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Newbie Help please - Hartke A25 Bass Amp combo

Started by Psabin8951, May 05, 2017, 09:01:24 AM

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Psabin8951

Hi all,
I have a Hartke A25 Bass combo that has stopped working.
Having checked the usual fuses etc (which were ok) I find that Q8 (output stage?) is a charred mess.
So firstly as type number is burnt off, does anyone know what type Q8 is so I can replace it and secondly what would the likely cause be for it to go in the first place?
Any help or even a schematic would be greatly appreciated.
Thanks in advance.

g1

I think this schematic should be for both A35 and A25.  In the lower right corner it indicates parts marked with * are for A25.
Looks like Q8 should be 2SD2058, same as Q7.  What is Q7 in your amp?

tonyharker

If Q8 is blown, then its likely Q5, 6, &7 are as well.  Initially find what caused them to go in the first place.

Psabin8951

Thanks all to replies so far. As Tony mentioned Q5,6,7 & 8 are indeed blown. Can anyone suggest a likely cause? 220R feedback res across Q7 & 8 are ok as are  the various power resistors.

DrGonz78

Perhaps Q9 went first and took all the others with it? Have you tested Q9 yet? Also test Q2-Q4 if you have not yet and even those diodes D13 & D14.
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

Likely cause is stuff happens, we usually don't know the ultimate cause of a blow up.  A part could have been stressed 6 months ago, and just now fails.

Psabin8951

Ok quick update, Q2,3,4,5,6,7,8,9 now replaced. R97 is now drawing too much current through it.
Any ideas?

phatt

If you are not measuring the Current at R97 but assuming it's passing too much current because it's getting Hot then don't fret as that resistor will run warm as long as it's not turning black from overheating then it's likely ok.
As long as the amplifier is working and passing signal with little DC at speaker output then you may have fixed it. If it bothers you you can change it to a 2 Watt resistor which will run cooler.
Phil.

Psabin8951

Well I'm getting sound out albeit very quiet but the resistor is running so hot it's melting it's solder joints so definately not right.

Enzo

OK, so what COULD draw a ton of current through that resistor?

But first, what voltage is on the ends of it?  I don't know what V+ and V- are, but I'd guess around 25v, so if you have 25v on the right end of R97 (as seen on schematic), what is on left end?  If a huge drop, then that explains it all.

I see C61 to ground. 

Are other parts heating?  Like R98?

Is R52 open?  (near output jack)

Left end of R97, does it measure any lowish resistance to ground while power is off?

Psabin8951

No other parts heating.
R52 is reading 10R so that's ok
Left end (junction with R98) reads around 2R to ground.
I will try lifting C61 and see if that makes a difference.

Psabin8951


Enzo

Find the 2 ohms, that is your trouble.  Look for a bead of solder touching a copper trace maybe.  There is no component that can take that trace to ground, so for my money it has to be a short on the board.  Start at that resistor, and visually follow the trace everywher it goes, looking for something else touching it.

Psabin8951

Doesn't appear to be any short. When Q4 is removed however R97 does not get over hot.

g1

Like Enzo said, that 2 ohm reading is the key.
What happened to the 2 ohm reading when Q4 was removed?