Hi Guys!
I was out to buy some used speakers from a local guy and got a NAD C272 and a AKAI AM-55 amp with it.
Both very dirty but in working condition.
I like to use the NAD as additional amp for my stereo setup, so I'll take care of the AKAI later.
I'm by far no electronics expert, but it should be fair enough to refresh worn parts, check components/circuits, follow schematics etc.
At least for this aged amps.
So please forgive me some stupid questions, and that I don't understand everything immediately.
I've opened the case and cleaned the inside with compressed air and
found very dirty nicotine caked PCB's and components.
The pre owner must be a chain smoker...
I've checked the amp on the bench and found only a scratching line in pot.
Apart from the higher cleaning effort it should be worth a refresh/rebuild.
Well it is 15 years old but should be still a good sounding amp.
This is my todo list so far:
Clean every PCB/components
Renew all electrolytic caps
Renew line in pot.
Renew all Trimpot's
Renew all sliding switches
Renew speaker/standby relays
Renew standby switch
Because nicotine goes everywhere inside of components I added the sliding switches and relays to the list.
I want to refresh the amp so that it should last for another 10-15 years.
Is there anything else that i should add to the list under this aspect?
Maybe rectifiers, regulators, diodes?
At first my thoughts was to replace every part which are under heavy thermal load which accelerates aging.
Especialy some of the 2/3W resistors get very hot. But also the 2SA872A/2SC1775A transistors on the R/L amp boards has left heat tracks on the PCB.
I have no experience with aging or accelerated aging of semis etc. and I'm grateful for every advice.
I know it's hard to find low noise substitutes for such semis with right/matching hFE.
But if its woth it I'll go for it.
I looked already for replacements PSU caps which seems not easy to find.
Maximum diameter is about 35mm for a 10000uF 100V 105°C snap in cap.
40mm will not fit.
NAD changed the caps from 80V to 100V after a few was blown.
The shematic shows still 80V
NAD has spare parts, but they want 65€ per cap.
So far I only found chineese copys/fakes on aliexpress etc which would fit.
The PSU board has space/holes for 6 caps but only 4 are fitted.
Before I'll get fake/copy caps i'd rather use 6 smal caps of 6700uF caps which i could get with good quality.
Maybe someone has a source for good genuine 35x max ~110mm long caps with 10000uF 100V 105°C?
You can find the Service Manual on Vintageshifi
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-C-272-Service-Manual.pdf
I was out to buy some used speakers from a local guy and got a NAD C272 and a AKAI AM-55 amp with it.
Both very dirty but in working condition.
I like to use the NAD as additional amp for my stereo setup, so I'll take care of the AKAI later.
I'm by far no electronics expert, but it should be fair enough to refresh worn parts, check components/circuits, follow schematics etc.
At least for this aged amps.
So please forgive me some stupid questions, and that I don't understand everything immediately.
I've opened the case and cleaned the inside with compressed air and
found very dirty nicotine caked PCB's and components.
The pre owner must be a chain smoker...
I've checked the amp on the bench and found only a scratching line in pot.
Apart from the higher cleaning effort it should be worth a refresh/rebuild.
Well it is 15 years old but should be still a good sounding amp.
This is my todo list so far:
Clean every PCB/components
Renew all electrolytic caps
Renew line in pot.
Renew all Trimpot's
Renew all sliding switches
Renew speaker/standby relays
Renew standby switch
Because nicotine goes everywhere inside of components I added the sliding switches and relays to the list.
I want to refresh the amp so that it should last for another 10-15 years.
Is there anything else that i should add to the list under this aspect?
Maybe rectifiers, regulators, diodes?
At first my thoughts was to replace every part which are under heavy thermal load which accelerates aging.
Especialy some of the 2/3W resistors get very hot. But also the 2SA872A/2SC1775A transistors on the R/L amp boards has left heat tracks on the PCB.
I have no experience with aging or accelerated aging of semis etc. and I'm grateful for every advice.
I know it's hard to find low noise substitutes for such semis with right/matching hFE.
