Yesterday, I was given a nice little bass practice amp, a US made Peavey MicroBass, probably a 1994. It precisely matches the photo on the 1994 manual and I've read the imports do not. Extremely heavy (30lbs) 1x8" combo. The tolex work is stellar. The amp works and it's surprisingly loud for 20W solid state.
What worries me is a faint hum from the chassis while the power button & indicator are OFF. It only stops when I unplug it from the wall. It's audible only when I put my ear to the front or back of the chassis. Not the speaker or headphones [which do hiss slightly at idle with power switched ON and no instrument plugged in - that's normal, right?]. Presumably a transformer hum? But how, unless there's power bypassing the switch?
I haven't pulled the chassis, because I'm concerned about charged capacitors. Should I be, it's 20w solid state? At any rate, I'm no tech or EE. All the problems I know to look for are bulging electrolytic caps and anything charred or loose.
I have attached a schematic, though it might as well be the Code of Hamurabi to me. FWIW: I've read that import versions don't always follow Peavey's schematics.
Like I said, the amp does work. With the mids turned down it sounds good. So, if I'm describing a normal situation, please let me know.
Thank you!!
What worries me is a faint hum from the chassis while the power button & indicator are OFF. It only stops when I unplug it from the wall. It's audible only when I put my ear to the front or back of the chassis. Not the speaker or headphones [which do hiss slightly at idle with power switched ON and no instrument plugged in - that's normal, right?]. Presumably a transformer hum? But how, unless there's power bypassing the switch?
I haven't pulled the chassis, because I'm concerned about charged capacitors. Should I be, it's 20w solid state? At any rate, I'm no tech or EE. All the problems I know to look for are bulging electrolytic caps and anything charred or loose.
I have attached a schematic, though it might as well be the Code of Hamurabi to me. FWIW: I've read that import versions don't always follow Peavey's schematics.
Like I said, the amp does work. With the mids turned down it sounds good. So, if I'm describing a normal situation, please let me know.
Thank you!!
Big difference between hiss and hum.
Simple amplifier likely doesnt have a mute circuit or output relay for startup or shutdown.
So yes many simple practice amplifiers can pop, on turn on or turn off
Or can hum or hiss after shut down because it takes awhile for the power supply capacitors to drain.
Dont know if TDA2040 is self muting.
Otherwise a strange ground loop can occur within the wiring
Either house or amp chassis. Rather rare. Turned off
More notable with florescent lights or any noisy device on same circuit.
Have you tested on another outlet that is for sure not on same house breaker showing issues.
Simple amplifier likely doesnt have a mute circuit or output relay for startup or shutdown.
So yes many simple practice amplifiers can pop, on turn on or turn off
Or can hum or hiss after shut down because it takes awhile for the power supply capacitors to drain.
Dont know if TDA2040 is self muting.
Otherwise a strange ground loop can occur within the wiring
Either house or amp chassis. Rather rare. Turned off
More notable with florescent lights or any noisy device on same circuit.
Have you tested on another outlet that is for sure not on same house breaker showing issues.
Since there's no standby transformer to buzz or hum, looks like it could be a faulty mains switch. Or faulty mains voltage selector switch. Definitely a fire hazard!
If your line and neutral were reversed at the outlet you are plugged into C29 can draw current even with the power switch off. It is supposed to be a VERY small current.. and silent. If C29 (or 30) were making ANY noise or under 120V/60 Hz excitation they are faulty, a fire hazard, and should be replaced. Try taking C29 out - it is safe without it, but might not be full FCC-compliant. If it quits making the noise, check your house wiring! That would be a safety hazard if that is what is happening. Then replace C29 and 30 with new Panasonic “X capacitors”. There were some brands (ie, Rifa) that liked to go up in smoke. Panasonic are cheap, available, and won’t short unless there is a direct lightning strike on the line.
I set about following all of the great advice I received. I pulled the chassis to look for the offending caps and Well, well, well…I didn't expect this.
Based on the schematic, I expected to see C29 & C30 near the mains connection, but no. Unless they are the pair of orange drops. And the PCB is labeled on the underside. The electrolytic caps look good to me, however this amp may have been stored a long time. They can dry out without looking blown, afaik. On the other hand, how can the amp still be working? Since I'd have to pull the PCB & remove each cap to test it, I guess I might just order replacements for them all before unbolting the PCB & disturbing the heatsink paste. Any advice on replacing the transformer? I haven't noticed people getting picky about transformers on solid state amps.
Here's a pic of the rest of the board. Sorry, I should've pulled the chassis before asking. Thanks!
