was more of an issue with lead free solder. No lead free solder on this board. To be sure, I reflowed all the solder joints and nothing changed. Then, based on Conrad's suggestion I connected the 'scope to the base of TR-606 and fed in a 1k Hz sine wave. When the popping occurs, the sine wave alternates between jumping with some vertical lines and blinking out and back on. This distortion occurs with decreasing frequency for a couple of minutes then settles down and behaves normally. The voltage at the speaker terminal follows a similar pattern, starting quite high then settling to a normal value.
TR606, TR605, D602, C607, C604 are all new. I don't have the proper zener diode on hand to replace D601 but I might try swapping it with the other channel.
- Pete
TR606, TR605, D602, C607, C604 are all new. I don't have the proper zener diode on hand to replace D601 but I might try swapping it with the other channel.
- Pete
ghazzer
Senior Member
Let me kick a few things around:
You swapped L/R boards and the problem followed the sick board. So there is no hint of this problem in the good board? That at least isolates your search to one board.
Is this summary correct – In post#8 you swapped out the C1583s with 4 C1845s and that fixed everything. Then in post#15 you replaced R603&4 and it was soon back to “its original tricks”. Is that what you had at post#1, or after post#8. Not questioning you, just trying to follow correctly.
In Post#20 you observed spikes at the bases and emitters of the DPs. Was that just the bad bd? Assuming you have a dual trace scope have you monitored the same point(s) in both boards? Hopefully there is no evidence of any spike in the good board, but it could be that the good bd is able to handle the event and the bad bd cannot.
What are the nominal times you have had to wait for it to ”reset”?
Gadget suggested monitoring some component voltages in both channels. Any imbalance may clear after the pops are gone, but this is still a good idea.
Does the S9910 have a protection chip similar to the HA12002 in my G-6700, and have you checked the specs for it? I am also surprised that the speaker relay would close with DC Offset of 1.5V.
Are any spikes associated with the pops? I realize that you will not hear any pops until the speaker relay closes, but you said that you only get pops in the first few seconds after power on. My relay doesn’t close until 4-5 seconds after power on. Increasing the time constant of the protection circuit may help your speakers.
Like I said, just throwing a few things at the wall to see if any of it sticks - - -
You swapped L/R boards and the problem followed the sick board. So there is no hint of this problem in the good board? That at least isolates your search to one board.
Is this summary correct – In post#8 you swapped out the C1583s with 4 C1845s and that fixed everything. Then in post#15 you replaced R603&4 and it was soon back to “its original tricks”. Is that what you had at post#1, or after post#8. Not questioning you, just trying to follow correctly.
In Post#20 you observed spikes at the bases and emitters of the DPs. Was that just the bad bd? Assuming you have a dual trace scope have you monitored the same point(s) in both boards? Hopefully there is no evidence of any spike in the good board, but it could be that the good bd is able to handle the event and the bad bd cannot.
What are the nominal times you have had to wait for it to ”reset”?
Gadget suggested monitoring some component voltages in both channels. Any imbalance may clear after the pops are gone, but this is still a good idea.
Does the S9910 have a protection chip similar to the HA12002 in my G-6700, and have you checked the specs for it? I am also surprised that the speaker relay would close with DC Offset of 1.5V.
Are any spikes associated with the pops? I realize that you will not hear any pops until the speaker relay closes, but you said that you only get pops in the first few seconds after power on. My relay doesn’t close until 4-5 seconds after power on. Increasing the time constant of the protection circuit may help your speakers.
Like I said, just throwing a few things at the wall to see if any of it sticks - - -
restorer-john
Addicted Member
Have you replaced D651, the STV3? Is it a 'blob' version (spider's egg) or a tombstone/horseshoe?
They are notorious, troublesome little b#stards. Use 3x1N4148 is series.
They are notorious, troublesome little b#stards. Use 3x1N4148 is series.
One board is good, no hint of this issue. The bad board is bad in either the left or right channel position and the good board is good in either position.
Now I'm not sure it was ever completely fixed. It may have not have cooled down enough to cause the issue to occur after swapping out the C1583s.
No spikes on the good board. Yes, I'm using a dual trace scope to reference the good board.