But if its woth it I'll go for it.
I looked already for replacements PSU caps which seems not easy to find.
Maximum diameter is about 35mm for a 10000uF 100V 105°C snap in cap.
40mm will not fit.
NAD changed the caps from 80V to 100V after a few was blown.
The shematic shows still 80V
NAD has spare parts, but they want 65€ per cap.
So far I only found chineese copys/fakes on aliexpress etc which would fit.
The PSU board has space/holes for 6 caps but only 4 are fitted.
Before I'll get fake/copy caps i'd rather use 6 smal caps of 6700uF caps which i could get with good quality.
Maybe someone has a source for good genuine 35x max ~110mm long caps with 10000uF 100V 105°C?
You can find the Service Manual on Vintageshifi
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Nad-C-272-Service-Manual.pdf
When I worked for a national TV rentals company and the odd TV came in covered in nicotine, I used to remove the main PCBoards, wash them in washing up liquid and hot water (ensuring there were no porous parts to get affected) and then thoroughly dry them off for a couple of days in a warm dry place.
Reassemble the cleaned chassis and test. After testing and any repairs, the PCBoards were lacquered with PCB varnish on the solder side only!
They were then fit for re-rental.
Always fit the same type and value components. The designers knew a lot more than most others!
Reassemble the cleaned chassis and test. After testing and any repairs, the PCBoards were lacquered with PCB varnish on the solder side only!
They were then fit for re-rental.
Always fit the same type and value components. The designers knew a lot more than most others!
I'm sorry, I forgot to mention that solder-/desoldering equipment is available.
As long we are not talking about SMD soldering my skills shoult be sufficient.
Thanks for your reply.
I disassembled the amp and cleaned every PCB so far.
Before cleaning I removed all pots, relays and switches.
I'm looking for same/comparable replacement parts, but some are hard to find.
Low noise audio transistors of this age are no longer available. And it seems that there are many fakes.
But the biggest problem so far is to get the PSU caps.
The only ones wich I found are all fakes.
They are sap in's with 35 mm diameter and 10000uF 100V and 105°C.
NAD replaced the 80V (on the pics) with 100V caps as part of a service action.
I think the pre owner didin't know about this service action and the original caps were never replaced.
Here are some pics during disassembling.
Because of the dust the nicotine coating was not visible at first.
After cleaning with compressed air and removed amp boards.
Next step is to remove all electrolytic caps.
Regards
Mike
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I did another little project in the meantime, so the NAD had to wait a bit.
Finally I used the last week do get the most things done.
I couldn't find any fitting genuine 100V main PSU cap.
The main PSU delivers measured 68V with no load after rectifying.
There could be more headroom, but 80V cap's should be sufficient.
I found a statement, that there was a bad charge of the 80V Jinghai original caps which caused the 80 to 100V cap swap by NAD. Some devices were also repaired with good 80V caps.
That's why I decided to go with good quality Mundorf 80V PSU caps with same diameter.
After cleaning and stripping down the NAD, I found some bad capacitors and a thyristor (1 of 2) in the main power supply which gave me strange readings from my LCR-TC1 component tester.
I also got a hint from a guy who works for a local NAD service partner to replace all small transistors of the input stage power supply which are running very hot and failing frequently.
I've replaced this 2SA872A/2SC1775A transistors with Fairchild/ON Semi KSA992/KSC1845 which seems to be the best substitute.
On the search for replacements for the other semis I found a local source with some NOS parts for a good value.
So I decided to replace all TO-92, TO-126 and TO-220 Transistors on all boards with same type and brand. I saw the original boxes with lable and Date etc. It matches with the date code on the parts, so i think there are genuine.
In the "hot" areas I also replaced every resistor with a metal film type.
As you probably already thought, I decided to rebuild than just to replace the caps.
All other mainstram parts like caps, switches, pots, resistors, rectifiers etc. I've ordered from mouser.
I choosed a mix of Nichicon audio grade caps because there are not all voltages, temperatures and capacities available by one cap series.
