why is the negative rail so low? that spells trouble...
all your voltage measurements are with respect to chassis ground?
all your voltage measurements are with respect to chassis ground?
Its just a light hum for about 10 seconds on mine.
Valves can take a while to settle down.
If the hum is still there after about 30 seconds I would suspect something is wrong.
You could attach your scope then power on for a little while and then power off before any damage is done. As others have said disconnect the speaker in case there is DC on the output. DC on output probably means SS amp is faulty.
I think it is a chip amp on mine but I haven't had it apart to see.
My Marshall 100watt amp is definitely a chip amp, I had to take it apart to replace a noisy cooling fan.
YES, see above.
You lost filtering on the -LT rail, so either its cap (2200x50) is open, has a cracked leg or cracked solder.
Start by resoldering it, add a little fresh solder.
If that does not solve it, get a new one.
Oh!!, and as repeatedly said above , you have a non-limiting bulb, it's way too large, will happily pass enough current to fry your amp and then some.
40/60W would be way more approppriate.
Ok I'll replace the bulb with a 60W incandescent. Can you explain a little more about what you mean by a non-limiting bulb? I was under the impression that all incandescent bulbs provided some current limiting, and I was hoping to cover all of my bases (and future amp wattages) by purchasing a 250W bulb.
I'll resolder C12 and recheck the rails. If that doesn't fix anything I'll replace it.
I'll resolder C12 and recheck the rails. If that doesn't fix anything I'll replace it.
JM what you mentioned in your first reply (post #2) a shorted output transistor could still be a possibility.
also, but less likely, is a failed diode in the bridge rectifier.
Cheeto have you looked at the solder/foil side of the pcb to verify it's not solder joint problems?(assuming you've dis-assembled it that far)
also, but less likely, is a failed diode in the bridge rectifier.
Cheeto have you looked at the solder/foil side of the pcb to verify it's not solder joint problems?(assuming you've dis-assembled it that far)
the wattage you select is what gives the "dim bulb tester" it's current limiting ability.
set the limit too high you risk frying things.
it's roughly equivalent to a fuse but rather then going open when the current limit is exceeded it robs the "device under test" of voltage.
set the limit too high you risk frying things.
it's roughly equivalent to a fuse but rather then going open when the current limit is exceeded it robs the "device under test" of voltage.
simul posting
the lamps you linked to in your last post don't say what the filament or type is.
dim bulb testers rely on "hot wire" filament lamps like tungsten (standard incandecent) for their operation.
the lamps you linked to in your last post don't say what the filament or type is.
dim bulb testers rely on "hot wire" filament lamps like tungsten (standard incandecent) for their operation.
Hi,
You can do a simple test by checking the +/- rails voltage with the Voltmeter in AC . If you have a high voltage reading means you need to replace the capacitors.
You can do a simple test by checking the +/- rails voltage with the Voltmeter in AC . If you have a high voltage reading means you need to replace the capacitors.
Replaced 250W flood bulb with 60W bulb outlined in post #27. Bulb now glows dimly when amp powered on and slightly brighter during initial power up (progress!).
Took out power amp PCB and inspected solder joints, especially around C12 and bridge rectifier. C12 negative leg looked a little iffy, so I reflowed it and added a little fresh solder (repeated for 3 other legs of C11 and C12).
Remounted PCB and checked rails at R33 and R36 again (wrt chassis ground). Rails measured +/- ~33 VDC (progress!).
Checked DC at speaker output and measured ~0VDC during idle and shut down (hurray!).
If no one sees any red flags, I'm going to plug in my speaker cab and check status.
Thanks to everyone for the guidance and expertise so far.
Took out power amp PCB and inspected solder joints, especially around C12 and bridge rectifier. C12 negative leg looked a little iffy, so I reflowed it and added a little fresh solder (repeated for 3 other legs of C11 and C12).
Remounted PCB and checked rails at R33 and R36 again (wrt chassis ground). Rails measured +/- ~33 VDC (progress!).
