Hello all,
This is my first post/question. I am choosing the tube forum for this question even though the amp is a hybrid because I know you are all highly intelligent and tasteful people.javascript:smilie('
')
A (good!) friend of mine recently gave me a Moscode 600 amplifier which I have listened to on a
couple of occasions but have experienced a fairly loud 60 cycle hum and a loud turn off thump. I know my friend retubed the amp not long before he gave it to me so I don't think that is the problem. Do any of you own this amp and have experienced this problem? If so, is it fairly easy to fix or should I seek professional help(no jokes)?
If I do have to have it repaired I would like to do so in the Phoenix, AZ area as it is extremely heavy. Does anyone know of a reputable repair shop which could handle this beast?
Any help you give is greatly appreciated. It seems a shame to leave this amp languishing in the closet.
This is my first post/question. I am choosing the tube forum for this question even though the amp is a hybrid because I know you are all highly intelligent and tasteful people.javascript:smilie('
')A (good!) friend of mine recently gave me a Moscode 600 amplifier which I have listened to on a
couple of occasions but have experienced a fairly loud 60 cycle hum and a loud turn off thump. I know my friend retubed the amp not long before he gave it to me so I don't think that is the problem. Do any of you own this amp and have experienced this problem? If so, is it fairly easy to fix or should I seek professional help(no jokes)?
If I do have to have it repaired I would like to do so in the Phoenix, AZ area as it is extremely heavy. Does anyone know of a reputable repair shop which could handle this beast?
Any help you give is greatly appreciated. It seems a shame to leave this amp languishing in the closet.
moscode hum
DentAudio,
Thanks for the info. Any idea where I can find those power supply caps? I looked on the Moscode website's parts section and they only have the caps for the high voltage supply(or are those the caps you are referring to?).
There is a ceramic cap strapped across the power switch. The switch was arcing when I first turned it on after a long period of not being used so I replaced the switch. Perhaps the cap was damaged when this happened causing the turn off thump.
I'll keep surfing around and see if I can find these gigantic caps.
DentAudio,
Thanks for the info. Any idea where I can find those power supply caps? I looked on the Moscode website's parts section and they only have the caps for the high voltage supply(or are those the caps you are referring to?).
There is a ceramic cap strapped across the power switch. The switch was arcing when I first turned it on after a long period of not being used so I replaced the switch. Perhaps the cap was damaged when this happened causing the turn off thump.
I'll keep surfing around and see if I can find these gigantic caps.
MattV
The thump is normal for a Moscode 600, but not the buzz.
This guy mods and fixes Moscode in AZ.
http://www.thuntek.net/~bk11/home.htm
Quinnling
The thump is normal for a Moscode 600, but not the buzz.
This guy mods and fixes Moscode in AZ.
http://www.thuntek.net/~bk11/home.htm
Quinnling
Quinnling,
Thanks for the input on the amp. I do remember my friend using this amp (circa 1991) on a pair of Mirage speakers and there was no thump at turn off. I did follow your link to the repair shop in Tucson but all of the links to the various portions of his website seem to be dead. Maybe I'll try giving him a call later. If all else fails I'll follow the "parts replacer" credo of electronics repair and put in new power supply caps and hope that solves the problem. I think I found some reasonable replacements on the Newark website for about $30 each.
Thanks for the input on the amp. I do remember my friend using this amp (circa 1991) on a pair of Mirage speakers and there was no thump at turn off. I did follow your link to the repair shop in Tucson but all of the links to the various portions of his website seem to be dead. Maybe I'll try giving him a call later. If all else fails I'll follow the "parts replacer" credo of electronics repair and put in new power supply caps and hope that solves the problem. I think I found some reasonable replacements on the Newark website for about $30 each.
It should not be a problem as long as the voltage is the same or Higher.
BTW, My Moscode 600 and 300 always thumped when the amp turned off. I notice the amount that it thumped depends on the speaker. George Kayes has a modification on the protective relays that caused, I'm not sure if mod removes the thump of not.
BTW, My Moscode 600 and 300 always thumped when the amp turned off. I notice the amount that it thumped depends on the speaker. George Kayes has a modification on the protective relays that caused, I'm not sure if mod removes the thump of not.
