This is interesting. All of the caps I've tested recently have been nearly spot on the stated value. Granted, they're mostly smaller values, but I was surprised.
Kenwood 700M - replace filter caps?
- Thread starter davstev
- Start date
This is interesting. All of the caps I've tested recently have been nearly spot on the stated value. Granted, they're mostly smaller values, but I was surprised.
I’ve restored/upgraded 3 amps and 1receiver and noticed a big improvement in bass response in all 4 units. Not saying it was all filter cap related but i like it now much better. I look at it this way, if you’re replacing other 45 y/o components why not the big ones too?!
I am OCD about this stuff so maybe that’s part of it.
I am OCD about this stuff so maybe that’s part of it.
corbin
Well-Known Member
Kenwood used very high quality capacitors, replacing caps that measure in spec is unnecessary.
Vintage audio equipment is like cars in that they are only original once. I don't agree with the shotgun approach of replacing all caps. Better to be more conservative and preserve the original unit with reliability upgrades such as re-seating output transistors, setting bias, and selectively replacing only out of spec parts.
Unquestioned recapping is popular here on AK, but a lot of techs don't agree with it.
Vintage audio equipment is like cars in that they are only original once. I don't agree with the shotgun approach of replacing all caps. Better to be more conservative and preserve the original unit with reliability upgrades such as re-seating output transistors, setting bias, and selectively replacing only out of spec parts.
Unquestioned recapping is popular here on AK, but a lot of techs don't agree with it.
Mike Sweeney
Well-Known Member
Ok.. a few points.. these caps are 35 plus years old. Past their state life span with unknown stresses. For the really big filter caps, many ESR testers are useless. Some will go to 22000uf but quite a few of the cheap imports can barely get to 6800uf and most are less. And ESR really doesn't tell the story. What you need to test is leakage. I've had more than a few filter caps test "ok" on the ESR and absolutely fail a leakage test. When the cap is rated at 80 volts and cannot get past 25 volts without leaking like a sieve, thats a real problem. Even more of the smaller caps that test good on ESR fail a leakage test. And some that seem to be good have really high internal resistance. I just put up a few images from a Marantz where the tone board has some 3.3uf caps. They worked.. sorta.. ESR was over 20 ohms.. the new wima caps are .2 ohms and within 2% vs the 40% of the OEM caps. The tone now sounds clear and bright and the pots actually make a difference 
And these WERE quality caps in the day.. but old is old.
The difference between vintage cars and audio gear is if they original oil seal breaks, it doesn't burn down your house like a shorted main filter can do. I pulled one unit apart and found the fuse wrapped in foil
What it CAN do is destroy an irreplaceable block like burning up irreplaceable boards in your audio gear. I've restored cars.. many of them and 95% of the owners want working parts at least on the internals and systems like brakes. They don't care if the seals or valves or other bits are original or not. They do care a lot of about OEM paint or interior, neither of which will kill them if it fails. Very good money is paid for a "survivor" and rightly so. The 5% that want a period correct authenticate restoration that wont be driven.. ever is a different topic. Which is why even worn parts for planes and cars can be valuable to that type of collector. I just saw a "telephone" system at a very old movie house that was all tubes. Wont ever work again but they had the right parts and the right tubes since it was display only.
It comes down to what you plan to do with it. Is it going to be a display hanger queen? turned on very occasionally to show off the lights? Thats one thing. It's quite another if you plan to really use it as a "daily driver" or a demo piece to show off what "old" audio gear can really do
I fall into the camp that any gear I own has to work and work safely. If that means a shotgun cap replacement, so be it. Thats my own choice. I carry that over into gear I work on. I don't "repair" old gear. It's either restored to working order or I pass on it. I don't need the liability of old parts failing after I touched it. Again, it's my personal choice.. your milage may vary.
