stevie Posted March 28, 2008 Share Posted March 28, 2008 Anyone familiar with the old OHM GA125 bass combo? I recently acquired one that was crackling. I resoldered a few connections on the PCB and managed to fix it. Resoldering and replacing components is about my limit, I'm afraid. The amp seems to be working now but I am concerned that two of the wirewound resistors on the preamp board are getting very hot. The amp is split into a preamp section (front) and a power amp section (back) with an umbilical cord between them which seems to carry the power from the power supply (on the power amp board) to the preamp. The two resistors are the first components in the preamp circuit at this point. The original resistors were 470 ohm, 5W wirewound, but as they were visibly burning the board, I changed them to 10W wirewound and spaced them away from the board. However, even these ones are still getting very hot. By hot, I mean you can touch them but you can't keep your finger on them for more than a fraction of a second. Is this normal? Second question for the experts. I have taken the amp out of the combo and installed it in an amp sleeve. The graphic section of the preamp is now closer to the mains transformer and is picking up losts of hum. Moving the preamp away from the transformer reduces the hum. So, I've turned the preamp round 180 degrees so that the input section is next to the transformer and the hum has gone (the power amp is now upside down in the sleeve, if you follow me). I'm not very happy about the power amp being upside down. I would have thought that the input section of the preamp would be more susceptible to hum than the graphic section. Any ideas what I should be looking for here, or is this normal? Finally, the compressor operates very oddly and degrades the sound of the bass. It's basically unusable. Is this also par for the course? Taking the amp to a repairman would cost more than the unit is worth. Any help in saving it from the landfill would be much appreciated. Stephen Quote Link to comment Share on other sites More sharing options...
DHA Posted March 28, 2008 Share Posted March 28, 2008 [quote name='stevie' post='165142' date='Mar 28 2008, 11:58 AM']Anyone familiar with the old OHM GA125 bass combo? I recently acquired one that was crackling. I resoldered a few connections on the PCB and managed to fix it. Resoldering and replacing components is about my limit, I'm afraid. The amp seems to be working now but I am concerned that two of the wirewound resistors on the preamp board are getting very hot. The amp is split into a preamp section (front) and a power amp section (back) with an umbilical cord between them which seems to carry the power from the power supply (on the power amp board) to the preamp. The two resistors are the first components in the preamp circuit at this point. The original resistors were 470 ohm, 5W wirewound, but as they were visibly burning the board, I changed them to 10W wirewound and spaced them away from the board. However, even these ones are still getting very hot. By hot, I mean you can touch them but you can't keep your finger on them for more than a fraction of a second. Is this normal? Second question for the experts. I have taken the amp out of the combo and installed it in an amp sleeve. The graphic section of the preamp is now closer to the mains transformer and is picking up losts of hum. Moving the preamp away from the transformer reduces the hum. So, I've turned the preamp round 180 degrees so that the input section is next to the transformer and the hum has gone (the power amp is now upside down in the sleeve, if you follow me). I'm not very happy about the power amp being upside down. I would have thought that the input section of the preamp would be more susceptible to hum than the graphic section. Any ideas what I should be looking for here, or is this normal? Finally, the compressor operates very oddly and degrades the sound of the bass. It's basically unusable. Is this also par for the course? Taking the amp to a repairman would cost more than the unit is worth. Any help in saving it from the landfill would be much appreciated. Stephen[/quote] Good move upgrading the resistors to 10W, I would say that if you can touch them at all then all is well. The tuned circuits in the graphic section are most likely picking up the 50Hz. You could try adding an earthed metal plate between the pre-amp and power section as this should block the 50Hz signal and hence remove the hum. No idea about the compressor. Dave Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted March 29, 2008 Share Posted March 29, 2008 I won this a while back, should be arriving soon (had some fun with that). [url="http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=190199467160&ssPageName=STRK:MEWN:IT&ih=009"]OHM head[/url] Is it similar to that? Will be paying attention to the things your mods achieve. Quote Link to comment Share on other sites More sharing options...
