Acoustic Control Corporation TO-220 transistor identification

[thintheherd]

Hello, I have an ACC 120 solid state amplifier that has blown outputs. I need help in identifying one of the TO-220 packaged transistors. It is an RCA #480067. I cant find any reference to it online. I tried doing a cross-reference with NTE and nothing came up. Again this is a TO-220 package not the TO-92 style. (I know ACC used a germanium TO-39 #480067) I figure it maybe an in house number but since its RCA it may not be? Any help would be really cool. Any ideas? Thanks.

[Enzo]

RCA, Motorola, whoever, when they make large numbers of parts for various equipment manufacturers, they often are requested to print them with the OEM's part number instead of 2N3055 or whatever. That doesn;t make them any less a RCA or Motorola part, and their name stays on.

Can you post or link the schematic, and which part is this in the circuit? My schematics skip from 118 to 124.

Or can you tell us what the board number is? Starts with 17.

[thintheherd]

Thanks Enzo, here is a link to the schematic. It is a pdf file and the transistor is the one dead center of the schematic.

ampix - Acoustic/Acoustic 215B-120

Thanks!

[Enzo]

The bias transistor? No reason for that to be a TO220 type. It should never see more than a couple volts across itself, other than when massive circuit failures occur. And it need not carry much current either, less than an amp. Not saying a TO220 is bad, it can work just fine.

On a lot of Acoustic amps that part would be a 480023, which was a 2N2484, which might as well be a MPSA009. 2N2484 looks similar to the 2N4248 they used a lot - so watch carefully.

I can;t find a 480067 in my Acoustic files at all.

That is a hand drawn schematic from one of our members, so if it was drawn from a particular unit, that unit may not be stock.

Is it mounted on the heat sink with the output transistors?

Geez, it could be most anything.

I got numbers on a bunch. Up in the 50-70 range I see:
480055,56,57,58,59,60,62,63,64,65,71,72 and so on. My list is only from the schematics I have here.

[thintheherd]

Sorry, the transistor is in fact mounted to the heatsink, between the two output transistors. It is the only T0-220 on the board. I guess the number on the T0-220 I have determined to be shorted, has the number 480067 stamped on it. I guess my confusion is I have seen T0-39 transistors in Acoustic amps that are marked with the same number. The schematic is the only one I could find so that is what I am stuck with. Sorry for the confusion Enzo.

[thintheherd]

I am looking at this schematic and normally I can identify things pretty quickly but I am having trouble trying to figure out where that T0-220 is on that schematic? I guess I need to pull the board again and investigate and try to isolate it.

[Enzo]

If it is on the heatsink, then I believe it is the bias transistor indeed. If they changes a particular part number from TO39 to TO220, I would consider that odd, but it would be their call after all. Normally I woulod expect the new part to get its own part number and the circuit would be updated to include the change.

But we got what we got, eh?

Now I am confuced. You earlier claimed this transistor was the one in the center of the drawing. I assumed that meant the one marked 480067 and with the 1k trim pot at its base. Now you indicate you can;t find it? Not sure where we are getting confused now. In the drawing I see the 480067 with trimmer as the bias transistor. You report the part in the amp is also 480067, and is mounted to the heat sink. SO that to me means we are all looking at the same part.

Does it measure a dead short? Or just low resistance or "leaky?" Remember, ther are resistors between the base and the other two leads. 2.2k to the collector, and up to 1.3k to the emitter. They will reduce the junction reading. If you want to verify the conection, without pulling the board, you can measure continuity to the bases of the two drivers from the collector and emitter of the device.

But next time you DO pull the board, see if you can find a "17" number on it. 170080 or whatever. COuld also be 17-80 or whatever. Especially since the schematic has ??? for me. In particular, the output stage shows both transistors as NPN in quasi-complementary, but next to them are two different part numbers. He has MJ150023 and MJ150024. Assuming he means MJ15023 and MJ15024, one is PNP and the other NPN. Not to mention they didn;t have those parts 30 years ago.


If the bias transistor shorts, the result is that the amp runs cold and crossover distortion increases. But it won't hurt it. On the other hand, if the outputs and the drivers burnt up, then it could have been damaged by that.

[Steve Conner]

I think SS amp designers sometimes use a TO-220 for the bias transistor because it comes with a tab for easy mounting to the heatsink. Not because it needs to dissipate heat, but because it needs to sense it.

Almost any TO-220 NPN transistor should do.

Having said that, why did we think this transistor was bad? It would be pretty hard to blow it up: it's overrated for what it has to do, and kind of out of the line of fire of domino effect failures. If you suspect a fault in that area, it's more likely the wiper on the 1k pot went open, turning the bias up to "transisto-roast".

The circuit is indeed quasi-comp, so both output transistors should be NPN. The MJ15003, MJ15004, and MJ15024 would probably all work...

[thintheherd]

Thanks guys. I know the transistor is bad because I pulled it and tested it out of circuit, it is indeed bad. Its shorted. With that said, the other reason I am replacing the output section is there are a couple of roasted resistors in line with the outputs. Yes, the part # is 480067 and it is a T0-220, the only reason I brought up the thing about the T0-39's is because I have seen T0-39's with the same number. So you say any T0-220 npn should work for this? I will give it a go. Thanks!

[thintheherd]

Ok, so I reinstalled new output transistors, replaced the T0-220 with similar tip29b, found 2 charred 100 ohm resistors replaced those and checked drivers, decided to fire it up on the variac. There is some serious dc coming through the speaker output. The variac was buzzing and the speakers themselves were blaring hum when brought up to 10 volts. I am using a current limiter but nothing is happening with the bulb, I am a being a bit cautious with bringing the voltage up. Any ideas? Thanks!

[loudthud]

That circuit won't balance if the incoming DC is too low. Until the positive rail is over 15V or so, there is not enough current going through the 12K resistor feeding the diff pair to allow the 480059 to turn on. The output swings to the positive rail.

[thintheherd]

Hey Loudthud thanks for the reply, I found a shorted pnp driver which I guess I overlooked. Go the head powered on and getting output without the dc but, now I have a starving output. In other words, something is starving the output and only responds if the strings of the guitar are hit really hard. This leads me to believe the bias transistor I put in is not what it should be. I tried using a tip29b and I thought the pinout was right but I am going to try and use something else. From what I read is it necessary to mount the bias transistor to the heatsink? because if it isnt couldnt I just use a standard npn to-92?

Edit: I just looked over steves post about sensing heat. It does need to be mounted...

[Enzo]

All TO220s have the same pinout.


When you are first applying power to a repaired amp, do not connect a speaker. COnnect a voltmeter to the output to monitor for DC or not, and watch the mains draw current. Once you find you can bring power up all the way with no excess mains current and no DC on the output only THEN connect a speaker.

If there is 20v for example sitting on the output, with no load, zero current flows from the output. If you connect a 4 ohm speaker load, then 5 Amps will flow.

[thintheherd]

Thanks for all the info everyone. Finally getting back to this beast and still having issues with severe crossover distortion and a starving output. I checked several transistors within the circuit and they are reading fine. The preamp out is working great so i know this issue is within the power amp. There are two, what seem to be varistor symbols on the schematic that are labeled E24. I am getting a reading of .6 on each one of them. At this point I am not sure what these are, but they seem to be reading high from what they should actually be. Does anyone have any insight?

Schematic:

ampix - Acoustic/Acoustic 215B-120

Thanks!

[thintheherd]

One more thing I forgot to mention is, when I play a chord on the guitar and simultaneously power off the amp, the output goes from distorted and starving to clean and normal sounding until it fades out to power off.

[Jazz P Bass]

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]

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???