Troubleshooting bass amp

[mfacen]

I own a hartke HA2000 bass guitar amplifier and I'm having some problems with it:
Some months ago the amp started loosing some punch in the bass and treble, the sound is as loud as it used to be but is lacking presence and definition in the low frequencies and clarity in general. The amp is not distorting at all, it still sounds ok but not as good as it used to.
I have some experience fixing amplifiers but I'm having a really hard time to find the problem with this one. Most of the amps I fixed where the usual burnt power transistors, or major failures where the amp will not work at all. I service the equipment for all the bands I play with, I live in southern mexican pacific coast, here there are not good technicians for this kind of work.
How can I troubleshot such a subtle problem in this amp ? I have an osciloscope and function generator, it is allright for most of cases but when I test the amp at diferent frequencies it seems to work allright but it just doesn't sound quite right when I play the bass trough it. I believe is a problem in the preamp, I connected the effect send output to my other bass amp and it didn't sound right either.
Any tips about how to pinpoint the problem with this amp ?

[jaybombalous]

Your on a good start if you ask me... Now that u think its the pramp why not check the speakers too. Once you rule these out then maybe u can look at other things.

I am also thinking cables can have a lot to do with the problem, especially if you are talking about it being another device and not the amp.

[phase_accurate]

Did you check the bass as well, maybe by playing through another amp that you know well ?
For how long are you playing the bass ?

Regards

Charles

[AndrewT]

Hi,
are you able to measure the ripple on the supply rails when the amp is delivering power at various frequencies?

[Zero Cool]

These amps have problems with input grounding. Check to make sure the input section has a solid ground. They ground the input stage at the pre-amp. if they ground is weak, the input stage looses its reference and the amp can do all sorts of odd things. there is no direct ground to the output section or power supply from the input. there is a resistor in line to break the ground loop currents.

Also check for broken pots. many can develop hairline cracks that "look" ok but have failed. also check for bad solder connections. these amps have that issue as well.

I have had problems where a component had a broken lead. but the lead was broken where it went through the PCB and from the topside it looked fine. but electrically it wasnt connected to the bottom side.

I have fixed many of these and had all sorts of odd problems with them....


Zc

[mfacen]

Of course I checked cables, diferent speaker cabinets, etc. I've playing the bass for about 12 years, I love drumming too and I've been playing drums for 20 years. Hey I'm not that old, I just started early !

Andrew:
I checked the ripple and is allright. I suspected that as the head I'm using now is a old Peavey Mark III from '78 which I rebuilt, the power amp section was completly useless so I built a amplifier board and hooked it up to the Mark III preamp ( I just love this preamp ). It has a very nice punch. That one had exactly the same problem and was a bad filter capacitor in the power supply. It is possible to test properly a 10000 uF capacitor ?


zero Cool:
I checked the ground in the preamp section and found no problems there. I also rechecked the pots, once I was playing in loud rock-ska band and my amp head fell down from the cabinet and I broke a couple of pots and had to change them. I re-checked everything and it seems ok. Beside the amp was sounding fine after I fixed it for about 1 year.
Is true also what you say about broken leads and bad or cold soldering, I thought maybe the problem might be some bad capacitor in the preamp but I don't know how to test them properly. A series resistence meter would help ? Maybe I can get one from ebay if is really worth it.

[unclejed613]

the only way to properly test the 10,000uf caps is with an impedance bridge, and test the impedance from 20hz to 20khz. if it's a hartke, then i gather there's a preamp tube in there? like a 12ax7? that's a definite possibility for that kind of change in sound in a hartke amp. the same manufacturer makes samson amps, and they are 100% solid state, but the hartke line has at least 1 tube in the preamp.

[mfacen]

uncle:
You are right, it has a valve in the preamp but I allways used just the solid state input as I like the cleaner sound it gives.

[AndrewT]

Hi,
DC blocking caps act as high pass filters. Can you check/replace them?

[mfacen]

Still no luck with it. I uploaded the schematics for it in case someone needs them. I was looking for them in the web and couldn't find them so I wrote to Hartke and they kindly sent me the file.
The schematics are here:
http://www.puertobungalows.com/HA_2000.pdf