Fender Stage 112 se blowing fuses and transistors.

[g1]

The main filter caps are not grounded in this design. It is a bit unconventional.
The power transistors connect to ground through their emitter resistors. Make sure you have a good ground there.
The output is taken from the C.T. (where main filter caps would normally be grounded).

[The Dude]

...... there is hum on both sides of the 45V filter caps. I can't see how else you would get grid hum in the circuit unless either the filter caps or the rectifier diodes are bad. Is this reasoning correct?

If you have "hum" on both sides of a filter cap, you are missing the ground connection to the cap.

[gilmo789]

I'm trying to fix the horrible sound coming from both clean and dirty channels. I don't know where it's coming from.

This is what I know:

• Both channels sound bad from speaker with and without LBL
• Clean is much quieter than drive
• Voltages seem good across power xistors
• Signal tracing the components of pre-amp produce clear tone
• Signal tracing components of power amp produce bad tone even with new filter caps
• Power amp signal sounds like it's overlaid with 50hz hum

I'm assuming the terrible sound is caused by a grid hum somewhere. My thinking is that a slightly bad rectifier diode would allow grid frequency into the power circuit. My reasoning being that there is only hum on one side of the 16V filter caps whereas there is hum on both sides of the 45V filter caps. I can't see how else you would get grid hum in the circuit unless either the filter caps or the rectifier diodes are bad. Is this reasoning correct?

[g1]

Do you hear hum when you plug into the FX return? Not really sure what you are trying to fix here.
I thought the power amp was fixed and you were moving on to the preamp? If power amp is not humming, your supplies should be ok.

[gilmo789]

fooled around with the signal tracer. Clean preamp seems clean, but every stage of the power amp has hum on it. Pretty sure its grid hum. tried new and old filter caps: identical problem.

If i take my probe to the 16V, 1000uF filter caps then I get a hum on one leg and silence on the other. With the 45V, 3300uF filter caps I get hum on both sides even with new components. Could this be rectifier diodes? It's hard to believe they've failed. They're enormous (6A4) and the test OK, but I guess the meter test voltage is tiny...

Diode resistance test (forward is with black probe to band)

# F B
1 2M .5M
2 2M 107k
3 2.5M .6M
4 3.5M .8M

[gilmo789]

I have tried the amp, briefly without the LBL, it doesn't really sound all that different, clean sounds muddy, "underwater", driven and way too quiet. I think I'll have to try using a signal tracer

[Enzo]

In J1, R3 and R4 are in parallel for 16k, and that is in series with the input. In jack 2, R3 is grounded (by J1 contacts) and the input is through R4. The junction between then is the feed point into the system. Thus R3, R4 form a voltage divider that slices your signal level in half. A simple 6db pad. This is the exact same circuit that is on a million old Fender inputs with paired jacks. Just thos one used jacks with extra contacts and looks more complex. But it isn't.

[g1]

Input 2 should be quieter than input 1. If both are used at the same time they are equal volume. So that is normal (see owners manual).
You can not get any proper volume when using the bulb limiter. So any listening tests on the bulb are not valid. It is for checking DC idle conditions only.

You could run the FX send to another amp if you want to listen to the preamp. But you would have to disconnect the speakers on the Stage so it is not drawing any major current through the bulb. And this is assuming the power amp is no longer faulty.

[gilmo789]

more wierdness....
Input 2 is noticeably quieter than input 1. the only functional difference are resistors: r3 & r4, both 33kO. I'm getting 16kO across both but tha ts not surprising seeing as they're in parallel.
both inputs are brand new btw, i just replaced them with the correct, genuine part.

[gilmo789]

Hi, I'm not off the LBL yet, I really don't want to sacrifice any more xstors. Are you thinking this would contribute to the drive on the clean? I haven't noticed this before. The quietness of the clean is strange though. This amp should be unbareably loud above 3 which the drive is but I can turn the clean up to 5 and it's a quiet practice level and flat thereafter

I'm not the first to have this problem
https://www.tdpri.com/threads/need-h...please.159369/
https://www.harmonycentral.com/forum...m-please-help/

Will check components later
edit:
-16 and +16 confirmed on resistors
Diodes all test at 0.48V

[g1]

Well, at least those big differences in idle current may have been why some transistors were blowing and some were not.
Have you checked all the components in that bias string? (R92, CR13, CR14, CR15, CR16, R93)
Are +/-16V to R92 and R93 correct?
You are off the bulb limiter?

[gilmo789]

Anticipating the obvious question I've replaced Q3 and 8 so the pairs now all match

Base to Emitter (Pins 1-3)
142s
Q3: 1.17V ¦ Resistor: 1.1mV
Q4: 1.17V ¦ Resistor: 1.1mV
Q5: 1.17V ¦ Resistor: 1.1mV

147s
Q6: 1.165V ¦ Resistor: 1.1mV
Q7: 1.166V ¦ Resistor: 1.0mV
Q8: 1.168V ¦ Resistor: 1.0mV

Clean now sounds more wierdly distorted than ever. Dirty channel is driving at the lowest gain level.

I'm very confused!

[gilmo789]

Hi,
Transistors arrived so I got round to this today!
I've rebuilt the amp installing new TIPs where needed and I've replaced the Filter caps too just in case

Voltages across power transistors (note Q3 and8 are original parts)
Base to Emitter (Pins 1-3)
142s
Q3: 1.132V ¦ Resistor: 9.8mV
Q4: 1.142V ¦ Resistor: 1mV
Q5: 1.144V ¦ Resistor: 1mV

147s
Q6: 1.185V ¦ Resistor: 1.1mV
Q7: 1.186V ¦ Resistor: 1.1mV
Q8: 1.180V ¦ Resistor: 8.7mV


Also there is still a problem on the clean. It's still way to quiet and it now sounds distorted.
Could there be an opamp problem?

Thanks

[g1]

The hot spots around the 5W zeners and resistors are normal. They run hot, that's why they are higher power rating components.
Not sure what you mean about pot with terminals arranged differently, they are always the same. Double check your work and that none of the pads/traces are cracked at the pot.

When you are testing with less than all the output transistors (without bulb), it can not handle the same load impedance, so don't run at the minimum load, or at any significant power, you will just blow output transistors.
Once you get them all installed again, measure the DC mV (at idle) across each of the output transistors emitter resistor. Post results.

[gilmo789]

Thanks for all your help so far,

Some more things I've noticed:
1: There appears to be a hot spot beside two of the diodes in the main power rectifier, Is this normal? thi is without LBL
The hottest components seem to be CR41 and 42, and R114 and 115, part of the 16V supply. getting hot to touch.




2: On the clean channel, I had to replace the bass pot because the shaft was broken. I found one that fit and also had a D shaft. Its the correct value, 15A50k, and it measures correctly. However it seems to be functioning like another volume knob. This is the source of the quiet clean channel btw. If I turn both the bass and volume up I can get good volume. Don't want to push it though.
Would this happen if the terminals were arranged differently?