First check for the output tube screen regulated voltage, I recall it's around 300VDC.
That would be the most likely cause of no signal in both channels. The output tube
bias current would then be 0mA as well.
If that is ok, with only a dummy load and scope, try an input signal to only the positive input,
and ground the negative input. Then try the reverse. Does either work?
That would be the most likely cause of no signal in both channels. The output tube
bias current would then be 0mA as well.
If that is ok, with only a dummy load and scope, try an input signal to only the positive input,
and ground the negative input. Then try the reverse. Does either work?
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Thanks Rayma. Actually it turns out nothing was toasted. It has been my lack of focus this whole week (holidays? or just plain stupid?, the jury is still out). Today I started tracing the circuit on the back pcb to figure out the pinout on the tubes; and then I realized I removed the wrong tube 
. So again without V1 and V2 with balanced input the amp measured fine on the scope.
So I tried it in my audio system. Well I got a little taste of the 1975 ; in mono so far since only the left channel works beautifully. I have to say the sound was not what I expected. I use diy Pass class A amps. In my Burning Amp 3 mono blocks I had actually tuned distortion to my taste, and I thought this tube amp would have even more H2 distortion and sound warmer on voices. But this amp sound is crystal clear!; but what surprised me even more is how the amp rules the speaker top to bottom; (I experienced the same before when I changed from Aleph30 to BA3b amp with Elsinores speakers, but did not expect another amp being so noticeable after BA3bs on LX521s). Mind you, here the amp only runs the top portion of the lX521 speaker down to 120Hz, but the midbass is already playing noticeably better than with my BA3s. I have 12inch subs under run by a Dclass ICEamp. The highs are also more pronounced.
> WOW, had the right channel been running right off the bat it would have been addictive, (cannot wait now to be able to give it a proper listening session in stereo).
Ok, about the right channel: the noise seems to be an issue. It has a terrible hum; when the volume gets turned up the music starts to show on top of the hum. Without the right channel input connected the noise is lower but still there. The other channel input has no bearing on it. I tested both D150 channels with the same preamp channel output and the same speaker to make sure the amp was the only variable. Not sure yet how to go about troubleshooting this?
. So again without V1 and V2 with balanced input the amp measured fine on the scope.So I tried it in my audio system. Well I got a little taste of the 1975
; in mono so far since only the left channel works beautifully. I have to say the sound was not what I expected. I use diy Pass class A amps. In my Burning Amp 3 mono blocks I had actually tuned distortion to my taste, and I thought this tube amp would have even more H2 distortion and sound warmer on voices. But this amp sound is crystal clear!; but what surprised me even more is how the amp rules the speaker top to bottom; (I experienced the same before when I changed from Aleph30 to BA3b amp with Elsinores speakers, but did not expect another amp being so noticeable after BA3bs on LX521s). Mind you, here the amp only runs the top portion of the lX521 speaker down to 120Hz, but the midbass is already playing noticeably better than with my BA3s. I have 12inch subs under run by a Dclass ICEamp. The highs are also more pronounced.> WOW, had the right channel been running right off the bat it would have been addictive, (cannot wait now to be able to give it a proper listening session in stereo).
Ok, about the right channel: the noise seems to be an issue. It has a terrible hum; when the volume gets turned up the music starts to show on top of the hum. Without the right channel input connected the noise is lower but still there. The other channel input has no bearing on it. I tested both D150 channels with the same preamp channel output and the same speaker to make sure the amp was the only variable. Not sure yet how to go about troubleshooting this?
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Good you're getting somewhere, never work when distracted or preoccupied, especially on 600VDC.
Both channels are the same, with a common power supply. Look at and check again the new wiring.
Check all the DC voltages in the bad channel and compare with the good channel. Then test on
the bench with a scope, generator, and dummy load. Probably related to the input connections.
Both channels are the same, with a common power supply. Look at and check again the new wiring.
Check all the DC voltages in the bad channel and compare with the good channel. Then test on
the bench with a scope, generator, and dummy load. Probably related to the input connections.
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Today I could not do much. I cut PS lines for my quiet fans; I thought maybe some switching noise or small transformer/rectifier noise was making it over to the amp signal line. Nope. Still, when I turn the amp on, about maybe 6 seconds later a strong constant noise in the right channel starts (probably right after the big electrolytic caps have charged up). I could get a mic (or voltage) measurement through FFT to confirm the frequency but it must be grid frequency related.
