K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21623] |
Fri, 03 October 2014 10:45 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Jerrybass1955
Messages: 45 Registered: August 2014 Location: NC
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Hi Folks,
I am still working on my K100 which now appears to be a version from prior to Rev. 3 since it has no Q131. I am struggling to get the dead half of my power amp working. I have tried several things including replacement of the output transistors, drivers and most all of the resistors in the neighborhood. I may try replacement of the bias diodes next, but here are a few symptoms/questions for you guys:
* At the base of Q126, collector of Q124 I am reading about 26.6 volts. This looks pretty consistent with the K100-1 schematic (same circuit), but the K100-2 schematics show 32.8 at the same location. Is 32.8 wrong? Seems odd that it would show up as 32.8 on the rev. 3 drawing and not have been corrected by the rev. 4 drawing - especially when it is 26.9 volts on the K100-1 schematics.
* The problem I have is that the channel driven by Q128 and Q2 is not working. If I drive the output relatively hard and look at the signal at the base of Q2, I get approximately square wave of maybe 1/2 volt or so and no real contribution at the speaker terminal. (good news, is that I am getting about 10 watts out of the other channel operating class A until I hit asymmetrical distortion. Earlier stages don't appear to be distorting too soon or asymmetrically.)
* This is the same thing that happened with the original driver transistors. I replaced Q127 with a 2n3440 and have tried the original transistor as well as a 2n25415 and a 2n5416 for Q128. Currently the 2n5416 is in and when it was installed I immediately got a high current draw situation with things getting hot. I shut down the amp and need to address that. But otherwise, I get the base drive signal mentioned above which is the basic problem. Any reason these transistors shouldn't work?
* I disconnected the protection circuit at the diodes to see if that was a contributor but there was no change (except that perhaps not having it in could have contributed to the high current situation when I later tried the 2n5416).
* I replaced the 1 ohm resistors which had drifted with a pair of matched resistors. I also replaced the 95 ohm resistors in the protection circuit with 100 0hm 1% (they were 5% in my circuit - should I try to get closer to 95 ohms?). R205 and R207 were replaced with 1% 2 Watt resistors. Most if not all relevant resistors have been replaced with metal or carbon film resistors of correct value.
* It appears that the Q1 side is working well and putting out about 10 watts into 8 ohms before asymmetrical distortion sets in. No sign of unreasonable asymmetrical distortion in stages prior to the driver.
* It seems to me that Q2 and Q128 must be turned on suggesting a bias problem. Perhaps the diodes are bad. They test good in-circuit, but I know that is not always reliable. I shorted out all the diodes which I think should have likely led to an output from Q2 (albeit with crossover distortion), but saw the same signal at the base of Q2.
* I replaced the power transistors Q1 and Q2 with matched MJ15015s for good measure and nothing seems wrong there. They test the same when I pull the wires as they did out of the circuit and Q1 is obviously working like a champ.
* Some DC readings Q126 collector =-3.6V, Q128 Base =-0.7V, Q124 Base = 47mv, emitter=0.6V, Q125 base=54mV. I forgot to bring other voltage readings with me.
I am at a loss at this point. Any suggestions would be appreciated. I have double checked my replacement resistors to assure myself that I didn't do anything wrong, but if something seems suspicious, I'll triple check.
Thanks,
Jerry
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21625 is a reply to message #21623] |
Fri, 03 October 2014 12:05 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4758 Registered: June 2004 Location: NY
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I do not have the time right now to go over what you have posted in detail right now, but I will latter, but I need a clarification from you as to what you mean when you say that on one channel you get hardly any power output, but on the other you get some 10 watts.
The K100 -2 only has one channel, is this a pA head?
[Updated on: Fri, 03 October 2014 12:06] Report message to a moderator
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21630 is a reply to message #21627] |
Fri, 03 October 2014 15:28 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Jerrybass1955 wrote on Fri, 03 October 2014 12:14Q1 seems to provide up to about 10 watts of output prior to starting to clip and Q2 essentially contributes nothing to the output.
I'm still confused, Q1 is working up to 10 watts and Q2 is off? From your posts, you are looking at the signal with an oscilloscope right? Then what does the waveform look like at the output, top and bottom of the sine wave both there?
What are the voltages at Q2 and at Q1?
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21647 is a reply to message #21635] |
Sat, 04 October 2014 09:07 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Jerrybass1955
Messages: 45 Registered: August 2014 Location: NC
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Here is a bit more data - DC voltages:
Q1 E+.06, B+.62, C+40.1
Q2 E-39.9, B-39.3, C-.03
Q126 E-33.3, B-25.8, C-3.6 -> is B at -25.8 OK, some schematics say -32.8, others about -26?
