Help! Need a Acoustic G60 - 112 schematic

[Dog]

Hi All..Newbie here..Great forum..

Have an Acoustic G60 - 112 amp left for dead so I grabbed it. Speaker blown out. Power supply OK with + & - 35 volts. LED's for Ch1 & Ch2 will lite when switched. Drivers and outputs checked OK with transistor tester. Connected 8 ohm speaker and fed audio generator to input. Nothing out. My cross reference book dated 1991 does not list any of the 4 IC's installed - 3 TL07ICP's & 1 LM380N-8. I assume they are some sort of op amp. Are they still available if needed? My book lists only some of the transistors on the board - some are JFET's. I want to start checking for signal path with scope but am in dire need for a schematic. Checked AcousticControl website and they only list tube amp schematics. Any one know what year this amp was made?

Thanks for any help ahead of time,

Dog

[vintagekiki]

May be help for the beggining?
http://www.schematicheaven.com/newam...ustic_g60t.pdf
http://www.schematicheaven.com/post70s.htm

[Dog]

vintagekiki,

Thanks for the info links but --- the 60T is a tube amp and the post 70's link is silent also.


Dog

[J M Fahey]

Hi Dog.
Yes, that G60/112 is a SS amp. Not much info available because it was made in the last chaotic months when Acoustic went bankrupt.
Your ICs are very common, although the LM380 might be alittle more difficult, being obsolete.
Anyway, *do not* "shotgun", that's to say, do not start changing parts at random, tat's a waste of time and a sure fire way to render the board unrepairable.
1) Turn it on and off. ¿Absolutely no sound? No click, pop, hiss, hum?
Check that the speaker works (duh) , follow the path (wires/tracks/connectors) from its terminals to the corresponding pads on the board.
One should have perfect continuity to the PSU ground, the other to one of the output transistors emitter or collector.
Post a couple pictures of the board, specially one showing the output transistors and another the wires that leave it towards the speaker.
Good luck.

[Dog]

JM Fahey,

Thanks for the response - I did find some old catalogs with the IC's pinouts. The LM 380N-8 is an audio amp that I assume is in the reverb circuit. It has 15VDC at Pin 7. The TL071CP is listed as a low noise JFET op amp.I could only find a TLO71CN which I assume is an N channel JFET op amp. The pin outs at pin 4 & 7 have 15VDC. I took another look at the PC board and someone has bridged a 100 ufd electroletic cap around a 70 ufd cap which I assume has opened. Obiviously they didn't want to take the time to remove the PCB and do it the right way!
Now the clencher - I took your advice and checked for continuity at the out puts. Some one has removed a component on the board - I see 2 small wire stubs sticking out where the brown output lead comes off the board and goes to the ext speaker jack. With this component removed there is no connection to the outputs. Without a schematic I can only guess it is a load resistor or maybe a fuse device. But this is the problem - no connection to the outputs. I guess when the speaker blew this device fried also. Another tidbit - someone clipped the ground pin on the line cord! Amasing. I will try to attach some pics of the board maybe today. Thanks again - I'm getting close!

[Mark Black]

Are the wire stubs for the missing component a little thicker than say a resistor lead?

Reason I ask is it would be very common for a transistor output stage to pass through a coil of 20 to maybe 16 gauge wire as one of the last components before the output jack. Actually it would be a coil and a large-ish wattage resistor, in fact it's pretty common on older amps to see the coil actually wrapped around the body of the resistor and soldered as a parallel unit. That makes up part of what most of us refer to as a Zobel network and is involved in keeping the output stage happy driving a reactive speaker load.

[J M Fahey]

The LM 380N-8 is an audio amp that I assume is in the reverb circuit. It has 15VDC at Pin 7.

Right on both counts. You were lucky to have the 8 pin version; the early one was 14 pin and *very* difficult to get; anyway it's probably fine.

I could only find a TLO71CN which I assume is an N channel JFET op amp.