I finished rebuilding all boards yesterday and tested the PSU board so far.
PSU works as expected and all Voltages are in the range as they should be.
I can't test ISC circuit and input buffer completly without the AMP boards, but it looks like there are no problems.
Because ISC ajustment is not so easy, I set the ISC trim pot to the same resistance as the old pot to have a good starting point.
I'll check the AMP boards later with dim bulb in the line and do some testing with all boards on the bench if everything is ok.
Finally I used the last week do get the most things done.
I couldn't find any fitting genuine 100V main PSU cap.
The main PSU delivers measured 68V with no load after rectifying.
There could be more headroom, but 80V cap's should be sufficient.
I found a statement, that there was a bad charge of the 80V Jinghai original caps which caused the 80 to 100V cap swap by NAD. Some devices were also repaired with good 80V caps.
That's why I decided to go with good quality Mundorf 80V PSU caps with same diameter.
After cleaning and stripping down the NAD, I found some bad capacitors and a thyristor (1 of 2) in the main power supply which gave me strange readings from my LCR-TC1 component tester.
I also got a hint from a guy who works for a local NAD service partner to replace all small transistors of the input stage power supply which are running very hot and failing frequently.
I've replaced this 2SA872A/2SC1775A transistors with Fairchild/ON Semi KSA992/KSC1845 which seems to be the best substitute.
On the search for replacements for the other semis I found a local source with some NOS parts for a good value.
So I decided to replace all TO-92, TO-126 and TO-220 Transistors on all boards with same type and brand. I saw the original boxes with lable and Date etc. It matches with the date code on the parts, so i think there are genuine.
In the "hot" areas I also replaced every resistor with a metal film type.
As you probably already thought, I decided to rebuild than just to replace the caps.
All other mainstram parts like caps, switches, pots, resistors, rectifiers etc. I've ordered from mouser.
I choosed a mix of Nichicon audio grade caps because there are not all voltages, temperatures and capacities available by one cap series.
I finished rebuilding all boards yesterday and tested the PSU board so far.
PSU works as expected and all Voltages are in the range as they should be.
I can't test ISC circuit and input buffer completly without the AMP boards, but it looks like there are no problems.
Because ISC ajustment is not so easy, I set the ISC trim pot to the same resistance as the old pot to have a good starting point.
I'll check the AMP boards later with dim bulb in the line and do some testing with all boards on the bench if everything is ok.
Hi,
sorry for my english, it's not as good as I would like!
You should replace 2 caps in your circuit: C301 and C401; 10µF, for DC blocking. Replace them by MKP type or your prefered cap like Muse, Silmic,...
Have a look to input buffer supply; IC702 and IC703 who are LM317 and LM337. take care about caps around to replace them with "better" like Panasonic FC.
There are 3 parts in your amp circuit:
buffer supplied by +/- 18V
preamp supplied by +/- 68V
amp supplied by +/- 64V
sorry for my english, it's not as good as I would like!
You should replace 2 caps in your circuit: C301 and C401; 10µF, for DC blocking. Replace them by MKP type or your prefered cap like Muse, Silmic,...
Have a look to input buffer supply; IC702 and IC703 who are LM317 and LM337. take care about caps around to replace them with "better" like Panasonic FC.
There are 3 parts in your amp circuit:
buffer supplied by +/- 18V
preamp supplied by +/- 68V
amp supplied by +/- 64V
Hello Nicozero, thanks for your reply!
I thought about MKP caps but there is not enough space for it, and I don't like to tinker around to put them anywhere else. So I changed C301 and C401 to Nichicon Muse.
The voltage regulators IC702 and IC703 are new and all caps in all power stages are Nichicon Audio or Mundorf Audio grade types.
All PSU voltages are fine, like mentioned above. The main PSU for the amp supply has two different rectified transformer inputs with diffenent voltages which are swiched over by Thyristor TC701 and TC702.
The first one from B701 is about +/-46V (Not stated in the schematic) and the second from B702 is +/-64V. I measured about +/-50 and +/-70V without any load. With amp boards connected and no other load there are +/-48 and +/-68V left. I think with a bit more load they will drop near to nominal value as mentioned in the scheme.
