Schematic: http://shermanr.web.prw.net/Dr. Tube A500 Version 2.gif
Had the "A" channel phono go bunk on me over lunch. Low gain and no bass present in the signal. Had a few minutes to spare so I did a quick troubleshoot. The problem becomes a bit intermittent and will function normally at first (read, cold) but after 3-4 mins will completely cut out. Something is heating up and not happy.
If I feed the amp with my phone at the tape monitor input or any of the aux inputs on the back, I get the same fantastic sound I'm used to. So that means everything downstream of the phono is bueno.
Switched the two 12ax7's around, still resides in chan A. Good to know the tube isn't bad.
If I feed the phono with the correct input for my TT, channel A is as described above. Switching the RCA inputs around makes no difference, still issues with chan A. If i feed the phono in (hi inputs) with my phone (yes i know its not a RIAA blah blah blah....wanted to test another source) same thing happens, chan A is kapput.
Everything from the back to the front side of the 10K on pin7 ohmed out. Out of curiosity I disconnected the wire going to the chan A input at the rear and jumpered it to the chan B signal. Thought maybe something could be bad with the wiring on the way up. Nope, still does the same thing....okay we're getting somewhere.
Looking at all of the components, nothing appears to have let out the magic smoke. I'm suspecting it's in one of the capacitors. Quite possibly in C3 or C35. I'm thinking that if I've got low gain and no bass response that sounds an awful lot like if someone chose the wrong cathode bypass cap. The next bit is speculation but I'm thinking C35 because of the HPF in the NFB loop. If there's more low frequency being passed through, then it will cancel out those frequencies in the NFB (aka the HPF is not working) and would also lower gain because of the increased feedback.
In any event I'll be pulling the caps out and testing them but I'm afraid It'll have to wait a few days. Couldn't really find anything specifically like this with the search so I wanted to get it out there in case anyone else has the same issue.
thanks guys
Had the "A" channel phono go bunk on me over lunch. Low gain and no bass present in the signal. Had a few minutes to spare so I did a quick troubleshoot. The problem becomes a bit intermittent and will function normally at first (read, cold) but after 3-4 mins will completely cut out. Something is heating up and not happy.
If I feed the amp with my phone at the tape monitor input or any of the aux inputs on the back, I get the same fantastic sound I'm used to. So that means everything downstream of the phono is bueno.
Switched the two 12ax7's around, still resides in chan A. Good to know the tube isn't bad.
If I feed the phono with the correct input for my TT, channel A is as described above. Switching the RCA inputs around makes no difference, still issues with chan A. If i feed the phono in (hi inputs) with my phone (yes i know its not a RIAA blah blah blah....wanted to test another source) same thing happens, chan A is kapput.
Everything from the back to the front side of the 10K on pin7 ohmed out. Out of curiosity I disconnected the wire going to the chan A input at the rear and jumpered it to the chan B signal. Thought maybe something could be bad with the wiring on the way up. Nope, still does the same thing....okay we're getting somewhere.
Looking at all of the components, nothing appears to have let out the magic smoke. I'm suspecting it's in one of the capacitors. Quite possibly in C3 or C35. I'm thinking that if I've got low gain and no bass response that sounds an awful lot like if someone chose the wrong cathode bypass cap. The next bit is speculation but I'm thinking C35 because of the HPF in the NFB loop. If there's more low frequency being passed through, then it will cancel out those frequencies in the NFB (aka the HPF is not working) and would also lower gain because of the increased feedback.
In any event I'll be pulling the caps out and testing them but I'm afraid It'll have to wait a few days. Couldn't really find anything specifically like this with the search so I wanted to get it out there in case anyone else has the same issue.
thanks guys
I was worried about a resistor as well OR having one of them break down once it gets into voltage. I'm more hoping that its a cap so I don't have to try and replace as many
Just got back home after a trip and the guns all soldered up! Going to take a look at these.
I forgot to mention that I did run through and voltage probe everything. I had a similar thought about maybe having too low a plate voltage in one of the stages. Everything checked out on voltages
I thought something similar as well. Although I didn't clean them (yet) I did go through and exercise everything around just for that reason. I had originally thought this to be a source of the problem after the problem became intermittent when I was switching sources. Don't think it's the issue but I doubt they'll complain getting some TLC
Kind of like the LDO and 5W resistors I've got in there already eh?
I may or may not have stumbled upon that advice that was given to another member working on an A500 by none other than yourself. It was one of the first things that was changed when I got the unit
I forgot to mention that I did run through and voltage probe everything. I had a similar thought about maybe having too low a plate voltage in one of the stages. Everything checked out on voltages
I thought something similar as well. Although I didn't clean them (yet) I did go through and exercise everything around just for that reason. I had originally thought this to be a source of the problem after the problem became intermittent when I was switching sources. Don't think it's the issue but I doubt they'll complain getting some TLC
Like many a H/K unit, the A500 employs the "Cheap Charley" method for DC phono section heaters.
Since you are already "under the hood", consider installing a proper, regulated, 12 VDC supply for the 3 small signal tubes and 2X separate RC bias networks for each channel's PP O/P pair.
Kind of like the LDO and 5W resistors I've got in there already eh?
I may or may not have stumbled upon that advice that was given to another member working on an A500 by none other than yourself. It was one of the first things that was changed when I got the unit
For those of you that had money on a resistor....winner winner!
The 470K on the input let the smoke out. Last night I started to disassemble everything and was just dead set on replacing it all. Luckily I got to my 2nd resistor in and found it. Was pulling the resistors out and this sucker just clean came apart. Tested ok in the amp but I imagine once things started to heat up it would get angry.
Did a full test with a DSOTM mofi, both sides of GRACE, and some big-band I have in a box set. held up great Of course now that I say this I'll have it fail sometime next week
The 470K on the input let the smoke out. Last night I started to disassemble everything and was just dead set on replacing it all. Luckily I got to my 2nd resistor in and found it. Was pulling the resistors out and this sucker just clean came apart. Tested ok in the amp but I imagine once things started to heat up it would get angry.
Did a full test with a DSOTM mofi, both sides of GRACE, and some big-band I have in a box set. held up great
Of course now that I say this I'll have it fail sometime next week An externally hosted image should be here but it was not working when we last tested it.
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