Q127 E+.62, B+1.2, C 39.9
Q128 E-.023, B-.62, C-.39
Power supplies were at about 40.5 and 8.3 or so last night. Varies a bit with my line voltage which runs a little high.
Seems like something is turning Q128 and Q2 on pretty hard I guess, but can't figure what it is unless it is the diodes.
I reconnected the protection circuit on the scope, but I couldn't see anything that looked overly suspicious. Any thoughts?
Jerry
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21654 is a reply to message #21647] |
Sun, 05 October 2014 00:19 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Looks like Q128 Collector voltage has a decimal that shouldn't be there. What are the voltages at Q124 and Q125?
The -26 volts at the base of Q126 is bothersome to me as the differential voltage between the base and the emitter should be less than a volt, you've got 6 volts.
Have you tested other resistors in the power amp? How about the tantalum caps?
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21682 is a reply to message #21623] |
Fri, 10 October 2014 11:27 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
Jerrybass1955
Messages: 45 Registered: August 2014 Location: NC
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Good News! Doing the repairs based on my last post seems to have the power amp going. I read just over 20Vrms at the output with an 8 ohm load. That is just over 50 watts, so I guess it is pretty much working now. I didn't bother re-measuring all of the DC voltages, but I am reading about 26 volts at the emitter of Q124 and about 32v at the emitter of Q125. VBE is around .6 or so for both. Oddly, I still see the small square wave-like signal at the base of Q2, but the output looks pretty good and sounds clean.
One significant power amp section issue remains - It seems that Q124 is a bit unstable. A touch of the emitter with a voltmeter probe sends it into oscillation at about 18Khz. I am also seeing a bit of oscillation or ringing at one side of the output signal just before it clips. Tracing back, it seems to start at Q124 (nothing at Q108 - recall I don't have a Q131), so I suspect the two symptoms may be the same problem. Q126 has been replaced with a 2n3440, so I guess the load is a little different, but it may just be a bad transistor at Q124 or something. Anyone have thoughts about this phenomenon? I am going to replace Q124 (and probably 125 to get a reasonable match), but if that doesn't work, what do you suggest (or what do you suggest instead). And should I use 2n4249s or a more modern transistor (note the voltages are a bit high in this area - Q125 has over 30 volts from collector to emitter.)
Thanks guys and sorry about the earlier wild goose chase.
Jerry
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21684 is a reply to message #21623] |
Fri, 10 October 2014 13:01 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Glad to hear that it's working again. The oscillation is probably due to the replacement transistors that you have used.
I've had this happen in the larger heads when all of the outputs and drivers have been changed to modern parts. It doesn't always happen just sometimes when the moon is full and the frost is on the punkin.
I figure that the newer transistors have been built using modern technology that allows them to work at higher frequencies than the old transistors could. So what was a stable circuit now has ultrasonic parasitic oscillations that are caused by lead dress, pc layout, etc.
I usually add snubber caps on the drivers and it will go away. That reminds me, did you check C140 on Q126? Do you have a C140 on your version of the board?
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21686 is a reply to message #21623] |
Fri, 10 October 2014 15:37 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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I add the snubber from C to B on the drivers and that usually fixes it, but I don't remember if I've ever done this on K100 head or not. I use a 47 or a 56 pf ceramic cap. Like I said this doesn't happen that often, but it does happen.
As for the higher value on the pre-driver, it might help, but I don't know when it will start to effect the high end response of the amp.
Matching the two differential inputs will help to reduce any output stage offset voltage, but I don't think that it will help to stop the oscillation.
And of course be sure to check all of the grounds and C139 for that matter. Remind me what parts have been replaced in the power amp section so far, the output ballasts and the voltage dividers in the protection circuit, the driver transistors, what else?
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Re: K100-2- Pre-Rev-3...no Q131...1/2 the power amp [message #21687 is a reply to message #21686] |
Fri, 10 October 2014 16:29 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
Jerrybass1955
Messages: 45 Registered: August 2014 Location: NC
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This part of the amp was rather extensively rebuilt.
Q124 and 125 replaced with same transistor type but matched
Q126 and 127 replaced with 2n3440
Q128 replaced with 2n3416
Q129 and 130 original
Q1 and Q2 replaced with matched MJ15015
CR107 and 108 replaced with 1n4148
All polarized caps replaced with Panasonics
Most resistors replaced with carbon film or metal film.
Ceramic caps are original.
1 ohm resistors replace with matched set
R207 and 205 replaced with metal film 2Watt resistors (after one fried while the amp was not working right).
Q108 was replaced with a 2n2222 (I believe).
Voltage divider resistors in protection ckt was replace (R213 and 214 are now 100K 1%, R209 and 212 are 510 ohm 5% carbon films that I selected to match).
Heat sinks added to Q127 and 128.
Jerry
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