Fender Stage 160 hum

[catstrat]

I have a Fender stage 160 transistor amp on my bench that not only has the dreaded hum that i hate to encounter in a solid state amp, the 3 tip 147 outout transistors have no collector voltage at all. I found and replaced one tip 142 and one tip147 that were shorted. I tested all of the rectifier diodes for the high voltage side. They all tested good. Replaced both high voltage filter caps because one was bad. Any ideas? thanks.

[The Dude]

So have you restored collector voltage to the TIP147's yet? Also, that hum is most likely DC, so unhook any speaker(s) until you get rid of it.

[52 Bill]

...the 3 tip 147 outout transistors have no collector voltage at all.

Are you measuring to ground or to the "CT" point? Remember that this amp is not wired in the traditional way.

[J M Fahey]

^^^^^^ what b52bill says.

Don't have the schematic before me, but if it's a grounded emitters circuit, where speaker "hot" is actually filter capacitors midpoint, then "no (0) voltage on the TIP147 collectors" actually means they are either shorted or full tilt driven/saturated which means that filter caps center point is some 45V above ground which means *speakers* are getting some +45V DC .
As deadly as a .357 Magnum to the chest.

Can somebody link the schematic here so we all refer to the same part numbers?

[Audiotexan]

As you wish:

http://support.fender.com/schematics..._schematic.pdf

=)

[Audiotexan]

...the 3 tip 147 outout transistors have no collector voltage at all.

This could mean you're missing the -45vdc rail.

Checked D59?

Have you checked D56/D57 and C64 in the secondary of the PSU?

Also, verified P1/P2/P3 that supply the pos./neg. rails for the output section?
(red/yellow/red from the PT's secondary outputs)

[loudthud]

the 3 tip 147 outout transistors have no collector voltage at all.

That can be normal for this circuit when there is +45V on the output and +90V on the TIP142 Collectors. One of the TIP 147s might be shorted, or there is some other fault that causes the outut to go to the rail. Disconnect the speaker until you can get the output to go to zero Volts.

[olddawg]

Have you checked the power supplies for symetry and ripple? First thing I would look at is if the main filter caps have become disconnected.

[TomCarlos]

What Dawg said.... Check the soldering. I had a Stage 160, the worse electro-soldering you can imagine. Take your time, look at the solder side of the PC board, and make sure everything is well connected. Once I did that, it was a nice amp! I have a Stage 100 combo and Stage 100 head- love 'em!!

[catstrat]

fender Stage 160 hum

Sorry for the delay getting back to you guys, fighting an illness. I still have no voltage at the tip147s. The first thing i found early on was a filter cap had un soldered it self. I have plenty of those caps in stock so i just replaced them. I also checked and replaced the flyback diodes. I thought maybe one was leaking. I replaced them with 4007s. 52 Bill, i don't follow you when you say these are not wired the normal way. Can you elaborate on this ? I don't remember seeing any caps in the output section. I checked the o/p for dc voltage and don't remember having anybut i'll check it again. I've removed all of the output transistors and tested them with a meter but since these are Darlington's, can they be checked with a meter? If so then they test ok. Now i did find two shorted output trans. early on, one tip142 and one tip147.
This has two small transistors mounted in a recess in the heatsink, i don't remember the P/N off hand. What do they do? The other thing, when i test the amp none of the o/p transistors get hot to the touch. I've had bad zeners show similar symptoms. I'm thinking bad solder joints even though i haven't found any, yet. Thanks everybody.

[catstrat]

52 Bill, i measured the voltage to both chassis and tranny center tap with the same reading. No voltage. The transformer is outputting ac voltage on it's windings. thanks

[The Dude]

Check the voltage directly on the anode of either/both of D56 & D57. That is where the voltage for your TIP147's comes from. If you have it there, you have a connection issue or a burnt trace and the voltage isn't making its way to the outputs.

[Jazz P Bass]

Yeah that.
And make sure that all of the TIP147 collectors are physically tied together.


Ohmmeter time.
Don't even need to plug it in.

Note: I think what Bill was refering to (not a normal output section) is the fact that the output emitters are grounded.
The current flows through the CT.
There is also that little note that the +45 & -45Vdc voltages are referenced to CT, not chassis ground.

[loudthud]

This is not a conventional amp in the sense that the speaker does not connect directly to the output transistor emitters. Look at the emitter resistors of those TIP142/147s. That point goes to ground. The speaker connects to the center tap of the power supply, a point called "C.T.".

Q10 and Q11 are temperature sensors that set the bias on the output transistors. If one of those is not conducting, the output transistors on that side will be on and the C.T. point will be +/- 45V. The collector Voltage on that side will be close to zero. Look for the Emitter to Collector Voltage of Q10 and Q11 to be about 1.08V. Check the diodes in series with the Collectors D22, D23. Those are Schottky diodes, they will measure "funny" on a DVM.

Q12 and Q13 monitor the current through the output transistors and turn on if too much current flows. If one of the .47 Ohm 5W resistors is open, Q12 or Q13 will turn on distorting the output.

Pins 1, 2 and 3 of U6 should be around zero Volts. If they are not, it does not mean you have a bad IC. You have to determine if the opamp output is obeying it's inputs. If pin 3 is higher than pin 2, the output will be about +14V. If pin 3 is lower than pin 2, the output will be about -14V. If U6 has control of the power amp, pins 2 and 3 will be within a couple of milliVolts and should be very close to zero Volts.

[52 Bill]

Well, everybody has already explained that this amp is what Fender calls a "Flying Rail" design. It really is not that much different from a more typical design, just as long as you know how to measure voltages.

Like the others said, try and find out why you are missing the voltages on the one side of the output transistors. Either there is an open circuit that is not letting the voltage get there, or there is something shorted that is pulling down the voltage.

Are you running this on a limiter?