I did not measure any voltages yet. I cannot see what could be wrong with the two wires and the shield on that channel's balanced input as all is identical. Will check connectivity again.
p.s. there is btw a slight buzz coming from the encapsulated PS transformer (with ear fairly close and no music of course). Then again if the PS is common and the left channel is quiet there should be nothing to this.
I did not measure any voltages yet. I cannot see what could be wrong with the two wires and the shield on that channel's balanced input as all is identical. Will check connectivity again.
p.s. there is btw a slight buzz coming from the encapsulated PS transformer (with ear fairly close and no music of course). Then again if the PS is common and the left channel is quiet there should be nothing to this.
There is definitely a difference between left and right channel:
I loaded each channel (one at a time) with 8 Ohm resistor and fed 1V pk-pk at ~1kHz in through the balanced input.
The nominal 350Vdc at the removed V2 pins 1/6 reads about 342V for both channels, but the anode voltages picked up by wires after R26/R27 to the output stage tubes are: 136.5V and 135.7V for the left (good) channel, and for the right (noisy) channel they read: 156V and 226V. All voltages read as dc by multimeter relative to amp (chassis) ground.
@Rayma: What do you make of this lack of symmetry? it looks to me that these voltages should be brought back to symmetrical and then the tubes used by the right channel should be rebiased (i.e. I should probably turn the bias down on the tubes on that side right away with before we figure out the adjustments needed, right?). That trimmer on the constant current source tube, does that need adjustment? It should have been factory set. I am not happy to mess with that.
p.s. The back (input stage) tubes on that side feel a little loose in their sockets compared to the other side, but that should not be able to contribute to this, right?
I loaded each channel (one at a time) with 8 Ohm resistor and fed 1V pk-pk at ~1kHz in through the balanced input.
The nominal 350Vdc at the removed V2 pins 1/6 reads about 342V for both channels, but the anode voltages picked up by wires after R26/R27 to the output stage tubes are: 136.5V and 135.7V for the left (good) channel, and for the right (noisy) channel they read: 156V and 226V. All voltages read as dc by multimeter relative to amp (chassis) ground.
@Rayma: What do you make of this lack of symmetry? it looks to me that these voltages should be brought back to symmetrical and then the tubes used by the right channel should be rebiased (i.e. I should probably turn the bias down on the tubes on that side right away with before we figure out the adjustments needed, right?). That trimmer on the constant current source tube, does that need adjustment? It should have been factory set. I am not happy to mess with that.
p.s. The back (input stage) tubes on that side feel a little loose in their sockets compared to the other side, but that should not be able to contribute to this, right?
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The difference is likely within normal variation. You can check by exchanging the driver tubes
between channels, but be sure they end up back where they belong. The AC balance
adjustment is sensitive to the tube, and can only be made with suitable test equipment.
Also I would test using a much smaller input voltage, say 0.1V p-p. Tube sockets won't affect this.
between channels, but be sure they end up back where they belong. The AC balance
adjustment is sensitive to the tube, and can only be made with suitable test equipment.
Also I would test using a much smaller input voltage, say 0.1V p-p. Tube sockets won't affect this.
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thought i should mention on the side that I brought the pigtails for balanced inputs out through the holes for the RCA connectors which I removed.
As far as the measurements: Ok I will switch the driver tubes and recheck the same (anode) voltages if they reversed. I am thinking also of connecting a junk speaker and getting something up on the scope with signal generator input. I want to establish if I can reproduce the noise on top of the signal from the generator instead of connecting with my audio sources. Not sure what else I can do..
As far as the measurements: Ok I will switch the driver tubes and recheck the same (anode) voltages if they reversed. I am thinking also of connecting a junk speaker and getting something up on the scope with signal generator input. I want to establish if I can reproduce the noise on top of the signal from the generator instead of connecting with my audio sources. Not sure what else I can do..
Update:
Shorting the input makes the noise worse.
Swapping driver tube makes no difference.
Swapping constant current tube makes no difference.
With input shorted and with the resistor load:
On the bad channel:
the cathode currents on the constant current source tubes are not symmetrical: voltage drops across R28 and R29 are 7.2V and 5.1V (ratio ~1.4). The cathode voltages ratio is ~1.15 (423V/368V).
