Can an amateur refurb and update a KA-7100?

neevo

Super Member
I've been into vinyl since I was 15 although EDM mainly and a few albums of my dads. Bizarre to hear of the affection of SL1210 MKII's as I have a couple of workhorses upstairs. However the latest project is to setup a snug at the front of the house, expand out the vinyl collection and turn it into a music room with some vintage equipment at the heart of it (it's also reignighted the fire for buying vinyl, LP's this time though). I've already sourced a Pioneer PL-155E turntable (not high end but will do until I can get the AR ES-1) and I've added a Kenwood KA-7100 to the setup too (grabbing it today):



Given the amp appears in good condition externally, I'm keen to put some time and effort into it internally. It seems to be a great amp with fantastic reviews, so I plan on keeping it for a while, plus I'm also planning some cosmetic upgrades too like CNC handles:



I'm getting my head around all the great threads on the recap/upgrade process and have come across a few:

Great write up with pics:
http://audiokarma.org/forums/index.php?threads/kenwood-ka-7100-recap-refurb.610823/

Another great write up with a parts list:
http://www.audiokarma.org/forums/in...er-kenwood-ka-7100-recap.462314/#post-5961606

An upgrade thread for minimising arcing in power relays:
http://www.audiokarma.org/forums/in...orth-5-and-an-hour-or-so-of-your-time.504673/

My plan is to:
1. Get a parts list together for replacing all the caps
2. Check the power relay when I get the amp, plus get parts to do the soft start mod
3. Have a go at doing it all

I'm pretty confident around electrics, I've wired a custom bike I'm building at the moment plus have built an RFID starter for it too. However this thread is here so I can document my journey and have a place to ask questions along the way vs clogging up someone else's thread.
 
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I did one about a year ago as my second ever recap, a KA-5700 being my first. It's a pretty easy one for a beginner, which I am. I really liked the fact that the main board was silk screened on the solder side. A good quality service manual is important. I usually look on the bay for an original, and if I can't find one a high quality copy from www.stereomanuals.com is my next choice. The downloadable PDFs aren't usually very good from those I've looked at.

I did exactly as outlined in this thread, including the full parts list. http://www.audiokarma.org/forums/in...er-kenwood-ka-7100-recap.462314/#post-5961606 I also added a relay for the power switch.
 
Great to hear, that has me a little more confident. Did the parts list miss anything or was it comprehensive?
 
I have also been thinking about the possibility of rebuilding a amp myself too. I am a Kenwood fan and I have a spare room in our house that I could put another smaller stereo in. I hope you will keep a thread going here on AK as you progress as it will give me a bit more insight from a beginners standpoint to help gage if I could handle a project like this. Will be following! :lurk:
 
. . .

My plan is to:
1. Get a parts list together for replacing all the caps . . .

My two cents: When it comes to ordering parts, don't rely completely on service manuals or someone else's parts list. Do your own careful visual inspection of every board and see what is actually in your unit, "on the ground." You may find different part values and marking errors on boards. Generally, go with what's actually installed instead of what's on the parts list.

Plus, a thorough inspection can reveal damaged components, evidence of prior repairs, bad solder joints, heat damage, possible fit problems with new parts, etc. And you get to know your new toy this way.
 
That's a really good idea. Will use the list as a guide and check off, validate when I get the amp.
 
As C.coyle said, do not rely on the parts lists there are sometimes different versions and different components on your amp, sometimes.
I am a newbie myself and already re-caped 5 amps and tuners without problems. Take your time and do one board at a time, testing results
in between each boards you re-cap. Watch for non-polarised caps too, when ordering!.
 
Thanks for linking my thread neevo, and welcome to AK :thumbsup:

I will look in on this thread as it proceeds, and do my best to answer any questions as I am familiar with the KA-7100. You have received good advice so far. I would add to that by suggesting that you do your refresh in sections, with testing and listening time after each chunk of work. That way it would be much easier to track down any problems that may develop. As mentioned by c.coyle, get to know your particular unit before proceeding. That should include ample listening time and checking of all functions before any parts are replaced.
 
Thanks Roger, I loved reading your thread as I wait for my amp to arrive, gives me ideas on what to do, however I have to admit its been a slow process getting my head around all the terminology and I am a long way away (maybe never) from understanding the circuits and what they do. Good advice on the listening part, I had planned on a fair amount of listening before any work anyway, mainly because I am excited to plug it in and hear it, plus it will also provide a baseline sound that I can maybe compare the final result to.

