Hi, I recently pulled my 212r out of storage, its been unused for about 3 years, I turned it on and got a loud hum, after a few seconds I turned it of, for the next few hours I tried to find any problems, found nothing, the next day I turned it on again and left it for a longer time, after about 4 seconds the hum disappeared , I plugged my guitar in and sure enough it worked perfectly, all settings, both channels, no problems, now when I turn it on, it produces that loud hum for 2 to 3 seconds and works well after that. I asked a tech what could possibly do this, he told me its more than likely the 2 x 4700uf electro's he said that they are probably drying out and take that little bit longer to charge, so replacing them could very well fix the problem.
Peter
Peter
Hi, definitely NOT the 4700uf caps, I replaced them and nothing changed,
I need to clarify my previous post.
Ok, turn amp on with NOTHING plugged in, I get a loud hum that does NOT go away.
When I turn amp on WITH guitar plugged in, the loud hum goes away after about 2 to 3 seconds.
I am guessing the Hum is to do with the "Mute" circuit.
The fact that the amp works well with a guitar plugged in tells me its not the output stage.
When the 2 inputs and the PA input have nothing plugged in, the "Mute" circuit is grounded, when a plug is inserted, it lifts the earth connection.
Which is why I thinks its to do with the "Mute" circuit, I f I have missed anything please let me know.
Regards Peter
I need to clarify my previous post.
Ok, turn amp on with NOTHING plugged in, I get a loud hum that does NOT go away.
When I turn amp on WITH guitar plugged in, the loud hum goes away after about 2 to 3 seconds.
I am guessing the Hum is to do with the "Mute" circuit.
The fact that the amp works well with a guitar plugged in tells me its not the output stage.
When the 2 inputs and the PA input have nothing plugged in, the "Mute" circuit is grounded, when a plug is inserted, it lifts the earth connection.
Which is why I thinks its to do with the "Mute" circuit, I f I have missed anything please let me know.
Regards Peter
Well, if you have reason to think the mute is the cause, explore the mute circuit. It probably isn't the jack switches. I don't have the schematic up, but is the mute that little circuit in the corner of the power amp area with three or four transistors? Start by looking at voltage levels through that circuit. Look for something that takes the same 2-3 seconds to settle.
Hi Enzo,
Thanks for your replies, Ok, I have gone over that part of the circuit several times over the weekend, all voltages are close to the spec, I haven't found anything out of the norm. Basically the amp doesn't Mute when turned on with nothing plugged in, which makes me wonder why it mutes when something is plugged in ......I know this isn't all that helpful but I am really clutching at straws here . I have read posts from others who seem to have the same prob, but no-one seems to know where to go from there.
Some just like to rubbish the amp, which I think is very unfair, hey but that's me
Peter
Thanks for your replies, Ok, I have gone over that part of the circuit several times over the weekend, all voltages are close to the spec, I haven't found anything out of the norm. Basically the amp doesn't Mute when turned on with nothing plugged in, which makes me wonder why it mutes when something is plugged in ......I know this isn't all that helpful but I am really clutching at straws here . I have read posts from others who seem to have the same prob, but no-one seems to know where to go from there.
Some just like to rubbish the amp, which I think is very unfair, hey but that's me
Peter
I think you are confusing "mute" with silence. You say it mutes when something is plugged in, but you also say the amp sounds OK after the first few seconds. If it sounds signal, then it is not muted. The amp can be muted and still make noise, but it ought not pass signal when muted. Of course what triggers the mute is a lack of input connection, so it is hard to say if it is mutted or not, A defect in the circuit that makes noise doesn't mean it is not muted, if you get me.
I agree it sounds like your mute system. What I was looking to test was not voltages per se, but changing voltages that followed your 3 second thing. Once the amp is ready and working, then I expect all the voltages will be OK too. What might be useful is watching for voltages changing in those first few seconds. It may require turning the amp on and off many times, because you only get 3 seconds each time.
But lets work on it with no input, so if I understand, the amp sits there humming. If we fix that, everything else will probably be OK.
