Hello everyone!
I have been given a Laney RBW300 to check (I'm not a professional but I have electronics as a passion e built some amps, mainly tubes).
The amp was recently used during a concert, I was not present but the issue was described as a temporary swing in volume (getting low then back up etc).
When I've received the amp it had the MV knob at 10.
I've turned it on and before connecting a bass it was making at "midrange sound".
Then after connecting the bass (and lowering the volume to human levels) that sound disappeared and now it doesn't do it even without any instrument connected and volume at max.
That's it I was unable to reproduce the problem, but I've just played for 20 minutes or so (the amp was used much longer during the concert).
Now, I've found this vid
So I'll clean all jacks and pots but I was wondering:
Can this (up and downs of volume) be caused by old electrolyct caps too? Not able to keep the powersupply voltage during long sessions at max volume?
Thanks
I have been given a Laney RBW300 to check (I'm not a professional but I have electronics as a passion e built some amps, mainly tubes).
The amp was recently used during a concert, I was not present but the issue was described as a temporary swing in volume (getting low then back up etc).
When I've received the amp it had the MV knob at 10.
I've turned it on and before connecting a bass it was making at "midrange sound".
Then after connecting the bass (and lowering the volume to human levels) that sound disappeared and now it doesn't do it even without any instrument connected and volume at max.
That's it I was unable to reproduce the problem, but I've just played for 20 minutes or so (the amp was used much longer during the concert).
Now, I've found this vid
So I'll clean all jacks and pots but I was wondering:
Can this (up and downs of volume) be caused by old electrolyct caps too? Not able to keep the powersupply voltage during long sessions at max volume?
Thanks
No.
Broken/parts do not self heal at random, then get sick again in minutes.
I expect some contact problem, say worn/dirty pots - jacks - internal connectors, etc.
Of course, if no problem is visible on Service bench, a common thing to happen, no diagnosing or repair can be carried, just guessing.
But some contact cleaner (NOT WD40) at strategic points never hurts, so ...
Also, your friend´s amp might have been very hot at the moment, and some thermal protection would act ... although I´d expect speaker to cut off, not just lose volume.
Broken/parts do not self heal at random, then get sick again in minutes.
I expect some contact problem, say worn/dirty pots - jacks - internal connectors, etc.
Of course, if no problem is visible on Service bench, a common thing to happen, no diagnosing or repair can be carried, just guessing.
But some contact cleaner (NOT WD40) at strategic points never hurts, so ...
Also, your friend´s amp might have been very hot at the moment, and some thermal protection would act ... although I´d expect speaker to cut off, not just lose volume.
Only read your post #1, so apologies if I'm duplicating, but:
Are there any IC's in the signal path that are in sockets?
Sadly I had a long-standing problem with a Moog parametric that was nothing more than an inexpensive IC socket that (small wonder) failed after 30-some years. And it was highly intermittent! All it took was a slight loss of spring pressure on the IC leg. Ooo-oo-oof !
Just noticed -- please don't use WD-40 on electrical things.
And, No, electrolytic caps do not generally exhibit the symptom you described.
Cheers
Are there any IC's in the signal path that are in sockets?
Sadly I had a long-standing problem with a Moog parametric that was nothing more than an inexpensive IC socket that (small wonder) failed after 30-some years. And it was highly intermittent! All it took was a slight loss of spring pressure on the IC leg. Ooo-oo-oof !
Just noticed -- please don't use WD-40 on electrical things.
And, No, electrolytic caps do not generally exhibit the symptom you described.
Cheers
Sorry but to be sure.... I dont mean normal WD40 I mean the one specific for electric connections
Are you warning me about this one or the normal one?
I'm not at home right now but I dont remember any IC socket but I will check for sure, thanks!
Well, 😱 -- you've shown me something I was not aware of -- I had no idea they broadened the brand to include Contact Cleaner.
Cheers
Cheers
The WD40 company has 35 products now, probably more soon. Anything you can put in a spray can, plus 5 gallon drums of stuff. Salad dressing? Sex-lube? Whipped cheese? Party balloon gas?
It's not just denatured kerosene any more.
"WD-40" has to be one of the 10 most recognized consumer brands, right after Coke and Toyota. WD-40 has distribution into EVERY large and small hardware and most general-goods stores in the US. Unlike 'Rick's Rocket Oil' with Rick cold-calling thousands of outlets and then struggling with delivery, WD-40 just puts a check-box on an order form and they have a new product, and half of retailers will take it un-tasted.
It's not just denatured kerosene any more.
"WD-40" has to be one of the 10 most recognized consumer brands, right after Coke and Toyota. WD-40 has distribution into EVERY large and small hardware and most general-goods stores in the US. Unlike 'Rick's Rocket Oil' with Rick cold-calling thousands of outlets and then struggling with delivery, WD-40 just puts a check-box on an order form and they have a new product, and half of retailers will take it un-tasted.
Yes, specific contact cleaner is fine, of course.
It´s just that everybody and his brother plain squirts normal "lubricant oil" WD40 onto contacts (not that bad) and potentiometers (horrible).because they have it within arm´s length.
Worst is that it "works" (most of the time) because of the built-in solvents but it leaves behind a ton of oil, which makes an ugly paste with crud, dust, lint, cigarette ash and what-not was there causing trouble.
Solvents not made for that purpose sometimes can unglue resistive carbon strip from phenolic backing, and it flakes out in pieces 😱 ... you don´t want that.
Even pulling a connector and reinserting it in the same socket often scratches away that.
It´s just that everybody and his brother plain squirts normal "lubricant oil" WD40 onto contacts (not that bad) and potentiometers (horrible).because they have it within arm´s length.
