Fender Stage 112 se trouble

[Enzo]

TP19 is the mute. Look at the upper left, input jacks. See the mute line at the top? Now follow that down through the two input jacks and then WAY over to the power amp in jack. Plugging into any of those jacks opens that line. Now see by the power amp jack, R1 is a 1meg resistor up to +16v. So whenever nothing is plugged into any of those three jacks, that +16v through R1, meets the -16v through 1meg R65 at TP19. And they cancel. Plug something into one of those jacks, and I bet TP19 falls into place.

[mikeaaron_1999]

Yep! that worked just like you said! Thanks again Enzo! When I plugged into either of the input jacks or the power amp jack, I received -15VDC! Now I've got to figure out tp 24 and tp 25. Don't know why those two would be out of whack. Still waiting on the TIP 147, we'll see when that comes in.

[mikeaaron_1999]

Updated for tp 24: I get +0.029 with drive selected and +1.151 with clean selected. On tp 25: I get 0.000 with reverb off and -1.11 with reverb on. Thanks again for any help! It appears that both of those are related to U6B, I'm thinking U6B is toast? Ok, I'll quit thinking now!

[Enzo]

TP17,18 will tell you if TP24 and 25 are OK. There is no exact voltage specified on 24 and 25, just higher-than and lower-than voltages per condition. Those are comparator circuits for the channel and feature switching. If the channels switch OK, and the reverb turns off and on, then all that stuff is working.

[mikeaaron_1999]

Ok, Thanks again Enzo! I was just going off of what the schematic said for readings. I guess I just figured those were specified voltages and when I didn't get it, I was concerned. So any way, I'm still waiting for the TIP 147 to come in and once it does, I'll have it back on the board and start testing everything else. I doubt I'll be back on here until after the 4th, so I hope you have a great 4th weekend and anyone else who might happen to be around here. You guys take care and be safe!

[mikeaaron_1999]

Well, the147 came in, I installed. It and started to check things out. With the variac at 10VAC, the 142's bases were all about 1.1VDC emmiters were 0.6VDC and collectors were 0.00VDC. The 147's read base: -0.6VDC emitter: -1.8VDC and collectors: 0.00VDC. I then put the variac at 40VAC. The 142,s read base: 6.2VDC emitter: 0.00VDC collector: 0.00. The 147's base: -5.89VDC emitter: -33.010VDC and collector: 0.00 VDC. There was also -15VDC on the + or white speaker site, speaker not connected. After a while and checking the TIPs, the 142's collectors went to +15VDC and the 147's collectors went to -15VDC and I lost all voltage on speaker lead. I turned it off let it sit 10-15 min. Turned it back in and up to 40VAC . The TIP's read collectors: 15VDC on 142's and -15VDC on 147's. Turned it back to around 10VAC and the 142's collectors were back to. 6Vdc. I then checked the speaker and found -15VDC again. I had to shut everything down because it was quitting time. Just wanted to give an update. Take it easy fill you on more on Tuesday.

[Enzo]

Please understand the use of the variac. While readings through the amp at 40v on the mains might have occasional value, mostly they mean nothing. What you REALLY need to watch is mains CURRENT as you dial up the variac. I am WAY more concerned that the amp is trying to draw 10 Amps at 40v on the mains than I am over what voltages are where.

[mikeaaron_1999]

Alright, I've dialed up the variac from 0VAC to 80VAC and watched the amps through the ammeter connected in place of the fuse. The amps never went above 0.14 at 80VAC. I turned the variac down and off because I got called away for a moment. When I returned, I powered up, slowly dialed up the variac from 0 to 50VAC and stopped there because I had -20.65VDC on CP13 or white speaker (+) wire and amps were at 0.05. This is with the variac at 50VAC. Is the -20.65VDC on speaker wire a concern or should I turn the variac to operating voltage and continue to monitor amps and voltages? I don't want to smoke any more components.

[Enzo]

Amps usually are not stable at low mains voltage. The current is what to worry about. Work without a speaker load. An amp can put out 75v on the output and still draw no current if there is no load. Slap an 8 ohm speaker across that 75v and all of a sudden it wants tons of current.

If you advance the variac to 50v and it shows 20v on the out, as long as the mains current is still minimal, keep going and see if it snaps back into place a few volts higher on the dial.

You know how some amps go whump when you turn them on, or off? When you flip the switch, the amp transistions from full power to no power really fast. Imagine now that that whump is drawn out over a minute, now it will no longer be a pulse, it will be a long voltage swing. I suspect that is what you are encountering.

[mikeaaron_1999]

Ok, variac placed at 100VAC for 35 to 40 minutes, amps were at 0.35 and slowly over the course of time reached 0.37 with speaker connected and while sitting idle with no input, the heat sinc reached approximately 112 degrees F. I'll work with it more tomorrow, dial up to a full 120VAC and monitor the main current.

