Hi all 
I wondered if you good people could give me some ideas for renovating my amp. It is a Nakamichi PA7 from the late 1980's. It is a 200W per channel design running in class A so it gets very hot. What type of things should I check and/or change and/or mod?
On the power supply side it is a BIG transformer into standard bridge rectifiers then some 33,000uf caps. There are very few caps on the output board itself (about 10 per channel). The design has no input or output caps, but has protection circuits for excessive DC. Channels have output relays.
Some ideas I have are:
Changing power supply caps & bypassing (any1 know any sources for 33,000uf caps? Or should I go with more smaller paralleled?)
Upgrading standard rectifiers to low noise types (any suitable part suggestions?)
Changing relays (suggestions on parts?)
Changing internal wiring
Replacing input RCAs and output speaker jacks
Any other thoughts?
One other thing, one channel runs hotter than the other, I've checked the bias and that is the same on both sides, is this normal or should I be worried about life expectancy. I have the service manual but its not scanned in...)
Cheers for any suggestions
I wondered if you good people could give me some ideas for renovating my amp. It is a Nakamichi PA7 from the late 1980's. It is a 200W per channel design running in class A so it gets very hot. What type of things should I check and/or change and/or mod?
On the power supply side it is a BIG transformer into standard bridge rectifiers then some 33,000uf caps. There are very few caps on the output board itself (about 10 per channel). The design has no input or output caps, but has protection circuits for excessive DC. Channels have output relays.
Some ideas I have are:
Changing power supply caps & bypassing (any1 know any sources for 33,000uf caps? Or should I go with more smaller paralleled?)
Upgrading standard rectifiers to low noise types (any suitable part suggestions?)
Changing relays (suggestions on parts?)
Changing internal wiring
Replacing input RCAs and output speaker jacks
Any other thoughts?
One other thing, one channel runs hotter than the other, I've checked the bias and that is the same on both sides, is this normal or should I be worried about life expectancy. I have the service manual but its not scanned in...)
Cheers for any suggestions
I have a PA7 and mine ran very hot as well until I adjusted the idle current and set the Dc offset.
you need to use shorting rca connectors on the inputs and remove the load on the output stage(speaker wires) and let the amp stabilize for about 30 minutes.It is best to use two DC MilliVolt meters as you adjust one it effects the other.Adjust the DC offset to as close to 0 as possible then set the idle current to at least 40Mv.(Mine was at over 200Mv)You will have to make these adjustments several times .I would recommend replacing both of the trim pots with multiturn sealed types.The original ones are crap.Extremely hard to adjust and collect everything in the air,as they are not sealed.Yours may be defective.quality pots here will make a big improvement in stability as well as sound.When all is done you should have a 40Mv at TP101/102 and 201/202 and 0-Mv. at both amp outputs.The amp should run only very slightly warm due to the large heatsinks.
Get the heat problem resolved before you attempt any other mods to this amp.Both channels should run the same temp.Check your speaker cables especially the hot running side.
I use vampire OFC input rca and Eichman cable pods with terellium copper.Both are far superior to the stock versions.And make a huge improvement in sound quality
I am also in the process of replacing the input and output wiring with homemade continuouse cast copper braids with unbleached cotton insulation.(I may be offering a complete mod kit for this amp soon)
The rectifiers are hard to find(as far as high speed types,these types are best utilized in digital circuits,wher the speed is really needed) since they need to be able to pass alot of current.I would worry more about the wiring now, then look for upgrade parts.
Unless the filter caps need replacing(swollen or leaking) all I would do with them is place matched bypass caps on them to improve high frequency response.Multiple filter caps tend to fight one another for current.
you need to use shorting rca connectors on the inputs and remove the load on the output stage(speaker wires) and let the amp stabilize for about 30 minutes.It is best to use two DC MilliVolt meters as you adjust one it effects the other.Adjust the DC offset to as close to 0 as possible then set the idle current to at least 40Mv.(Mine was at over 200Mv)You will have to make these adjustments several times .I would recommend replacing both of the trim pots with multiturn sealed types.The original ones are crap.Extremely hard to adjust and collect everything in the air,as they are not sealed.Yours may be defective.quality pots here will make a big improvement in stability as well as sound.When all is done you should have a 40Mv at TP101/102 and 201/202 and 0-Mv. at both amp outputs.The amp should run only very slightly warm due to the large heatsinks.
Get the heat problem resolved before you attempt any other mods to this amp.Both channels should run the same temp.Check your speaker cables especially the hot running side.
I use vampire OFC input rca and Eichman cable pods with terellium copper.Both are far superior to the stock versions.And make a huge improvement in sound quality
I am also in the process of replacing the input and output wiring with homemade continuouse cast copper braids with unbleached cotton insulation.(I may be offering a complete mod kit for this amp soon)
The rectifiers are hard to find(as far as high speed types,these types are best utilized in digital circuits,wher the speed is really needed) since they need to be able to pass alot of current.I would worry more about the wiring now, then look for upgrade parts.
