Just swap the tubes in the bad amp and see if the supplies change..don't put the KT88 in the good amp in case it causes problems.
I have to sign off now..Hopfully you have enough info to go further..If needed.
Regards
M. Gregg
Okay Kevin I think I will try your suggestion and replace the Opamp. I live way out in the country [Nacogdoches Texas] so it will probably take at least 3 days to get the parts. Pony express is kinda slow. I will post once I finish the job. One more question. Is the servo opamp the same? A TL071cp. Thank you for your help.
Tom Wild
Tom Wild
The servo op-amp is U1 on the schematic and as memory serves is also a TL0-71. I have seen these blow up particularly if there is a problem with the ~ -250V supply.
Good luck! You'll soon understand why I hate these amps with a passion even as good as they sound..
Zeners again!
I'm back,
I have been reading about op amps and checking pins of this particular one for their Voltage. One thing that is baffling me is the reference voltage at pin 2. I should be getting 6.9volts but I am not. If the op amp is bad is that going to effect the reference voltage? Kevin I know you said that the zeners are more of a circuit breaker for safety purposes. However there are two here One they don't show in the schematic. It is under the board, looks very similar to the string of zeners that I originally talked about, is axial and looks like an afterthought more than something that was in the original design. The other is a LM239B and a very precise zener. The info on it says that it will operate over a .5 to 15 mA range with a reverse breakdown voltage of 6.9 volts Min. 6.6volt Max. 7.25 volts. . When I measure across the zener underneath which I suspect is the fail safe, I only get 6.02 volts. What is going on? Can the opamp be affecting this [I think not] or are one of these zeners bad? How should I proceed? I'm thinking pull the tubes pull the fail safe zener underneath out of the circuit and see what voltage I get. If it is still low pull the other zener and check the voltage. Any input would be greatly appreciated.
Thanks everyone,
WILD1
I'm back,
I have been reading about op amps and checking pins of this particular one for their Voltage. One thing that is baffling me is the reference voltage at pin 2. I should be getting 6.9volts but I am not. If the op amp is bad is that going to effect the reference voltage? Kevin I know you said that the zeners are more of a circuit breaker for safety purposes. However there are two here One they don't show in the schematic. It is under the board, looks very similar to the string of zeners that I originally talked about, is axial and looks like an afterthought more than something that was in the original design. The other is a LM239B and a very precise zener. The info on it says that it will operate over a .5 to 15 mA range with a reverse breakdown voltage of 6.9 volts Min. 6.6volt Max. 7.25 volts. . When I measure across the zener underneath which I suspect is the fail safe, I only get 6.02 volts. What is going on? Can the opamp be affecting this [I think not] or are one of these zeners bad? How should I proceed? I'm thinking pull the tubes pull the fail safe zener underneath out of the circuit and see what voltage I get. If it is still low pull the other zener and check the voltage. Any input would be greatly appreciated.
Thanks everyone,
WILD1
Last edited:
Hello, I just replaced the three Zeners (100V 5W) off the 6550 regulator on my Dual-75 ARC amp and the amp biases up and works fine now.
Unlike you, my regulator tube (6550) wasn't putting out the required 320v - 325v (only140V). I located those three zeners on PS board (looked like hell) changed all three (1N5378B) and all is good now...
I know you have an entirely different amp, but just my 2 cents...if you need a few 1N5378B ZDs, lemme know - I bought 24 of them...
Unlike you, my regulator tube (6550) wasn't putting out the required 320v - 325v (only140V). I located those three zeners on PS board (looked like hell) changed all three (1N5378B) and all is good now...
I know you have an entirely different amp, but just my 2 cents...if you need a few 1N5378B ZDs, lemme know - I bought 24 of them...
Hi John,
I thought long and hard about replacing them but decided to take Kevin' s advice and replace the opamps first. If that doesn't cure it I'll go for those two zeners. Wasn't that hard to get them out so I went ahead and did all five in the sick amp. TI who makes that opamp sells an upgrade for it so I went with that one. Also bought machine sockets for them.Hope they are not too difficult to install. Now just sitting around waiting for Pony Express to arrive. Thanks for the two cents and I will let you know how it goes.
WILD1
I thought long and hard about replacing them but decided to take Kevin' s advice and replace the opamps first. If that doesn't cure it I'll go for those two zeners. Wasn't that hard to get them out so I went ahead and did all five in the sick amp. TI who makes that opamp sells an upgrade for it so I went with that one. Also bought machine sockets for them.Hope they are not too difficult to install. Now just sitting around waiting for Pony Express to arrive. Thanks for the two cents and I will let you know how it goes.
WILD1
Hi everyone,
I have an update. I replaced all five opamps and no difference at U6, pin 2 and pin 3 were still off and the voltage at the anode of the 12AT7 tube was still low. So I pulled the opamp out and checked the resistor at pin 2. It checked out. I then replaced the C72 capacitor and no change. Next I disconnected the Zener below the board [hooked up parallel to the capacitor] which I had commented looked like an afterthought and does not appear on the schematic. Everything fell into place. The voltage at pin 2 came up from 6.1 V to 6.5 V and the voltage at pin 3 settled in at 7.13 V. I now have 320V at the anode of The 12AT7 tube.
A little history on these amps. After I bought and received them I hooked them up and right away I knew there was a problem with the 6550 regulator. It was way too bright and hot and it started oscillating when I was listening to it so I sent the one back. I don't know if this was their Techs idea of taming the problem with the 6550 but I suspect that he possibly added this Zener. When I got the one amp back the second time and I realized that there were still problems I called them and told them either discount the price and make me a helluva deal or send me a UPS return label. So I now have a very cheap pair of M100s with a multitude of problems. But one less. Thanks everyone. By the way I am starting a new thread with my latest problem " M100 output tube problems". Life goes on and it's the trials and tribulations that make it so interesting.
Hasta lluego mi compadres,
WILD1
I have an update. I replaced all five opamps and no difference at U6, pin 2 and pin 3 were still off and the voltage at the anode of the 12AT7 tube was still low. So I pulled the opamp out and checked the resistor at pin 2. It checked out. I then replaced the C72 capacitor and no change. Next I disconnected the Zener below the board [hooked up parallel to the capacitor] which I had commented looked like an afterthought and does not appear on the schematic. Everything fell into place. The voltage at pin 2 came up from 6.1 V to 6.5 V and the voltage at pin 3 settled in at 7.13 V. I now have 320V at the anode of The 12AT7 tube.
A little history on these amps. After I bought and received them I hooked them up and right away I knew there was a problem with the 6550 regulator. It was way too bright and hot and it started oscillating when I was listening to it so I sent the one back. I don't know if this was their Techs idea of taming the problem with the 6550 but I suspect that he possibly added this Zener. When I got the one amp back the second time and I realized that there were still problems I called them and told them either discount the price and make me a helluva deal or send me a UPS return label. So I now have a very cheap pair of M100s with a multitude of problems. But one less. Thanks everyone. By the way I am starting a new thread with my latest problem " M100 output tube problems". Life goes on and it's the trials and tribulations that make it so interesting.
Hasta lluego mi compadres,
WILD1
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