Basic Black distorted sound

[pablovera87]

Hi everyone, my name is Pablo and i'm trying to repair my SWR Basic Black combo. The signal comes out very distorted (kind of broken) and very low, i can't get a loud output even with the gain and master volume pots at 10. I've dealing with this for a couple of weeks and i'm running out of ideas.

So far, it's not the tube, it's not the speaker, and i'm pretty sure that the problem comes from the power amp stage, since i've tried recording the signal from the send effects output and it sounds very clean, and then i soldered a 1/4" jack from the preamp out and the signal is fine.
DC voltages from the power supply are OK and i already replaced a couple of electrolythic capacitors, burnt resistors and a NPN transistor.

Any ideas?

Thanks in advance,
Pablo.

[52 Bill]

Welcome Pablo.

SWR amps are fairly straightforward in design, and you should be able to fix it. What transistor did you replace, an output, a driver?

Two things that I see a lot of in these amps are broken leads going to the transistors. Check to make sure that all of the transistors are ok.

[pablovera87]

Thanks for the reply!

I replaced just one MPSA56 that looked burnt, but it wasn't actually.

When you say leads you mean tracks? Sorry for my english... If that's what you meant, i will do that, i checked with the voltimeter but i'll check again, i may have missed something, since it has tracks on both sides of the PCB, with makes it a little bit more difficult.

I was actually thinking of replacing the output transistors, these are 2SC3264. It is a bit risky since they aren't cheap, but honestly i replaced a lot of capacitors and resistors and nothing seems to bring it up. I did not change or replace anything on the power supply, but since i'm getting the correct voltages from it, i guess that is not where the issue is.

Thanks again.
Peace.
Pablo.

[52 Bill]

When you say leads you mean tracks?

No, the legs coming off of the transistors. Often the legs will break off where they enter the plastic transistor body.

If you search the forum, Enzo has posted the schematics for your amp in a thread from last year. If you print them out we all can discuss your amp using the same reference points.

[pablovera87]

Ok, so i will check the status on the legs coming off the transistors.

About the schematic, i've been following it with the circuit and analized it, that's where i got the resistors and capacitor values, for the ones that had blown. I also e-mailed SWR requesting the schematic, and they sent me one, but it isn't exactly the one for this PCB, since it has another model number. So, i've been following the one posted by Enzo on the previous thread.

Do you think that the output transistors may be causing this problem?

Thanks
Pablo.

[52 Bill]

Anything is possible, but normally the outputs on these amps will fail by shorting. You replaced some resistors, which ones?

Check all of the transistors and diodes on the board with your meter.

Also pay particular attention to the start up mute circuit at the upper left side of the schematic. If it doesn't turn on, the amp will not work.

[pablovera87]

Well, the thing is that there's a 2SC4382 on the schematic that wasn't on the board. I guess it was replaced by someone on a previous repair, but they replaced it for a TIP31C... which isn't exactly a correct replacement. In addition to this, the resistors and capacitors around this incorrect replacement were all burnt out, so i replaced them all (180ohm, 100ohm, 820ohm, 470ohm and 100pF capacitor).

I did not use the 2SC4382, but i replaced it for a 2SC3298 as it says on the semi-conductor database. If that can be a problem, i will have to travel to the other side of town to get the correct replacement but i'll do it anyway.
I also replaced a couple of MPSA56.

About the mute circuit, i'll check that but i can get a sound from the amp, it's distorted and very low, but the signal goes through, so i don't think that could be the issue. However, i will check.

Thanks,
Pablo.

[52 Bill]

So you replaced Q3 and related parts? The 2SC3298 isn't rated as high as the C4382, but it should still work ok.

Double check your own work as well as the work done by the previous tech. Make sure that the transistors have been installed correctly and that there are no problems with the pc board, like broken traces, etc.

Have you checked the rest of the transistors? Did you adjust the bias control?

[pablovera87]

OK, so now the amp is working, honestly i don't know which of all the modifications brought it back on, i replaced some of the capacitors from the preamp section and some of the power amp and it started working but i still have a little bit of distortion in the background and i think it does not sound as loud as it should, since it's a 100W combo.

Could it be the 2sc4382 replacement, the 2sc3298?

Also, i replaced both 2SC3264 (output transistors) just in case, they weren't that much expensive, but they overheat pretty bad... after 5 or 10 minutes, i can't even touch the heatsink. In the datasheet it says that they go up to 150°C, but i'm still not sure about this, so i don't let it run more than 5 minutes.

One thing i should mention is that i'm testing it with a guitar, so i guess the response isn't quite the same. I'm going to try to get a bass guitar from someone in order to make a more accurate soundcheck.

Thanks a lot.

Pablo.

[pablovera87]

Thank you all for your help, the amp is fully working now. It was a problem with a broken track and some capacitors that i hadn't check. I received the amp in a very very bad condition, and a great number of components had blown, so i was almost as if i had to re-build the amp.

Thanks again for all your messages.

Saludos desde Argentina (greetings from Argentina)

Pablo.

[J M Fahey]

Hola Pablo, cualquier duda o problema que tengas, comunicate nomás.
Un abrazo.
Juan Manuel Fahey.