Yes, I changed the buffer output caps C104 and C204 also to Nichicon Muse.
C103/C203 and C612/C613 are already metallized polyster film caps by default.
Maybe there are better types, but I wanted to keep them.
Good Morning!
Made some progress yesterday.
First power on with amp boards and dim bulb in the line was without problems.
After a few basic measurements I took all boards together on the bench.
The amp swiched on and came out of protect mode as it should be.
So far so good, time to ajust DC offset and idle current.
I replaced the basic trim pots with sealed 5 turn precision pots which made setting the DC offset very easy.
Then the first problem came up during rebuild.
As I tried to ajust idle current I found that it is not responding to the pot setting as it should.
I set the pots for the first start to the lowest idle current.
That was 3mV on both channels. If I try to ajust it up to 6-7.5mV as it should be, it came only up to 3.4mV.
Both channels behave exactly the same way and stay cool after power up.
Hmm, very strange.
All replaced parts are same type and all Transistors have same brand and gain class and I mached them as good as possible.
I double checked if there any parts blown or at the wrong place, but I didn't found any.
Then I connected speakes to hear if there is something unusual audible.
But there is nothing and musik sounds normal so far.
At a bit higher volume the amp heatsinks started to get warm.
So apart from the idle current problem, the amp sems to be ok.
I guess the problem could be related to some of the replaced parts who do not behave like the old ones even though they are of the same type.
So what is the best way to solve the problem?
Change related parts back to old ones?
Modify bias resistor to check if it has any affect?
Other suggestions?
Made some progress yesterday.
First power on with amp boards and dim bulb in the line was without problems.
After a few basic measurements I took all boards together on the bench.
The amp swiched on and came out of protect mode as it should be.
So far so good, time to ajust DC offset and idle current.
I replaced the basic trim pots with sealed 5 turn precision pots which made setting the DC offset very easy.
Then the first problem came up during rebuild.
As I tried to ajust idle current I found that it is not responding to the pot setting as it should.
I set the pots for the first start to the lowest idle current.
That was 3mV on both channels. If I try to ajust it up to 6-7.5mV as it should be, it came only up to 3.4mV.
Both channels behave exactly the same way and stay cool after power up.
Hmm, very strange.
All replaced parts are same type and all Transistors have same brand and gain class and I mached them as good as possible.I double checked if there any parts blown or at the wrong place, but I didn't found any.
Then I connected speakes to hear if there is something unusual audible.
But there is nothing and musik sounds normal so far.
At a bit higher volume the amp heatsinks started to get warm.
So apart from the idle current problem, the amp sems to be ok.
I guess the problem could be related to some of the replaced parts who do not behave like the old ones even though they are of the same type.
So what is the best way to solve the problem?
Change related parts back to old ones?
Modify bias resistor to check if it has any affect?
Other suggestions?
Hello Mooly, thanks for your Information!
So do you think the voltage drop through R345/R445 and R346/R446 + VR301/401 is too low that Q415 has too little control over the result?
If i got this right, do you have a proposal for the resistance of R445 and R446?
I had a brief glance at the schematic.
Check:
R345 and R445 = 1k2
R346 and R446 = 820r
The pots replaced VR302 and 402 = 500r
And VBE multiplier components as Mooly says
Hello avtech23!
I've checked this resistors and pots again. All resistors are in 1% tolerance and are on the right place. The pots are working evenly without dropouts etc. and have the right resistance.
I know how destructive it could be to do something wrong on VBE multipler.
I'll have to be very carefull and use the dim bulb as well.
You can alter either of the resistors R445 and R446, whichever is easiest. Either replace one or add another in parallel.
R446 could be decreased to 680 ohm as a test.
You know that 820 ohm alone (when the pot is on min resistance) isn't getting the bias high enough. 680 ohm plus the 500 ohm of the pot (when on max resistance) is way over the resistance that would generate any appreciable current and so is perfectly safe as a starting point.
