Charliegibbons Posted January 29, 2013 Share Posted January 29, 2013 Hi everyone I recently picked up an old OHM head, an old english company that used to make PA gear. This particular amp seems to be for bass/ keyboard however and while it's sounding great so far one problem is that whenever I power the amp on or off it makes a very loud crackling popping noise through my cab for a few seconds which doesnt seem to be dependant upon volume. The thing that i've found to negate this is to only plug my cab after the head has powered up and to unplug it again before powering down the unit. Any idea what might be causing the problem? Also, while on the topic the amp doesnt seem to work well with speaker cables and won't send a signal through one at all. A normal guitar lead seems to work fine however, anybody got any ideas? Quote Link to comment Share on other sites More sharing options...
icastle Posted January 29, 2013 Share Posted January 29, 2013 Cor, I haven't seen OHM kit for about 30 years! To deal with the second part of your question first, either the output sockets on the amp have become loose (a guitar lead might have a different shape tip that just manages to touch the socket contact whereas your speaker lead tip might not) or your speaker cable is faulty. The first part of the query is a little harder to answer without the unit in front of me, but... 1) Is it just a THUMP when you hit the power switch? 2) Is it a prolonged series of crackles? If 1) then I seem to remember that OHM used a supressor cap wired to the mains switch - it needs replacing. If 2) then it could be anything from a dry joint on the PCB to a faulty capacitor in the power supply section of the amp - that'll probably require an amp tech to diagnose and replace. Quote Link to comment Share on other sites More sharing options...
Phil Starr Posted January 29, 2013 Share Posted January 29, 2013 I think icastle has it. jack plugs weren't really a standard size and there used to be considerable variation so some pugs will just make better contact than others, replacing the output socket will help but you might improve things by using a spray of switch cleaner onto the socket contacts, Find a plug that works and make up a speaker cable with that. I find gold plated plugs do often make a cleaner contact but replacing the socket is the cure. If the crackle persists I'm afraid it could be any one of a number of caps or almost any joint inside the amp or even worn or dirty potentiometers. Quote Link to comment Share on other sites More sharing options...
bremen Posted January 29, 2013 Share Posted January 29, 2013 And if it's a valve output, don't switch it on without the speaker connected! That's very likely to release the magic blue smoke. Quote Link to comment Share on other sites More sharing options...
icastle Posted January 29, 2013 Share Posted January 29, 2013 [quote name='bremen' timestamp='1359494761' post='1955781'] And if it's a valve output, don't switch it on without the speaker connected! That's very likely to release the magic blue smoke. [/quote] It's not. All the OHM range were solid state. Quote Link to comment Share on other sites More sharing options...
icastle Posted January 29, 2013 Share Posted January 29, 2013 [quote name='Phil Starr' timestamp='1359494626' post='1955776'] If the crackle persists I'm afraid it could be any one of a number of caps or almost any joint inside the amp or even worn or dirty potentiometers. [/quote] If it were here in front of me then my first action would be to see if the crackling changed by rotating the controls during switch on, then try tapping the front panel during switch on and see if there's a particular place that makes a difference (gives you a clue where to look inside). While you're in there then giving the pots a squirt of switch cleaner wouldn't hurt - they've been sat there for the best part of 30 years after all. If that didn't solve it then having a physical look at the underside of the PCB looking for bad joints - especially the big capacitors by the power supply. That generation of amps had big (by todays standards) electrolytics and could exert a fair strain on the small solder connections causing them to crack. All this with the mains lead unplugged of course! Quote Link to comment Share on other sites More sharing options...
Monckyman Posted January 29, 2013 Share Posted January 29, 2013 Had an Ohm120 head with a graphic on it. Nice amp. Quote Link to comment Share on other sites More sharing options...
icastle Posted January 29, 2013 Share Posted January 29, 2013 [quote name='Monckyman' timestamp='1359501854' post='1955976'] Had an Ohm120 head with a graphic on it. Nice amp. [/quote] They were groundbreaking in their day. The cabinets were CNC machined (cutting edge technology back then) and had a really smart grey textured paint finish. The grilles of the cabs were laser cut and the amps were all MOSFET (again, cutting edge back then). Quote Link to comment Share on other sites More sharing options...
brensabre79 Posted January 30, 2013 Share Posted January 30, 2013 wow there's a blast from the past! My dad had an Ohm combo, it weighed a ton and then some, great sounding bit of kit though. I seem to remember his always made a rather loud thud when powering up/down. I think that part is fairly normal for an amp of this type/vintage. The crackles though... I think icastle has it spot on. If those checks don't work, take it to be looked at, these amps are from an era when stuff was designed to be repaired, not replaced. You might find its just an aged connection, or indeed the jack socket. [b]But I would also reiterate, its not good practice to turn the amp on/off without a speaker connected. Its a surefire way to shorten the life of the amp![/b] Quote Link to comment Share on other sites More sharing options...
Charliegibbons Posted January 30, 2013 Author Share Posted January 30, 2013 Thanks for all the help! Some great things to look into. First things first, the second part of my question was easily remedies when I tested the brand new lead I bought and found it was defective so I went and got it replaced today. There's a nice little example of Occam's razor in action! Quote Link to comment Share on other sites More sharing options...
icastle Posted January 31, 2013 Share Posted January 31, 2013 [quote name='Charliegibbons' timestamp='1359561740' post='1956692'] Thanks for all the help! Some great things to look into. First things first, the second part of my question was easily remedies when I tested the brand new lead I bought and found it was defective so I went and got it replaced today. There's a nice little example of Occam's razor in action! [/quote] Excellent, you're half way there then. Quote Link to comment Share on other sites More sharing options...
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