Here in the midwest we have had some rather severe weather in the last month or so. A few weeks ago a lightning strike exploded a transformer close to the house. The only victims were a microwave, and my Kenwood 700M amp, which was plugged into a non-protected outlet whist I changed some bias pots. Amp shut down and would not power back on.
Fuses looked good, and I eventually discovered that the 4-ohm 10W resistor used to step-start/limit inrush for the power supply was opened. Additionally, the right channel had one NPN and one PNP output device shorted dead (can't seem to figure out how that happened, but...??) Replaced the step-start resistor, and changed out all the TO-3 output devices (12 of 'em) with Toshiba 2SB554/2SD424 transistors, since I don't have the original NEC 2SB539/2SD287 handy. Also diode-tested the drivers, and they looked OK from that perspective.
This brought it back to life, and all seemed cool. However, I get a very intermittent 'pop' in the right channel...kinda sounds like a popcorn kernel when cooked (best description I can come up with). It is not super-loud, but clearly audible above the music at moderate listening levels. The 'pop' might happen 10 minutes after turning it on, or it may work great for 5 hours, then 'pop', and not happen again for hours.
I suspect that a driver has a problem. I thought capacitor, but all the electrolytics have been replaced with new Panasonic FC's before the transformer fiasco. Seems doubtful that a cap is the problem.
Going to order some 2SA1930/2SC5171 TO-220 devices and replace the drivers with these, hoping to cure it that way, but if I have to replace parts at random this could take forever.
Any other thoughts/ideas?
Thanks guys!!!
Fuses looked good, and I eventually discovered that the 4-ohm 10W resistor used to step-start/limit inrush for the power supply was opened. Additionally, the right channel had one NPN and one PNP output device shorted dead (can't seem to figure out how that happened, but...??) Replaced the step-start resistor, and changed out all the TO-3 output devices (12 of 'em) with Toshiba 2SB554/2SD424 transistors, since I don't have the original NEC 2SB539/2SD287 handy. Also diode-tested the drivers, and they looked OK from that perspective.
This brought it back to life, and all seemed cool. However, I get a very intermittent 'pop' in the right channel...kinda sounds like a popcorn kernel when cooked (best description I can come up with). It is not super-loud, but clearly audible above the music at moderate listening levels. The 'pop' might happen 10 minutes after turning it on, or it may work great for 5 hours, then 'pop', and not happen again for hours.
I suspect that a driver has a problem. I thought capacitor, but all the electrolytics have been replaced with new Panasonic FC's before the transformer fiasco. Seems doubtful that a cap is the problem.
Going to order some 2SA1930/2SC5171 TO-220 devices and replace the drivers with these, hoping to cure it that way, but if I have to replace parts at random this could take forever.
Any other thoughts/ideas?
Thanks guys!!!
It is possible that a diode may have been damaged. I had the same problem with a Zener diode. It would change a half of volt when it heated up. It had the same intermittent popping. It was the last thing I changed after all the transistors.
Reply appreciated. I would also suppose that a transistor junction could do the same thing.
Anyone else ever experinaced anything like this or have any other ideas?
Anyone else ever experinaced anything like this or have any other ideas?
Is it related to temperature? Maybe you have a dry joint or dodgy component moving once it gets warmed up?
Not sure. Again, it could happen at any time. I turned it on last night...speakers connected but no input to the amp. Sat and surfed the web while I listened. Popped after about 1 hour on, and again about 2 hours later. Today I turned it on, and it popped after about 10 minutes, and not once more for the next 4 hours.
Not sure what to think. The reason I suspect drivers is that...on almost every amp I have worked on where the outputs got smoked, there was usually a driver (or all the drivers) that needed to be replaced also.
Might start doing some serious resoldering on the board, but I still suspect damaged component, as all was fine until the lightning strike...
Not sure what to think. The reason I suspect drivers is that...on almost every amp I have worked on where the outputs got smoked, there was usually a driver (or all the drivers) that needed to be replaced also.
Might start doing some serious resoldering on the board, but I still suspect damaged component, as all was fine until the lightning strike...