And these WERE quality caps in the day.. but old is old. The difference between vintage cars and audio gear is if they original oil seal breaks, it doesn't burn down your house like a shorted main filter can do. I pulled one unit apart and found the fuse wrapped in foil
What it CAN do is destroy an irreplaceable block like burning up irreplaceable boards in your audio gear. I've restored cars.. many of them and 95% of the owners want working parts at least on the internals and systems like brakes. They don't care if the seals or valves or other bits are original or not. They do care a lot of about OEM paint or interior, neither of which will kill them if it fails. Very good money is paid for a "survivor" and rightly so. The 5% that want a period correct authenticate restoration that wont be driven.. ever is a different topic. Which is why even worn parts for planes and cars can be valuable to that type of collector. I just saw a "telephone" system at a very old movie house that was all tubes. Wont ever work again but they had the right parts and the right tubes since it was display only. It comes down to what you plan to do with it. Is it going to be a display hanger queen? turned on very occasionally to show off the lights? Thats one thing. It's quite another if you plan to really use it as a "daily driver" or a demo piece to show off what "old" audio gear can really do
I fall into the camp that any gear I own has to work and work safely. If that means a shotgun cap replacement, so be it. Thats my own choice. I carry that over into gear I work on. I don't "repair" old gear. It's either restored to working order or I pass on it. I don't need the liability of old parts failing after I touched it. Again, it's my personal choice.. your milage may vary.
I would like to replace mine, and will once I find by good authority the proper replacement. A few hundred bucks is nothing compared to what this amp means to me. I have owned it since new and plan to be playing it right up to the end (a very long time from now with any luck). One thing I don't want to do is run out and get just any big ass cap that might work or might not.. Not being that techy and living far out away from anyone who might have the skills I am content to wait.. Hopefully someone will give out the part numbers of filter caps that do indeed work well with the 700MOk.. a few points.. these caps are 35 plus years old. Past their state life span with unknown stresses. For the really big filter caps, many ESR testers are useless. Some will go to 22000uf but quite a few of the cheap imports can barely get to 6800uf and most are less. And ESR really doesn't tell the story. What you need to test is leakage. I've had more than a few filter caps test "ok" on the ESR and absolutely fail a leakage test. When the cap is rated at 80 volts and cannot get past 25 volts without leaking like a sieve, thats a real problem. Even more of the smaller caps that test good on ESR fail a leakage test. And some that seem to be good have really high internal resistance. I just put up a few images from a Marantz where the tone board has some 3.3uf caps. They worked.. sorta.. ESR was over 20 ohms.. the new wima caps are .2 ohms and within 2% vs the 40% of the OEM caps. The tone now sounds clear and bright and the pots actually make a difference
The difference between vintage cars and audio gear is if they original oil seal breaks, it doesn't burn down your house like a shorted main filter can do. I pulled one unit apart and found the fuse wrapped in foil
It comes down to what you plan to do with it. Is it going to be a display hanger queen? turned on very occasionally to show off the lights? Thats one thing. It's quite another if you plan to really use it as a "daily driver" or a demo piece to show off what "old" audio gear can really do
I fall into the camp that any gear I own has to work and work safely. If that means a shotgun cap replacement, so be it. Thats my own choice. I carry that over into gear I work on. I don't "repair" old gear. It's either restored to working order or I pass on it. I don't need the liability of old parts failing after I touched it. Again, it's my personal choice.. your milage may vary.
Thanks for your input!
Mike Sweeney
Well-Known Member
The schematic is available on HiFi Engine.. 170 watts per channel? nice
The key caps are here. C301 AND C302 which are 22000uf at 80Volts ( big assed caps
) and C304 and C303 which are 1000uf at 80 volts. All four are electrolytics are for filtering the main powerMouser has 30 in stock for the 22000uf 80 volts.. assuming they fit and these are screw tops. I'm assuming that is what you have.. a lot of assumptions here.
https://www.mouser.com/ProductDetai...=sGAEpiMZZMtZ1n0r9vR22e9Nsyw693MC3Tj3u4L9oEE=
A bit pricy at 50 bucks a cap, the 1000uf 80 volts will be significantly cheaper. The schematic is nicely laid out so it would be pretty easy to build a shopping list from it. Add some DeOxit for the pots and some D100 liquid DeOxit for socking the switches, thats half the battle. It looks like a very nice amp on paper.
Blue Shadow
Waiting for Vintage Gear from this century
The op mentioned a 3" diameter cap he was thinking about. I linked a different 3" cap. But I don't know the diameter and no one has said that the originals are 3" in diameter or posted a length limit. something to consider when replacing the big caps.
Anyone know the size of the big caps in the 700M and if the unit can easily use a different size cap?
Anyone know the size of the big caps in the 700M and if the unit can easily use a different size cap?
All I can say is sumb$&ch that’s a big cap!
3” diameter! Crazy.
3” diameter! Crazy.
A few questions from a now concerned 700M owner.
In general how often do these big filter caps fail and cause damage to the rest of the amp? Is it an eminent threat? Or something less?
