Sparky Mark Posted yesterday at 17:45 Posted yesterday at 17:45 20 minutes ago, Rosie C said: My friend is a knowledgeable amp tech She explained it to me (though I've since been to Google to make sure I get it right here)... The 50Hz AC mains is rectified, converting it into a series of positive DC pulses, and the capacitors smooth the pulses into a steady DC voltage. If the capacitor isn't working properly, the ripples aren't fully smoothed and you can hear a ripple frequency in the speakers. But because it's between the peaks of the rectifier wave the frequency has been doubled to 100Hz. I found this drawing which sort of shows the frequency doubling... That's good information; no disrespect intended towards your friend. I recently replaced some swollen but still working capacitors in a 1980s NAD amplifier because I could see they were damaged. The good news is that new capacitor technology has moved on and you should be able to find replacements in smaller packages. 2 Quote
Rosie C Posted yesterday at 17:49 Posted yesterday at 17:49 1 minute ago, Sparky Mark said: That's good information; no disrespect intended towards your friend. I recently replaced some swollen but still working capacitors in a 1980s NAD amplifier because I could see they were damaged. The good news is that new capacitor technology has moved on and you should be able to find replacements in smaller packages. None taken! Nice work! I did look on ebay earlier for new capacitors, but immediately realised that I don't know what sort of pins they have soldered to the PCB - some of the new ones have wires, while some of the larger values have flat blades - so I can't progress until we open the amp again and this time look at the underside of the PCB. 1 Quote
Sparky Mark Posted yesterday at 18:06 Posted yesterday at 18:06 (edited) 3 hours ago, Rosie C said: None taken! Nice work! I did look on ebay earlier for new capacitors, but immediately realised that I don't know what sort of pins they have soldered to the PCB - some of the new ones have wires, while some of the larger values have flat blades - so I can't progress until we open the amp again and this time look at the underside of the PCB. Cheers. Hopefully you won’t need to do too much disassembly to get to the underside of the PCB. Old solder joints and connectors can be rather fragile and crack when disturbed, causing more issues. Desoldering large caps can also be a bit of a PITA, due to needing a fair amount of energy from the soldering iron to melt the solder sufficiently to remove without lifting the PCB track/pad. You are very lucky to have a friend with the knowledge to help fix your amp. Edited 23 hours ago by Sparky Mark Quote
tauzero Posted 18 hours ago Posted 18 hours ago 9 hours ago, Rosie C said: While we had the case open, I took a photo of the power supply capacitors. There's a bit of 100Hz hum on the speakers and we're suspecting dried out capacitors. There's a bit of work to do to remove them though. That's either "5 go mad with the heated glue gun" or there was a bukkake party while they were building the amp. Quote
Rosie C Posted 11 hours ago Posted 11 hours ago 7 hours ago, tauzero said: That's either "5 go mad with the heated glue gun" or there was a bukkake party while they were building the amp. I know! For bonus fun there's that little capacitor beside them, just asking to be damaged Quote
SimonK Posted 2 hours ago Author Posted 2 hours ago Those capacitors were the first thing we changed on the GP12 (picture from a few pages back) but it didn't stop the hum... 1 Quote
Rosie C Posted 2 hours ago Posted 2 hours ago 5 minutes ago, SimonK said: Those capacitors were the first thing we changed on the GP12 (picture from a few pages back) but it didn't stop the hum... That's a shame! I'll probably still give it a go. It wouldn't be a problem on stage, but I only need the main volume knob at '1' for home practice and it's a little annoying. Quote
DGBass Posted 1 hour ago Posted 1 hour ago The GP12's are reasonably complicated amps. Probably why I prefer GP7's but I do have a series 6 GP12 and a 250 GP12 SMX, both hum free. If I had a penny for every minute I spent trying to track down a hum on a GP12 I'd probably have quite a few quid by now. There are at least three power supply points that have smoothing caps, the output board, the tube circuit, and the pre-amp board. Dublier caps on the output boards are in my experience very durable. Unless they are obviously leaking or bulging they don't usually cause problems even after 20-30 years. Not to say they can't cause issues. Worn dirty jacks and loose or tarnished ground points are very common noise generators on these amps after many years and IMHO always worth a check, especially the back board, before getting into scraping hot glue from boards and components! 🙂 1 1 Quote
Rosie C Posted 23 minutes ago Posted 23 minutes ago Thanks for that - it's definitely worth trying, and would fit with the suspicions that the foot pedal switches need cleaning - if they do, maybe other jacks and switches do Quote
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