I have a feeling maybe a component is bad now from one of those POPS...also check all you wireing if you had things apart....I also realized later on that a cap was blown in my cossover causing the pops and it did eventually destroy a driver
Kenwood amps of that vintage used to use uPA63 input dual jfets which might be the cause of your problem, assuming it uses this fet for the differential. The uPA63 is no longer available you could try a 2SK109.
Regards,
Jam
Regards,
Jam
Love the avitar Joe! 🙂
The amp is driving four Dynaudio 20W75's as part of a biamp setup...no crossover after the amp.
Jam, this one uses PNP bipolars for the diff-pair. Going to post the redrawn schematic if anyone wants to take a look...
BTW, can anyone speculate as to why the diff-amp current is set so low? Looks to be about .5mA (250µA per transistor). Seems exceedingly low to me...
The amp is driving four Dynaudio 20W75's as part of a biamp setup...no crossover after the amp.
Jam, this one uses PNP bipolars for the diff-pair. Going to post the redrawn schematic if anyone wants to take a look...
BTW, can anyone speculate as to why the diff-amp current is set so low? Looks to be about .5mA (250µA per transistor). Seems exceedingly low to me...
Look at the diode strings. They are known to go intermittent. You should be able to replace them with discreet diodes. I have had some sucess replacing them with 1N4007's.
Regards,
Jam
P.S. I would still look at the diff pair.
Regards,
Jam
P.S. I would still look at the diff pair.
Really? Hmmmm....
I assume you are speaking of the biasing diodes. I actually have some NOS STV-3H thermal diodes here. Might replace the two that are in the suspect channel.
Have ordered some replacement drivers also. Going to wait till the parts come in and start shotgunning. If this wasn't such an rare and expensive amp I'd probably just sell it, but when working it sounds too good to not attempt to track down the problem.
BTW, the drawing has a minor error in the text concerning the diodes..D5 is thermal coupled to Q11, and D4 thermal coupled to the Q1 output transistors....renamed the transistors after drawing and forgot to change text....no biggie.
I assume you are speaking of the biasing diodes. I actually have some NOS STV-3H thermal diodes here. Might replace the two that are in the suspect channel.
Have ordered some replacement drivers also. Going to wait till the parts come in and start shotgunning. If this wasn't such an rare and expensive amp I'd probably just sell it, but when working it sounds too good to not attempt to track down the problem.
BTW, the drawing has a minor error in the text concerning the diodes..D5 is thermal coupled to Q11, and D4 thermal coupled to the Q1 output transistors....renamed the transistors after drawing and forgot to change text....no biggie.
You could also swap suspect parts between left and right channels and when the problem switched channels.....
Regards,
Jam
Regards,
Jam
Well, input pairs need swapped anyway, as DC offset is a bit high on one channel (60mV, too much for me), so in go a few pairs of 2SA872A's (have not added the DC offset circuit as shown in the schematic yet).
The transistors in the VAS stage have the odd discoloration on the legs, so I plan on replacing with 2SD756. I already ordered drivers (2SA1930 / 2SC5171), so that about covers the main amp board. Only the compound diodes and the IV limit transistors will remain, as well as the caps and resistors. Might go ahead and replace the compound diodes whle I'm in there...tired of pulling this thing apart. Easy to work on and all, but I'm about to wear out the damn screws.
The transistors in the VAS stage have the odd discoloration on the legs, so I plan on replacing with 2SD756. I already ordered drivers (2SA1930 / 2SC5171), so that about covers the main amp board. Only the compound diodes and the IV limit transistors will remain, as well as the caps and resistors. Might go ahead and replace the compound diodes whle I'm in there...tired of pulling this thing apart. Easy to work on and all, but I'm about to wear out the damn screws.
Since you have it open you could remove the VI limit for better sound. I could be wrong but my bet is one of the compound diodes is defective.
Regards,
Jam
P.S. A 10uf quality cap across the bias network could also help.
Regards,
Jam
P.S. A 10uf quality cap across the bias network could also help.
Well, don't know if anyone really cares, but I went on my hunch (keeping in mind your suggestions Jam), and replaced the pre-drivers Q9 and Q10 with 2SB649A and 2SD669A, and replaced the drivers Q11 and Q12 with 2SA1930 and 2SC5171. Had the amp on for two days and heard nothing wierd so far...
...think I might have got it.
...think I might have got it.