[mikeaaron_1999]

Update: Conditions are, no input, sitting idle, all controls at mid-position, clean channel, drive select off, 8 ohm speaker attached. Dialed variac up from 0VAC to 120VAC. Through the dial up, amps rose normally, or what I would believe normal, as there were no spikes and a smooth steady fluctuation in a forward or positive swing until reaching 120VAC on variac. Once there, the amps started at 0.57 and slowly over the course of sitting idle for 30 minutes, the amps gradually increased to around 0.95. I took a temperature reading of the TIP’s, they were all around 140 degrees F. to around 180 degrees F. The heat sinc and TIP’s read cooler from the outer edges to warmer closer to the center. After about 35 minutes total ON time, the amps started increasing faster and in five minutes went from 0.97 to 1.20. I turned the variac off as it started “smelling” hot. Immediately after removing power, I took temperature readings again and the center TIP’s were 200+ degrees F. the outers were right about 180. So, my thought is that something is causing them to heat up, break down and short out. How much heat can those handle before they burn up? What would be causing them to heat up like that? Is there anything I can do to prevent the heat from reaching that level? If only after about 35 – 40 min this thing is ready to burn darlingtons, there has to be a cause. Any ideas? Thanks again, I appreciate it.

[Enzo]

yeah, that's too much current. SOunds like the thing is not thermally tracking well. The four bias diodes are in little holes in the heat sink. Check the voltage drop on them. Any of those climbing with warm-up?

They heat up because they are turned on too hard and are conducting too much current. The amp outputs should be biased just at the point of conduction. If the bias is a bit high, then both sides conduct at once to some extent, and that is just power going into heat.

Just thinking here: The four bias diodes, two above and two below the feed point from the op amp. If we were to tack a short across one diode on each side - say C13 and C16 for example - so there would only now be two diodes biasing things, I bet the amp would run tons cooler. There would be an increaase in crossover distortion. Shorting the diodes is more a diagnostic tool than a repair procedure, of course.

[mikeaaron_1999]

Yay! I think I've got it. I checked the pads (again) for the diodes, and just for extra measure, soldered jumpers to the diodes in place of the runs (or more like with the runs) and put things back together. Through the variac, dialed into 119.7VAC, ammeter in place of fuse, no inputs, all controls at mid-position, speaker connected, no input. The ammeter never went above 0.07 while running idle, only jumped to 0.90 when switched to "ON" position one time, all other times it would hit 0.70 or lower. Ran it for about a half an hour, no amperage creeping up, heat on the heat sinc was never above 90 Degrees F. All bases were 1.11VDC +/- respectively, all collectors were 46.7VDC - 47.0VDC +/- respectively and all emmiters were 0.008VDC +/- respectively. I did after a while inject a radio signal through input 1 and 2, varied all controls with no jumps or swings in amperage. In fact, with a signal applied, the amperage actually went DOWN to around 0.04. So I'm going to let it sit IDLE for a while longer next time and then use an instrument if it idles good for a period of time. I'll post again on my success or failure! Just to let everyone know how it went. It may be tomorrow (wed.) or Thurs. before I can test it again fully. But I want to give another thank you to all of you guys for the help and guidance. I am keeping my fingers crossed and thinking this did the trick! So THANKS A BUNCH for everyting up to this point! You people are AWESOME!!!!

[mikeaaron_1999]

UPDATE!!!! Alright, I connected an MP3 player to Input 1, all controls very low, let it play for just over an hour with no heat up problems, no blown fuses, no blown TIP's. All at 119.8VAC through the variac. So, I connected a fender squire copy to it and played through it quite a bit varying the controls, "punching it" a few times watching amps, they never climbed above 0.70 when I really got into it with heavy bass on my "low E" string. I am satisfied that I have made this amp good again. And as I've said before, Thanks to Enzo and 52 Bill. You guys rock and I wouldn't have been able to do it with out you. Now, the only other thing I'm wondering about is the reverb. When I connect the pedal the amp automatically switches to "Drive" channel and by selecting "reverb" on the pedal, it doesn't really sound like reverb. I thought I would find some demo of this amp on youtube but, not so much. I found where this amp was being used, but not alone and not using the internal reverb. I'm going to mess with it more tomorrow and try to figure it out, it doesn't have a user manual, so I may have to scout the 'net to find one. Well, any way, Thanks to you guys! You made this very enjoyable and easy, I wouldn't have been able to do this without your assistance. I can not thank you all enough. Wish I could buy ya some beers or something. Take care all and for any new comers, listen to these guys! They know what they are talking about!!!!!!!!

[Enzo]

Unplug the footswitch. Without it, the reverb should be ON. Now how does it sound?

Are you using the ACTUAL Fender footswitch for this amp? This is not a simple two button stomp switch, there are actual circuits in it.