Acoustic 126 Schematics Needed

[TigerAmps]

Hi - I am looking for the schematics for an Acoustic 126 combo amp.
I loook around and did not find them posted anywhere ...

The board numbers are:
preamp Board 170077
EQ Board 170076A
Power Board 170080

The amp powers up and does not blow the fuse.
DC offset was around 35ma and measuring about 77V on the large can cap.
I have distorted output; the preamp out is clean when pumped into a Peavey Monitor 300 amp.
so I figure the problem is on the power board (170080) ....

Questions:
1) a resistor is marked as follows: .27J250 - how do you interpret that?
2) 3 of the output transistors are TCG130 (which I have never seen before) and 1 is an RCA marked 48-15 7747 (strange)
What would a modern equivalent be for these transistors?

Thanks in advance!

[Steve L]

Schematics

Here are two schematics for the power amp. That is likely where the problem is.
The output transistors are common 2N3055 devices. Don't worry about the "special" designation on the schematics. The Acoustic part number was 480015. 7747 is the date code. (late 1977)

Some matched 2N3773 transistors would be a better choice for replacements.

The resistor looks like it would be a .27 ohm value, however the schematic calls for a .24 ohm value. (If we are talking about the same resistor)

Check the wiring to the output transistors (usually a tightly pulled solid conductor wire). This was a common failure point in those amps as the wire would break. I usually rewire the module.
Check the rest of the transistors, diodes, etc. These modules usually went bad in a big way, some even burned holes in the board.

Acoustic 170080 Power Amp (Rev.A) Schematic.pdf
Acoustic 170080 Power Amp (Rev.B) Schematic.pdf
Acoustic 170077 Bass Preamp Schematic.pdf

Steve

[TigerAmps]

Steve - Thank you for the schematics.

I was unable to get the 170077 pdf; I was able to click on the 170080 image and it did download.
Could you please re-post the Preamp schematic.

My bad - the resistor is marked as follows: .27J250 is actually a capacitor (the white things that look like "Chiklits gum" on the board);
I measured them with my meter.

I will replace the four output transistors with a new set of (4) MJ15003's (because I have them on hand and I read in the forum they will work)
and post the results in the next day or two.

Thanks - Keith

[g1]

If Steve can zip those schematics, then they will work.

[vintagekiki]

Acoustic 126 schematics

Acoustic 126 and Acoustic 220 have identical boards in preamp (DWG C1595), eq (DWG C1575) and power supply (DWG C1613)
The difference between Acoustic 126 and Acoustic 220 is in the power amp boards.
Acoustic 126 use power board 170074 (DWG C1571) or 170080 (DWG C1592).
Acoustic 220 use power board 170079 (DWG D1598).

http://www.ampix.org/albums/userpics/10075/Acoustic_220_service_manual.pdf
Acoustic 220 service manual


http://unofficialacousticcontrolcorpmessageboard.yuku.com/topic/2296/
Acoustic 126 Bass Amp inductors

[TigerAmps]

Thank you for the files and info!

I replaced the 4 output transistors with MJ15003's and found that C4 10uF/100V was bad as well and replaced it.
Now I have loud, but distorted output. If you put a low level signal into the amp it sounds clear
but if you increase the signal, it still distorts. Also, after replacing the 4 output transistors and the C4 cap,
the DC offset not less than 2mV.

Anyone have any ideas regarding the distortion?

I do plan on replacing the remaining electrolytics on the power board as they are 30+ years old.
The diodes appear to be good (in circuit testing) but I will remove them and test them individually as well.

Thanks in advance ...

[vintagekiki]

Acoustic 126 no offset voltage, because the output transistors are isolated from the speakers with a coupling capacitor (C14 / 1000uF), so that in case of any defect of the output level of the speaker stays protected.

Instead offset voltage accuracy of the output stage is performed to measure the emitter voltage of the transistor Q6 or + connection of capacitors C14 / 1000uF. The measured voltage is + 36V ⁺/_- 5% or ½ of the supply voltage

Half of the supply voltage (+ 36V ⁺/_- 5%) can be adjusted to the required value by changing the resistor R4select (PCB 170080 / DWG C1592) or R206select (PCB 170074 / DWG C1571)

Quiescent current of the output transistor Q6 is 25mA ⁺/_- 5% is set to the required value by changing the resistor R15 / R16 (PCB 170080 / DWG C1592) or R214 / R215 (PCB 170074 / DWG C1571)

Low quiescent current is manifested in loudspeaker distorted signal at low power, and distorted sine wave on an oscilloscope.

My knowledge of the language https://translate.google.com/

[Leo_Gnardo]

Thanks Steve L & Vintage Kiki ! By coincidence an Acoustic 220 showed up & needed ASAP repair. The schematics & manual helped immensely. Much appreciated! LG.

[TigerAmps]

Quick Update:

After checking over the ALL components on the power board, all appeared to meter out OK.
so, I decided to "chop stick" test the power board. When I hit one of the driver transistors I started to get clean output.
After removing the driver, re-soldering it AND fixing a questionable looking trace going to that driver it now works!
This would be the first time I "chop stick" tested a solid state amp; I usually end up doing it in in tube amps.

I have a little more work to do one the amp, but I amp "out of the woods" now; the rest is pretty basic.

Thank you to all who posted and provided the schematics!!!

[vintagekiki]

Links to Acoustic (ACC) schematics

http://music-electronics-forum.com/t40939/