I put in a set of 47kuf CDE caps,whcih are a perfect fit. These are fine. This was in the last version of the 600 that came out, the one with the rifa(?) film caps beside the big cans. I moved him up to a 50A 600V rectifier at the same time, the original being 25A or 35A rated
The thump may be the relay circuit not functioning properly, as the relay kills the signal ridiculously fast in that amp, when shutting it down. I don't like the relay being there, but I've seen it save many a speaker at my friend's place where cab drivers sit outside and key their mike on their radio..and he has a full tube system, except fo the moscode. The +400W rf-AM-af blast through the system is not pretty. The Moscode kills it via the relay before it gets through. It may be that the voltage sensing system for theis protection circuit is failing in some manner, the relay might be sticking, etc. If the relay does not fire immediately at the moment when you hit the power switch to the off position....then there may be your issue. (I'm talking about somewhere in the area of a 1/4 second or so after turn off..that relay should slam shut) Dirty contacts on the relay can also allow turn on thumps to come through, as it's job is to cut the tube circuit out of the hitachi fet's input until the tube circuit has stabilized..and to shunt the Hitachi MOSFET power circuit to ground...to keep it quiet and stable..this..until the tube front end is ready and stable. This takes about minute, via a timer circuit. If that relay or it's socket is dirty, some signal may be getting through to the MOSFETs at turn on. This is likely your 'thump' culprit.
Clean and tighten the plugs for the circuitry as well..and consider getting rid of the plugs altogether, via new wiring and solid-soldered connections. It makes a big difference.
The amp also likely needs a bit of DC and offset checking/resetting, as the amp has tendacy to drift over the long term. It likely needs new tubes as well. There are two regulator tubes in there, that should be sourced as soon as possible, they are becoming fairly rare. Use matched sets if you can, of course. Grab 4 or so of those regulators, if you can. That particular tube will never be manufactured again.
IIRC, the entire board is 'floated' via connectors, so no stable low-impedance ground is actually available..thus the potential rises for thumps at turn on. Just a guess.
The thump may be the relay circuit not functioning properly, as the relay kills the signal ridiculously fast in that amp, when shutting it down. I don't like the relay being there, but I've seen it save many a speaker at my friend's place where cab drivers sit outside and key their mike on their radio..and he has a full tube system, except fo the moscode. The +400W rf-AM-af blast through the system is not pretty. The Moscode kills it via the relay before it gets through. It may be that the voltage sensing system for theis protection circuit is failing in some manner, the relay might be sticking, etc. If the relay does not fire immediately at the moment when you hit the power switch to the off position....then there may be your issue. (I'm talking about somewhere in the area of a 1/4 second or so after turn off..that relay should slam shut) Dirty contacts on the relay can also allow turn on thumps to come through, as it's job is to cut the tube circuit out of the hitachi fet's input until the tube circuit has stabilized..and to shunt the Hitachi MOSFET power circuit to ground...to keep it quiet and stable..this..until the tube front end is ready and stable. This takes about minute, via a timer circuit. If that relay or it's socket is dirty, some signal may be getting through to the MOSFETs at turn on. This is likely your 'thump' culprit.
Clean and tighten the plugs for the circuitry as well..and consider getting rid of the plugs altogether, via new wiring and solid-soldered connections. It makes a big difference.
The amp also likely needs a bit of DC and offset checking/resetting, as the amp has tendacy to drift over the long term. It likely needs new tubes as well. There are two regulator tubes in there, that should be sourced as soon as possible, they are becoming fairly rare. Use matched sets if you can, of course. Grab 4 or so of those regulators, if you can. That particular tube will never be manufactured again.
IIRC, the entire board is 'floated' via connectors, so no stable low-impedance ground is actually available..thus the potential rises for thumps at turn on. Just a guess.
Moscode turn off thump & hum
As the Moscode designer and inventor I think I can shine some light on this subject.
First, The turn off thump. On a stock 300 or 600 there is a time delay relay in series with the speaker. In both amplifiers it is controlled by a LM555 timer. It remains open for 45 seconds, disconnecting the speaker in order to allow the tubes to settle to their operating voltages. Without the relay you would get a rather large woofer excursion which could damage the woofer.
