Acoustic Control Corporation TO-220 transistor identification

[Jazz P Bass]

Acoustic Control 220

Can you measure the Vdc of the driver bases /40409 & 40410.
Also the 47Uf/50V caps right in back of the bias resistor (on the scheme) have to be good.
Bootstrap caps.

[thintheherd]

Sure, I will check it out. I will also check those caps. To me and what I have experienced with ACC amps before, this sounds like a starving transistor. So, I will check those voltages there and hopefully we can pin this down. Thanks!

[52 Bill]

Maybe Vintage Kiki will chime in here, but what I understand E24 to mean is a 0.24 ohm resistor.

What does the part look like?

[thintheherd]

It appears to be just a large wire wound resistor, I guess the schematic symbol was just a little confusing.

***generic reference below, not same rating:

http://media.digikey.com/photos/Ohmi...tos/90J10R.JPG

that is what it looks like...I figured it was just a resistor but my concern was it is reading .60 ohms as opposed to near .24 but they both read the same so this tells me they are probably ok?

[thintheherd]

Well checked voltage at the drivers (40410 and 40409) and I am getting -49.5 and 49.5 at each heat sinks of the transistors respectively and I am getting NO voltage at the other pins of each transistor! I believe the heat sinks are the collectors on the drivers, so I am getting no voltage at the base of either of the drivers. I did substitution test with the 47uf caps and that did nothing for me. Any ideas??? Thanks!!!

[Jazz P Bass]

Bias Voltage

Look at the schematic fot the NPN driver.
Its base is 4.7K & 470 ohms from the collector.
Does it measure that?

Are the limit transistors o/k?
The PNP is marked 2N4248. there is no marking on the NPN.
There job is to rob the drivers.
Which is what you are seeing.

[gbono]

Acoustic 120 Guts - Trouble-Shooting, Repair and Restoration - New Forums - unofficial acoustic control corp forum - Message Board - Yuku

Link to pictures of the 120 power amp - img212[1].pdf schematic for the power amp (ACC used this same board in many designs).

Anyone have a schematic for the 120 preamp?

[thintheherd]

here is what i noticed today. if i left the heat sink for the output transistors not screwed down i would get voltages to the power board, only until i let the heat sink touch the chassis did my voltages on most components go near zero. so essentially as soon as i ground one of those outputs voltages drop to near zero for most all components.

[gbono]

You mentioned that this amp originally didn't work since the output devices were damaged. I'm assuming you never had the amp working in the first place.

I'm wondering if there is a grounding issue with your amp that existed before the output devices were damaged? Are the input and output jacks isolated from chassis ground - with insulating washers?

I was looking for the preamp schematic for your amp so I could compare it to the G120-112 (model 123). I have a nasty hum on this amp unless I remove the insulating washers on the input jack???

[thintheherd]

hey gbono, i just noticed there are insulating washers on the output jack completely isolated the output jack from ground and there are washers on the input jacks too. why would the output jack be isolated from ground?

[Steve Conner]

While we're on the subject... This TO-220 bias transistor that you replaced, maybe it needs an insulating washer between it and the heatsink, or maybe the insulating washer you installed is shorted.

If the tab of that transistor were shorted to ground, that would rob all drive from the output stage, sure enough.

[gbono]

I don't see a sil-pad or anything else under the TO-220 transistor on the PA board I have. Are you sure the center tap connection on your PS transformer is tied to chassis ground?

Why insulate the output jack from chassis ground? Think of the grounding scheme like a river system. The smaller streams (low level audio "commons") should flow into the larger river (PS common connections, mains earth, etc). You want to avoid the opposite pattern of flow to keep large noisey current pulses out of the low level (and sensitive) audio path.

Anyone have a schematic for the preamp section???