I will also make sure to play the amp after each board has been done as you suggested, to make sure I can isolate any issues if they arise.

A coupe of quick questions on that:

1. I've seen a few amps plugged in with the top/bottom covers off, I assume this is safe to do as long as you don't touch anything in the amp? I assume risk of electrocution is high if you go poking around in there? If so what is it best to adjust bias pots with etc? I would say a well shielded electrical screwdriver at least or maybe a plastic implement of some sort?
2. When working on amplifiers, how is it best to ensure the caps are discharged? Some of them have a fair bit of storage capability and I am keen to make sire they've had the opportunity to discharge properly prior to any work being done.

edit: a third one for you actually if that's ok! Why did you replace the trim pots in your amp? Was it to replace them with ones that were easier to get fine adjustment? I think I remember some adjustments you had to do to legs of them, but I got a bit lost at that point. So I wasn't sure why to replace them and what challenges you had to overcome when you put the new ones in.
 
I bought some inexpensive ceramic screwdrivers from the bay for adjusting trimpots. I also have a favorite metal screwdriver that I covered with heatshrink tubing

I use a 100W incandescent bulb to discharge the big caps, screwed into one of these with alligator jumpers. http://i.ebayimg.com/images/g/oggAAMXQ0pNRn2Vk/s-l300.jpg

...which is also part of my dim bulb tester, which I recommend you research and make (easy, cheap). I always power up through the DBT after making changes before I plug it straight into the wall
 
1. I've seen a few amps plugged in with the top/bottom covers off, I assume this is safe to do as long as you don't touch anything in the amp? I assume risk of electrocution is high if you go poking around in there? If so what is it best to adjust bias pots with etc? I would say a well shielded electrical screwdriver at least or maybe a plastic implement of some sort?


When the amp is powered up there are different types of voltages present. There is incoming AC that comes from the wall plug and is routed through the power switch and is present a couple other places. That is @ 120V in the US, don't know about OZ. That is where there is the greatest possibility of physical harm to you. There are also DC voltages, usually at least two different ones, that are created and used inside the amp. With those the most likely problem is shorting a voltage to ground or to a lower voltage. Remember that electricity only flows between two points. So when touching a voltage, whether with a finger or with a metal tool, actual electrical current does not happen until that voltage finds a path to ground (or another point of lower voltage), which could be a kitchen sink, metal pipe, concrete floor (etc) in the house, or some other component or metal part inside the amp.

For adjusting bias or DC offset trimpots, the ideal tool is a small plastic (or ceramic as gort69 suggested) screwdriver. Second choice would be a small common metal screwdriver with electrical tape insulating all but the very end. For these adjustments (bias), probably the most common mistake is to use regular alligator clips, that typically are supplied with a DMM, to connect to test points inside the amp. Much, much safer to use mini-grabbers, the best known brand of which is Pomona. Please google those, and get a pair. Other brands do exist in case Pomona is not available to you.

2. When working on amplifiers, how is it best to ensure the caps are discharged? Some of them have a fair bit of storage capability and I am keen to make sire they've had the opportunity to discharge properly prior to any work being done.

Measure voltage across the main filter caps. " Across" means measuring between the positive and negative terminals of the caps. Discharging caps can be done with resistor. Google Advanced Search using "audiokarma.org" as site or domain can be your best friend for more about this and practically anything else you may want to know more about. (i.e. I am not certain what size of resistor to suggest off the top of my head)

edit: a third one for you actually if that's ok! Why did you replace the trim pots in your amp? Was it to replace them with ones that were easier to get fine adjustment? I think I remember some adjustments you had to do to legs of them, but I got a bit lost at that point. So I wasn't sure why to replace them and what challenges you had to overcome when you put the new ones in.

Trim pots are components that tend to get flaky with age. Trimpots are in key spots inside the amp's circuitry. Replacing them is best practice when restoring an amp. Old trimpots are usually open frame, meaning that their conductive parts inside are exposed to air, and that means that corrosion can (and does) develop over time. Modern Bournes pots, as used by many here, are sealed and they are also higher quality in other ways vs original vintage era trimpots. Adjusting offset and bias are much easier with new pots than with the originals.

In fact this reminds me of another thing to suggest before diving into a refresh project. IMO it is good thing to adjust bias and DC offset with the amp in its original state before doing any new work. This could possibly reveal existing issues. Generally, it is best to ID and resolve existing issues before replacing parts, that way you would not be 2nd guessing your new work. In addition to that, adjusting the amp in original state will give you a point of comparison once the new trimpots are in place.
 