In the 600 there is also a DC offset detector that will disconnect the speaker for about 3 seconds if there is more than a 4 volt offset for more that about a tenth of a second. I see in one of the posts a 600 is picking up some mobile radio signals. This may be causing the circuit to trigger as the RF signal may be being fed into the amp through the speaker leads. Remember the negative feedback circuit is also an input. Or the RF signal may be directly triggering the safety circuit.
In any case if there is a small amount of DC offset in the output circuit you will hear a thump when the amp is turned off. There is a small amount of time delay before the relay disconnects when the amp is turned off to keep the speaker connected during a brownout so that the timer circuit doesn't go through its 45 second warm up mute. During this time the High Voltage power supply is discharging and that may cause a transient that gets through to the speaker before the relay drops out.
My upgrade to the amp eliminates the relay in series with the speaker and replaces it with one that shorts the drive from the tube circuitry to ground during warm up. This mod takes the relay out of the circuit while the amp is playing and eliminates the signal degradation caused by the relay contacts and relay socket. However because the relay shorts the gates of the Mosfets to ground during warm up and turn off the mosfets are not fully biased on until a few seconds after the relay opens (disengages) and the bias shifts which causes a pop on turn off.
In all cases the turn off thump is benign and will not harm the speaker unless there is a fault in the output stage-blown mosfet for instance.
I was able to design this transient out of the Moscode 401HR and the soon to be released 402Au.
The hum you are experiencing is most likely due to a faulty high voltage regulator circuit or defective filter caps for the hi voltage regulator. There is a high voltage test point on both sides of the board and it should measure around +320 volts. If it is high, like around 450 volts the regulator is not working. If it is low then probably the filter caps are defective. If you are technically adept you can replace the 2 800 mfd filter caps in the middle of the board.
There is also a possibility there is a bad input ground in your wiring or a heater cathode short in one of the tube, most likely the ones on the outside edges of the 600 or the 2 in the middle of 300.
I hope this helps. If you need more help contact me through my websites.
I service all the classic Moscode products and you can find my service site at http://moscode.home.att.net/ or www.Moscode.com which has a link to the service site.
As the Moscode designer and inventor I think I can shine some light on this subject.
First, The turn off thump. On a stock 300 or 600 there is a time delay relay in series with the speaker. In both amplifiers it is controlled by a LM555 timer. It remains open for 45 seconds, disconnecting the speaker in order to allow the tubes to settle to their operating voltages. Without the relay you would get a rather large woofer excursion which could damage the woofer.
In the 600 there is also a DC offset detector that will disconnect the speaker for about 3 seconds if there is more than a 4 volt offset for more that about a tenth of a second. I see in one of the posts a 600 is picking up some mobile radio signals. This may be causing the circuit to trigger as the RF signal may be being fed into the amp through the speaker leads. Remember the negative feedback circuit is also an input. Or the RF signal may be directly triggering the safety circuit.
In any case if there is a small amount of DC offset in the output circuit you will hear a thump when the amp is turned off. There is a small amount of time delay before the relay disconnects when the amp is turned off to keep the speaker connected during a brownout so that the timer circuit doesn't go through its 45 second warm up mute. During this time the High Voltage power supply is discharging and that may cause a transient that gets through to the speaker before the relay drops out.
My upgrade to the amp eliminates the relay in series with the speaker and replaces it with one that shorts the drive from the tube circuitry to ground during warm up. This mod takes the relay out of the circuit while the amp is playing and eliminates the signal degradation caused by the relay contacts and relay socket. However because the relay shorts the gates of the Mosfets to ground during warm up and turn off the mosfets are not fully biased on until a few seconds after the relay opens (disengages) and the bias shifts which causes a pop on turn off.
In all cases the turn off thump is benign and will not harm the speaker unless there is a fault in the output stage-blown mosfet for instance.
I was able to design this transient out of the Moscode 401HR and the soon to be released 402Au.
