Output Clipping Too Early Mesa 2:90 Help!!

[swla71276]

Ive had a 290 boogie power amp on the bench the last few days. Complaint of a distorted and weak sounding ch b. So i changed all the tubes in the pre and power. Didnt make any difference. Started scoping and running signal generater throught the circuits. I found that CH A works fine, stays clean till about 70 percent power, then clips evenly on top and bottom. Ch B starts to clip much earlier, at about 30 perecent volume. But, only on the bottom potions of the output sine wave. Im scoping these readings from the output jack and inputing the sig. gen. at the input jack by the way. The top half waveform stays clean till about 70 % just like ch A. Any ideas? Output trans? Basically the ouptut on Ch b just cant seem to swing negative very much.

[Marina]

Supply voltage?

Is the supply voltage ok to that channel?

[swla71276]

yep.. checked all volts at all tubes.. about 468 plate, 465 screen, -54 bias, Did find something new tho.... The total voltage output at the speaker jack wide open 1 khz sine 1.5 colts input results in and output of 58 vac on ch a (the good ch) and only about 28 vac on ch b.... wide open the clipping seems similar on both ch's.. just one has a much lower output.... im starting to fear the trans.... even tho the primary resistance and secondary resistance check out ok on the trans... and is equal on both... i dont have a way to check for henries and etc tho.... also strangely enough i notice that the sine wave is a lil distorted throughout the pre section... well more specifially it seems a lil more rounded on the bottom and a lil more pointed on the top.. as it come off the plates of both PI's ....even tho im putting in a perfect sine wave

[swla71276]

did some basic trasnformer testing... center tap to each plate for ohms, and tested eache plate to gound... all reading came back good.... maybe its not the trans..... its almost like its stuck in half power.... after about 3 on the volume know the output will not get any louder... just starts clipping more and more... like its clamped... almost seems like its running 50 watts instead of 100?

[swla71276]

well finally had a strike of common sense... i swapped out trans from channel to channel and fired it up.. guesss what... the low volume problem switched sides as well... now ch a is low... does this mean the trans is toast ? any other checks i can do?

[swla71276]

o yeah!!!!! popped the bellhousings off the trans... looks and smells like black tar and burnt toast... definatly fried.... thanks guys...

Lance

[Marina]

Looks like you figured it out pretty well by yourself. Congratulations for isolating the problem! Your posts will probably help someone else who encounters that as well.