Working on the premise I had not adjusted the trimmers properly, I rolled back the adjustments and measured voltages again to confirm they were the same as previously. I then measured other parts of the circuit and found I was getting about 0.585vdc across one of the polystyrene treble caps and other points in the circuit that did not seem correct. This wasn't present on the left channel treble caps. These caps don't check as shorted, and measured accurately with my LCR meter. Offset at the output was still hunting around as before, and fine adjustments of each trimmer didn't improve the measurements.
Having bought new LS replacements transistors on anatech's proposition that the transistors are malfunctioning, I replaced one right channel matched dual at a time and tested/adjusted DC offset at the output. I don't know where to measure offset at only the tone circuit and didn't want to damage anything. I was only able to get offset down to about 20mV with slight incremental adjustments of the trimmers - line first, tone switch out, then tone trimmer, tone switch in, but the signal was really distorted (looked to me like oscillation at the top/bottom of square wave), not at all clean like with the original transistors. Measuring voltages again, I had about the same voltage across both treble caps - 0.585vdc. No adjustment of the trimmers could further reduce offset, even with trimmers down to just about 100R from the baseline of 1.2k with the original transistors.
Working on the indication that the replacements from Linear Systems have different characteristics and I don't know how to adjust the circuit to make them work, I then re-installed the old transistors, and as I was doing so, found two broken solder joints - one at a mica cap, and the other at a resistor. I then found a bad joint at the volume pot. These were not visible to me when I first examined the board, but I just recently bought a new magnifying visor with good glass lenses, and damned if I didn't see these little bad joints. After fixing these, voltages on those polystyrene treble caps went down to about .002V, same as left channel. I then re-flowed all of the joints in the tone and line sections, and most of the rest of the board.
This brought DC offset down to 0.03mV with the tone out, and 0.05mV with the tone in. Square-wave signals looked excellent at the output of the unit. Thought it was on the way to being fixed, but I noticed that changing the volume causes a jump in offset that settles within a second or two - from about 28-90mV back down. I hooked it up to one of my amps, turning the volume controls causes the amp to go into protect mode. Left channel is worse - very little rotation causes amp protection to pop. Right channel more stable, but at about 1/2-3/4 rotation, amp protection pops. Back on the bench, I found that the offset was now about 20mV on the left channel, and about 5mV on the right channel. I also found that the left channel volume pot appears to have a bad trace at the upper end, which shows on the DMM but not on the scope. But definitely getting a rise in offset with adjustment of either pot.
My next thought was to pull the mica and polystyrene caps and check them out of circuit. All the e-caps are new Nichicon. I haven't yet ordered a non-polar for that 470uF but it's in my Mouser cart. It seems like something is passing DC through the pots that the circuit is trying to re-balance, as the offset drops after it jumps.
All original parts are back in the unit, all trimmers set very close to originally measured settings. I haven't damaged anything. I work with an ESD safe soldering station with digital temp control, on grounded static-free pads, with a static wrist strap and I take my time. I've built 22 tube guitar amps, 2 mono block audio amps, repaired amps, preamps, built power supplies, repaired a bunch of other electronics, build equipment and devices for our work, and have soldered for the past 50 years. I know how to follow instructions if they are clear and specific. Ignorance isn't the same as incompetence. I just don't have the knowledge yet to figure this out, so I appreciate any constructive help anyone cares to offer.
Having bought new LS replacements transistors on anatech's proposition that the transistors are malfunctioning, I replaced one right channel matched dual at a time and tested/adjusted DC offset at the output. I don't know where to measure offset at only the tone circuit and didn't want to damage anything. I was only able to get offset down to about 20mV with slight incremental adjustments of the trimmers - line first, tone switch out, then tone trimmer, tone switch in, but the signal was really distorted (looked to me like oscillation at the top/bottom of square wave), not at all clean like with the original transistors. Measuring voltages again, I had about the same voltage across both treble caps - 0.585vdc. No adjustment of the trimmers could further reduce offset, even with trimmers down to just about 100R from the baseline of 1.2k with the original transistors.
Working on the indication that the replacements from Linear Systems have different characteristics and I don't know how to adjust the circuit to make them work, I then re-installed the old transistors, and as I was doing so, found two broken solder joints - one at a mica cap, and the other at a resistor. I then found a bad joint at the volume pot. These were not visible to me when I first examined the board, but I just recently bought a new magnifying visor with good glass lenses, and damned if I didn't see these little bad joints. After fixing these, voltages on those polystyrene treble caps went down to about .002V, same as left channel. I then re-flowed all of the joints in the tone and line sections, and most of the rest of the board.