It completely settles down in 4 or 5 minutes then it has to sit overnight, 10 or more hours to get it to fully occur. I've checked it a number of times after sitting for 2 or 3 hours, it doesn't show the spikes but the DC offset is still high, as much as 300 to 500 mV.
I'll have to revisit this.
It has a discrete protection board. No IC.
The spikes and voltage fluctuations occur as soon as it powers up. You don't hear the pops until the speaker relays kick in. By then the number of pops has decreased.
Today I swapped the zener diodes left to right with no change and I replaced C607 again for good measure.
The issue may be getting worse. When starting up cold and the spikes are occurring, it will occasionally cause the DBT to brighten up. My reflex is to shut it off at that point.
- Pete
Those diodes are not mounted on the driver boards. they mount on output transistor heat sink some distance away. So they do not stay with the driver board when swapping the boards left to right. So it's unlikely they are causing the problem.
- Pete
I put this one aside for a few weeks but now I'm back at it with no resolution. I've been chilling the troublesome driver board in the freezer to provoke the issue. Checking the resistors with the board freezing cold turns up an odd discrepancy. R612 starts out measuring around 32k and slowly climbs to it's rated 47k ohms. Measuring the same resistor on the good driver board shows the correct 47k resistance. I've verified all the solder solder joints on tr601, the differential pair and replaced R612, C603, C605, C606 and checked the associated resistors. I just don't see what could cause a temperature related change in that resistor's value. Any ideas?
- Pete
- Pete
FWIW I've never had a resistor measure lower than it should. But I don't see anything which would cause that measurement to occur. My guess would something conductive on the board (like solder whisker suggested earlier) but not as obvious. Try a good cleaning around the resistor, and maybe scraping blank areas. (stupid question - does resistor measure low removed from circuit, but cold....) very strange.....
elnaldo
Lunatic Member
Resistors measuring low and climbing are usually in parallel with a large capacitor charging. When you mesure the other channel, the capacitor is already charged.
Or, something is discharging a cap in one channel faster than normal.
Any difference between channels deserves further investigation.
Or, something is discharging a cap in one channel faster than normal.
Any difference between channels deserves further investigation.
This does make some sense. The value of R612 is crucial in maintaining a low DC offset and if it's the wrong value it will cause problems. Is this the original resistor or a replacement?
Yes, sounds plausible. If the problem is due to an area of muck around that resistor condensation would make it worse. A good clean would certainly do no harm.
Yes, that's true. Unfortunately there's a lot of stuff in parallel with that resistor to confuse the issue.
I'd suggest removing the resistor from the board and checking it in isolation.
Thanks for the feedback. Sometimes I'm too smart for my own good. I thought it was pretty clever to put the board in the freezer to provoke the issue. What I didn't consider was condensation. When the board is dry the resistance is stable.
I gave the foil side of the board a good scrubbing with Isopropyl alcohol and a tooth brush, then examined it again with a lighted magnifier. I touched up a couple of traces and plugged it. The DC offset dropped quickly to under 10mV. I played it for a couple of hours and the offset was stable. Unfortunately when I turned it on the next morning, it was back to it's old tricks.
This unit has separate driver boards for each channel. One works fine and the other is the problem child. The problem follows the board when they are swapped left to right. I've been checking voltages around the differential pair and comparing the values left to right and there are definite discrepancies but I just don't see why.
Diff pair emitter voltage, bad channel around 250mV, good channel 640mV. The good value matches the voltage call out on the schematic
Collector voltage at TR 602, bad channel around 9.5V, good channel 9.8V. The called out voltage is 10.6V
Voltage at the junction of diff pair base and R612, Bad 170mV, Good 60mV
Voltage at the junction of R607 and C603 is zero on both boards.
Voltage at the junction of diff pair base and R601, Bad 240mV, Good 70mV
Voltage at the junction of R601 and R602, Bad 230mV, Good 60mV
I've verified the values of R601, R602, R605, R607 and R612. None are out of spec. C601, C602, C603, C605 and C606 are all new. All the transistors have been replaced on both boards. All the small diodes have been replaced on the bad board. The zener diode is original but I've compared the voltage to the good board and they match pretty closely.
