Need help fixing guitar amp

[maurycy]

I have Washburn BD40R guitar amplifier. The amplifier turns on, lights work but there is no output on the speaker. Actually with no load (speaker detached) and no input the speaker terminals show 6.2V DC (yes, that's volts not mV). I have very limited knowledge of electronics so I would need somebody to help me diagnose the problem. Basically I would like to know what should I measure/check next.

Here is the schematic: link

Visually the D3 and D4 LEDs work. D1 and D2 do not light up but I am not sure if they are supposed to or not. D3 and D4 are on the panel and D1 and D2 are on the PCB so no visible when amp is assembled.

Anybody willing to help me?

Thanks.

[djoffe]

The basic tactic is to split and trace. Cut it in half, at a convenient point, and see what's going on.

First though, check the power supplies...B+1, B+2, B-1, and B-2. Unfortunately, I can't see nominal voltages on the schematic for B+/-1.
B+2 and B-2 should be plus and minus 12 Volts.

D1 and D2 light to indicate overdrive...If the front bits of the amp are working, then a hot signal and cranked gain should light D1 and D2. If it doesn't, you may have power supply problems.

If D1 and D2 seem to with a loud signal, then probably power supply is ok, at least for the low voltage stuff. The next place to check is at the send jack...put a source into the input, and connect the send jack to the input of another high impedance amp...If you get a signal out, then you've just cleared most of the preamp.

Finally, you can check just the power amp...Jack into the return jack with a source, and see if you can hear a source from there in teh speaker.

Now...as to your 6 volts at the output of the amp...that does say the power amp is probably unhappy...just how unhappy and why, it's hard to say...measure B=1 and B-1, and report back. There are other measurements to do, but we need to get calibrated on your comfort level before we suggest any more.

[visioninscience]

with the unit off & unplugged, test the voltage on the filter caps and carefully drain them with a resistor if needed.

then,

test q3 & q4 (tip42c,tip41c transistors), and protection diodes d5 & d6.

the tda2030a is most likely ok, but the outputs seem damaged due to lack or decent overload protection. check r45 & r46 to see if they are visibly smoked. then, measure them for shorts or opens.

do the same with r44 & r47.

[maurycy]

Thanks for all the replies. Sorry it took me that long to reply but with two kids and full time job, I do not have as much time that I could devote to my hobby as I would like.

Anyway, I did some basic measurements and the B+1 on the power supply part (lower left corner of the schematic) shows as 12V but B-1 shows as only 4.8V. Shouldn't B-1 value be the same as B+1?

As to the pre-amp testing, the amplifier does not have the RETURN and SEND jacks that are listed on the schematic. I will have to remove the board and come up with something to test the signal.

[visioninscience]

you don't have to worry about signal testing for now, but yes 12v and 4.8v is certainly a problem. and yes, whatever they should be, they should be the same.

b+1 and b-1 should be higher than 12volts anyways, probably 18 to 20 volts per rail as the zener diodes for the low voltage side are 16 volts and the source needs to be higher including voltage drops through resistors r60 and r61.

test all the parts i mentioned in the previous post. test transistors and diodes with the diode test and resistors obviously with the ohm test.

the tda2030 ic could be damaged internally, but probably not likely.

if the unit is old enough, it could also be the filter caps, c45 and c46, but I feel more confident that it's something else loading or partially shorting the supply rails.

[visioninscience]

went back in browser and it double posted.

[visioninscience]

he is right they are about 12 v +, - ..,but they should be the same on each rail. parts to check are still the same.

[maurycy]

Isn't the drop in voltage on the power supply part of the circuit? I will test the amp parts but shouldn't I start with the power supply first?

Thanks.

[prairiemystic]

When troubleshooting, you can start at the input or the output or the power supply. It helps to be lazy and follow the shortest route
If you have 6.2V on the speaker, that points to the power amp section having problems and you should check Q4, Q5 and IC5 for shorts (the power amp is not DC coupled to the rest of the circuit). The power amp runs off B+1,B-1 and IC5 can take max. +/-22VDC. I would guess IC5 has failed and Q4 or Q5 might have gone as well.

[maurycy]

Again, took me a while but I have replaced few parts with new ones. Unfortunately, all I get now is a buzz (before was just a silence). No actual sound from the guitar thru the headphones. The parts I have replaced are:

TDA2030A (IC5)
TIP41C (Q3)
TIP42C (Q4)
1R5 (R44)
1R5 (R47)
0R22 (R46)
0R22 (R45)
IN4004 (D5)
IN4004 (D6)
68K (R39)

So at least I know that they should be working as I got new parts from Mouser. Any ideas what could I test now?

Thanks.