Xpress Debug

[Billy Batz]

I got the Xprs wired up this weekend but Im having a problem thats pretty new to me. I cant crank the amp at home so Ive wired in a simple PPIV Matchless type master just to be able to play around and debug any initial problems. The problem Im having is Im getting a rather intense PO. Its not the preamp. Though it happens regardless I have the preamp shorted. My problem now seems to be centered around the PI. As I turn up the master I quickly start getting a very audible and very peircing oscilation that soon breaks into a solid low frequency that sounds like a fog horn. It comes quickly. The high freq squeel comes around 1.5 on the master and the fog horn comes at about 2 and it stays regardless of the volume setting. The volume doesnt intesify the noise. Without the master its always there. It seemed to respond to the presence control so I bypassed the presence to ground and it went away. Ive since hooked up a pot between the PI and ground and it seems almost any resistance will start it up. Also, disconnecting the NFB kills it as well. This has got me scratching my head a bit. Everything looks fine accept the voltages are a bit higher because of the PT Im using but other then that I cant find anything. Anyone know what this is about?

[toddyjoe]

My guess is that you have your negative feedback loop on the wrong tap of the output transformer. Putting it on the wrong tap creates positive feedback instead of negative feedback and results in squealing. This would be consistent with your notes that the the oscillation seems to be in the phase inverter and removing the negative feedback loop and/or grounding the presence kills it. Switch the negative feedback loop to the other secondary tap on the output transformer and see if that works. Good luck!

[Billy Batz]

Problem with that is this is an old Fender OPT and the only secondary lead is the 8ohm and ground. The only other thing I had in mind to try was to swap the primarys.

[Omar]

Dan - I agree with toddyjoe. When I fired up my wreck for the first time I had a very similar problem with oscillation in the power amp section. I thought it was some kind of wiring problem but it turned out to be much similar. I would flip the primaries.

Omar

[toddyjoe]

Yes, flipping the primaries should work too. Good luck!

[Billy Batz]

Yes, flipping the primaries should work too. Good luck!

Well it did work. Thank you fellas. On to the good stuff. Tweaking the tone. Its a little noisier then I would like but that should be simple considering theres not many grounds to play with. This is a pretty simple amp.

[Billy Batz]

I may have spoke too soon. Withthe preamp shorted and the presence all up I can open the amp up a bit but if I start turning the presence down I start getting an oscillation that sounds like machine gun fire or a jack hammer. Bypassing the presence pot to ground remedies this and disconneting the PI from ground causes it as well. It seems any resistance between the PI and ground will start it up.

Now with the preamp in and the amp wired up correctly it doesnt just come on. When I open the amp up and hit a note then it starts.

Anyone familiar with this at all?

[Chill]

Sounds like motorboating, which is a low end oscillation. Easy cures are smaller coupling caps or series resistance. Layout and lead dress are another possibility. Do you have any pics of the chassis guts?

[wrecked]

Hi Billy!

Since the problem you're describing goes away when you ground the second input of the PI across the presence control, you can be pretty sure the fault has something to do with the negative feedback. I'd suggest checking the routing of that feedback wire first (try to get it as far away from everything else as possible).

Matt