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Thanks all, loads there to get going on. I found a pretty good page for reading up on a DBT:

http://antiqueradio.org/dimbulb.htm

Found it the easiest to understand with avert simple wiring diagram. I'm not sure why they chose to put the switch after the bulb, I thought putting it before would make more sense.

Will certainly look at adjusting bias etc beforehand Roger. Will grab some of those mini clips and a plastic screwdriver to cover my arse in the safety department.
 
Well after a fair wait (I also went back to the UK too), I'm now finally in possession of my Kenwood amp. Initial impressions:

1. It blows the previous setup I had out of the water. Much more gain, detail and the highs/lows are much more to my liking with a little boost on each through the tone control.
2. It's not as perfect as the images suggest but it's stuff I can tidy up. The front is fairly good and some minor touching up could get it back to perfect.
3. Switches and pots need a deep clean. I've got an intermittent left channel drop out but I suspect it's linked to the tone control switch so I will see what a clean on the switches and pots yields against that issue.
4. One of the a speaker posts doesn't tighten up. Any thoughts on how to fix or potentially upgrade all the speaker connections to something more modern?
5. One of the feet on the amp is broken (I knew this beforehand though). Anyone have an old spare lying around? I would like to get the amp back to great condition.
6. Some of the knobs could do with a refurb as they have some Alu corrosion in some spots. Whats the best solution for getting those back in top form without ruining the finish on them.
7. Some of the lettering on the front has worn off. Can in fill in the recesses for the lettering with any ink of some sort to get them back in A1 condition?
 
I couldn't bear to listen to the amp with an intermittent left channel so this evening I took it into the shed and decided to have a tinker.

Up on the bench:



Thought I would tackle some of the grime on the switches, knobs and faceplate:





I think this may be paint, not the lettering worn off:



Bit of corrosion on this polished inner ring:



Behind the faceplate was even more dirt and grime to be found:



Pulling off all the knobs and switches I found 2 that were apart. These were fixed with expoxy glue:



All the faceplate parts got a good scrubbing:



Shiny!



Whilst I had it open I thought I would try my hand at some maintenance:



I had a go at balancing the voltage and doing the bias with my new ceramic screwdriver set:



I was surprised that the voltages on the left and right were about 5-6v but I tried my best to get them down to zero. Pretty much an impossible task with the adjustment range on the trim pots but this is as good as I could get (left then right):





Next the bias and I was happy to get them both bang on 20 as per the service manual.

Lastly I cracked out the Deoxit and set to cleaning the pots and switches. I ran out of the stuff only having done the left side, but I am happy to report after plugging it in the left channel is now working!

Not bad for an evenings work. Next up I need to do the rest of the pots plus crank up the air compressor to clean out the insides of all the dust.

When I do that I will list all the parts in the machine ready for my recap!
 
Nice pictures. You have a good, methodical approach to your project. I don't know how much help I can be on your current batch of questions, especially those relating to cosmetics as that is not my primary area of interest. But I can offer a couple thoughts...

Well after a fair wait (I also went back to the UK too), I'm now finally in possession of my Kenwood amp. Initial impressions:

1. It blows the previous setup I had out of the water. Much more gain, detail and the highs/lows are much more to my liking with a little boost on each through the tone control.
2. It's not as perfect as the images suggest but it's stuff I can tidy up. The front is fairly good and some minor touching up could get it back to perfect.
3. Switches and pots need a deep clean. I've got an intermittent left channel drop out but I suspect it's linked to the tone control switch so I will see what a clean on the switches and pots yields against that issue.
4. One of the a speaker posts doesn't tighten up. Any thoughts on how to fix or potentially upgrade all the speaker connections to something more modern?
5. One of the feet on the amp is broken (I knew this beforehand though). Anyone have an old spare lying around? I would like to get the amp back to great condition.
6. Some of the knobs could do with a refurb as they have some Alu corrosion in some spots. Whats the best solution for getting those back in top form without ruining the finish on them.
7. Some of the lettering on the front has worn off. Can in fill in the recesses for the lettering with any ink of some sort to get them back in A1 condition?


1. Good!

2.

3. ...also input selector switch, speaker selector switch...any switch really, can cause trouble. If you have isolated the issue that is great. But is still a good idea to do at least all of the switches to which you can reasonably gain access. (If I recall correctly, there are a couple that are sealed).

