Acoustic Control Corps B220 - Low, distorted output

[wayward bluesman]

I've got an Acoustic B220 on the bench with low output and heavy distortion that starts far too early. This was the original problem. Early in testing, one of the output transistors failed and I replaced all four of them. So now I'm back to the original problem.

I could not located schematics for the power section of this amp. I've attached a JPG of schematics I drew after having mapped out the power section; all of the components "looked" original, but you never know. This circuit is very similar to the G120-112, which I used as rough guide. I've also attached a JPG of the schematic which includes some of my voltage readings while running a 1500 Hz, ~0.23 Vp-p test signal (both DC and AC).

I think the problem lies just downstream of what I'm assuming is the voltage amp, #480059, which is spitting out the signal at close to 20VAC (which is roughly the level I think the final output of the amp should be) which then drops to less than 4V immediately after the 470 Ohm resistor.

Here's what I know:
-All transistors have either been replaced or test good out-of-circuit
-The 470 Ohm resistor in series with the emitter of 480059 tests right at 470 Ohm in-circuit
-The diode in series with that 470 Ohm also tests good in-circuit
-Test signal is normal coming from the pre-amp section

Do any of my voltage readings jump out as being funny or as pointing to something obviously at fault in the circuit?

Thanks!

[gbono]

Acoustic G120-112 Service Manual[1].pdf

Attached G120 manual - what type of distortion - is it symetrical? are your AC voltages peak to peak? Are you sure replaced transistors are the correct type?

[mozwell]

i would check the current limit circuit, 2N3440 & 2N5416. If you take out the two 1N4003 diodes in series with these transistors, you disable the current limit protection circuit. If this circuit was activating early, you woudl get low distorted output.
If this is the problem, check all components between the transistors & the 7W resistors & replace as necessary.
Also measure the 4x 7W 0.24 ohm power resistors in series with each output transistor.

Did you repalce the driver transistors (40409 & 40410) along with the output transistors. If the output transistors let go, they can often take out the driver transistors as well. MJE340 & MJE350 are good as drivers, not sure if they will be ok in this case, but worth a look if you cant find other ready replacements.

[Enzo]

Scope the output. KNowing what the distortion looks like is a big clue. SYmmetrical clipping is one thing, while a missing half of the waveform is another.

[wayward bluesman]

Gbono - see attached for output distortion. As is, the amp would make a great 40 lb fuzz pedal. And the AC voltages measured on the schematic are RMS, measured from my meter.



Transistors I replaced:
Output - 2SC1586 - replaced with NTE388 (x4)
Driver - 40409 - replaced with 2N3440 (per this thread: http://music-electronics-forum.com/t19430/)
Driver - 40410 - replaced with 2N5425 (see above)
Current Limit - 2N3440 - replaced with 2N3440
Current Limit - 2N5416 - replaced with 2N5415
Bias - 480067 - original but tested OK out-of-circuit
Voltage Amp - 480059 - replaced with 2N3440

All other components are testing OK so far. I will lift the 1N4003s in series with those x-istors and see what difference that makes...
MPSA56 - replaced with MPSA56

[Jazz P Bass]

First thing is the SM states a 600mv signal into the power amp.
I would insert a 100mv signal at the input & measure the Vac at the input to the power amp.
If it does not exceed 600 mv, scope it.
In other words, is the power amp receiving a clean signal.
If so, then I would suggest checking the static voltages of the power amp.
The base voltage of all the transistors is a good piece of info to have.
For starters.
Remove the load while troubleshooting.

[wayward bluesman]

Distortion on the signal at the output is mostly symmetrical; however, I've realized that I've been a bit inconsistent about loading the amp while testing. Today, I've found that the amp passes a clean signal with no load, but when I add the 8 ohm speaker... I've got distortion all the way back at that first differential pair!

[Jazz P Bass]

Well, yeah it goes that far back because of the feedback.
Have you replaced the drivers?(the ones that drive the output transistors)
It sounds like the drivers cannot supply enough current.

[wayward bluesman]

Yes, I replaced the drivers when I replaced the output transistors. They're labeled as 40409 and 40410; I replaced with a 2N3440 and 2N5415, respectively.

[Enzo]

I hope you added heat sinks to those replacements - like the original 40409s had.

When the output looks clean with no load, but can't handle being loaded, that usually means the output stage cannot supply the load with current. WHile it could be a bad output transistor or drivere, it is just as likely ther is an open resistor in the circuit.

[wayward bluesman]

Definitely added the heatsinks to those drivers. Soldered them in tight enough so the heatsinks were sandwiched in nicely; I had good contact with the heatsink and the collector/shell of the transistor and so didn't solder them together for fear of killing the transistor with the heat from the iron. As I mentioned before, all of the output transistors and drivers are new so I'm making the assumption that they're operating correctly and of the right type. I'll double check the resistors around those outputs and drivers, even if I have to test 'em out of circuit.

[Jazz P Bass]

Monitor the base voltages of the 40409 & 40410 at idle.
This will help tell if the amp is stable.
Check those caps from B+ & B- to the driver bases.
I have seen these fail shorted.

[wayward bluesman]

Bias voltage on the base of the 40409 is 1.00 VDC at idle, no load. Bias voltage on the base of 40410 is -0.85 VDC at idle, no load. All 0.24 Ohm resistors are good. Snubber caps from rails to base of drivers are good. Rail voltages still good. Hmmm...

[wayward bluesman]

Final update - I rechecked the connection between the collector/housing on the drivers and the heat sink it's supposed to be connected to (I sounded fairly confident in that previous post - now I'm kicking myself). It was an intermittent connection at best, AS LONG AS I didn't apply hardly any pressure with the meter leads, which I can only assume was happening before.

What's interesting is that, on the circuit board, nothing actually connects to the collector "leg" poking through; all the traces connect to the heat sink mount (which, for the original 40409 and 40410, was integral to the transistor housing/collector). So, even though the heat sink was sandwiched in with the new transistor, the connection wasn't good enough (or, sometimes not there at all) for the collector to be connected to anything. I'm still wary of soldering the transistor housing directly to the heatsink for fear of toasting the transistor before I got anything hot enough for solder to flow. I ended up wiring the collector leg to the heat sink mount directly under the board and VOILA ... amp was working as it should.

Thank you all for the help. It's a little disappointing to realize that the problem, in the end, was due to my own faulty installation, but it's been a hell of a learning experience.

[gbono]

I knew that output waveform looked familar. I once spent several weeks chasing a similar issue with an ACC 470. I had replaced the missing plastic covers on the output transistors and I wasn't making contact with the heatsink on some of the transistors