Worst is that it "works" (most of the time) because of the built-in solvents but it leaves behind a ton of oil, which makes an ugly paste with crud, dust, lint, cigarette ash and what-not was there causing trouble.
Solvents not made for that purpose sometimes can unglue resistive carbon strip from phenolic backing, and it flakes out in pieces 😱 ... you don´t want that.
Check/clean any metal to metal surface contact, atmosphere, meaning oxygen, weird gases,moisture, are all corrosive to a degree and along years oxidize or attack them, destroying contact.
Even pulling a connector and reinserting it in the same socket often scratches away that.
So, the problem was worse than I thought.
After cleaning and let it dry for 2 days (just to be sure lol) I've just let played the amp at max volume for a while and it suddenly stopped working... I've immediately switched it off
I've tryed to switch it on with a variac to gradually rise the main voltage but I've at very low main voltage (and front panel volume to 0) I hear a buzzing sound from the speaker
With an oscope on the output I see a 100hz signal.
I put here the schematics I've found online.
Any suggestion would be helpful
After cleaning and let it dry for 2 days (just to be sure lol) I've just let played the amp at max volume for a while and it suddenly stopped working... I've immediately switched it off
I've tryed to switch it on with a variac to gradually rise the main voltage but I've at very low main voltage (and front panel volume to 0) I hear a buzzing sound from the speaker
With an oscope on the output I see a 100hz signal.
I put here the schematics I've found online.
Any suggestion would be helpful
Disconnect speakers and measure DC at speaker out.
Up to, say,100 milli Volts (0.1V DC) is fine, but I fear you have a much higher vootge.
If so, power amp is blown and it will also blow speakers.
Measure and post values.
Build a lamp bulb limiter for safe troubleshooting, use a 40-75W filament bulb (not LED, CFL, etc.); still available as a "high efficiency" type, showing a small olive sized quartz bulb inside of a normal lightbulb "bottle".
Up to, say,100 milli Volts (0.1V DC) is fine, but I fear you have a much higher vootge.
If so, power amp is blown and it will also blow speakers.
Measure and post values.
Build a lamp bulb limiter for safe troubleshooting, use a 40-75W filament bulb (not LED, CFL, etc.); still available as a "high efficiency" type, showing a small olive sized quartz bulb inside of a normal lightbulb "bottle".
This is what I see under the poweramp pcb
BTW the fuses are ok 🤔
Wait, you mean with full mains or via variac?
BTW the fuses are ok 🤔
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When you hear the buzz, measure DC; is 100mV or less leave speaker connected; fi 1V or higher disconnect it and remeasure.I've tryed to switch it on with a variac to gradually rise the main voltage but I've at very low main voltage (and front panel volume to 0) I hear a buzzing sound from the speaker
With an oscope on the output I see a 100hz signal.
Show a screen capture or picture so we also see that waveform.
Indicate values (say, 1 Vpp or whatever).
Yes, that will do fine.
We need a small piece of red hot tungsten wire inside, which exhibits PTC current limiting effects: the more current through it, the hotter it gets, it drops more voltage, which opposes current increase.
If/when these disappear, we can still use pencil quartz lamps, although mounting is slightly more inconvenient.
We need a small piece of red hot tungsten wire inside, which exhibits PTC current limiting effects: the more current through it, the hotter it gets, it drops more voltage, which opposes current increase.
If/when these disappear, we can still use pencil quartz lamps, although mounting is slightly more inconvenient.
So,
I've connected the oscope to the output (speakers connected), and a voltmeter on the powersupply +rail.
I've then started and begun increasing voltage through the variac (passing through bulb limiter of course).
This is what I see with just 2V on the V+ rail.
As you see the buzzing sound is very audible already (amp volume is at 0)
I'm in Italy so my wall voltage is 50hz, and here is a 100hz wave
I've connected the oscope to the output (speakers connected), and a voltmeter on the powersupply +rail.
I've then started and begun increasing voltage through the variac (passing through bulb limiter of course).
This is what I see with just 2V on the V+ rail.
As you see the buzzing sound is very audible already (amp volume is at 0)
I'm in Italy so my wall voltage is 50hz, and here is a 100hz wave
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Ok, with just 2V on rails,amplifier is not even ON.
Disconnect speaker for now (we don´t need it at this moment, we can see the output voltage) and rise variac voltage so as to have some real world rails, at least +/- 15 or 20V each, or as high as possible (30-40V?) ... what do you see and measure at the output?
Weird almost sinusoidal waveform so far, I would have expected sawtooth ripple or rectified sinewave or even raw 50 Hz ... not that.
But repeat testing as suggested.
Disconnect speaker for now (we don´t need it at this moment, we can see the output voltage) and rise variac voltage so as to have some real world rails, at least +/- 15 or 20V each, or as high as possible (30-40V?) ... what do you see and measure at the output?
Weird almost sinusoidal waveform so far, I would have expected sawtooth ripple or rectified sinewave or even raw 50 Hz ... not that.
But repeat testing as suggested.
Ok I'll try that, hoping to not burn anything.
The rail I think would get to more or less 50V, transoformer gives 36Vrms for windings.
In the meantime I post some pics of the power board I've taken
This joint looks quite burned... lol (also the board has a lot of white paste, flux I guess, around joints)
Joint is for -rail reservoir cap
EDIT. Also the joint for the other reservoir looks... "cracked"?
If that cap is bulging (look like) it needs replacing, do the other one while your at it. You might want to increase there temp. rating, there right next to the heat sink of an amp that runs at high power. Like full volume.
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I think it's only the reflection...
Here's another pic
To me they look completely flat on top