The hum you are experiencing is most likely due to a faulty high voltage regulator circuit or defective filter caps for the hi voltage regulator. There is a high voltage test point on both sides of the board and it should measure around +320 volts. If it is high, like around 450 volts the regulator is not working. If it is low then probably the filter caps are defective. If you are technically adept you can replace the 2 800 mfd filter caps in the middle of the board.
There is also a possibility there is a bad input ground in your wiring or a heater cathode short in one of the tube, most likely the ones on the outside edges of the 600 or the 2 in the middle of 300.
I hope this helps. If you need more help contact me through my websites.
I service all the classic Moscode products and you can find my service site at http://moscode.home.att.net/ or www.Moscode.com which has a link to the service site.
Wow, I started this thread about a year ago and have since fixed(by replacing the power supply caps) and sold the Moscode and moved 3 states away! But thanks to all who replied and specifically to "Mr. Moscode" for taking the time to answer. That 600 was the best sounding amp I have ever owned and I wish I had it back.
I've always got one in the house. right now, it is a Moscode 300, with the schematic being needed, as there is a hum problem that appears to be involving the regulator circuit for the HV for the tubes. I can likely fix it without the schematic, but..it is nice to have. Considering that the circuit is that old, I find it a bit of a selfish crime that it (schematic) isn't out 'there' --but to each their own. On the other hand - there may be extenuating circumstances I'm not aware of surrounding the publishing of the given old Moscode circuits.
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KBK,
Sorry I can't furnish the schematic but it is entirely likely that the high Voltage filter cap has gone open due to age. You can replace it with any cap over 100 mfd and 350 volts that will fit. I use 390 mfd at 350 volts.
If you need more help call me - 802 257 5085 or George "a t" moscode.com
George
Sorry I can't furnish the schematic but it is entirely likely that the high Voltage filter cap has gone open due to age. You can replace it with any cap over 100 mfd and 350 volts that will fit. I use 390 mfd at 350 volts.
If you need more help call me - 802 257 5085 or George "a t" moscode.com
George
Mr. Moscode?
Are you serious George. I have not spoken to you since 1986. Went we had that long talk by phone. I still have all the schematics of the OTL's and the 300/600's you sent me. Back in 1986.
Yes, you are correct, replace the HV caps with good photo caps 200uf/330v. and look into the regulator circuit, for the hum problem. But also, do not forget the heater circuit. I believe it does use DC voltage. So check the ripple level as well.
Now the thump? The could lead to a long write up. I will do that another day
My problem, now..., is do still remember me. I worked for Counterpoint Electronics. I helped design and build all of their transformers, when the factory was in Vista, CA.
I left there in 1993, to pursue my career as a AeroSpace Electronic Engineer at Lockheed Martin. I have the drawing of each piece of equipment. My love "file".
Take care George
Ivey
Are you serious George. I have not spoken to you since 1986. Went we had that long talk by phone. I still have all the schematics of the OTL's and the 300/600's you sent me. Back in 1986.
Yes, you are correct, replace the HV caps with good photo caps 200uf/330v. and look into the regulator circuit, for the hum problem. But also, do not forget the heater circuit. I believe it does use DC voltage. So check the ripple level as well.
Now the thump? The could lead to a long write up. I will do that another day
My problem, now..., is do still remember me. I worked for Counterpoint Electronics. I helped design and build all of their transformers, when the factory was in Vista, CA.
I left there in 1993, to pursue my career as a AeroSpace Electronic Engineer at Lockheed Martin. I have the drawing of each piece of equipment. My love "file".
Take care George
Ivey
After all this time, I just noticed this, George! thanks for the reply.
The amp is still sitting around.
I bought the amp for a friend, at a good price..as it had a hum problem.
The issue is that some repair tech botched this part of the HV rail circuit, by McGuyvering a barstewardized repair into service, one that failed. So the original circuit isn't even there in the amplifier to analyze...
Thankfully ANOTHER friend bought a different and working one. I just borrowed it to take a look at, so I can finally repair this botched one. There are more than a few Moscodes in this town, as we had a decent and active Audio Dealer at the time when the Amps where spanky new..
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HUM IN UNIT REPAIR
KBK:
I am sorry to everyone on the forum, but my health is keeping me off the net. I am seldom active in or around my home. I do not get on the net much. My meds are so hard on me. I live for times like this when my mind and eyes are clear.
