| K-200B-2 Hum/feedback [message #25590] |
Mon, 26 September 2016 17:34  |
slk854
Messages: 192
Registered: January 2015
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Senior Member |
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When I power the amp up with no effects there is no loud hum. There is a hiss but that seams to be somewhat normal with these 200's. When I turn the Reverb on if I go much past the first pointer on the dial I will start to get a loud hum. This is with or without a guitar connected. It gets worse the more I crank the reverb knob. What is going on????
Steve
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| Re: K-200B-2 Hum/feedback [message #25591 is a reply to message #25590] |
Mon, 26 September 2016 19:01  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Does the reverb effect still work?
It is possible that one of the RCA cable plugs needs to be cleaned, or somewhere along the line the reverb tank was installed backwards with the tank output close to the power transformer. Of course there could be a circuit problem as well.
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| Re: K-200B-2 Hum/feedback [message #25594 is a reply to message #25590] |
Tue, 27 September 2016 02:59 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I think that, that is the correct way to have it plugged in. Right now I'm not positive though.
The intermittent switch may need to be cleaned. Try spraying a little Deoxit into it and switch it on and off a few times. Also spray each of the RCA plugs and jacks with a little and twist the plugs to burnish the contact areas.
To test the circuit, disconnect the plug from the tank output and short the tip to the shell. Now turn up the reverb control. Still humming?
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| Re: K-200B-2 Hum/feedback [message #25596 is a reply to message #25595] |
Tue, 27 September 2016 11:33 |
slk854
Messages: 192
Registered: January 2015
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Senior Member |
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Here is a close up of that cap soldered to the pot and looks like a transistor. Doesn't look right to me, but you guys would know for sure.
I was not sure what end of the rca cable you were referring to disconnect so I did it both ways. When I disconnected it from the circuit board and touched the tip to the shell there was no sound/hum at all. When I disconnected it from the reverb tank side and touched it to the shell it was loud, scared me. I was not expecting it to be that loud. There was no way I could have turned the reverb dial up any more. It was just on #1.
I just opened up my other 200B-2 and that cap was not there. So was that some sort of revision that went out or someone's idea of an improvement? If it is not needed can I cut it out of there??
Steve
[Updated on: Tue, 27 September 2016 14:15] Report message to a moderator |
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| Re: K-200B-2 Hum/feedback [message #25597 is a reply to message #25590] |
Tue, 27 September 2016 18:11 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It doesn't look original to me. It may be somebody's quick and dirty fix or mod, done without removing the pc board from the the chassis to solder the parts in.
If the part is replacing something that should be on the board, then cutting it out might be a problem. If this is a mod, then removing it would probably not create a problem. I'm not sure what it is connected to. I'll try and pull out one of my heads to see what it might be doing.
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| Re: K-200B-2 Hum/feedback [message #25599 is a reply to message #25590] |
Tue, 27 September 2016 23:24 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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That noise seems to be pretty much normal. If the added cap is increasing the gain of the reverb return circuit, it might be louder than normal.
I was hoping that you could test the ground connections with that test. The shield of the return cable should be grounded so that when the reverb is turned up there should be less hum. If the connection at either end of the cable is dirty or if the cable itself is bad, then when the control is turned up there will be hum.
The ground connection on the send side of the tank is not connected at the circuit board side of the cable. The shield is connected to ground at the tank end through the return cable ground. If both ends of the wires were grounded at the circuit board side, then there would be a ground loop that could create additional hum.
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| Re: K-200B-2 Hum/feedback [message #25601 is a reply to message #25590] |
Wed, 28 September 2016 19:19 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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ok , a took a peek into my amp which is a 1968 , and my pc303 board is a rev-1.
That cap you have is going from the pot to the emitter of pNP transistor Q319 and should not be there , but that being said I do not think it's doing any harm as the emitter it's tied to the + 8 volt power supply rail and my gues is someone added it to try and filter 120 HZ hum from that power supply rail , and it did not work / change anything and left it there .
The RCA female on the left goes to the end of the pan marked output .
[Updated on: Wed, 28 September 2016 19:20] Report message to a moderator |
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| Re: K-200B-2 Hum/feedback [message #25606 is a reply to message #25590] |
Thu, 29 September 2016 09:51 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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The output side of the tank should be on the side of the amp that is away from the power transformer, is that the way it is?
The loud buzz that you get when you touch the tip of the output cable is normal and will take place if the reverb control is turned up, as it's the same thing as touching the end of your guitar cable with the channel volume turned up.
If you short out the end of that cable with a clip lead or something to make a good connection then if the amp is ok then turning up the reverb control will add no hum!
[Updated on: Thu, 29 September 2016 09:56] Report message to a moderator |
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| Re: K-200B-2 Hum/feedback [message #25609 is a reply to message #25608] |
Thu, 29 September 2016 12:31 |
slk854
Messages: 192
Registered: January 2015
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Senior Member |
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I just did check the continuity of the cable and it checks out ok.
Now let me add something very strange to this equation. I just tried the head with my 2-15 cab and the hum was more or less gone. I then connected back to the 3-15 cab and if I turn the reverb up I am getting like a wave length hum. Freaky sound like the outer limits TV show. "Don't touch your dial" The head seams to not like the 3-15 cab.
Steve
Ok a little update. I just tried a different 200B head with the 3-15 and everything is just fine. So I connected the head that we have been talking about back to the 3-15 and when I turn the reverb knob up the noise/hum does get worse even without the guitar being played but connected.
[Updated on: Thu, 29 September 2016 12:37] Report message to a moderator |
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| Re: K-200B-2 Hum/feedback [message #25610 is a reply to message #25590] |
Thu, 29 September 2016 14:22 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Try this. Pull both of the plugs on the pc board keeping the tank plugged in. Now test the continuity of the grounds from one cable plug to the other.
It sounds to me like the head is oscillating when the reverb is turned up. The 3-15 cabinet is lower in impedance than the two, so the power amp may be effected by the lower output load.
On the back panel where the speaker jacks are, are the blue wires that connect the speaker jacks to the pc board pushed close to the tank?
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| Re: K-200B-2 Hum/feedback [message #25615 is a reply to message #25590] |
Fri, 30 September 2016 06:10 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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There seems to be a strange grounding issue taking place, for kicks try this, pull all the knobs off of the front of the amp for that channel and tighten up all the nuts on the front of the pots and check for any improvement.
[Updated on: Fri, 30 September 2016 06:11] Report message to a moderator |
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| Re: K-200B-2 Hum/feedback [message #25629 is a reply to message #25590] |
Sun, 02 October 2016 01:41 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Then it was feedback and not a hum. From what you described, I assumed that it was a hum. More like when you have a cord plugged into the amp that is not connected to anything.
The hanging cover could have been part of the cause. Loose springs or loose suspension fittings could also contribute to the problem.
In any case, you got it working, so it's all good.
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