KA-8006 Re-cap & Repair Cost

39cross said:

Neat stuffing job! How do you re-attach the can top to the base?

I also liked those pictures of the filter caps with the new wire extensions, that's a handy trick.

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I used JB Weld to put the two pieces together. Then i ground it down a bit after it cured and gave the entire can a shine with a scotch bright pad. Prior to cutting I made several sharpie marks so I could line it back up properly. The pink stuff on the inside of the base is Bondo. I wanted to make sure there was a little rim of something to back where the metal-metal joint would be. It also provided a little bottom weight since the caps had to be stacked, and covered some "replacement" metal tabs I needed to fashion since some of the twist-lock tabs broke trying to get it out of the chassis. I didn't have a hot enough iron at the time to do a great job of desoldering the massive blobs from the factory.

The extensions of the snap-in caps really works well using 14/2. Just the right gauge to fit the existing holes, and being soft copper, you can bend them a little like the straight leads of other caps for a little extra mechanical stability prior to soldering. Then just snip the excess.

Thanks for the explanation. I wondered about the pink stuff, couldn't tell if it was original or not.

OK, back to the 8006...

I wish I lived closer to Myrtle Beach. There's a KT-8007 KA-8006 combo for sale on CL for a decent price.

Since I own a couple of 8006's, once since new, I am subscribing to this thread! Looking forward to the progress on this project.

I'm Such a Dummy

I pulled out my Pioneer SX-9000 today, with the intention of connecting the 8006 to it via those 4 Channel out jacks just to see what would happen.

I haven't used the Pioneer SX-9000 in awhile so decided I better make sure that is working correctly before confusing the situation by adding the Kenwood 8006. The input device is a Sony TT PS-4750 that I purchased with the KA-8006. I'm getting a little hum on the right channel of the Pioneer SX-9000 that I was also getting on the Kenwood 8006.

So, I'm thinking it must be the TT. Checked the ground wire connection, still getting hum on the right channel. Switch L/R RCA from the turntable, and that puts the hum on the left channel. So it is the turntable. I took the Shure M93E cartridge off that came with the turntable and put one of my own on. No more hum and now everything sounds delightful, also on the Kenwood 8006.

So all this messing around had nothing to do with the amp, it was a bad phono cartridge. That's what I get for trusting a yard sale turntable and cartridge to test things with. I might have messed up the cartridge, it was filthy and I gave it a few sprits of contact cleaner.

I think I may go ahead with replacing caps on the 8006 power supply board, but it sounds great the way it is. Having never done this before, I'm wondering if this should be the unit I make my first attempt on?

I see better guinea pig candidates just about every weekend at yard sales. Like a $5 Sherwood S-2660-CP receiver that I passed on last week.

I'd pick up the cheap yard sale item, unless you have a guinea pig already, and do a little desolder/solder practice on it. You don't even have to buy any parts. Just put the ones you pull out back in until you get the hang of getting in and out with the solder iron without lifting traces, etc.

Then, I'd go ahead and at least recap the power supply on the 8006 for longevity purposes. I look at recapping as preventative maintenance more than really looking to improve the sound, though that can happen as a side benefit if a bunch of components are out of spec. And I wouldn't say it's common for caps to just "let go" and explode inside a piece of equipment, but... there are some unobtainium parts in these vintage gems that you're protecting in the process. I'd also seriously consider replacing those trimmers I suggested a post or two back before attempting to set the BIAS. I have left some in on other restores and it can be pretty frustrating some of the wild swing changes you get with the old single turn trimmers. It's almost like you just think about turning the screwdriver to get slight changes. The multi turn trimmers are definitely the way to go.

Since the amp is working, when you take each old one out, measure the resistance from each end to the middle pin. Then preset the replacement as close as you can before soldering it into place. That way you're starting at a known "working" setting and can adjust back to spec from there.

jheu02 said:

Just curious as to why? I've seen threads where it's recommended to add some solder to them to ensure a good connection after many years. If that's good advice, what is the difference between wire wrap and soldered connection that you wouldn't want to change it? IOW, why were some connections wrapped and others soldered? Inquiring minds want to know.

Click to expand...

They are solid-core wire. They get work-hardened and brittle. If one breaks inside the insulation, you will have very hard time finding it. Also, it stresses the solder joints at the base of the pins weakening/compromising them.

