dc voltage at the output [message #22351] |
Wed, 04 February 2015 10:35 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
smackoj
Messages: 67 Registered: September 2013
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hi gents; I have a K200B4 that I have been running with the PC403 preamp board disconnected because it was inop when I got the amp. I took a look at it lately and found the transistors Q400 and 401 had been replaced by someone who did the job bass-ackwards. I replaced these two trans. with suitable replacements installed correctly and the channel is now working but there is a bad buzz in it. I measured the dc at the output and found .033 vdc there. Is that too much, not too much or ???
thanks, jack
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Re: dc voltage at the output [message #22352 is a reply to message #22351] |
Wed, 04 February 2015 12:42 ![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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Not sure I understand what you mean. Where are you measuring the voltage at the output?
As for the buzz, try and isolate the source by testing a few things. Does the buzz change volume when you adjust the volume control? Do the tone controls change the sound of the buzz? What about the clipper, does the clipper control change the buzz?
You see what I'm saying, try and see if you can find out where in the circuit the buzz is coming from.
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Re: dc voltage at the output [message #22355 is a reply to message #22351] |
Wed, 04 February 2015 15:06 ![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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Having 0.033vdc on the output isn't too bad. If you had 0.33 I'd be a little concerned, but even that level is fairly normal.
Now for your buzz, what happens with a guitar plugged in? Now try turning the guitar's volume control all the way down.
If the amp's volume control changes the buzz, then the problem is located at the input jacks and wiring or in the first stage of the preamp, where you changed the two transistors.
What transistors did you use to replace Q400 and Q401?
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Re: dc voltage at the output [message #22357 is a reply to message #22351] |
Wed, 04 February 2015 16:39 ![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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Those transistors should work as replacements. If you have another 2N3904 you can try replacing Q402 as well.
Does your meter measure capacitance? Resistance?
I would start by checking the resistances of all of the resistors around Q400-Q402 and if your meter can test them, C403 and C404.
There are many multiple resistance paths in the circuit, so don't expect to get exact readings that match the schematic, but look for opens or dead shorts. When somebody else has been in there, you can't assume that everything is the way that it should be. You may find broken pc traces or traces with solder bridges, etc.
And just to be certain that you know this, these tests should be done with the amp unplugged from the wall and the filter caps discharged.
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Re: dc voltage at the output [message #22366 is a reply to message #22351] |
Thu, 05 February 2015 11:10 ![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: 4767 Registered: June 2004 Location: NY
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Since the volume control is right after the channels first gain stage if turning it down does not kill the hum then you are chasing down the wrong trail.
This does not mean that the jacks may not be wired wrong, but you have the other channel to compare that to!
Use your voltmeter to step back thru the signal chain from Q416 and look for dc voltage where there should be none.
Resistors are not a likely cause, more likely would be a leaky electrolytic coupling cap, or a leaky transistor, and also take note that all dc voltages are taken with all the controls full up and with no speaker load or test load on the amp as noted on the schematic!
Also just for kicks just check the two 8 volt preamp voltage readings as if they are not pretty close two each other then that can make for hum also!
[Updated on: Thu, 05 February 2015 11:15] Report message to a moderator
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Re: dc voltage at the output [message #22367 is a reply to message #22351] |
Thu, 05 February 2015 12:32 ![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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Jack, if you go to the technical section and look at any of the schematics for the K200B series amps and not the individual pc boards you will find the schematics for the input jacks as well as the power supply and footswitch jacks, etc. Go to the schematics by amp model list and then click on the link for the model number.
I would think that any problem with the power supplies would show up in both preamps.
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Re: dc voltage at the output [message #22371 is a reply to message #22351] |
Thu, 05 February 2015 17:35 ![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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Both sets of inputs should be wired the same way. What does the black wire connect to at the jacks and at the board side?
If it's an additional ground wire, it might be creating a ground loop.