OK, I may very well be talking to myself, but that's nothing new around here.
After replacing the drivers, I didn't hear anything odd at all for 3 or 4 days. Thought for sure I had it fixed. Then late one evening, 'POP!'. Crap! My heart fell...I have hammered and hammered on this thing, and the 'pop' just keeps coming back.
Alrighty, taking Jam's lead, I swap the left and right amp boards. Figure that if the multiple-junction diode is bad on the amp board (there's another one on the main heatsink) the problem will change to the left channel. Power it on and in less than an hour I get a 'pop', still in the right channel.
OK then, perhaps the multiple-junction diode on the output transistor heatsink is flaky. So I swap the entire heatsink assembly from left to right. Power on. It takes hours, but I finally hear a 'pop'. Crap!
So now I've eliminated the amp boards and output devices, as well as determining that the multiple-junction diodes are fine. All that is left is the active input high-pass filter (subsonic, one transistor per channel configured as an emitter-follower with a switchable path through the filter caps or around them for full range), the two power supplies (+/- 70V, and +/- 65V), the output relays/ Zobel network, and the internal wiring.
Replaced the output relay. No dice. Popped after a couple of hours listening.
So I spend a week thinking about it. When the whole thing first blew up, and I had to replace the soft-start resistor, I had to unscrew the rear panel and tilt it back to get to the resistor. What if the a ground or signal wire is broken/shorted/damaged somehow? As a test, I disconnected the input to the amp boards, and provided a proper ground for the diff-pair (they get their ground from this cable). Two days of listening, and no 'pop'. OK, now I'm homing in...the 'pop' is originating at the input somewhere.
Pull the amp apart once again
and unscrew the back panel. Sure enough, the shield ground for the right channel is broken. Very small solid wire, almost impossible to see. I resolder it up, and for the last two days all has been cool.
Hot Damn!!
Bottom line: Old amps are filled with solid wire that breaks easily. Lots of fussing with the amp can easily break wires that are seriously tough to find later. Check the sucker good before reassembly, might save you from going half-nuts like me...
After replacing the drivers, I didn't hear anything odd at all for 3 or 4 days. Thought for sure I had it fixed. Then late one evening, 'POP!'. Crap! My heart fell...I have hammered and hammered on this thing, and the 'pop' just keeps coming back.
Alrighty, taking Jam's lead, I swap the left and right amp boards. Figure that if the multiple-junction diode is bad on the amp board (there's another one on the main heatsink) the problem will change to the left channel. Power it on and in less than an hour I get a 'pop', still in the right channel.
OK then, perhaps the multiple-junction diode on the output transistor heatsink is flaky. So I swap the entire heatsink assembly from left to right. Power on. It takes hours, but I finally hear a 'pop'. Crap!
So now I've eliminated the amp boards and output devices, as well as determining that the multiple-junction diodes are fine. All that is left is the active input high-pass filter (subsonic, one transistor per channel configured as an emitter-follower with a switchable path through the filter caps or around them for full range), the two power supplies (+/- 70V, and +/- 65V), the output relays/ Zobel network, and the internal wiring.
Replaced the output relay. No dice. Popped after a couple of hours listening.
So I spend a week thinking about it. When the whole thing first blew up, and I had to replace the soft-start resistor, I had to unscrew the rear panel and tilt it back to get to the resistor. What if the a ground or signal wire is broken/shorted/damaged somehow? As a test, I disconnected the input to the amp boards, and provided a proper ground for the diff-pair (they get their ground from this cable). Two days of listening, and no 'pop'. OK, now I'm homing in...the 'pop' is originating at the input somewhere.
Pull the amp apart once again
and unscrew the back panel. Sure enough, the shield ground for the right channel is broken. Very small solid wire, almost impossible to see. I resolder it up, and for the last two days all has been cool.Hot Damn!!
Bottom line: Old amps are filled with solid wire that breaks easily. Lots of fussing with the amp can easily break wires that are seriously tough to find later. Check the sucker good before reassembly, might save you from going half-nuts like me...
Echowars...I hate to say it but I posted in another thread about a similar problem with my sony receiver
.....Its hard to keep reiterating but I`m happy you have it resolved
DIRT®
.....Its hard to keep reiterating but I`m happy you have it resolvedDIRT®
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