Just remember to set the preset to max resistance before switching on. That willl give minimum bias current.
R446 could be decreased to 680 ohm as a test.
You know that 820 ohm alone (when the pot is on min resistance) isn't getting the bias high enough. 680 ohm plus the 500 ohm of the pot (when on max resistance) is way over the resistance that would generate any appreciable current and so is perfectly safe as a starting point.
Just remember to set the preset to max resistance before switching on. That willl give minimum bias current.
I had only a 500 ohm resistor wich gave me 980 ohm total with the pot at max resistance.
But that gave me only 0,1mV more. And this was already at the begining of the pot range around 780 ohm total.
There was no change anymore below 750 Ohm.
This couldn't be in a working Vbe multipler, there must be another problem.
I stopped there and took the old resistor back in place.
Than I checked the voltages again from the +/- power rails on the way to Vbe Multipler transistor and found still nothing wrong.
That brought me back to check the replaced parts and i checked the shunt regulators IC401/402 again.
They were ok and working, but then came the moment where I hit my head.
Why the f*** is there an "L" at the end of the part ID!
I ordered AZ431 BZ shunt regulators but i got AZ431L BZ which is the low voltage version.
As I first checked the new parts I didn't notice the "L" and looked at the second row for the right BZ variant.
The low voltage version only delivered the half of the actual voltage, there was simply not enough juice to get the right idle current.
I soldered back the old regulators and the problem was gone!
Time for a beer!
I'm sorry, this was missleading.
I used the DBT only for the first powerup with the amp boards.
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That's ok, I've read about people having problems biasing because they still had the DB tester in circuit.
Glad you figured it out!!
Good Morning!
Still some work to do. I use the time until the new shunt regulators arrive to clean the case and prepare everything for reassembly.
But there are two bad things I found after I changed the shunt regulators.
I noticed a lot of hum without the RCA line connected.
I didn't notice it before, because the RCA cable was connected with my computer during bench testing.
I took the pcb's temporary in the case and the hum was gone.
Good, there seems to be no problem.
The second thing is not so easy. I let the amp heat up an play some music to measure tempreatures as last check of my rebuild.
The most warm/hot parts of the PSU and the amp boards are in the range of 30-40°C.
Some of the fusible resistors are at 50-60°C some at 60-70C.
The very hot resistors R334,R344 and R434,R444 of the amp boards which I changed from 3 to 5 W are running with 80°C way more cooler as the old before.
Everything fine so far, but then i measured the transistors of the 68V PSU rail which I already mentioned above.
Q710, Q711 and Q712 are running at 105°C with medium load and open case!
They will be very close to maximum during summertime and some load.
That seems to be the reason why they are failing as the NAD service told me.
I'll definitely buy some to-92 heat sinks if there is enough space for it.
Or is there a way to modify the circuit to reduce the temperature?
Still some work to do. I use the time until the new shunt regulators arrive to clean the case and prepare everything for reassembly.
But there are two bad things I found after I changed the shunt regulators.
I noticed a lot of hum without the RCA line connected.
I didn't notice it before, because the RCA cable was connected with my computer during bench testing.
I took the pcb's temporary in the case and the hum was gone.
Good, there seems to be no problem.
The second thing is not so easy. I let the amp heat up an play some music to measure tempreatures as last check of my rebuild.
The most warm/hot parts of the PSU and the amp boards are in the range of 30-40°C.
Some of the fusible resistors are at 50-60°C some at 60-70C.
The very hot resistors R334,R344 and R434,R444 of the amp boards which I changed from 3 to 5 W are running with 80°C way more cooler as the old before.
Everything fine so far, but then i measured the transistors of the 68V PSU rail which I already mentioned above.
Q710, Q711 and Q712 are running at 105°C with medium load and open case!
They will be very close to maximum during summertime and some load.
That seems to be the reason why they are failing as the NAD service told me.
I'll definitely buy some to-92 heat sinks if there is enough space for it.
Or is there a way to modify the circuit to reduce the temperature?