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Re: dc voltage at the output [message #22373 is a reply to message #22371] |
Thu, 05 February 2015 17:57 ![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) |
smackoj
Messages: 67 Registered: September 2013
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I removed the black wire and made sure both sets of input jacks are wired the same. The buzz completely stopped but it's still not acting quite right. The black wire was on the shaft ground lug of the input jack and up to a solder pad about an inch directly below the solder pad for the blue signal wire.
revised 30 mins. later.
OK, It is working better but now the buzz is still there although quite a bit less at low volume but increases as I turn up the volume.
thanks friends.
[Updated on: Thu, 05 February 2015 18:22] Report message to a moderator
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Re: dc voltage at the output [message #22381 is a reply to message #22351] |
Thu, 05 February 2015 23:52 ![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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Well, it sounds like the wrong person was working on your amp. Does it look like there have been any other changes done to the board? You may need to check the board against the schematic and see if anything else has been messed with.
I think that the board itself is grounded by the pot mounting nuts on certain pots. Have you cleaned the chassis and star washers at the mounting points? How about the where the pot mounts to the pc board?
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Re: dc voltage at the output [message #22388 is a reply to message #22386] |
Fri, 06 February 2015 08:47 ![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) |
smackoj
Messages: 67 Registered: September 2013
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I will check all the ground points and post an update. I believe that all 4 input jacks are shorting hot to ground. Maybe cleaning up the contact areas will quiet it down? I don't see any other spots on PC403 that look tampered with. All components and solder joints look orig.
thanks, jack
[Updated on: Fri, 06 February 2015 09:01] Report message to a moderator
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Re: dc voltage at the output [message #22395 is a reply to message #22351] |
Fri, 06 February 2015 12:49 ![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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It sounds like you are on the right track now. The added black wire was probably the other guy's attempt to fix the grounding issue.
Also don't forget to look for cold solder joints on the board.
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Re: dc voltage at the output [message #22406 is a reply to message #22351] |
Sat, 07 February 2015 00:39 ![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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The only 200B head I have open right now is a B-1, but I'll take a look at how things are grounded. Later on if I get a chance, I'll pull out my B-4 and check that.
Edit: I checked and one channel has a black wire twisted with the blue input wire that grounds the board to the input jacks.
[Updated on: Sat, 07 February 2015 02:39] Report message to a moderator
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Re: dc voltage at the output [message #22408 is a reply to message #22406] |
Sat, 07 February 2015 04:52 ![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) |
smackoj
Messages: 67 Registered: September 2013
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I will install the black wire again and see what happens. I can tell the blue wire for that channel was obviously twisted at one time.
I am suspicious of the on-off switch wiring. I see both black wires from the thermostat to the on-off switch. I see one wire shown on the diagram in "technical" but are both supposed to go to the switch?
results later today. thanks
[Updated on: Sat, 07 February 2015 05:05] Report message to a moderator
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Re: dc voltage at the output [message #22411 is a reply to message #22351] |
Sat, 07 February 2015 11:56 ![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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The thermoswitch is wired in series with the power transformer. One wire goes to the ac/polarity switch and the other goes directly to one end of the transformer primary winding. That's the way it's supposed to be wired in any case.
I've seen black/black wires as well as black/white and white/white-black stripe wires depending on the era of the amp. They are always twisted pairs.
Double check the wiring that you have, maybe it just looks like both wires go to the power switch.
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Re: dc voltage at the output [message #22412 is a reply to message #22411] |
Sat, 07 February 2015 12:57 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
smackoj
Messages: 67 Registered: September 2013
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Member |
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ok; finally moving towards the goal. The power switch was wired wrong and I got that put back to correct. I added the black grounding wire back from PC403 brd to the ground lug on the input jack. It sounds 80 percent better. Still has some buzz that changes with the position of the Bass and Treble pots on the PC403 side but there is a 'sweet' spot on the pots that gets the amp to a buzz level I can ignore it's so minimal. When I try the PC303 side it is a little quieter but the Treble pot does effect the buzz, the Bass pot does not and the vol. pot is quiet until around 8-9 on the dial.
thanks, jack d
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