Acoustic 140 issue

[Nelson200]

Weird situation a bandmate has an acoustic 140 that he uses at practice and last practice it started cutting out and distorting so I offered to take a look. I am no expert but familiar with amp and pedal circuits. Anyway i dig into this and have isolated the issue to the power module. R311 was toasted. Replaced that amp sounded better but its still not right as i tested voltage across. That resistor and im getting 74v on both sides. Should be 75 on one side 40 on the other. So somewhere it appears this resistor is essentialy shorted but it measures fine. Any ideas? Thanks!!

[Jon Snell]

If you have 75volts on both sides and the old one was overheated, I would suspect Q301 or Q302.
This is the same circuit as the Tannoy 70 amplifier from the early 60's.

[Nelson200]

Thanks. I shall replace both. Other thing i notice is that one of the thermistors reads like 20 ohm instead of 100 i doubt that is the problem but i might as well replace that too only ones i can find are 1/2 watt i think the originals were 1 watt i think. They run in parallel w 6.8k so i suspect they wont get too thrashed and will work fine

[Nelson200]

Ok all the transistors are in order swapped out the bad thermistor and amp sounded pretty distorted. R311 was getting hot and
R312 and 313 were hot enough for me to shut it all down… it was uncomfortably hot. Somebody is corrupt so I guess its time to go door to door with my investigation and interrogate every resistor and cap or maybe its not the right thermistor

[Jon Snell]

Why did you replace those transistors and if they were faulty, not fix the cause of their failure?
Transistors in a well designed circuit, don't fail without reason.
Throwing parts, probably not identical parts, at it, will acheive nothing more than a damaged piece of junk. A replacement semiconductor may have different parrameters and must be matched with the circuit.
Voltage readings would be a good start.
Centre point reading is?

R312 & 313 will run very hot, dissipating 9.3 Watts of heat!
R319 must sit around 37volts.
DO NOT change the thermistors! They run in parallel with R314 & 315 6.8 OHMS NOT kilo OHMS.

R312, 313, 316 & 317 provide a DC bias for the output stage. The transformer windings produce the signal to drive the output transistors.

Is the signal on pins C & D clean and distortion free?
Have you got about 14volts on Q301s emitter? If not, why not.
Have you got a clean signal on Q302s collector?

[Nelson200]

Sorry for the confusion i oulled every transistor initially one tested bad and i almost ordered a replacement but i retested on another unit and it checked out ok. So i double checked again. All good and placed back in circuit. I will check those volages in a bit. Thanks for the help.

[The Dude]

Please upload or link a schematic.

[Nelson200]

Some voltages. On the 75 v side of r 311. I am getting 75. On the 40v side only 14. Collector of q303 and 304 are also 75v but collector of q305 and 306 are only 28mv. Supposed to be 37v. Q301,14 v at collector and 13 at emitter just like its supposed to be. Gotta be a partial short somewhere in here. I will link the schematic. Thanks. This is weird.

[Nelson200]

Schematic ..

[Nelson200]

Sorry that was sideways !

[Nelson200]

So everything else checked out ok so i disconnected the transformer and tested the windings. Based on previous posts those are red to blue i got 45 ohm. Green to yellow i got 3.8 ohm and greenblack yellowblack i got 3.7 ohm. Does this sound like it is functional ? Thanks!!

[TomCarlos]

Nelson....It looks like you are measuring the "Driver" transformer and not the power supply. Is that what you intended to do?

There is another thread on the Driver Transformer - https://music-electronics-forum.com/...ge-amps/49751-

[Nelson200]

Yep correct thanks for the link. I am not fully understanding the issue here. R312 is being shorted in the circuit. The resistor itself is fine. Also collector to base of q303 (and of course 304 because they are connected) measures 4 ohm they should measure like 400 ohm. what the heck?

[g1]

Attaching a more legible schematic. Sounds like Q303 or Q304 is shorted. Disconnect them and measure R312 again.

[Nelson200]

Great idea 303 was shorted! I had replaced 2 already. That was the third. Might as well replace to other as well. Now i am getting the correct resistance i will go over everything tmro and fire it up. Fingers crossed thanks!!!!!