Kenwood KA-8100 issues

[Dom123]

Hi Everyone

I just bought a KA-8100 off the bay in Germany. It was sold as defective but is in great near perfect cosmetic condition. Inside it is completely 100% original but full of tons of gunk around each pot & switch.

I've been collecting Sansui's but was so impressed with my KA-6100, I decided to look for a KA-7100 or KA-8100.

I've managed to fix a few Sansuis, Maranantz & Pioneers and it's amazing how each amp type true to form has a characteristic failure cause & remedy as described in various threads in Audiokarma. I'm new to Kenwood, and was looking for advice how to fix my sick amp.

Symptoms : On the DBT, the 60W bulb shines bright but gradually dims, a little more than optimal for my liking.
The relay kicks in after about 10 seconds. Moving the volume knob without load causes the relay to click off but once the volume knob is static the relay kick-in again after a few seconds.
Offset at the speakers is fluctuating like mad on both channels but in the tens of millivolt range, not more.

Connecting up speakers, sound is coming through both speakers, but the volume is low even at a high volume position.
The tone control-in switches for low & high frequency cut the sound immediately as soon as they are activated.

Removing the jumpers, and connecting a pre-amp from another amp into the main-in, the sound is loud and clear. The issue seems to be on the control amp board.

I can see signs of heat damage on the control board. The zener diode D1 just above Q13 is dead, although D2 looks OK.

Question : D1 is a EQA01-25R, so 25V zener. I was hoping to buy a replacement from Reichelt but they only have 24V or 27V zener. Do I need to stick to 25V and is it acceptable to use a higher wattage one ?
If I need a 25V one, can someone suggest a suitable replacement ?

Concerning one zener being bad, would this cause the cutting out of the sound if either the bass or treble turnover switches are activated ? Also why would moving the volume knob cause the relay to disconnect even though the power amp works fine fed by an external pre-amp ?

Sorry for the dumb questions, but I am hoping some of you guys recognise these symptoms on the KA-8100 & can help me.

 

[HWKI]

Good work on isolating to your preamp. Mouser carries 1N5253B. 25V. 1/2W. You could also use a 12V + 13V (1N5242B + 1N5243B).

The Zeners after many yrs can go bad or get noisy. I would replace both. But your unit, your call.



D1 sets your Qi13 Base voltage so your pos. Control rail is a diode below. ~ +24-25V.

D2 does same Qi14 but sets the negative rail ~. -24-25V

Could you go lower, yes. Could you go higher yes, but mind your rail caps and other filter caps that see these voltages so use appropriately rated caps. But the 25V zeners are available in various forms. Your unit, your call. Be mindful that these +/- 25V rails are used
elsewhere in your unit, such as the EQ/Phono section. So whatever you do in the control can directly have an impact on other circuits. Study the circuit diagrams, this can help you make wise(r) choices.

I would check the caps ci53-56 as well. Nothing wrong in replacing these as well. May as well replace.

The pass devices Qi13 and Qi14 are complimentary. Verify they are okay. KSC2073/KSA940. TO220 package.

With what you said above.
I suggest you go through all switches and pots and clean the dickens out of each one. Deoxit/Fader lube and actuate 30-50 times while doing so.

Also, what are your main filter caps measuring? should be approx +/- 50VDC steady.

Clean up your power supply rails as we discussed above . Then see where you are after this.


Post some pictures or this never happened.

 

[Dom123]

Many thanks HWKI for the advice ! Gives me a good starting point of where to look.

Think I will stick to the correct zeners & replace both Di1 & Di2. Wanted to avoid paying shipping charges at Mouser (Europe) but I need to get a bunch of stuff anyway so will go for the minimum order to reach free shipping. Will order the caps at the same time for this amp.

I checked the voltages at E of both Qi13 & Qi14, and measure 0.6V & -24.1V respectively. Definately an issue on the negative rail.
Di1 is shorted (0V both ways on the DMM diode check) while Di2 appears to be working but voltages are lowish.