Based on the schematic, I expected to see C29 & C30 near the mains connection, but no. Unless they are the pair of orange drops. And the PCB is labeled on the underside. The electrolytic caps look good to me, however this amp may have been stored a long time. They can dry out without looking blown, afaik. On the other hand, how can the amp still be working? Since I'd have to pull the PCB & remove each cap to test it, I guess I might just order replacements for them all before unbolting the PCB & disturbing the heatsink paste. Any advice on replacing the transformer? I haven't noticed people getting picky about transformers on solid state amps.
Sorry, I was editing the post for over 30 minutes and got blocked. Here is the edited text:
I set about following all of the great advice I received. I got the same hiss plugging into 3 different circuits. One of which definitely had nothing else plugged in and the receptacle tester showed it was wired "correct". We rewired for a kitchen remodel around 2000 and it all passed inspection. FWIW, I recorded the hiss and even with massive preamp & software gain, the only peak I could tease out was 60hz. There is no voltage selector on this unit. The mains switch has full continuity when ON and 0 when OFF. That's good, right?
I pulled the chassis to look for the offending caps 29 and 30 and well, well, well…I didn't expect this. What could do that to a transformer?!
Based on the schematic, I expected to see C29 & C30 near the mains connection, but no. Unless they are the pair of orange drops. But, I thought those type don't age out. I use Sprague orange drops in all my guitars. And the PCB is labeled on the underside. So I don't know where 29 & 30 are. The electrolytic caps look good to me, however this amp may have been stored a long time. Electrolyte can evaporate in storage. How can the amp still be working? It actually sounds pretty good. Except my p-bass pickup hums very loud, if I take my hands off the strings and it really buzzes near my cell phone. Same issues with my other tube & SS amps, but not nearly to the same degree.
I set about following all of the great advice I received. I got the same hiss plugging into 3 different circuits. One of which definitely had nothing else plugged in and the receptacle tester showed it was wired "correct". We rewired for a kitchen remodel around 2000 and it all passed inspection. FWIW, I recorded the hiss and even with massive preamp & software gain, the only peak I could tease out was 60hz. There is no voltage selector on this unit. The mains switch has full continuity when ON and 0 when OFF. That's good, right?
I pulled the chassis to look for the offending caps 29 and 30 and well, well, well…I didn't expect this. What could do that to a transformer?!
Based on the schematic, I expected to see C29 & C30 near the mains connection, but no. Unless they are the pair of orange drops. But, I thought those type don't age out. I use Sprague orange drops in all my guitars. And the PCB is labeled on the underside. So I don't know where 29 & 30 are. The electrolytic caps look good to me, however this amp may have been stored a long time. Electrolyte can evaporate in storage. How can the amp still be working? It actually sounds pretty good. Except my p-bass pickup hums very loud, if I take my hands off the strings and it really buzzes near my cell phone. Same issues with my other tube & SS amps, but not nearly to the same degree.
How did you measure the switch? They can break down under voltage in the off position, especially if you hear a hissing sound. It might measure fine at DC on your meter. wg_ski is right also, it could be C29 if polarity was reversed. Since mains goes to the PCB, it could also be tracking underneath the board. You'd need to disassemble it to get a look at that side of the PCB.
Hmm, interesting. You're right, the way I tested the switch wouldn't detect continuity between mains and the circuit in the off position. With regard to polarity, I thought the question was the polarity of my house wiring which is not reversed. Are you proposing that if the previous owner plugged into a bad wall socket it would have damaged something, so that the amp now hisses no matter where it's plugged in? Maybe fried a filter cap? But that wouldn't stop the amp from working, just make it a fire hazard?
And what about the transformer? The copper shielding came apart where it had been soldered together. It's a bass combo, so maybe the joint vibrated loose. But, one of the steel covers is also warped. Seems like heat damage to me. Which brings something to mind! This is a closed back combo. The chassis isn't vented. Nor is the speaker. No airflow at all. The speaker is mounted to the baffle from the front. Yes, the bare metal side of the spider screws onto the wood and the padded rim is exposed behind the grille cloth. The back of the speaker is further separated from the chassis by an internal baffle, with one little hole for the speaker wires. And the rear panel of the cab is integral to the enclosure. Five sides of the cube are sealed in one seamless sheet of tolex. Even the bottom. The PCB is stood off from the chassis floor, but has one transistor screwed to the center with heat transfer paste, which I think degrades over time. Heat can only radiate from that one point to the control panel & rear of the chassis. And when I pulled the chassis, there were brown cobwebs inside. The outside & inside were plenty dusty and I assumed the brown was from the dust. Now I wonder if the brown was from heat!