Checked DC at speaker output and measured ~0VDC during idle and shut down (hurray!).
If no one sees any red flags, I'm going to plug in my speaker cab and check status.
Thanks to everyone for the guidance and expertise so far.
Hello Cheeto333,
So you see, step by step and we can follow what you do and eventually give the correct tips.
Still I like to ask you to have a closer look at the foil side of the pcb.
Inspect the solder joints at all the jacks, potentiometers and while it's there bare naked, all other solder joints. These amps have the tendency to have , at the said places, bad solder joints.
Good to see that it's up and running again.
Cheers,
So you see, step by step and we can follow what you do and eventually give the correct tips.
Still I like to ask you to have a closer look at the foil side of the pcb.
Inspect the solder joints at all the jacks, potentiometers and while it's there bare naked, all other solder joints. These amps have the tendency to have , at the said places, bad solder joints.
Good to see that it's up and running again.
Cheers,
a magnifiying glass is a helpful tool in finding bad/cracked solder joints.
if the voltages have come back to "normal" then hooking up a speaker(i harvest old drivers to use as "sacraficial lambs") to the output to see if the buzz/hum is gone is next.
note that the dim bulb tester's brightness may increase with a load hooked up.( this is the time i move to a higher wattage lamp)
if the voltages have come back to "normal" then hooking up a speaker(i harvest old drivers to use as "sacraficial lambs") to the output to see if the buzz/hum is gone is next.
note that the dim bulb tester's brightness may increase with a load hooked up.( this is the time i move to a higher wattage lamp)
i also am curious to know if you gained a little more understanding as to why your original lamp "wattage" was a bad choice.
Ok I'll look closely at the jack and pot solder joints on the PCBs. Probably wouldn't hurt to reflow all of them. I'll check functionality with my speaker cab once I do.
To your question, Turk, I understand now that a larger wattage bulb can pass enough current to damage an 100W amp if there is a short. I'll use the 60W bulb unless dealing with a >~150W amp.
One question I was thinking about for the future: how does one check DC on the speaker output for an all-tube amp, since it's a bad idea run them without a load?
To your question, Turk, I understand now that a larger wattage bulb can pass enough current to damage an 100W amp if there is a short. I'll use the 60W bulb unless dealing with a >~150W amp.
One question I was thinking about for the future: how does one check DC on the speaker output for an all-tube amp, since it's a bad idea run them without a load?
To answer your last question:
99.99% of all tube amps have a transformer between the supply voltage and the load.
There is no dc voltage on the outputs.
And you have to connect a load to a tube amp; either a speaker (and become deaf) or a resistive load.
And clean your pots to. Use a deoxidant spray something.
99.99% of all tube amps have a transformer between the supply voltage and the load.
There is no dc voltage on the outputs.
And you have to connect a load to a tube amp; either a speaker (and become deaf) or a resistive load.
And clean your pots to. Use a deoxidant spray something.
For tube guitar amps (which seem to always use an output transformer, and transformers work on AC), it's pretty hard to get DC on the speakers unless something goes wrong with a negative feedback leg, I think.
EDIT: As Tarzan said as I typed....
But, no doubt the amp experts here have come across this very problem at some time ??
BTW, it seems to me that 'straight DC' (not pulsating) wouldn't cause a hum or buzz in the speakers anyway -just a single 'thunk' like you get when testing speaker phasing with a dry cell or battery.
as an educational exercise with a cheap bipolar supply and amp; removing a filter cap and hearing what diode noise/rectification "sounds" like (all the while running on a dim bulb tester) is worthwhile, once you've heard that kind of noise you'll know it instantly the next time you encounter an open filter or bad solder joint on it.
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It depends how good the power supply is.
I have a test rig for testing amplifiers after I have built them.
Occasionally something goes wrong and DC is output.
There is usually a thump followed by a light humming sound.
The speaker loading the power supply causes a little hum.
The original complaint was buzzing/humming?
But the first bunch of diagnoses seemed to focus on "DC on the outputs", which confused me.
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