4. If something is stripped it may not be possible to fix. Replacement binding posts, banana type usually, have been done on lots of amps and reported on many times here. Not sure if Ka-7100 has been done. Search can help...and google advanced search is more powerful than AK's search, especially when you start using multiple search terms. But I like AK's search for searching thread titles, i.e. "binding posts", maybe "speaker jack", should give you some hits.

If I was keeping the amp for myself, I would probably just use the B speaker (assuming the A side has the stripped part). But I understand that it is also nice to have things as perfect as possible.

5. I don't have a spare foot, sorry. Maybe someone else has one. If you were a subscriber, you would have access to Barter Town...this type of request is common there.

6. Don't know....again suggesting search if you are a perfectionist. In my project Ka-7100 I did use Deoxit D100L and small, stiff, plastic brushes on the face plate and knobs & switch covers...a highly unusual approach but it worked for me. YMMV

7. sounds reasonable if done with care. Can not comment further though.



Hopefully others will chime in with some additional thoughts....
 
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I have a fantastic ability to start a small job and end the day going way into the deep end. Today was no exception.

I started off the day with the left channel problem rearing its head again. Appears the tone switch fix was not the cause, I played around a bit with the amp and discovered the problem was coming from the source selector. I don't have any Deoxit at the moment but some vigorous use got the channel working again, so it's a decent proof of concept for what I need to fix, however I have plans to sort all the remaining pots, switches, so I assume this one should be problem of the past soon.

I was keen to clean the insides today like I had done with the outsides last night. I also noted the clever way the front and back panels flipped in the service manual so I kicked off today's amp session with some nudie pics:



The grime was pretty thick, fluffy and icky and the foam supports for the main board had perished on one side. Will need to source a suitable replacement material:







I used my air compressor, an old kids toothbrush and a rag to clean out the amp as best as possible. I got it pretty damn clean except for some areas of the boards where I didn't want to damage anything, so there I used air alone.

Flip style for better access:



The amp is in great condition inside, except for these, which look like they've had some time underwater:



I hope they are fine to run, or should I look to find replacements (are they even available)?

This is where I took a left turn instead of buttoning it back up. I thought to myself "since I've got it open this far, I should make a list of all the caps I need". I had been given a brilliant list from 39cross and I thought it best to check against all the caps in my amp to make sure there were no changes.

So I grabbed my laptop (Excel) and opened her up even more:





This is where I had a tiny whoopsie moment. I was trying to get the earth lead from the pre amp off the chassis and my soldering iron work was not accurate enough and I ham fistedly broke this cap on the lead:



Well that's it then! I'm stripping it right down for recapping now! I've got to check the caps anyway, I may as well list them, order the parts (along with the broken cap) and get it back together once it's all done.

So out came the tone board too:



That's where I finished the evening. Now I'll be checking all the parts to order and get some money over to Mouser asap so I can get them here ready for the big job.

I've also ordered some other parts to do an upgrade on the speaker terminals. Hopefully they come in soon so I can do everything and have this slab of amp goodness in prime condition for another 30 years service.
 
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First question on parts ordering. Anyone know why there are multiple options for the same capacitor? Im searching for Nichicon PW's on Mouser, 100uf 35 VDC for example and multiple options turn up with some twice the price as the top one. No idea why and what I should be buying.
 
Anyone know why there are multiple options for the same capacitor?

You'll often find this, but if you look closely, for a given capacitor manufacturer/range, there will be small variations in diameter, height, lead spacing (pitch), whether they are surface mount or wire ended, and also packaging options, these many options will sometimes cause it to appear that the same component may be listed several times.

For that particular component there are 4 choices, and it seems to be packaging that is influencing the price, there is a 5mm pitch version, the others being 3mm pitch. Personally I would think about the 5mm pitch one as it may more closely match the pitch of the original, failing that the one that isn't on 'cut tape' as it is cheaper. Overall size can be very important but the variations here are so small it shouldn't matter.
 
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Ditto what Hyperion said. The big parts distributors primarily serve industrial/institutional sized customers. Some things that could be relevant to those customers would mean nothing to audio hobbyists, except for odd price variations which do not make apparent sense from our perspective.
 
No problem. Thanks all. It was a bit confusing so in the end I looked for 105 degree caps, then after that I picked the top one generally. Certainly not a simple process for the uninitiated.

Let alone the choice of cap manufacturer, model etc.

In the end I went with Nichicon mainly, with a few Elnas dropped in where it had a more suitable option for uF/v.

Hopefully ready to pull the trigger in the next few days.
 
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