I'm going by memory now, I once tested a Dynaco preamp (Pat 4) using the isn's and out's on the 8006. The preamp worked fine however it seems the volume control on the 8006 was still active. Simply speaking, if the volume control on the 8006 was turned down, no sound would come out. With it turned up, the Pat 4 would work and I could adjust volume with the Pat 4 volume control. The 8006's switch was set to "separate", so the 8006 acted like a power amp with a level control. Hope that makes sense.

On KA-4006 and likely the KA-8006...

The 4-channel jacks do not separate the pre-amp and power amp like typical pre/main jacks. When you plug into the 4-ch input the first thing the signal sees is the volume pot then the tone/preamp board then the power amp.

When you have something connected to the regular inputs and you take a signal from the 4-ch output it has been through the selector switch and if it is phono it has been through the phono amp. If it is anything other than phono you are essentially just passing the input signal unchanged.


EDIT: the above applies when the 4-channel switch is in the "Separate" position.

jheu02 said:

I wish I lived closer to Myrtle Beach. There's a KT-8007 KA-8006 combo for sale on CL for a decent price.

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You should go there. These beautiful units will become rare in a few years. They are almost impossible to find here. Been looking for a KA-8006 for long time and those who have it here are not willing to sell it.

JamVal said:

I think I may go ahead with replacing caps on the 8006 power supply board, but it sounds great the way it is. Having never done this before, I'm wondering if this should be the unit I make my first attempt on?

I see better guinea pig candidates just about every weekend at yard sales. Like a $5 Sherwood S-2660-CP receiver that I passed on last week.

Click to expand...

KA-8006 is a bit of a beast for a first project. On the other hand, any receiver adds complexity. IMHO the ideal first project is a small integrated.

You could always just follow your instinct which is to do the power supply caps only. Optionally, (while in that board) follow the lead of that external thread by replacing the resistors with 3-watters. And if price is not too much of a concern follow pete_mac's lead and use those nice filter caps. (Or if you prefer, simplify hold off on doing the big filter caps for now.)

Re-assess after this is done. You may be experiencing information overload at this point. I agree about replacing the trim pots but those are probably on different boards. It's all good information in this thread. But it would be very easy to go much deeper (this is a problem I have, lol).


my .02¢

I decided to check the DC offset. Set the KA8006 input selector to Aux, turned the volume down to min (0) and turned the amp on and let it warm up. Speaker selector on Set A, no speakers connected.

DC voltage meter set to 200mv (low as it will go). the speaker (A) terminals read 28mV to 32mV.

I guess ideally it should be 0.

Will the DC offset be corrected by changing the caps? (I assume the answer is no for changing caps on the power supply board) But how about on the main amp boards? Or is higher than ideal DC offset related to other components; transistors, resistors?

When I get a chance, I'm going to remove the covers and see what the bias readings are on the main amp boards. I'm going to follow the advice given in this thread and check voltage instead of current. I found this Kenwood document http://akdatabase.org/AKview/albums/userpics/10007/Kenwood Audio Adjustment.pdf

The bias current for the KA-8006 is 30mA. The pdf above gives a formula for converting to mV. (.47ohm + .47ohm) x mA = mV so (.47ohm + .47ohm) x 30mA = 28.2mV. That other thread said to check voltage across pins 8 and 5 and adjust to 30mV (rounded 28.2 up to 30?). I just need to look at the schematics to see where the .47ohm is coming from. This document covers an extensive list of Kenwood amps and receivers so I'm wondering what's common about the .47ohms.

That same document says to adjust the bias and DC offset voltage after changing transistors, and that some units have 2 pots for adjusting the offset voltage, one course and the other fine.

I'm a little confused, does this mean I shouldn't be tweaking those offset voltage pots until after I've changed caps, transistors, resistors? or is it ok to adjust the offset back to zero now?

Link to full Service Manual below in case you don't have one already. It is a must have for this type of work. This website asks you to register, but they won't nag you after you do.
http://www.hifiengine.com/manual_library/kenwood/ka-8006.shtml

I am not familiar with that other document you linked. I am guessing that the 0.47 in common among various Kenwood amps is the value in ohms of the emitter resistors.


The DC offset values you have are not ideal but they are acceptable and safe. Yes 0 is ideal but in most cases it is theoretical. EchoWars has the go-to excellent sticky thread on that subject, I don't recall which forum it is in, either General Audio, Solid State, or DIY.

