I couldn’t find this subject in the forums so thought I’d start it because I’m at a bit of an impasse with a recently bought amp.
Do we have any technically savvy members who might be able to offer assistance to those of us who are competent enough to handle a multimeter and soldering iron? I wouldn’t encourage folk to try such repairs if they don’t have experience working with electricity, and the caveat here is that any advice is checked and all responsibility is on the person doing the repair.
With that said…
I’ve just bought a HiWatt B40/12 (not the legendary era) that has a constant hum when switched on: it isn’t affected by volume or any other settings and is constant with or without an instrument plugged in. I knew this when buying it so got a good price and an amp that works well at volume.
My first port of call was YouTube (of course) but most vids were about ground loops, which was always about instruments/cables. I found one vid mentioning the possibility of two main capacitors. As a simple & cheap fix I replaced those, but it hasn’t made any difference.
When I did my basic electronics training (aged 16-1/2) I learned all about capacitors from an evil genius of a lecturer: he had a massive capacitor (only slightly smaller than a Coke can) that he had named Cecil, and he would charge it on the back of a CRT (cathode ray tube) tv then discharge it by shorting the terminals against a metal plate or cabinet. Basically spot welding it.
He also taught us about designing, printing and constructing circuit boards by building boxes that we called “strength testers”. You would squeeze the sides and get a small shock, but he showed us how the faster you released it the greater the shock - back EMF - so we would demonstrate it by squeezing and holding it, thus only getting a small tingle. Our victims - sorry, friends - wouldn’t spot the trick and got a nasty nip.
We also used to wind the legs around the body of smaller caps, charge them, then throw them over the partition between us and the next department: invariably somebody over there would either catch the mysterious flying object - if we shouted that a sweetie was coming over - or pick it up off the bench/floor. We always knew we’d found a target when we heard the scream.
Yeah, so I know not to let a capacitor discharge in your hand - and the bigger they are the more painful/dangerous.
Last nights YT research lead me down the AC hum path. Or possibly transformer hum - though I don’t think that transfers to the speaker.
I know I can buy a device to (probably) negate the ground loop and even AC interference) but at over £100 I’m not in a position to buy one - plus it’s a £30 amp (£50 would be market value) so not inclined to spend money on something that shouldn’t be happening.
I had the thing out of the cabinet and checked all solder joints, screws and cable connectors, but couldn’t find anything obvious. I might take it out again and start checking values on some components. It’s years since I did any electronics work so I only remember the basics.
Am I right in thinking that I can get a proper meter reading from individual components while still in situ?
No because they kinda cumulate, like if you have somewhere two resistors or two capacitors in parallel (which happens all the time) you won’t be able to read a correct value for each component.
If you have an oscilloscope, you can measure the signal path from the beginning of the preamp and so on, until you see where the noise appears. It would help you locate where the issue can be on the circuit.
I don’t really know anything about this amp but from pictures online it looks like it has two inputs. High and low. My first guess would be to look at how the inputs are connected and maybe the high gain input is always on and picking up something. Don’t know if there are some contacts to switch the inputs but there might be. Maybe there’s an easy way to disconnect the high input too but since I haven’t seen the amp or the schematic I can’t really tell. Post some pictures if you do end up taking it apart again.
Does it work on battery ? I could be interested because mine is an USB one that needs my laptop, it’s very cool and useful but not always the most portable.
Yep. My specific one I would not recommend but there were plenty around that price IIRC. I just googled a bunch of random ones in that price range. This one is rechargeable:
It appears to be a mass produced generic one sold under many brands, as one does with cheap gear from China (which there is nothing wrong with):
I had no idea oscilloscopes were so cheap - and small - these days. If I’d had known I’d have bought a multimeter with built-in oscilloscope when I last bought a new multi.