This brought DC offset down to 0.03mV with the tone out, and 0.05mV with the tone in. Square-wave signals looked excellent at the output of the unit. Thought it was on the way to being fixed, but I noticed that changing the volume causes a jump in offset that settles within a second or two - from about 28-90mV back down. I hooked it up to one of my amps, turning the volume controls causes the amp to go into protect mode. Left channel is worse - very little rotation causes amp protection to pop. Right channel more stable, but at about 1/2-3/4 rotation, amp protection pops. Back on the bench, I found that the offset was now about 20mV on the left channel, and about 5mV on the right channel. I also found that the left channel volume pot appears to have a bad trace at the upper end, which shows on the DMM but not on the scope. But definitely getting a rise in offset with adjustment of either pot.
My next thought was to pull the mica and polystyrene caps and check them out of circuit. All the e-caps are new Nichicon. I haven't yet ordered a non-polar for that 470uF but it's in my Mouser cart. It seems like something is passing DC through the pots that the circuit is trying to re-balance, as the offset drops after it jumps.
All original parts are back in the unit, all trimmers set very close to originally measured settings. I haven't damaged anything. I work with an ESD safe soldering station with digital temp control, on grounded static-free pads, with a static wrist strap and I take my time. I've built 22 tube guitar amps, 2 mono block audio amps, repaired amps, preamps, built power supplies, repaired a bunch of other electronics, build equipment and devices for our work, and have soldered for the past 50 years. I know how to follow instructions if they are clear and specific. Ignorance isn't the same as incompetence. I just don't have the knowledge yet to figure this out, so I appreciate any constructive help anyone cares to offer.
Hi Greg,
Well, the volume control can't upset the DC conditions unless you have current leaking back through the control, or being passed through by the previous stage. For sure you have current flowing through the volume control. Capacitor leakage is the most common cause of that. But, just to show that even a weird fault can occur, I have had a couple of diff pairs leaking back. Very unusual and it took me too long to check it because I thought it was either impossible or highly unlikely.
Your drifting DC offset can be one of two faults. Either the leaky component is temperature sensitive, and this is very common, or the diff pair isn't well matched. You could always disconnect one lead from the connection that feeds te diff pair - but make sure you have a DC path to ground. and see if your offset still drifts. If it does, you probably have a miss-matched pair, so stick one of the new diff pairs back in, leaving the previous connection to the input open. If you still have drift, then it's time to check the Vas, or the transistors that come immediately after the diff pair. This would normally be the extent of checking since issues in the output buffer stage will typically cause either high current draw, or a drop in current flow through the output buffer. These conditions will open the feedback loop or compromise the authority of the voltage amp stage to control the output DC offset. These issues are certainly uncommon, but not so much that you couldn't be looking at that in your preamp.
Remember to work calmly and write down your observations. Be very careful you do not write down a conclusion, that's pretty easy to do. Direct observations only. I just had a two transistor gain circuit that was biased off. After disconnecting the components that could drag the bias down (in this case the input transistor was off) I still had the fault. Going under the PCB, very difficult in this one, I found a solder splash from some previous happy soul. Cleaning it up resolved the problem, but you can see how very unlikely issues can cause problems.
-Chris
Well, the volume control can't upset the DC conditions unless you have current leaking back through the control, or being passed through by the previous stage. For sure you have current flowing through the volume control. Capacitor leakage is the most common cause of that. But, just to show that even a weird fault can occur, I have had a couple of diff pairs leaking back. Very unusual and it took me too long to check it because I thought it was either impossible or highly unlikely.
Your drifting DC offset can be one of two faults. Either the leaky component is temperature sensitive, and this is very common, or the diff pair isn't well matched. You could always disconnect one lead from the connection that feeds te diff pair - but make sure you have a DC path to ground. and see if your offset still drifts. If it does, you probably have a miss-matched pair, so stick one of the new diff pairs back in, leaving the previous connection to the input open. If you still have drift, then it's time to check the Vas, or the transistors that come immediately after the diff pair. This would normally be the extent of checking since issues in the output buffer stage will typically cause either high current draw, or a drop in current flow through the output buffer. These conditions will open the feedback loop or compromise the authority of the voltage amp stage to control the output DC offset. These issues are certainly uncommon, but not so much that you couldn't be looking at that in your preamp.
Remember to work calmly and write down your observations. Be very careful you do not write down a conclusion, that's pretty easy to do. Direct observations only. I just had a two transistor gain circuit that was biased off. After disconnecting the components that could drag the bias down (in this case the input transistor was off) I still had the fault. Going under the PCB, very difficult in this one, I found a solder splash from some previous happy soul. Cleaning it up resolved the problem, but you can see how very unlikely issues can cause problems.