It is fine to dial in DC offset and bias before refreshing any components if you are able.

I didn't see a DC offset setting procedure in the manual but I only glanced at it briefly. Some amps have trimmer pots for this (like the ones for bias setting) and some amps do not. My KA-4006 does not. The single factor that most influences DC offset is the input differential pair - a pair of matched transistors in each amp channel that take input signal from the preamp and are also part of the feedback loop. (I believe that other things gone awry can influence DC offset but this is beyond my knowledge). New caps shouldn't have any effect on DC offset. They may possibly make bias readings more stable, not quite sure about that to be honest.

Once you use trimmer pots to adjust bias or anything else, you will see how finicky the original trim pots can be and why jheu02 suggests new ones. Old trimmers can usually be made to work but they can be a hassle compared to fresh ones. With the old trimmers a minuscule rotation of the pot can result in a jump in the reading you are watching. Multiple attempts and just plain luck are often required to dial in the value you want.


The Service Manual should be pretty interesting reading for you at this point

EDIT: I looked at the bias setting procedure in the SM and it makes no sense to me. Does not say where to connect the two meter probes. Taking bias reading is one of the things where you do not attach one meter lead to ground. Usually the leads are across one (or two) emitter resistor(s). So maybe, hopefully someone in the know will comment, or maybe this is discussed in an existing thread.....

Roger,
Thanks for the reply. Yes I have that service manual downloaded, it's where I got the list of capacitors for the power supply, but, I'm not good with reading the schematics, it's been a long time since I took a basic class in college.

Anyhow, I finally found the Vre1 pot on the schematic for adjusting bias, so now I now what the symbol for a potentiometer is. Looking at the main amp schematic that's the only potentiometer I see, so I guess there's nothing on the 8006 for adjusting offset voltage. Last time I was in there I thought I saw more than one.

You've put my concern to rest that my readings are acceptable. When I get my shopping list of what I want to go after I'm going to take your advice and get those new trimmer pots.

This is going on hold for at least a few weeks. I found out that I need some minor surgery next week followed up with pain killers for a few days; which may help the headache I'm getting from looking at schematics and refreshing myself with Ohm's law.

From one of the links earlier in the thread, but I didn't find the original source here on a quick search.


"All that was left to do now was confirm the bias current matched the specs in the service manual and this thing would be good as new. The manual gives you some pretty wonky instructions for measuring the bias current, but I found out through the help of folks at Audiokarma that you can alternatively measure the voltage across pins 8 and 5 on each amp board. Then you just turn the adjustment pot until you get 30mV. Easy."

Ope this helps.

Looks like Kenwood screwed the pooch on several aspects of the SM. The bias adjust description in the manual is totally nonsensical. Use the above mentioned procedure to adjust, but get minigrabber clips for your meter...or else. Monitor the bias for an hour and readjust as necessary.

Also, looks like KW reversed the collectors and emitters on the PNP output transistors on the schematic.

JamVal said:

pete_mac,
Thanks, great thread and beautiful work you did on that 8006.
three questions:
1. Where did you get the BHC caps?
2. I'm not a tech. I assume the BHC are the same value as the originals - 15000uf 63v, so why were they bypassed with the 2.2uf 250v caps?

and 3. unrelated to the large caps, the service manual shows a 10uf 50v cap on the power supply board that I can't quite see without taking everything apart, but it looks like an axial rather than a radial mount - is that correct?

Click to expand...

Sorry for the late reply!

1. The BHCs were sourced from RS Components here in Australia.

2. The BHCs were the same size as the original caps. I added bypass caps (as I do with many vintage restos) given my affection for Sansui designs, and they used bypass caps extensively.

3. The cap you refer to is indeed an axial cap

I found this thread while searching repairs for a Kenwood KA-8006 that I bought new while I was in college. Any recommendations regarding a quality repair shop in western PA or someone reputable shop that works via the mail? The unit exhibits the hum that some have mentioned and there's a bit of imbalance. Thanks for your help.

Once you get it sorted, you are going to love that amp. The 8006 is a beautiful piece of Kenwood work.
Wargo's repaired mine a couple years ago.

Very nice...here’s a pic of mine waiting to be sent to a clinic.