The anodes voltages ratio (on the trimmer pot ends) is ~1.04 (570V/547V).
Ratio of anode voltages on the driver tube is (other side of R28/R29) is ~1.16 (417V/360V).
The ratio of driver cathode voltages (after R26/R27) is ~1.45 (225V/155V).
On the good channel everything is symmetrical:
CCS tube cathodes: 362V/357V
CCS tube anodes: 546V/540V
Driver tube anodes: 353V/348V
Driver tube anodes: 136.8V/136.2V
All resistors from R20 to R29 seem to measure close enough to nominal values.
I tried trimming the bad channel (by turning CCS pot CW to reduce the difference in voltage drop across R28 and R29. The trimmer reaches stop after half turn and the difference is reduced from 2V to 1V but cannot be trimmed to balance. So trimmer returned to the original position.
I am waiting for a friend of mine to take a look at the schematic and these values.
If you have any suggestions, by all means please feel free to share. Thx.
Shorting the input makes the noise worse.
Swapping driver tube makes no difference.
Swapping constant current tube makes no difference.
With input shorted and with the resistor load:
On the bad channel:
the cathode currents on the constant current source tubes are not symmetrical: voltage drops across R28 and R29 are 7.2V and 5.1V (ratio ~1.4). The cathode voltages ratio is ~1.15 (423V/368V).
The anodes voltages ratio (on the trimmer pot ends) is ~1.04 (570V/547V).
Ratio of anode voltages on the driver tube is (other side of R28/R29) is ~1.16 (417V/360V).
The ratio of driver cathode voltages (after R26/R27) is ~1.45 (225V/155V).
On the good channel everything is symmetrical:
CCS tube cathodes: 362V/357V
CCS tube anodes: 546V/540V
Driver tube anodes: 353V/348V
Driver tube anodes: 136.8V/136.2V
All resistors from R20 to R29 seem to measure close enough to nominal values.
I tried trimming the bad channel (by turning CCS pot CW to reduce the difference in voltage drop across R28 and R29. The trimmer reaches stop after half turn and the difference is reduced from 2V to 1V but cannot be trimmed to balance. So trimmer returned to the original position.
I am waiting for a friend of mine to take a look at the schematic and these values.
If you have any suggestions, by all means please feel free to share. Thx.
well I will try my best to figure out what may be loading the circuit asymmetrically so the two halves within the tubes could not be trimmed back to balance. I have no access to AR and on previous inquiries they did not provide any useful info anyways (as I understand the old guy there, Lenard?, familiar with the older amps has retired). If I can bring it back to life; else it will be just a disaster as far as $ and time investment goes. I am already behind on my SS projects which need attention. I am thinking of building another speaker this year just to lift my spirits after CoviD150 )
)took more measurements with a guidance from a friend: switching all tubes no difference. no current through R14 to pin 3 of V4 input amp tube on bad channel. all resistors seem to measure ok. V4 tube base sockets to resistors have connectivity. So the main suspect is the V4 base not grabbing onto the tube pins (the tube does feel looser in the socket than the others). So I will take another look under the board and probably order a socket for the 12AX7 (unless someone can mail me one).
No idea yet how I am to replace the base. I do not want to remove the back board completely since it would take de-soldering many wires. With board just at an angle I may not be able to clean all the pins so the base could come out; and vandalizing the base to remove the pins separately may be difficult if the brown plastic is tough and cannot be crushed. So with this delicate vintage pcb it will be a challenge: hmmm..
No idea yet how I am to replace the base. I do not want to remove the back board completely since it would take de-soldering many wires. With board just at an angle I may not be able to clean all the pins so the base could come out; and vandalizing the base to remove the pins separately may be difficult if the brown plastic is tough and cannot be crushed. So with this delicate vintage pcb it will be a challenge: hmmm..
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If there's no voltage drop across R14, that would certainly be a problem.
Make certain this is the case before dismantling it. Try the probes at several places,
like both top and bottom of the socket pin, etc. These boards are extremely fragile
to desolder parts from. The traces will come right off.
Make certain this is the case before dismantling it. Try the probes at several places,
like both top and bottom of the socket pin, etc. These boards are extremely fragile
to desolder parts from. The traces will come right off.
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I desoldered enough simple components by now to know that it is super fragile. I am not happy at all about the prospect of messing with the tube base.