The measured voltage values do move around a little. The unit is on a DBT with a 100W bulb. Since the symptoms are temperature sensitive, I'm just turning it on long enough to take the measurement then turning it off in an effort to slow down the board warming up.
I am out of ideas.
- Pete
I gave the foil side of the board a good scrubbing with Isopropyl alcohol and a tooth brush, then examined it again with a lighted magnifier. I touched up a couple of traces and plugged it. The DC offset dropped quickly to under 10mV. I played it for a couple of hours and the offset was stable. Unfortunately when I turned it on the next morning, it was back to it's old tricks.
This unit has separate driver boards for each channel. One works fine and the other is the problem child. The problem follows the board when they are swapped left to right. I've been checking voltages around the differential pair and comparing the values left to right and there are definite discrepancies but I just don't see why.
Diff pair emitter voltage, bad channel around 250mV, good channel 640mV. The good value matches the voltage call out on the schematic
Collector voltage at TR 602, bad channel around 9.5V, good channel 9.8V. The called out voltage is 10.6V
Voltage at the junction of diff pair base and R612, Bad 170mV, Good 60mV
Voltage at the junction of R607 and C603 is zero on both boards.
Voltage at the junction of diff pair base and R601, Bad 240mV, Good 70mV
Voltage at the junction of R601 and R602, Bad 230mV, Good 60mV
I've verified the values of R601, R602, R605, R607 and R612. None are out of spec. C601, C602, C603, C605 and C606 are all new. All the transistors have been replaced on both boards. All the small diodes have been replaced on the bad board. The zener diode is original but I've compared the voltage to the good board and they match pretty closely.
The measured voltage values do move around a little. The unit is on a DBT with a 100W bulb. Since the symptoms are temperature sensitive, I'm just turning it on long enough to take the measurement then turning it off in an effort to slow down the board warming up.
I am out of ideas.
- Pete
Just a WAG, take it off the DBT. I'm sure that you're a little ticked off by now, nothing is shorted so you have everything to gain.
gslikker
Super Member
So this looks like when the amp output voltage is up so the voltage at R612 rises, then the raised voltage appears on the other side of R612? This would indicate the input circuitry behaving correctly even when offset is there?
Could the problem be something like the output circuitry not behaving, for example open circuit from a driver transistor emitter to output transistors base, or connection between speaker output to R612. It looks like if you would have open circuit at G, there is still some sort of DC connection with R612 through the resistors coming from off board output transistor emitters through R620 ...R623.
Just mind jogging, but I think it would be an idea at least check zero ohms continuity at C,D and G and traces to components.
Or check Q608/9 driver emitter voltages versus output transistors base voltages, as well as whether the voltage at the output transistor emitter resistors junction (R651....R654,L651) versus circuit card junction (R612,619,621,622), this all at the moment the offset is still there.
(Myself, I had a bad connection at a HK Citation 12 pluggable amplifier card because of it was not deep enough on connector pins, I had to sand off about a millimeter off the card edge to make it reliable, the unit gave loud bangs as there was no loudspeaker relay...so the very reason was poor mechanical tolerances by design or manufacture)
Could the problem be something like the output circuitry not behaving, for example open circuit from a driver transistor emitter to output transistors base, or connection between speaker output to R612. It looks like if you would have open circuit at G, there is still some sort of DC connection with R612 through the resistors coming from off board output transistor emitters through R620 ...R623.
Just mind jogging, but I think it would be an idea at least check zero ohms continuity at C,D and G and traces to components.
Or check Q608/9 driver emitter voltages versus output transistors base voltages, as well as whether the voltage at the output transistor emitter resistors junction (R651....R654,L651) versus circuit card junction (R612,619,621,622), this all at the moment the offset is still there.
(Myself, I had a bad connection at a HK Citation 12 pluggable amplifier card because of it was not deep enough on connector pins, I had to sand off about a millimeter off the card edge to make it reliable, the unit gave loud bangs as there was no loudspeaker relay...so the very reason was poor mechanical tolerances by design or manufacture)
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I think the OP said that the problem follows the driver board and that the driver boards are separate from the output stage...or something along those lines if I understood correctly. If so, then the issue would not be on the output side.
I am assuming you have replaced the DC bal trimmer (if there is one), right?
Sorry if I missed that....