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.
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?
If Q8 is blown, then its likely Q5, 6, &7 are as well. Initially find what caused them to go in the first place.
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.
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.
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.
Ok quick update, Q2,3,4,5,6,7,8,9 now replaced. R97 is now drawing too much current through it.
Any ideas?
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.
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.
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?
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.
No Difference with C61 out of cct.
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.
Doesn't appear to be any short. When Q4 is removed however R97 does not get over hot.
Like Enzo said, that 2 ohm reading is the key.
What happened to the 2 ohm reading when Q4 was removed?
ok so with Q4 removed the reading was as expected so Q4 now replaced and to be sure d13,14,q2,3,5 also.
I now get a hum when a guitar is plugged in, R97 doesn't get too hot but Q8 catches fire!! It literally glows.
I'm about to give up on this one unless anyone has any final ideas for me.
Sorry to hear,,, Q, Are you using a limiter? 8|
These are Direct Coupled high current circuits and obviously if the circuit is unstable you can blow transistors before a fuse even has time to get out of bed. :)
To circumvent instant destruction of all your hard work you need to limit the current and the best way to do this is with a very simple electrical trick all details to build a Light Bulb Limiter can be found on this forum.
So before you crucify anymore components you need to build a limiter which allows you to get voltage readings around the circuit to get an idea of where the problem exists. if the current tries to rise the bulb Absorbs the inrush and *Limits* voltage saving the transistors from INSTANT DESTRUCTION. :'( :'(
Light Bulb Limiter circuit and build notes at the top of this page;
https://www.ssguitar.com/index.php?board=2.0
Build that then some of the more savvy tecks can tell you where to take voltage readings and they will guide you through the process of rebuilding the circuit.
Don't give up just yet,, Phil.
And maybe we forgot, but I hope we made mention of not connecting a speaker until we know the amp is stable and not producing DC?