| Author | Message | ||
     Patrick Springman (pspring) Username: pspring Registered: 04-2013 |
I have 3 different oscillations. One in the output around 200khz. Can't hear it but you can see it with an oscilloscope. At the driver transistor collector, the voltage level of the oscillation is about 20v. Only happens when there is enough bias current to get the transistors going. Even happens with the output switch in standby. Starts when the tubes heat up and start conducting. I then put the proper driver op amp part in, LM1458, and now it only oscillates with the amp in Hi or Lo power with the master volume between 1 and 9. I replaced all the 20uf caps in the final stage and will replace the power supply caps in a few days when they come in. The second oscillation is when all the channel levels are at 10 and the master is 3 or more. It's low frequency, like 10hz. (poor speaker takes a beating). The third oscillation is when the bass level is low. The frequency is somewhere between 15-20K. Dog hates it... So any ideas? All the preamp opamps are original with the exception of the mixing stage. I've noticed some differences in the schematic off the MM Ernie Ball web site. Like no damping diodes on the output transformer. No limiting diodes on the preamp input opamp. | ||
| pspring (pspring) Username: pspring Registered: 04-2013 |
I put a .0022mf on the main volume pot wiper to ground and that took care of the high frequency oscillation in the output section. The second high frequency oscillation goes away when I wave my hand over parts or the preamp board. Parasitic capacitance. Arg. Placing a big hunk of sheet metal over the top of the chassis takes care of that. Since there is a sheet of wire mesh built into the wooden box roof I'll assume that oscillation will go away when it's all put together. Still have the audible low and high frequency oscillations that seem be in the preamp. It sounds pretty clean now. I've replaced all the power supply caps but I don't think the old ones were too bad. The new ones are about half the size. lol Any ideas would be great. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
I'm kind of at a loss that the oscillations are in the preamp. Don't remember running into THAT. It's oscillating with NO signal through it or WITH one? How MUCH signal? Can you HEAR them if you probe it in the front end or are you just seeing something? | ||
| pspring (pspring) Username: pspring Registered: 04-2013 |
Found the problems. The oscillations kinda look like they were everywhere, typical for an amp, and I made some wrong assumptions. I was watching a tube driver transistor output while poking at the preamp and trying 100pf caps here and there. This thing was so touchy that just moving a wire near the output jacks caused a change in the low frequency issue! lol So I found one of the speaker jacks was touching the chassis. Its insulator was not centered, got squished when installed allowing the jack to touch the chassis. Before that I found the output transformer black wire connected to chassis instead of power supply ground. That sure messes up things! lol So I disconnected it and checked chassis to power supply ground continuity after removing all the input jacks(because the grounds are connected at those points). That's when I found a short and started disconnecting and tracing all the chassis connections while a meter was used to watch for the short to open. Which occurred when one of the speaker jacks was disconnected. Nice sounding amp even though my speaker is junky. Looks nice and clean on the scope and has a lot of gain. Can pick up a radio station when I touch a cap in the preamp.  Thanks for the nice forum! There is a ton of good stuff here! |