Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: KevO on April 10, 2013, 11:48:08 AM
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Greetings,
I ran the first power test and had a failure with C4S board. Found poor connection, repaired, and passed voltage tests.
Hooked up to play! Noisy, scratchy, popping, quite dynamic sounding. Swapped tubes, noise did not move. Both channels.
Read and reread 'Basic Troubleshooting' section. Spent next hours re-flowing solder per instructions. Tested each component. resistors, caps up to 10 uf, transistors where the lowest ohm load I found was 109 ohms. But same on both transistors. Slept.
Went back through resistance checks. With tubes.
Reading on 4 was 5.3 ohms but the reading on 19 was 120 ohms.
Swapped 300b's and reading on 4 was 178 ohms and 19 was 5.3 ohms.
And I was beginning to think I had killed an electrolytic cap....
then again....
Warmed preamp up 3-10 minutes each time before turning on amp. Always had 2 loud, weird thumps from left channel and scratchy noises from right channel. Low to no volume each time.
Just a bad tube or a cap and a tube?
Thoughts?
Kevin
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I would reheat the center pin of each MJE5731A transistor.
Having scratching/pops from both channels really narrows things way down, there aren't many components common to both channels. Do all the inputs have these issues?
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...
Reading on 4 was 5.3 ohms but the reading on 19 was 120 ohms.
Swapped 300b's and reading on 4 was 178 ohms and 19 was 5.3 ohms. ...
4 and 19 should both read around 5 ohms, it's the sum of 4 ohms from the parallel3ed 8-ohm resistors plus the cold resistance of the 300B filament. This data suggests one of the 300Bs has a dead filament. You should be able to measure around 1 ohm from pin 1 to pin 4 (the fat pins) of the tube, with the tube out of the socket. If the tube is really bad, then that's the problem. If the tube measures OK then the problem might be with the socket not getting a good contact (or the wrong pins in the socket? That happens!)
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Thanks gentlemen,
I will follow your recommendations. I ordered a new tube since the bad reading followed the tube swap. I will test as outlined also.
Since the noise was loud, even with no volume, I will check the transistors.
-Kevin
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Kevin to piggy back on your noise, I'm having a popcorn, crackling noise in one channel that doesn't follow the tube but is specific to one tube. I reflowed the tube pins and the corresponding joints and tightened the tube pins. It went away for a while but if I move that tube at all as I did when taking measurement today, that crackling sound comes back for a few seconds and then disappears on and off throughout the day.
Thoughts?
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Gentlemen,
I tested the 300b's. One was 1.1 ohms and the other 36 ohms. New one arrives Monday.
Reflowed solder on transistors. While doing so, I found a cracked wire going to IA on C4S board. That was probably the source of the noise! Probably killed the tube too.
I hope that is all I have to fix. We'll see.....
I drank less wine while assembling this BeePre and I won't make that mistake again.
Thanks for the help.
-Kevin
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Greetings from Napa.
Scratching noise is gone. Probably the cracked wire.
Now I am experiencing a major oscillation. Both channels. Woofers are dancing! I powered it off quickly.
I will start again and recheck all my wiring. Could any particular component be the cause?
Any special test I need to perform?
I did use a Goldpoint switch and volume. (bad Kevin!) but I think I have them wired correctly.
Boy am I learning with this build.
Thanks,
-Kevin
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Is the oscillation like 60Hz, or higher or lower? Is it a smooth tone or buzzy? Or perhaps is it a ringing sound?
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Doc,
It is low frequency. I can see the grill cloth pulsing.
I am a bit afraid to hook it up again, but my amp can't kill my speakers, so I can if necessary for testing.
-Kevin
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Was the volume control on the preamp turned down all the way when this happened?
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Yes it was.
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OK, assuming the attenuator is installed correctly (which i can't say for sure since it is not stock) being turned down all the way should eliminate anything upstream of the preamp.
The oscillation is in both channels for sure? The preamp is more or less dual mono, there is very little that is common between the channels. You should probably recheck all of your voltages for starters. You might check your output jacks too, to see if there is measurable AC voltage across them. Let us know of any measurements that look way out of spec.
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Thanks Doc.
I will recheck and document all readings. When I check the outputs, should I have volume up or off or both?
-Kevin
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Turn the volume all the way down. To be extra sure you can short the input jacks too. If you have significant AC across the output jack with no signal on the grid of the tube then there is definitely some kind of signal being generated by the preamp itself. Open grounds, backwards caps or a super sensitive amp and speaker setup that is amplifying the microphony of the 300Bs to feedback levels are some possibilities.
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Here are the results. Input shorted, volume 0.
T1 - 5.09 T2 - 99.1 T3 - 0 T4 - 9.8 T5 - .007 T6 - 145.6 T7 - (-.1) T8 - T9 - 0 T10 - 184.7
T11 - 145 T12 - (-.05) T13 - 0 T14 - 0 T15 - 184.7 T16 - 5.06 T17 - 101.3 T18 - 0 T19 - 9.74 T20 - .008
T21 - .012 T22 - 211.5 T23(-.006) T24(-.006) T25 - 211.5 T26 - .05 T27 - .034 T28 - (-.054) T29 - 6.75 T30 - 6.75
T31 - 13.54 T32(-.002) T33 .001 T34 - 13.58 T35 - 6.77 T36 - 6.77 T37(-.041) T38 - 0 T39 - .035 T40 - 211.6
T41 (-.009) T42 (-.009) T43 - 211.3 T44 - 0 T45 - 0 T46 - 211.3
A2=0, A3= 95.7, B2=0, B3=100.5
C2 (-.006) D2 (-.009)
C3 - 4.02, D3 - 4.13
C7 - 145.6 D7 - 144.9
C9 - 145.2 D9 - 144.6
VOLTAGE ACROSS OUTPUT SWINGS (-.075 TO +.075) VDC WITH OUTPUT SHORTED AND VOL ON 0
Thanks!