-Chris
That is one of two things I was beginning to suspect; either bad caps or a problem with the other transistors feeding the diff pair(s). I've had DC on pots of guitar amps that use mica treble boost and tone circuit caps. Where I get into new territory is the transistors before/after the diff pair. I've got a new backlit photo that is helping me see the circuit layout better, and I'll map those connections for myself.
Hmm. Good detail you brought up. On the bench, offset is stable, but changed after sitting awhile, then hooking up to amp, and then after back on bench. Temperature differences could account for that. I'm prepared for mismatched diff pairs with enough replacements to handle the entire preamp and more, but am discouraged by their apparent different operating characteristics as noted by inability to adjust voltage with the trimmers.
OK, I can start with the Line diff pair with tone circuit off. Which connection? Gate 1? Jump ground to the gate? See attached PDF.
Gotcha. However, I'm still faced with the new diff pair not being able to be adjusted with the trimmer, and a bad oscillation. I can however, check to see if it fluctuates with volume adjustment, something I didn't do before.
OK, I understand what you are saying. I've had that happen with an amp.
Hah. You are speaking my language. Same comments I make to biologists when I'm giving a workshop or in individual discussions. Good field biology practice! I just need to be sure I'm measuring at correct locations so the data are relevant and valid.
Many thanks, Chris,
Greg
Correction :
please adjust "line" dc offset first, taken at the regular output, tone switch out.
Then after that "tone" dc offset with tone switch in.
Are you sure there is no dc present on your source input, routed to the volume pot ?
Is the dc fluctuation with different vol settings also present without any input signal ?
please adjust "line" dc offset first, taken at the regular output, tone switch out.
Then after that "tone" dc offset with tone switch in.
Are you sure there is no dc present on your source input, routed to the volume pot ?
Is the dc fluctuation with different vol settings also present without any input signal ?
Hi A.S.
Yes, that has been my approach.
I can get a stable, very low offset with it on the bench until I rotate the volume pots. I had it set so that with tone in or out, it was stable without adjusting the volume. It doesn't fluctuate around like it did before, just sitting with no signal.
Yes, rise then fall in offset with use of volume has been with or without input signal. Source inputs have been: 1) Nothing connected, 2) Shorted input, 3) Known good FIIO digital player, 4) Known good CD player. Testing on bench, and with amp, have been the same. On the bench, I saw the volume-induced rise and fall in offset, but wasn't sure it was enough to pop the amp protection until hooking it up to that.
OK, I've just taken new measurements, no input signal. Not the same both channels, not consistent, even when at same vol. pot resistance levels. But yes, I see it spike the most going from 0 Vol. to Mid, compared to from Mid to Full.
At startup, readings were substantially different than yesterday, when I had both channels adjusted to 0.02mV.
L: 10mV
R: -2.4mV
after 1 minute or so:
L: 6.5mV
R: -1.5mV
L Ch., Tone OUT:
Vol. 0 = 6.5mV
Vol. 0-Mid = 6.5mV
Vol. Mid-Full = 6.5mV
L Ch., Tone IN:
Switching in = spike from 6.5mV to 11.5mV, settling to 8mV.
Vol. from 0 to approx. resistance mid (not mechanical) = spike to 28.3mV, dropped to 2.6mV, then to -1.5mV, then rose to 4mV.
Vol. from mid to full = rose to 6.6mV, dropped to 4mV.
Vol. from full to 0 = spikes to 30mV
Right Ch., Tone OUT:
Vol. 0 = -1.5mV
Vol. to Mid = -1.5mV
Vol. Mid-Full = -1.5mV
Right Ch., Tone IN:
Switching in - spike from -1.5mV to 58mV, drops to -4mV.
Vol. from 0-Mid = spikes to 85mV, dropped to -4mV.
Vol. from Mid-Full = dropes to -10mV, rises to 3mV.
Repeating these tests 1x resulted in different numbers - this isn't consistent or stable when idling, or making adjustments.
At startup, readings were substantially different than yesterday, when I had both channels adjusted to 0.02mV.
L: 10mV
R: -2.4mV
after 1 minute or so:
L: 6.5mV
R: -1.5mV
L Ch., Tone OUT:
Vol. 0 = 6.5mV
Vol. 0-Mid = 6.5mV
Vol. Mid-Full = 6.5mV
L Ch., Tone IN:
Switching in = spike from 6.5mV to 11.5mV, settling to 8mV.
