Fender Frontman 212R DC on output

[booj]

Getting almost the full negative supply voltage at the output. Amp started out with making loud popping sounds. Fixed that by changing the 2SD1857 (Q14) and 2SB1236 (Q15) and re-flowing all the connections in the output section. I also changed the drivers Q18 and Q19, the outputs, Q20 and Q21. Now it produces negative DC on the output almost equal to the negative supply. Also R84 will smoke if the amp is left on more than a few seconds. The current flowing through R84 and D24 travels through Q10 and Q11 which have been changed with no effect. I'm out of ideas. Any jog in the right direction would be appreciated! fm212r.pdf

[booj]

Forgot to mention the reference designators for Q16 and Q17 are incorrect. They are not 2N4001 and 2N4003, but 2N4401 and 2N4403.

[Jon Snell]

I wonder what the original fault was.
If R84 gets hot check R87 is not open circuit and also check Q11 is the correct transistor. Some serious current is going somewhere.
Reduce the power supply by half and see where the voltage is going.
I would hazard a guess that you have fitted some faulty components.
Q14 & Q15 are protection clamps, they stop too much current from driving the output stage and should not cause popping.
Check all resistors for odd values and always replace any resistor that is faulty with the correct type and value. Fu means it is a fuseable resistor!

[Helmholtz]

Now it produces negative DC on the output almost equal to the negative supply.

I hope the speaker is disconnected.

Use a variac or bulb limiter between mains and amp.

[vintagekiki]

In order not to make a breakdown without need, between power supply and PCB pwr, in series with power supply voltage +42V and -42V set current limiter in form of resistors 100 Ohm / 50W.
So you can safely perform measurements and interventions to repair amp without fear that it may burn.
post #24

[g1]

Not saying it's your problem, but both sides of diffy pair should always be replaced together (Q9 should always be changed along with Q10). Especially in this series of amps where they are known to be problematic.

[Jon Snell]

In order not to make a breakdown without need, between power supply and PCB pwr, in series with power supply voltage +42V and -42V set current limiter in form of resistors 100 Ohm / 50W.
So you can safely perform measurements and interventions to repair amp without fear that it may burn.
post #24

Whoops ... wrong post.
This is a Fender amp, not a Marshall.

[vintagekiki]

Whoops ... wrong post.

Sorry. Not wrong post, you didn't understand post.
It is principled schematic that allows safely measurements and interventions to repair power amp without fear that something may burn.
Who understands what it is about, understands. Who does not understand the basic principles, him not help nor schematic for each amp individually.

Similar links (click on #...)
#7 ; #12 ; #18 ; #31 ; #33 ; #37

https://www.ssguitar.com/index.php?topic=4500.0
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[Jon Snell]

Sorry. Not wrong post, you didn't understand post.
It is principled schematic that allows safely measurements and interventions to repair power amp without fear that something may burn.
Who understands what it is about, understands. Who does not understand the basic principles, him not help nor schematic for each amp individually.

Similar links (click on #...)
#7 ; #12 ; #18 ; #31 ; #33 ; #37

https://www.ssguitar.com/index.php?topic=4500.0
You must log in or sign up

Not clear at all unless following the link.
It may lead to confusion for those that do not realise it is an example.

[vintagekiki]

Not clear at all unless following the link.

Repeating same thing thousand times leads nowhere.
I always start from an assumption
Who understands what it is about, understands.
Who not understand, should not accept blind job, because


1)

loudthud
WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.

[J M Fahey]

It is principled schematic that allows safely measurements and interventions to repair power amp without fear that something may burn.

Wow, you seem to be a fan of the resistor-in-power-rail method

I used it myself in the early 70īs, I used 1k resistors, but after learning the lamp bulb limiter, never looked back.

Essentially same thing, the BIG advantage of bulbs over resistors is that they are PTC, so they allow "almost" normal voltages which makes troubleshooting easier, but strongly limit high currents.

[vintagekiki]

Wow, you seem to be a fan of the resistor-in-power-rail method
I used it myself in the early 70īs, I used 1k resistors, but after learning the lamp bulb limiter, never looked back.

Bulbs, unlike resistors, have a lack, because from power of the bulbs depends how much current will be limit.
A 100 ohm resistor for example at 100V precisely limits the current to 1A, which withstands almost every slightly better transistor.
In addition current limitation at power amp, does not limit current to periphery of the entire amp.
By the way, we are similar age (y74)

[booj]

Yes, of course, a light bulb limiter. Have one. If it keeps R84 from getting hot I can leave the amp on long enough to troubleshoot.

[booj]

g1 They were
Jon Snell Popping noises. Thx for the insight.
J M Fahey Thanks for being nice

[booj]

Ok, so I just left the speaker wires dis-connected, which allowed me to leave the amp on for troubleshooting. Everything looked normal up to the output of U7, and then everything looked normal. Checked the DC offset at the speaker terminals. Zero volts. But when I unplugged the input, now I'd get 2.5 volts. Keep in mind I did nothing to the amp since the last time I looked at it. So I'm thinking maybe the problem is in the mute circuit. Changed the two 2N4403 transistors (They're labeled as 2N4003's on the schematic). No change. So I decide to dis-able the mute circuit by cutting R73 out of the circuit. Now the amp works normally except for normal channel noise when the amp is unplugged. (If the dist. channel is engaged and turned up you'll hear the hiss when the amp input is unplugged). The mute connection reads 10 ohms due to R1, which seems normal. D18 and D19 not shorted. Maybe I should change D18? Can't find RT2. Maybe it's buried with heatsink compound in the heat sink assy? Read 200 ohms between base and emitter of Q6. Positive lead on the base. Maybe that's off? Bunch of maybe's. Any advice welcome