I hope I'm wrong, because I like this little amp. But, an EE friend at Boeing has previously mentioned to me that it's a bad idea to put circuitry in an unvented enclosure - even if the enclosure itself is designed to work as a heatsink. In his opinion, that never works in the long run. Maybe because the heat paste degrades or the metal warps from heat and loses contact. And I read a 20 year old thread in which Peavey's new QC EE said years before he joined the company he had a MicroBass and had rearranged it into a head/cab. He didn't say why. But, he said more than once in that discussion that he couldn't reveal proprietary info. I wonder if he was implying that this thing is a fire hazard as designed? This is just me thinking as I write. But, I must say my 2006 Peavey Valveking Royal 8 has 2 transformers, both mounted so they hang down underneath the chassis with the tubes, exposed to the air. It's an open back combo. And my other bass combo, a Fender Rumble 100 V3 is a closed back. But the back of the chassis is vented and there's a warning label saying not to block the vents.
And what about the transformer? The copper shielding came apart where it had been soldered together. It's a bass combo, so maybe the joint vibrated loose. But, one of the steel covers is also warped. Seems like heat damage to me. Which brings something to mind! This is a closed back combo. The chassis isn't vented. Nor is the speaker. No airflow at all. The speaker is mounted to the baffle from the front. Yes, the bare metal side of the spider screws onto the wood and the padded rim is exposed behind the grille cloth. The back of the speaker is further separated from the chassis by an internal baffle, with one little hole for the speaker wires. And the rear panel of the cab is integral to the enclosure. Five sides of the cube are sealed in one seamless sheet of tolex. Even the bottom. The PCB is stood off from the chassis floor, but has one transistor screwed to the center with heat transfer paste, which I think degrades over time. Heat can only radiate from that one point to the control panel & rear of the chassis. And when I pulled the chassis, there were brown cobwebs inside. The outside & inside were plenty dusty and I assumed the brown was from the dust. Now I wonder if the brown was from heat!
I hope I'm wrong, because I like this little amp. But, an EE friend at Boeing has previously mentioned to me that it's a bad idea to put circuitry in an unvented enclosure - even if the enclosure itself is designed to work as a heatsink. In his opinion, that never works in the long run. Maybe because the heat paste degrades or the metal warps from heat and loses contact. And I read a 20 year old thread in which Peavey's new QC EE said years before he joined the company he had a MicroBass and had rearranged it into a head/cab. He didn't say why. But, he said more than once in that discussion that he couldn't reveal proprietary info. I wonder if he was implying that this thing is a fire hazard as designed? This is just me thinking as I write. But, I must say my 2006 Peavey Valveking Royal 8 has 2 transformers, both mounted so they hang down underneath the chassis with the tubes, exposed to the air. It's an open back combo. And my other bass combo, a Fender Rumble 100 V3 is a closed back. But the back of the chassis is vented and there's a warning label saying not to block the vents.
The transformer shield just came unsoldered because it got very hot. But if the amplifier is switched off and you still hear noise, that mean's it's not the transformer. It's the power switch or bad joints on the PCB underside. Both of these are fire risks, so don't continue to use this amplifier until the issue has been solved. Unplug it, then inspect the underside of the PCB. If there's nothing going on there then there isn't much left to blame except the mains switch.
That transformer didn’t get too hot. To get it THAT hot there would be other signs of damage. It looks like a poor assembly job. Those welded laminate transformers are ChEA-p (with emphasis on the ea, and a hard P at the end, because they really really really mean it). And they DO hum, it’s normal for them. What we need to do is find out WHY it’s doing it with the power supposedly off. Maybe somebody wired the power switch off the secondary instead of the primary (the way it’s supposed to in the schematic). Which would leave the primary energized all the time. That’s not dangerous per se, but it does waste electricity.
Thank you, wg_ski. I have just taken off the PCB. I'm not sure how to test whether the switch is wired backwards. It turns out to be a 3PST, 6 prongs to the PCB. S1 is the only switch on the schematic and it's a single pole, right? Nor do I find continuity between any pole and either mains wire, switched on or off. Yet, I find 0Ω between the mains, regardless if it's switched on or off. None of that makes sense to me.
As to the switch itself, it doesn't seem possible to mis-wire it. Each prong is soldered to its hole. I couldn't find any solder bridging on the board. So, the switch could only be reversed if the mains were reversed, which I can't tell. I DID find one anomaly with the switch! As illustrated below, I found 100kΩ across one pair in the off position. Is this the culprit?
PS: should I replace the transformer? Or re-solder the shielding?
As to the switch itself, it doesn't seem possible to mis-wire it. Each prong is soldered to its hole. I couldn't find any solder bridging on the board. So, the switch could only be reversed if the mains were reversed, which I can't tell. I DID find one anomaly with the switch! As illustrated below, I found 100kΩ across one pair in the off position. Is this the culprit?