Kevin
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Can you measure AC voltage with your meter? An AC measurement across the output could be helpful.
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It reads 0.022 VAC
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Doc,
I think the fuse blew before the AC readings I just took. I tested when I did the DC readings and it was very low.
-Kevin
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Does the fuse look really blowed up inside, or like it went fairly gently?
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Gently. a quiet demise. I just measured with a new fuse and it is zero VAC.
-Kevin
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Doc,
Thanks again for the help yesterday. Oscillation has stopped. Can't say why. Just glad it did.
Only one anomaly now. I power up the source, then the beepre, wait 2 minutes and turn on the usually well behaved amp.
I get 2 Bzzt's from the mid/tweets and then 2 full excursion pops of the woofers.
That is followed by beautiful music. Detailed. palpable... but I digress.
My other tube and SS preamps do not have this issue on this system.
I will test beepree with another amp for comparison.
Oh, amp is ATI 6012 and between amp and preamp is Linkwitz active analog crossover. I always leave the x-over powered on.
Any ideas? Oscillation lite?
Thanks!
Kevin
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Following our discussion yesterday, it sounds like the safety ground and signal ground may not have good contact with each other.
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Doc,
Thanks for the reminder.
Kevin
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Doc,
I reflowed the ground tabs and tightened both up more. Same issue, both channels and effects all frequencies. Woofers shake the cabinet. Afraid I may blow a tweeter!
I can disconnect speakers from amp, power up and reconnect. Bypasses the audible issues that way, but certainly no solution.
I hate giving up, so will review wiring.
Sound had been too bright and bloated in mids. Last night the preamp 'gelled'. About 25 hours of play. I had no idea it would sound this *&%$!! fantastic. We both sat and listened to album after album until sleep won out. We are delighted with the music. Palpable, natural and nuanced.
Thanks for a stunning design
-Kevin
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If everything else is on and stabilized, and the power amp gives a jump, one would suspect the power amp first. But you said it is well behaved with other preamps. The only thing that comes to mind is a possible DC bias on the output of the filter that is not there with other preamps. That's possible if the BeePre has some leakage around the output capacitor AND the filter has no input or output coupling caps. Can you post a schematic of the active filter?
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Thanks Paul,
I will look for the schematic. As you can see from my posted measurements I did detect a small amount of DC on the outputs.
I am really curious why the noise stops. I can warm up the preamp and as soon as the amp goes on, 2-3 seconds of noise and then all is well.
Maybe I need a much longer warm up for the beepre. Would a mute switch help?
Since this is Linkwitz proprietary, I can't post it. I'll PM you tomorrow and we can figure this out. I am also checking the Linkwitz forum.
-Kevin
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I downloaded the ATI 6012 manual, and I see that its input is AC-coupled. Therefor a DC offset voltage (if present) won't have an effect on the power amp. The time constant of the BeePre output 10uF/475K is 5 seconds, and it will settle to less than 1mV in 60 seconds.
My conclusion is there must be a power line transient when the big amp switches on. That seems on the face of it unlikely, since the ATI supposedly has a soft-start to prevent just that. Still, you might put an AC voltmeter across the power line and watch is as the power amp is switched on.
How rapidly does the speaker oscillate? i.e. what frequency is the short impulse? I know, that's hard to tell - but you can probably see if it's closer to one per second or 10.
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Paul,
The ATI is very well behaved. I don't suspect the power line. Could be an interaction with the Orion active crossover.
I think the best clue is the noise lasts 2-3 seconds, then is over.
The woofers are maxing out and I am afraid of blowing a tweeter. Each driver has a 60 watt amp.
I will try again after warming up the beepre for 10 minutes.
I have sent you a PM.
Thank you,
Kevin
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I've been lurking on this thread a bit, but I'll throw in my $0.02.
I'm guessing just the woofers do this?
It is possible that the crossovers have no filter at the input, and if this is what's causing your problems, the fix is easy.
I would recommend investigating this case by simply connecting your BeePre directly to the woofer amp, then going through your power cycling.
If this ends up being the case, a C-R filter at the input with a 0.1uF cap and 249K resistor may take care of you. You could also try lowering the 475K resistors on the BeePre to 100K (or maybe try both).
-PB
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PB,
Thanks for the help.
The noise involves all the drivers, but the woofers seem to be cycling to full excursion at around 2-3 hz for 2 or 3 seconds. I am new to schematics, but I am fairly sure there is no filter at the input.
I will run your test in a few minutes.
Thanks!!
Kevin
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Gentlemen,
Just tested without the crossover. 1 pop instead of 3-5. So it appears the active crossover is more sensitive.
I put the o-scope on Beepre out and watched. But I have no idea what any of it means. Time to learn more about using the oscilloscope.
I found a diagram of the c-r filter (high pass) here; http://www.learnabout-electronics.org/ac_theory/filters81.php
Not sure how to build it into the crossover since it is a circuit board, but can I put the parts on a M/F RCA jack and insert it to the end of the BeePre out cable? How much will it cut the bass? The bass sounds great right now.
Thanks!
-Kevin