Roland Cube 100 guitar amp

[Stewen]

Hallo everybody ! Have changed all electrolytic caps but the amp "tilts" and gets 4,4 VDC at the output and all other voltages crazy , when starting up the amp with the speaker connected . BUT ,
if I disconnect the speaker and starts the amp , everything is fine , all voltages fine and I can connect the speaker and the amps plays wonderful . Reverb and high gainchannel , everything works good.
I guess theres something wrong with the poweramp but donīt know what . All transistors measure OK as I try to check them in the circuit . There is a trimpot on the board that is not in the schematic.
Amp is dated 1981. Thanks for some advice.

[nickb]

First, don't connect a speaker until the DC offset is fixed. Measure the voltage on the output, Q2 base, Q1 base and the power rails. The schematic is annotated with several voltages around the power amp. How do yours compare?

I have the schematic but the upload is not working.

[Enzo]

ANy chance you are bringing the amp up on a variac? Or using a light bulb limiter?

[Stewen]

Well , here in Sweden we have 230 VAC . I have a 15W bulb in series with the power in , then a 40 W lamp . They glow full when amp is out of balance. Not when its started up without a speaker connected , neither when I
connect a speaker after start up . Guess one powertransistor is open full when i have +4,4V in the "middle" (output) and that this current is flowing through the speaker ...... not good for the voicecoil !
So I have only 15, 25 , 40 , 60 , 75 and 100W bulbs .... too choose , no variac . I have an oscilloscope with two channels and can check the ripple for example on the + and - rails . Even if i have new electr.caps all
over I guess one can be bad ...... I have checked all diodes and they seem to be OK . I have one resistorbox which can be set from 1 ohm to 1000 ohms . Letīs say I connect the box to the speakeroutput and set it at
500 ohms , start the amp and that it doesnīt "tilt" , voltages fine . Then switch off , lower the box to 250 ohms , start and so on .... until it tilts . Letīs say everything is fine down to 32 ohms . At that resistance the amp
"tilts" . Can such an experiment give any useable answer ? ( All voltages are very close to the schematic voltages , WHEN the amp is idling normally. Thanks for answers so far .

[g1]

Try it on the load resistor without using any light bulb limiter, straight 230VAC.