stevie Posted April 1, 2008 Author Share Posted April 1, 2008 Thanks for the advice, Dave. Most helpful. Stephen Quote Link to comment Share on other sites More sharing options...
stevie Posted April 1, 2008 Author Share Posted April 1, 2008 [quote name='Mr. Foxen' post='166188' date='Mar 30 2008, 12:41 AM']I won this a while back, should be arriving soon (had some fun with that). [url="http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&rd=1&item=190199467160&ssPageName=STRK:MEWN:IT&ih=009"]OHM head[/url] Is it similar to that? Will be paying attention to the things your mods achieve.[/quote] It looks very similar - like the guitar version of my bass amp. I didn't know they did head versions of these. For your info, I have noticed a few things about mine already. First, there is no proper support for the preamp PCB - it's supported by the input sockets only, which is poor engineering IMO. That led to dry solder joints on mine, although to be fair, it did take 20-odd years to happen. You might also like to check the two 5W wirewound resistors in the graphic section of the preamp to see if they are getting hot and burning the board. If you don't feel like replacing these with larger ones (I'd advise 17W), at least space them away from the board to improve cooling a bit. Then there is the hum problem I mentioned in my first post. If you feel that yours suffers from excess hum - and I'd be interested in finding out - take the preamp out (undo the screws and it just pulls straight out of the sleeve) and see if the situation improves. If it's problematic, you'll need to follow Dave's advice above. Finally, I'm wondering whether 8,800uF of power supply capacitance is really enough for an amp rated at 125/140 watts. I'd be tempted to double this, although it may be more of a problem for bass than guitar. I'd be interested to hear what the specialists think, though (Dave?), as the few instrument amps I've looked inside were also very stingy in this area. Stephen Quote Link to comment Share on other sites More sharing options...
FJ1200 Posted April 1, 2008 Share Posted April 1, 2008 (edited) This is my Ohm GA140 1 x 15" Combo. Interested to see the head version. Had this 15 years or so, and still love it though it's a bit heavy lifting it into the back of a Fiesta! Like busses - haven't seen another one in 15 years then 2 come along together! The compressor on mine is quite good I think, doesn't change the sound too much, but with the Bass POD XTL I bypass everything, just use the Effects send/return and let the POD do all the work. Edited April 1, 2008 by FJ1200 Quote Link to comment Share on other sites More sharing options...
Mr. Foxen Posted April 1, 2008 Share Posted April 1, 2008 Not sure if it is a guitar or bass head, the EQ goes down to 125hz, so I guess that means guitar. It asrrived today, but ain't had a chance to try it, might go for some biamp action. Quote Link to comment Share on other sites More sharing options...
stevie Posted April 2, 2008 Author Share Posted April 2, 2008 [quote name='Mr. Foxen' post='168041' date='Apr 1 2008, 11:45 PM']Not sure if it is a guitar or bass head, the EQ goes down to 125hz, so I guess that means guitar. It asrrived today, but ain't had a chance to try it, might go for some biamp action.[/quote] I just thought it was a guitar amp because of what it said on the eBay ad, but I think you probably do have a bass amp. It looks exactly the same as mine and FJ's as far as I can see. Ohm did produce a range of guitar amps at around the same time - so I wonder why they called these GA (and why mine is a GA125)? Stephen Quote Link to comment Share on other sites More sharing options...
FJ1200 Posted April 4, 2008 Share Posted April 4, 2008 (edited) [quote name='stevie' post='168280' date='Apr 2 2008, 11:46 AM']I just thought it was a guitar amp because of what it said on the eBay ad, but I think you probably do have a bass amp. It looks exactly the same as mine and FJ's as far as I can see. Ohm did produce a range of guitar amps at around the same time - so I wonder why they called these GA (and why mine is a GA125)? Stephen[/quote] GA = Graphic Amplifier? As opposed to parametric. Dunno otherwise. Does yours have a 6-band graphic on the front? It does after all say 'Graphic Bass Amplifier' on the front of mine.... Just a thunk. Edited April 4, 2008 by FJ1200 Quote Link to comment Share on other sites More sharing options...
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