George used photoflash caps in the HV supply, their part number for it was #00132808331, C15. It was 800uf @ 330volts, driving a HV voltage regulator of TIP50 transistors.
I do not know how much room you got to play with. But one100 or 250uf, will not cut the mustard for reducing the hum. You will require a bank of caps. You can get them at "All Electronics" for a $1.00 each. They are 270uf @ 330v., part # PPC-273. Four will do it.
The ones George used; were light blue, most photoflash caps are black.
One of the problems with the Moscodes were the parts used. None of them used heavy duty parts. Replace the TIP50's, with MJE13009's. The 2N3439, will need replacing as well, but with what, I have no idea. You will need a high voltage, high current transistor, with some gain.
The thump is caused by the failure of the relay circuit. There is one IC1 chip there. I do not know its type. It is a 12v relay circuit, which is powered from the same winding as the filament voltage windings. Remember that the tubes HV is only 240 to 250 volts. Check your zener string for shorts or burn outs on your high voltage regulators. And replace the relay drive transistor with a BD139.
Tip: If you have the room in the amp, and you can get your hands on defunk vertical drive boards for large flat screens. You can use the caps from them..., to build you capacitor bank. Some are pretty high in uf. I have a hand full of them.
If you need a drawing, I can get my grandson to help. It will take time thou.
Take Care
Ivey
KBK:
I am sorry to everyone on the forum, but my health is keeping me off the net. I am seldom active in or around my home. I do not get on the net much. My meds are so hard on me. I live for times like this when my mind and eyes are clear.
George used photoflash caps in the HV supply, their part number for it was #00132808331, C15. It was 800uf @ 330volts, driving a HV voltage regulator of TIP50 transistors.
I do not know how much room you got to play with. But one100 or 250uf, will not cut the mustard for reducing the hum. You will require a bank of caps. You can get them at "All Electronics" for a $1.00 each. They are 270uf @ 330v., part # PPC-273. Four will do it.
The ones George used; were light blue, most photoflash caps are black.
One of the problems with the Moscodes were the parts used. None of them used heavy duty parts. Replace the TIP50's, with MJE13009's. The 2N3439, will need replacing as well, but with what, I have no idea. You will need a high voltage, high current transistor, with some gain.
The thump is caused by the failure of the relay circuit. There is one IC1 chip there. I do not know its type. It is a 12v relay circuit, which is powered from the same winding as the filament voltage windings. Remember that the tubes HV is only 240 to 250 volts. Check your zener string for shorts or burn outs on your high voltage regulators. And replace the relay drive transistor with a BD139.
Tip: If you have the room in the amp, and you can get your hands on defunk vertical drive boards for large flat screens. You can use the caps from them..., to build you capacitor bank. Some are pretty high in uf. I have a hand full of them.
If you need a drawing, I can get my grandson to help. It will take time thou.
Take Care
Ivey
I am sorry for grandfather getting on line
My grandfather is ill. And should not get on line without me.
Moscode amps were not all build the same. They went through many changes and upgrades. My grandfather can not properly tell any one the correct repair without seeing or knowing about the equipment.
The IC1 chip he mention, was the SE555/LM555, providing a 30 second to 45 second delay for tube warm up.
On early models of the Moscode 300 and 150, 800uf @ 330v photoflash caps were used. Those are very hard to get, unless with a special order. 100 to 330uf @ 330v are more common. And for hum, check for ground loops and insure that all grounds are solid ones.
Thank You
Ivey's grandson
My grandfather is ill. And should not get on line without me.
Moscode amps were not all build the same. They went through many changes and upgrades. My grandfather can not properly tell any one the correct repair without seeing or knowing about the equipment.
The IC1 chip he mention, was the SE555/LM555, providing a 30 second to 45 second delay for tube warm up.
On early models of the Moscode 300 and 150, 800uf @ 330v photoflash caps were used. Those are very hard to get, unless with a special order. 100 to 330uf @ 330v are more common. And for hum, check for ground loops and insure that all grounds are solid ones.
Thank You
Ivey's grandson
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