Understanding that these big caps can't easily be tested for spec, is there something I should be listening for?
Can physical condition be a predictor of failure? If the caps are not leaking and otherwise look fine should there be less concern?
Getting new caps that are not identical in specifications and ratings make me nervous of screwing up my otherwise great sounding amp. What are the risks of killing the patient here either by bungling the replacement or not doing anything?
It is on my long term plan to replace these but just a little leery of rushing into something I will regret.
In general how often do these big filter caps fail and cause damage to the rest of the amp? Is it an eminent threat? Or something less?
Understanding that these big caps can't easily be tested for spec, is there something I should be listening for?
Can physical condition be a predictor of failure? If the caps are not leaking and otherwise look fine should there be less concern?
Getting new caps that are not identical in specifications and ratings make me nervous of screwing up my otherwise great sounding amp. What are the risks of killing the patient here either by bungling the replacement or not doing anything?
It is on my long term plan to replace these but just a little leery of rushing into something I will regret.
Blue Shadow
Waiting for Vintage Gear from this century
Reading Conrad's posts in the Capacitor Abyss, the big caps are the least likely to die and he has measured numerous ones that were just fine decades after initially installed. I don't know all the failure modes so can't say if it will put your unit in peril of further damage. It is not an eminent threat as a decrease on performance is probably what will happen. This will lead to less performance from the amp that probably won't be audible until you install new caps. Physical issues have been discussed as has the process of selecting caps that are going to be good substitutes, a bit higher voltage/capacitance, if needed. Replacing these caps is only going to be a regret if you
-get caps that don't fit right and look weird/cobbled together to make them work (buy the right size, and they drop right in)
-get caps that have too high a capacitance, well above the installed caps, causing in rush current to fill them on turn on to cause damage to the parts that current flows through
-don't hear a nice improvement in bass response or overall sound quality of the amp...but this is just a regret of the financial kind and not bad but it comes with the peace of mind that the change has been done.
-get caps that don't fit right and look weird/cobbled together to make them work (buy the right size, and they drop right in)
-get caps that have too high a capacitance, well above the installed caps, causing in rush current to fill them on turn on to cause damage to the parts that current flows through
-don't hear a nice improvement in bass response or overall sound quality of the amp...but this is just a regret of the financial kind and not bad but it comes with the peace of mind that the change has been done.
Thanks @Blue Shadow and @rjsalvi for the good info.. I have the M setting in a place with 3 inches of room above it, open on all sides and back and no heat producing boxes near it either. Adding to that, this is a very cool running amplifier! It is driving JBL L200's which are 8ohm, and quite sensitive, 80db @15 feet with 1 watt. not sure what that translates to in 1 meter/1watt, but probably in the mid to high 90 db's. Also I seldom play this amp anything over 1 watt really. A 10 watt amp would probably suit me fine with enough headroom, but I like the 700M anyway! Already paid for (with 1975 dollars no less) and I admit to a touch of nostalgia in wanting to keep this amp going.. It is my only amplifier at the moment and plays for several hours every evening without any audible issues at all. I do like dynamic music and it seems to deliver that with much authority, but again, easy duty with these speakers..
Part of my hesitation in this and what keeps me from doing this sort of work myself is due to my eyesight not being that good, particularly up close. That and soldering skills (which I shouldn't even need with replacing these) is lacking. Money isn't that much of an issue in this. Whatever is the best I can get is what I want.
So in looking for a cap, mechanical size, lead attachment, mounting, etc, Then rated voltage or above, rated capacitance or above but not too much as to stress the other components with inrush current on startup.
Then the issue of startup after the caps are installed. I presume I would turn the bias pots back to zero and use the DBT? Then do the bias adjustment procedure, check DC offset etc.
This looks like a next winter project as summertime is too busy
Part of my hesitation in this and what keeps me from doing this sort of work myself is due to my eyesight not being that good, particularly up close. That and soldering skills (which I shouldn't even need with replacing these) is lacking. Money isn't that much of an issue in this. Whatever is the best I can get is what I want.
So in looking for a cap, mechanical size, lead attachment, mounting, etc, Then rated voltage or above, rated capacitance or above but not too much as to stress the other components with inrush current on startup.
Then the issue of startup after the caps are installed. I presume I would turn the bias pots back to zero and use the DBT? Then do the bias adjustment procedure, check DC offset etc.
This looks like a next winter project as summertime is too busy