PS: should I replace the transformer? Or re-solder the shielding?
Yep - they changed it. One pair of poles is going to the LED indicator. The others don’t have the required clearances for mains wiring. The secondary is indeed switched, leaving the transformer primary energized any time it’s plugged in. So it sits there, hums, and uses a watt or two, even when turned off.
The 100k is probably there to bleed off the charge in the caps after switch off.
The shielding is supposed to be a shorted turn outside the transformer core, to cancel any stray radiated field. If it’s open, it’s not working an could induce hum into the audio circuit, or any others nearby. Might be needed for FCC compliance. I’d just solder it back together.
The 100k is probably there to bleed off the charge in the caps after switch off.
The shielding is supposed to be a shorted turn outside the transformer core, to cancel any stray radiated field. If it’s open, it’s not working an could induce hum into the audio circuit, or any others nearby. Might be needed for FCC compliance. I’d just solder it back together.
Thanks again. Your answer comes as a relief. At least the amp is working as designed. I've been considering giving to a friend, but not if it's going to burn his house down. After I posted those pics I noticed that my PCB is dated 1993 and the schematic is 1987. No wonder they didn't match! Couldn't find a 93 schematic anywhere online. And I'm glad the switch isn't worn out. I couldn't find a replacement anywhere. And Digikey has like 78,000 types of pushbutton switch.
I am about to solder that shielding back on the transformer. Now I'm learning about cheap. The copper barely overlaps, no wonder the joint failed. I'm planning to lay a few strands of bare wire across the joint to give the solder something to grab. Unless anyone thinks that's a bad idea.
Also, there's room to add a mains switch to the back of the chassis. SPST toggle on the hot, 15amp/120VAC ($2.95 at Jameco, Digikey has no such thing). Presumably there's a reason nobody suggested that. The board grounds to the chassis. So, would turning off a panel-mount toggle short-out & go boom? Like I say, I'm used to signal wiring and turning anything off means sending current to ground. I don't know if that's how mains switching works at all.
I am about to solder that shielding back on the transformer. Now I'm learning about cheap. The copper barely overlaps, no wonder the joint failed. I'm planning to lay a few strands of bare wire across the joint to give the solder something to grab. Unless anyone thinks that's a bad idea.
Also, there's room to add a mains switch to the back of the chassis. SPST toggle on the hot, 15amp/120VAC ($2.95 at Jameco, Digikey has no such thing). Presumably there's a reason nobody suggested that. The board grounds to the chassis. So, would turning off a panel-mount toggle short-out & go boom? Like I say, I'm used to signal wiring and turning anything off means sending current to ground. I don't know if that's how mains switching works at all.
Done! I didn't feel like leaving off to wait for a switch. Maybe later, unless it's a bad idea. I soldered the transformer shielding with some wire buried under a big ugly wad of lead. And holy smokes, I was able to check my email on the phone standing 2 feet from the amp with no hands on the strings and heard no cell interference. [by comparison, my 2007 KRK VXT6 studio monitors go nuts unless the phone is clear outside the room] 60 cycle hum also is no louder than I get from my other bass & guitar amps! So satisfying. I'd never opened an amp chassis before.
Thank you!!!!!!!!!!
Thank you!!!!!!!!!!
I’m sure Digikey has a switch…. Just a matter of figuring out WHAT to put in their search engine to find it. But I guarantee it will cost more than three bucks. You can go down to a local hardware store and find one of these and mount it to the back panel. Don’t blame the messenger, I got it like this. Like Spock’s beard…. Gives it character.
Attachments
One would not expect an amplifier with an old school transformer power supply to interfere with a cell phone. Even if it’s putting out a lot of stray field - it’s 60 Hz and harmonics, not RF. But it might induce hum in a mic preamp that’s sitting on or next to it. Equally-cheap switch mode supplies can be filthy dirty RF wise.
Be proud. I was going to hide it on the back where the mains come in. But you've inspired me. Perhaps a pull-chain keyless nailed to the top, with a 60W 'indicator' light bulb. Seriously, though. A SPST on the black mains wire won't short circuit to the case when turned off?
Anyway, I meant the cell phone digital noise blasting thru the amp's speaker. I don't know if the amp interferes with the phone. I can check for that next time I get a spam call while playing the bass.
Anyway, I meant the cell phone digital noise blasting thru the amp's speaker. I don't know if the amp interferes with the phone. I can check for that next time I get a spam call while playing the bass.
OK, here's the finished product. I wanted the switch in front, so it wouldn't be forgotten. And it does the job. Off means off, now! Looks factory, huh? 😏