I attached a mini hook test lead to the tube pin and the other to the corresponding resistor (for both cathodes and both anodes on V4) and I get connectivity beep on sticking the tube lightly into the socket with the probe jammed in between. But I have no means to tell once the tube goes all the way in. I can only attest that it feels looser then the others as the tip can easily be moved around. I will look under the board but I am really out of ideas here.
I am only going by what my friend suggested after we found 0Vdc (to chassis) at R14 to v4 cathode. R13 to the other cathode .98V. On V3 they are symmetrical about 0.82V if i remember correctly.
On two sides of R18 (plus signal side and minus signal side):
133V/213V on the bad channel, and
108V/111V on the good channel.
Swapping tubes makes no difference.
I cannot get access under small film caps. All resistors seem to check out.
I will take another look before starting to tear things apart.
Maybe I should solder a bunch of leads to an external V4 base and under the board and try? Not sure how doable this may be with high voltages around.
I attached a mini hook test lead to the tube pin and the other to the corresponding resistor (for both cathodes and both anodes on V4) and I get connectivity beep on sticking the tube lightly into the socket with the probe jammed in between. But I have no means to tell once the tube goes all the way in. I can only attest that it feels looser then the others as the tip can easily be moved around. I will look under the board but I am really out of ideas here.
I am only going by what my friend suggested after we found 0Vdc (to chassis) at R14 to v4 cathode. R13 to the other cathode .98V. On V3 they are symmetrical about 0.82V if i remember correctly.
On two sides of R18 (plus signal side and minus signal side):
133V/213V on the bad channel, and
108V/111V on the good channel.
Swapping tubes makes no difference.
I cannot get access under small film caps. All resistors seem to check out.
I will take another look before starting to tear things apart.
Maybe I should solder a bunch of leads to an external V4 base and under the board and try? Not sure how doable this may be with high voltages around.
The big news is I got the amp working!!!! I checked EVERYTHING again, then sanded down the pins on the V4 tube, tried cleaning the socket a bit (did not use deoxit), moved the tube around in the socket a couple of times with turning it on after each adjustment and Voila, at once it started working. I trimmed the channel to a closer balance than the other side now. That socket should probably be replaced but I will not be messing with it. It works now and I will not touch it. Enough time spent on this monster.
I gave it a listen. Just some material from youtube: Boston covers by Leonid & friends (a variety of instruments and mastered very well), some numbers Live from Daryls' house, Lexington Lab band and a few better covers by the HSCC.
I am pretty sure these may be the only LX521s driven by a D150 in the world. The way my speakers are setup right now: D150 makes them more forward sounding; not quite my cup of tea. The top runs too hot. It reminds me of Pass F5 amp. Since the amp is resolving detail fine with this EQ it gets brutal on less then perfect sound recordings. Mind you my setup is active, with miniDSP in the audio chain EQed with my previous setup; so I have no doubt I can improve things by going in and playing with EQ to bring it back, more in line with my taste. I may also be a bit too critical today since with family at home due to Covid times I have been listening a lot on a high end headphone setup these days (well months, almost a year gosh!(().
So what a journey restoring this vintage guy. WAF is low (already confirmed), so the amp will be for sale in Summer. I will be collecting interest till then. Thanks for those following and many thanks to Rayma for providing guidance!
I gave it a listen. Just some material from youtube: Boston covers by Leonid & friends (a variety of instruments and mastered very well), some numbers Live from Daryls' house, Lexington Lab band and a few better covers by the HSCC.
I am pretty sure these may be the only LX521s driven by a D150
in the world. The way my speakers are setup right now: D150 makes them more forward sounding; not quite my cup of tea. The top runs too hot. It reminds me of Pass F5 amp. Since the amp is resolving detail fine with this EQ it gets brutal on less then perfect sound recordings. Mind you my setup is active, with miniDSP in the audio chain EQed with my previous setup; so I have no doubt I can improve things by going in and playing with EQ to bring it back, more in line with my taste. I may also be a bit too critical today since with family at home due to Covid times I have been listening a lot on a high end headphone setup these days (well months, almost a year gosh!(().So what a journey restoring this vintage guy. WAF is low (already confirmed), so the amp will be for sale in Summer. I will be collecting interest till then. Thanks for those following and many thanks to Rayma for providing guidance!
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