The mute circuit is a couple transistors, and it works by turning Q8 off and on. Q8 completes the circuit to V- from the differential pair at the amp input. If Q8 is off the amp is muted. The mute pins turn off Q7, so the base of Q8 never turns on. Unground the mute line, and Q7 turns on, pulling up the base of Q8 and enabling the amp.
That is what should happen. Now while it is muted but humming, what voltage is on the base of Q8? Compare that to what is there once the unmuted amp settles in and works. When muted, we expect the base of Q8 to be pretty close to its emitter. If the mute doesn't completely turn off Q8, then the amp could be left partly functioning and stuck in a sort of half on condition, kinda like when you hear an amp noise at turn off.
Thermal sensor RT2 turns off Q6, which mutes the amp when the temperature is too high. You could disconnect RT2 to see if it is involved.
I agree it sounds like your mute system. What I was looking to test was not voltages per se, but changing voltages that followed your 3 second thing. Once the amp is ready and working, then I expect all the voltages will be OK too. What might be useful is watching for voltages changing in those first few seconds. It may require turning the amp on and off many times, because you only get 3 seconds each time.
But lets work on it with no input, so if I understand, the amp sits there humming. If we fix that, everything else will probably be OK.
The mute circuit is a couple transistors, and it works by turning Q8 off and on. Q8 completes the circuit to V- from the differential pair at the amp input. If Q8 is off the amp is muted. The mute pins turn off Q7, so the base of Q8 never turns on. Unground the mute line, and Q7 turns on, pulling up the base of Q8 and enabling the amp.
That is what should happen. Now while it is muted but humming, what voltage is on the base of Q8? Compare that to what is there once the unmuted amp settles in and works. When muted, we expect the base of Q8 to be pretty close to its emitter. If the mute doesn't completely turn off Q8, then the amp could be left partly functioning and stuck in a sort of half on condition, kinda like when you hear an amp noise at turn off.
Thermal sensor RT2 turns off Q6, which mutes the amp when the temperature is too high. You could disconnect RT2 to see if it is involved.
Today while continuing to look for the cause of this hum, I discovered that when turning the amp on with nothing plugged into the inputs/P.Amp in, Q21 gets very warm/hot after 30 sec. or so, Q20 barely gets warm, The weird this is when I plug into the inputs, effectively lifting the ground tab, I get the usual few sec. of hum then the amp is quiet. the next 30 or so seconds, Q21's temperature drops to a barely warm temp, just like Q20.
I did this several times to make sure it wasn't a one of. What would cause this strange behavior
I did this several times to make sure it wasn't a one of. What would cause this strange behavior
Hi, I re-checked my reading, here are the result.
Firstly with nothing plugged in.
Q18 B = -1.175v
E = -1.781
Q19 B = -3.435v
E = -2.862v
Q20 B = -1.686v
E = -2.165v
Q21 B = -2.861v
E = -2.338v
The following readings are with a lead plugged in
Q18 B = + 1.140v
E = + .540v
Q19 B = -1.147v
E = -.549v
Q20 B = + .560v
E = + .3mv
Q21 B = -.553v
E = -6.2mv
It seems Q18 and Q20 got to positive V when a lead is plugged in.
I hope this is helpful
Peter
Firstly with nothing plugged in.
Q18 B = -1.175v
E = -1.781
Q19 B = -3.435v
E = -2.862v
Q20 B = -1.686v
E = -2.165v
Q21 B = -2.861v
E = -2.338v
The following readings are with a lead plugged in
Q18 B = + 1.140v
E = + .540v
Q19 B = -1.147v
E = -.549v
Q20 B = + .560v
E = + .3mv
Q21 B = -.553v
E = -6.2mv
It seems Q18 and Q20 got to positive V when a lead is plugged in.
I hope this is helpful
Peter
Hi,
I do not what to think now because with the lead plugged the voltages reading looks normal. The reason why Q21 it is warm to hot it is because the bias is set to -3.45. In the idle state they should look like the reading with the lead plugged. One more check. Check the Q18 base voltage when you power ON the amplifier.
I do not what to think now because with the lead plugged the voltages reading looks normal. The reason why Q21 it is warm to hot it is because the bias is set to -3.45. In the idle state they should look like the reading with the lead plugged. One more check. Check the Q18 base voltage when you power ON the amplifier.
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