Fender Solid State Stage 100 Amp huge buzz Problem... help needed ...

[la-2a]

After the fuse blew in my Stage 100 DSP , it turned out that some Darlington power amp transistors were burned out. Well I replaced these and the voltage is now stable. The main filter capacitors in the +/- 45V power supply are also new.. Now I have a huge hum problem in the power amp section.
The board ground is connected to the ground of the chassis. It hums even without the plug being plugged in. The preamp output (send loop-in point) at the back of the chassis is clean. I connected it to the active loudspeakers at my measuring station .Seems to be in the Power Section.
I listen to the power amp at the LINE OUT, without a speaker. If I just touch Pin 2 OpAmp U7 (4560) with the measuring tip of my multimeter, the hum goes away. but this means that the resistors R110, R111, R112 start to glow.... a HF Frequency oscilation??? . This fender circuit is a bit beyond my knowledge. I have no experience with solid state guitar amps.Very weird design. As I said, I don't measure any DC at the speaker output. AC is around 1,3 V ....the hum is a very loud 100Hz signal. Any help would be very much appreciated....
cheers
.

 

[Philbo King]

Start by checking for AC ripple on the power supply.

 

[NoisyIndividual]

Are the new Darlingtons genuine?

 

[la-2a]

About the Darlingtons...i used the same types..TIP142, TIP147. AC Ripple PSU : there are only 2 Big Caps 3300µF. I replaced them. The rectifier diodes should be fine as I can measure the expected voltage......

 

[NoisyIndividual]

No, I meant did you buy them from a reputable vendor?

 

[scott2000]

I listen to the power amp at the LINE OUT, without a speaker. If I just touch Pin 2 OpAmp U7 (4560) with the measuring tip of my multimeter, the hum goes away. but this means that the resistors R110, R111, R112 start to glow.... a HF Frequency oscilation???

Is there supposed to be a speaker load when measuring?

 

[la-2a]

I bought the transistors here in Germany from a reputable dealer Conrad electronics. Comparable to Mouser. @scott2000 In contrast to tube amps, the transistor amp does not have to be connected to speakers for measurements..as far as I know but will do . I tried to measure the ripple. If I just touch the collector of Q18 (TIP 142) with the measuring tip, the hum goes away. The ground line of the measuring tip is connected to the chassis ground.

 

[sodderboy]

Yep, you have to check all power supply rails when replacing any burned silicon components, especially a rectifier at the beginning or output stage at the end.
Make sure all of your solder connections are solid and clean ALL flux remnants. All board connectors should be wiggled while listening if possible (chopstick or bamboo skewer are your tools here) to check for solid connections.
Mike

 

[la-2a]

Dear Mike thank you very much , I've already done all that... What could help me now would be a concrete suggestion... Isn't it strange that the hum disappears when I touch the collector with the oscillator test tip on the Darlington transistor the +45v rail .??If the rectifiers were damaged then I wouldn't be able to measure the right voltages right.??...

 

[NoisyIndividual]

It sounds like you may have a missing ground somewhere. Maybe when the amp blew, one ground was via a PCB trace and this trace got damaged/vapourised, or maybe when you repaired it you didn't put a ground back properly.

This amp is a bit unusual for a guitar amp in that the usual amp output is grounded (after all the emitter resistors).
The +/- 45v supply is floating, and it is the centre of this supply that is actually the amps output (designated CT, i.e. centre tap of the transformer winding providing the 45 volt rails.)
So the amp "drives the ground", and the 45v power supply centre tap actually drives the speaker.
(It's the same thing, really, just looks weird)


Go and check all the ground connections for integrity, and being connected to each other, and that this CT from the 45v should not be connected to any other grounds at all.

 

[NoisyIndividual]

Oh, and also, when you replaced the 45v power supply caps, make sure that the caps vias are still intact, (providing the connection between top and bottom layers of the PCB) as often designers rely on these connections for the circuit to work, and replacing the caps can damage them. So make sure the bridge rectifier is properly connected to each cap, and that each cap is properly connected to each other, and that the CT here properly goes to the speaker jack sockets via the Inductor L1.

 

[la-2a]

Great, super you think it is a grounding problem, yes I will work on that suggestion. I will check all Gound connections...The traces on the board are not in 2 layers. I had already resoldered the spots..but will check again

 

[NoisyIndividual]

I would suggest that when you scope a transistor and the hum goes away, that is pretty much definitive proof of a grounding issue, but I think you kind of knew that already.


(The scope is providing the missing ground reference, or less likely is stopping some HF oscillations)

 

[la-2a]

HF oscillation was my first thought, I already wrote that in my first post... Touching Pin2 on Opamp U7 has the same effect, only then the resistors R110 -112 start to glow... which is not the case when touching collector Q18 is.

 

[NoisyIndividual]

If there are HF oscillations of sufficient power to produce a large hum in the power supply, then the output transistors will be getting warm if not hot.
That kind of hum means the amp is producing a lot of power and you will feel that if not smell it.

 

[Tubetec]

Im familiar with the stage series by Fender , though not the DSP version ,
Several times when I had these amps on the bench I found an upright ceramic resistor had either a broken leg , internal damage or a bad solder joint to the board . Ive definately had cases where a bad ceramic resistor caused a large amount of hum at the output in these amps .

Often it was the insertion machine weakened the legs of the resistor , but only later after years of use the problem manifested itself . Ive seen caps fall off the board too in these amps .
Generally in later revisions these problematic components were secured with a blob of silicone or hot glue .

 

[la-2a]

Hello @Tubetec, so I desoldered all darlingtons and measured the diode test again ... all OK. For nowI only installed 2 of the transistors, one for each side. The problem is still there.! I took the opportunity to measure the 0.47 ohm big ceramic resistors again and they are all good. According to the wiring diagram, all connections to ground are measured, everything seems to be fine.
Well I'll do a visual check with a magnifying glass, maybe I can see a hairline crack. I've been soldering for 40 years and have built many of the projects here on our forums. A cold solder joint is unlikely. What gives me no rest is the measuring tip phenomenon that the hum disappears when I touch the collector of the output stage transistor.

 

[Chandra]

Check those small ceramic caps in the power amp. One might be open.
Regards.

 

[la-2a]

@Chandra.... have all checked nothing open... very strange the whole MF thing

 

[la-2a]

@Tubetech...any suggestions??...