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1987 Homebrew issues!


 
6/29/2005 8:15 AM
Scott
email
1987 Homebrew issues!
I was wondering if someone could help me with some issues on a Homebrew Marshall 1987 50w.  
 
1) The presence control does not function properly. If I turn down the presence (between 0 and say 3) and play a note on low E string, I get a weird low oscillation that steadily gets faster. The oscillation continues whether I play or not. The only way to stop it is to flick on the stand-by switch for a couple seconds. The feedback is taken from the 8 ohm tap to a 47K resistor them to the lug 1 of the presence control (5KL). There is a 0.68UF cap connecting Lugs 2 to 3, with Lug Three goung to a ground point (a Single star located at the first filtercap.)It almost seems like the presence works in reverse.  
 
2)If I turn the presence all the way up, it can play fine, however as I turn it it down, It reaches a point where it screeches/Squeals like a banshee in heat.  
 
3)When I am plugged into Channel 2, and have the volume for channel 1 turned up, I get noticable hiss. But if I turn it down it goes away. Bad lead dress? (I am using shielded cable with the input resistors at the tube.)  
 
4) When Fully cranked, the amp squeels and screams like a pack of Banshees in Heat! I assume that I need to move wires around?  
 
5) Lastly, Is it not a good Idea to twist the wires to the pots together? I did this for wires going to the same pot.  
 
Any help would be appreciated.
 
6/29/2005 8:45 AM
Chuck
email

It sounds like you may have a positive fedback loop instead of a negative feedback loop. Happens all the time. Try disconnecting the feedback loop. If the squealing stops then just reconnect it and swap the OT primary leads.  
 
As for the hiss when from the unused channel... Thats kinda how it is. It can probably be minimized with lead dress and good grounding. My 1959ri did it too. Wasn't a problem for me since I always had the channels jumped.  
 
If the hiss is real bad you may have a grounding issue on the input jack of the channel you aren't using. Check to see that the first grid lead for that channel is grounded while it's not plugged in.  
 
Chuck
 
6/29/2005 12:17 PM
Scott
email

Chuck,  
I'll try and switch the OT Primary leads tonight. That is easily done. Hopefully that will kill the squeeling/ Presence issue. Could it cure the Squealing at full volume as well?
 
6/29/2005 7:46 PM
Chuck
email

Maybe, probably...It can't hurt if they're backward now. Thats why I suggested lifting the NFB loop first. It's often an easier way to check if it's a positive feedback loop. If swapping the leads is no problem in your build then just do it either way. I sometimes have trouble swapping leads because I have them cut to specific lengths to keep them as short as practical. So swapping leads for me in the past has meant adding some lenth and a piece of shrink tube. FWIW make sure those leads are twisted together for as much length as practical. It helps to stave off parasitic oscillations.  
 
Chuck
 
6/30/2005 7:13 AM
Scott
email

Chuck,  
I went home last night and switched the primary leads and that was the ticket! Seemed to cure all of my problems. I get a littles bit of oscillation when fully cranked, especially when pushed with a pedal. I'll have to go in and moe some wires around.  
 
Thanks,  
Scott
 
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