Unless the filter caps need replacing(swollen or leaking) all I would do with them is place matched bypass caps on them to improve high frequency response.Multiple filter caps tend to fight one another for current.
I used to do warranty on these. I really like them, but they are not a class A amp. They are a stassis amp. High idle current and the feedback is taken before the output stage.
There was one modification that used small PCB's that affected the protection circuit (backed it off). This was a factory modification so the boards may possibly still be available if yours isn't done. Probably at a cost at this point.
Unless you jacks have worn plating I doubt audio jewllery will help the sound, might make you feel better. The same applies for internal wire types. Just my 2¢. I've looked at many that have been hacked with "special" parts. Request: fix by returning to stock.
HiFiguy2, I don't have my manuals any more, if you scan it would you sent me a copy? Thanks.
-Chris
There was one modification that used small PCB's that affected the protection circuit (backed it off). This was a factory modification so the boards may possibly still be available if yours isn't done. Probably at a cost at this point.
Unless you jacks have worn plating I doubt audio jewllery will help the sound, might make you feel better. The same applies for internal wire types. Just my 2¢. I've looked at many that have been hacked with "special" parts. Request: fix by returning to stock.
HiFiguy2, I don't have my manuals any more, if you scan it would you sent me a copy? Thanks.
-Chris
I beg to differ on the connector issue.Quality copper or silver connectors sound much more dynamic than brass/nickle.Also the cable pods remove the mounting nut from the audio signal and were designed for audio use,rather than the surplus stuff found in most equipment designed for general electronic applications.Adding the rca and binding posts were the best >$100 improvement I have ever made to any part of my system.
I replaced the binding posts on my Infinity reference Standard 2.5's as well with the Eichman cable pods.Bass improved(more speed) as well as better mid and highs.Much more musical now.
I moded one side at a time and there was a definate improvement on the moded side,not at all subtle.Pure copper is far superior to brass/nickle.
I replaced the binding posts on my Infinity reference Standard 2.5's as well with the Eichman cable pods.Bass improved(more speed) as well as better mid and highs.Much more musical now.
I moded one side at a time and there was a definate improvement on the moded side,not at all subtle.Pure copper is far superior to brass/nickle.
kilowattski said:
Why I never know that???Is it happened when Mr Pass was working with Threshold??
Hi HiFiguy2,
Have tried and not heard a difference. Have heard many other mods in connectors and wire and not heard a difference. Have corrected design faults and heard a big difference. Have serviced equipment that had big problems and the owners didn't know. They heard a big difference after the repair.
Have tried and not heard a difference. Have heard many other mods in connectors and wire and not heard a difference. Have corrected design faults and heard a big difference. Have serviced equipment that had big problems and the owners didn't know. They heard a big difference after the repair.
Hi Leolabs,
There was a connection. NP would be the best to respond to that. NP's IP none the less.
.
-Chris
There was a connection. NP would be the best to respond to that. NP's IP none the less.
.
-Chris
Never did do the renovation - am building some class Ds to replace it in my active system since it is too big now and I don't need that much power.
I too replaced the bias pots with sealed multiturn types. The only other thing I did was to replace the power resistors and caps on the soft start board as they it was taking too long to switch on and therefore the power resistors had blackened legs
It is a great amp for the second hand price, and recommended!
I too replaced the bias pots with sealed multiturn types. The only other thing I did was to replace the power resistors and caps on the soft start board as they it was taking too long to switch on and therefore the power resistors had blackened legs
It is a great amp for the second hand price, and recommended!
The Nak PA-7 is a great amp, I would not mess with it. Just as suggested make sure the DC offset and Bias is set ok. Factory settings for Bias is about 40 mV even though I run mine at 110mV.
Protection circuits are finicky but overall an excellent amp!
Protection circuits are finicky but overall an excellent amp!
I have recently (one year ago) bougth one of these great monsters, to drive a pair of magnepan 1.6 panels.... rahh lovely !!
Unfortunately, one of the poweron protection relays is starting to buzz annoyingly for 2/3 seconds after starting, then everything is ok.
No service tech in Paris, France... anyone have a clue, or relevant page in the service manual ??
Help appreciated.
Unfortunately, one of the poweron protection relays is starting to buzz annoyingly for 2/3 seconds after starting, then everything is ok.
No service tech in Paris, France... anyone have a clue, or relevant page in the service manual ??
Help appreciated.
Either too much voltage in the relay circuit (unlikely) or your smoothing capis dried out. Check/ replace any smoothing caps that feed DC to the relay.
Check the inrush current limiting resistors and their solder joints. Also, as K-amps suggested, check the health of the filter caps.
-Chris
-Chris
Well good luck, but dont hold your breath, the PA7 is much more complex than the KSA-50. Lets see 🙂
- Status
- Not open for further replies.