It's the same part; the last letters refer to the case type and the temperature they support, *all* of them work there.

The pin outs at pin 4 & 7 have 15VDC.

I *hope* you refer to plus 15V on pin 7 and minus 15V on pin 4.

checked for continuity at the out puts. Some one has removed a component on the board - I see 2 small wire stubs sticking out where the brown output lead comes off the board and goes to the ext speaker jack. With this component removed there is no connection to the outputs. Without a schematic I can only guess it is a load resistor or maybe a fuse device. But this is the problem - no connection to the outputs.

As Mark suggested, it's *very* probably an inductor in parallel with a resistor, which has the nasty habit of cracking its legs easily and removing itself from the board.
Did you see any of the "Saw" films? Well, something like that.
You can make a new one by taking a 10 ohm 2W resistor and, say, 4 or 5 inches of enameled transformer wire, around #18 or #20, as suggested, scratching some enamel from each end, winding it around the resistor body and soldering its ends to the resistor leads.
Then you solder that kludge into the appropriate board holes. Just for testing you can use a short link of wire to replace it, but whenever possible do it right.
Good luck.

I'm getting close!

Yes you are.

[Dog]

Hey JM & Mark,

I found a 10 ohm 2 watt resistor in my stash and temp wired it in - low and behold the Acoustic gods are with me! The amp lives again - both channels and reverb to boot. I need to construct the RL component as you both sugested, replace the electrolytic that's bridged, replace the line cord plug and see about getting the speaker re coiled. Should I be concerned with proper bias settings on the finals as some one has had their hands in this unit? If so any idea what idle current should be?

Thanks both for your excelent coaching - guess I needed a little kick start!

Regards
Dog

[J M Fahey]

Good!!
Remember to add the coil in parallel with your 10 ohm resistor or you'll murder it.
As for the biasing, I don't remember, those last split supply Acoustics were completely different from the regular single supply ones and carried pretty poor info.
They might even have no bias trimmer, being based on the classic "RCA 70W" popular in those days.
If in doubt, don't touch it, remember posting some pictures.
Good luck.

[Johnny1988]

I NEED HELP TO ONE TRANSISTOR BLEW OUT AND I HAVE NO IDEA HOW MANY OHM'S OR WATTS IT IS ..AND I HAVE THE REVER AND PRE AMP OUT OUTPUTS DISCONNECTED SOMEONE DE- SOLDERED THE CABLES PLEASE ..PLEASE WATCH MY ENTIRE VIDEO AND LET ME KNOW IF ANYONE COULD HELP ME ....

HERES THE VIDEO ......

[Mark Black]

wow...

[Phostenix]

I'm guessing this is your video?
YouTube - Acoustic G60 112 (model 109)

That looks like it was a resistor to me, not a transistor? Just curious, why didn't you speak on the video?

[Johnny1988]

Anyone know where i can get one of this ?

its a RCA HN 8121 Transistor ...im looking for the original but i've been looking everywhere and i cant find it ...i tried getting readings too to look for a non original replacement but its shorted ...i even tried contacting ACC but no answer ....can anyone help me ?

[52 Bill]

That is probably a transistor, but you have the part number wrong. The HN 8121 is the manufacturing code and not the part number. The 81 is the year it was made and the 21 is the week of the year it was made.

The Acoustic part number appears to be 480067. Without knowing how this is used or where it is in the circuit, we have no way of knowing that it is a transistor or something else like an SCR.

Is there another one of these in the amp?

[vintagekiki]

Anyone know where i can get one of this ? ... its a RCA HN 8121 Transistor ...

Not RCA HN 8121 Transistor. It is RCA 480067. Used as the bias transistor in some vintage Acoustics amps like Acoustics 215B-120, Acoustics B4 ... ...
You can replace with any TO220 NPN medium power transistor. May be need minimal changes resistor in the base polarization.

Acoustic Control Corporation TO-220 transistor identification
http://music-electronics-forum.com/t16021/