Vol. from 0 to approx. resistance mid (not mechanical) = spike to 28.3mV, dropped to 2.6mV, then to -1.5mV, then rose to 4mV.
Vol. from mid to full = rose to 6.6mV, dropped to 4mV.
Vol. from full to 0 = spikes to 30mV
Right Ch., Tone OUT:
Vol. 0 = -1.5mV
Vol. to Mid = -1.5mV
Vol. Mid-Full = -1.5mV
Right Ch., Tone IN:
Switching in - spike from -1.5mV to 58mV, drops to -4mV.
Vol. from 0-Mid = spikes to 85mV, dropped to -4mV.
Vol. from Mid-Full = dropes to -10mV, rises to 3mV.
Repeating these tests 1x resulted in different numbers - this isn't consistent or stable when idling, or making adjustments.
Last edited:
Hi Greg,
In your picture, that resistor should be left in circuit. Disconnect anything that connects to the same trace from that resistor to the left you see traveling downwards. That should isolate the gate and allow you to judge the balance of the pair. You need to measure while cold, then after about 1/2 hour as it comes up to operating temperature.
Interesting that it oscillates with the new diff pair. Normally that isn't a problem, even with a substitute part number. Same thing for the offset out of range, but that could very well be the oscillation.
Good luck on finding something interesting.
-Chris
In your picture, that resistor should be left in circuit. Disconnect anything that connects to the same trace from that resistor to the left you see traveling downwards. That should isolate the gate and allow you to judge the balance of the pair. You need to measure while cold, then after about 1/2 hour as it comes up to operating temperature.
Interesting that it oscillates with the new diff pair. Normally that isn't a problem, even with a substitute part number. Same thing for the offset out of range, but that could very well be the oscillation.
Good luck on finding something interesting.
-Chris
After my last post I made some progress, but the entire project has been on hold since.
The tone section matched dual (U404) was indeed, very poorly matched, with one side way out of specs, which I discovered after pulling it a second time to test. Previously, I had replaced it with a new production LSI replacement, and had observed severe oscillation. I tested all of the replacements and they all appear well matched, unlike the original.
After replacing the original and returning adjustments to baseline, I found one shorted diode and one suspected bad U1732, which I replaced with a new old stock item. This corrected the DC offset problem when engaging the tone switch (yeay!) and adjusting the pot from just off minimum to maximum, but at minimum to just above, offset jumped. I then jiggled the pot shaft at that position and offset jumped around substantially. Above that range, offset is stable. I subbed in a 50k audio pot and offset was stable across the entire range.
Over the past 2 months I've been searching for replacement pots. State Electronics wants $100 a copy to make them, and I've not been able to find any on the used or NOS market - Clarostat V388-Z 50K with 3/4" x 1/8" shaft, pc pins, and 4 support legs. Now I've got some smaller, round body Clarostat 50k linear pots coming that I'll have to add a resistor to fake an audio taper. Because of the design and construction of the chassis, I'll either mount the new pots to the panel directly or fabricate a bushing - either way it looks like I'll have to open up the hole in the chassis plate.
Thank you both for your input, suggestions and insights, which have been very helpful in getting a better understanding of this problem, and showing my gaps of knowledge that I've been trying to fill in by reading, testing and experimenting.
The tone section matched dual (U404) was indeed, very poorly matched, with one side way out of specs, which I discovered after pulling it a second time to test. Previously, I had replaced it with a new production LSI replacement, and had observed severe oscillation. I tested all of the replacements and they all appear well matched, unlike the original.
After replacing the original and returning adjustments to baseline, I found one shorted diode and one suspected bad U1732, which I replaced with a new old stock item. This corrected the DC offset problem when engaging the tone switch (yeay!) and adjusting the pot from just off minimum to maximum, but at minimum to just above, offset jumped. I then jiggled the pot shaft at that position and offset jumped around substantially. Above that range, offset is stable. I subbed in a 50k audio pot and offset was stable across the entire range.
Over the past 2 months I've been searching for replacement pots. State Electronics wants $100 a copy to make them, and I've not been able to find any on the used or NOS market - Clarostat V388-Z 50K with 3/4" x 1/8" shaft, pc pins, and 4 support legs. Now I've got some smaller, round body Clarostat 50k linear pots coming that I'll have to add a resistor to fake an audio taper. Because of the design and construction of the chassis, I'll either mount the new pots to the panel directly or fabricate a bushing - either way it looks like I'll have to open up the hole in the chassis plate.
Thank you both for your input, suggestions and insights, which have been very helpful in getting a better understanding of this problem, and showing my gaps of knowledge that I've been trying to fill in by reading, testing and experimenting.
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