Acoustic G60-212 with low output

[Alemaker7578]

I was asked by a friend if I could look at an Acoustic G60-212 solid state guitar amplifier, to see if I could get going.

Found the drivers/finals to be blown, and now it works but isn't very loud.

Replaced the finals Q5/Q8 with 2N3055G and drivers Q4 with 2N5415 and Q7 with 2N3440

Voltage measurements were taken with all pots at 12 o'clock, and respect to chassis ground. R6 is in its centered position 4.7K ohms.

I noticed If I turn R6 clockwise down to around 1.3K ohms, the amp really wakes up and sounds much better... But... the drivers get very Hot! 250F for Q7 and 200F for Q5

Another thing is the finals stay cool no matter the position of R6.

Not sure if this is a problem with the selection of said replacement parts or a resistor biasing issue?

I am a definite Newb for amps, but have worked on other electronics for most of my adult life.
(I know which end of the soldering iron to grab..)


Any insights are definitely appreciated!

Wade

[g1]

Did the original Q4 and Q7 have heatsinks? If so, did you re-use the heatsinks? When those RCA type transistors are used as drivers, they generally run very hot. Don't worry about that if you have the appropriate heatsinking. Get the voltages in order and check the idle current across R14.

[Alemaker7578]

I punched out the bad transistors and re-mounted the new ones in the 'heatsink cage' the best I could with heat sink compound.
I tried to upload a picture, but I got a "you've exceeded your limit for uploads.."

I forgot to mention, that I replaced Q3 (RCA 480067) with a TIP31CG


As far as voltages, I guess that's my biggest problem... I'm not sure what the correct voltages should be at various points in the circuit?

Thanks for your feedback!

[g1]

First off, I'm assuming you checked the pinout of all substitute transistors and re-oriented the position of any that did not match. Please confirm.
At speaker output you should have very close to zero volts DC. If not, disconnect speaker until amp is repaired.
Check that supply voltages out of BR1 are roughly + and - 31V. Post voltages for E,B, and C of Q4 and Q7 for a start.

[Alemaker7578]

- Yes, correct orientation of components that were replaced.

- Voltage at speaker reads 20mV

- Voltages out of BR1 are +35.5V -35.4V (I wondered about these....??)

Q4 E -118mV Q7 E 871mV
B -558mV B 1.3V
C -34.6V C 35.3V

Q5 E -35.2V Q8 E 17.8mV
B -34.5V B 763mV
C -13.7mV C 35.3V

[Alemaker7578]

Q4 E:-118mV B:-558mV C:-34.6

Q7 E:871mV B:1.3V C:35.3V


Q5 E:-35.2V B:-34.5V C:-13.7mV

Q8 E:17.8mV B:763mV C:35.3V

[g1]

Those readings seem pretty good, just maybe the outputs are turned on too hard. Check voltage across R14 (0.27 ohm 2W).
Adjust bias trimpot to reduce voltage across R14. This should also reduce voltage from E to B of both output transistors (currently around 0.7V each).

[Alemaker7578]

So right now I have the voltage set to 2.3mV across R14. (Adjustment pot will swing voltage from .4mV - 48mV) E to B Voltages on the driver transistors is around .62V & .63V

and the E to B voltage on the finals is around .46 & .61 Volts and are barely warm.

Are the finals not getting hot because the amp is idling and no signal is passing through it?
I just assumed with the amp on (even with no signal) that the finals would warm up a little.

I'm not a guitar player, but am borrowing a guitar from a friend and know a chord or two...
I played it pretty hard for 5 minutes and the finals barely warmed.

I just don't know how loud this amp is supposed to be I guess..., and am apparently really getting hung up on the fact that the finals aren't getting Hot !!

Thanks for your insights and patience !

[g1]

The amp puts out somewhere around 60Watts. Comfortable household listening level is probably in the ballpark of less than 1 watt.
Unless you are running into a load resistor and can crank up the level, I would not expect the outputs to get very hot.
2.7mV across R14 means about 10mA through Q5. I would suggest setting in the 3 to 6 mV range (across R14). See how that sounds and leave it on awhile, the finals may warm up a bit.