Seems difficult to me to get a coherant measurement of Qi13 & Qi14 in circuit, so I will need to remove them from the board. May as well replace them once they are off the board. Thanks for your advice about KSC2073 / KSA940 as subs for 2SC1419 & 2SA755. I also read that MJE15032G & MJE15033G are suitable replacements.

Mainly filter caps are in the range 55.5V to 55.9V, all 4 caps. When this amp was originally sold, mains voltage was 220V in Belgium, but nowdays it is typically 235V like most of Europe. Unfortunately the amp only has a 120V or 220V voltage selector switch, no 240V.

So I will replace the diodes and voltage pass transistors , check all resistors in the vicinity of this heat affected zone and probably change also the caps in area.
If the amp is working well after that, will do a recap elsewhere. Thinking of using Panasonic FM for most caps, except the signal path caps where I am thinking of Nichicon FG's

 

[claneg]

Unfortunately the amp only has a 120V or 220V voltage selector switch, no 240V.

Hi Dom,
check your tranformer type (your should be W type): you might use 120+120=240. Broesel02 showed this connections on a KA701 amplifier, very useful. I used myself.

 

[HWKI]

I checked the voltages at E of both Qi13 & Qi14, and measure 0.6V & -24.1V respectively. Definately an issue on the negative rail.

i think you mean pos rail. Congrats you have no upper rail on your entire pre amp section. Thanks for posting measurements . As you have found, Signal coming out is less than fulfilling.

Good idea. To meet the free shipping. I would buy a complete recap. New Silicon diodes,zeners and bipolars . As far as passives, new resistors where needed. Look at high power , high current locations. I would leave the Junction FETs alone if they test good. For grins a new relay as well.

There are a few back to back electrolytic’s that can be replaced with single bipolar. If i recall there is one or two in the protection block.

For signal path caps i like to use Deutschland WIMAs.
Next best thing to Haribo Gummi Bears.

temporarily you could put some 24V zeners in to see if this cleans up your issues. Get it working first before you do complete overhaul is a good policy. I can’t say i always do that. Largely depends on the unit And my mood.

 

[Dom123]

Thanks guys

HWKI - You are proposing the full works ! Not sure I have the courage for that ! My plan was to get it work following the previous discussions, and new then caps not forgetting to uprate their voltage. I forgot to mention that I do use Wima film caps for everything less than 3.3uf.
Firstly have to see what gremlins lurk after replacing the zener diodes and possibly the 2SC1419 & 2SA755, before going any further. Will report back in a week or so

There are signs of high heat generation is some areas of the control board. This board is really badly ventilated being squeezed in at the front near to the front panel. Will check all resistors here.

One issue bugging me is the volume pot which wobbles a little. On closer inspection it seems the tags that hold the first balance pot closed are bent a little open causing everything behind it to wobble a bit as a block. I'll take it off and squeeze them shut, still I hope they are still strong enough.

Thanks claneg for the tip - I will look ito it. Is it a problem if the voltage on the main rails is running 10% too high ? As mentionned I will update all caps to give them a bit more voltage headroom

 

[claneg]

Is it a problem if the voltage on the main rails is running 10% too high ? As mentionned I will update all caps to give them a bit more voltage headroom

Well, you have to imagine that all the not voltage stabilized components of the amplifier will be powered with more voltage, not just the capacitors that you will substitue with higher voltage ones. Your amp is around 50 years old, give him some help!

 

[Dom123]

I've been so busy & still haven't ordered the parts yet ! Definately going to do it today.
I already established the amp wasn't working to to an issue on the control board.
Disconnected the control board from the front panel to gain access to all caps & resistors. Made a few measurements and visual observations :

1) Painstakenly checked all resitors, admittedly in circuit, but everything looks (very) good
2) All caps visually look OK, but took out some of the blue Elnas and they are all reading in excess of 20% higher than their capacitance rating
3) Found one cap with a 360° halo around the lead on the solder side. This can't have been conducting much, if at all.
4) Removed the SA755/C1419 from the board. They seem to measure fine :

SA755 PNP bce
+e to -b = 0.542V
+c to -b = 0.530V
-e to +b = OL

C1419 NPN bce
-e to +b = 0.545V
-c to +b = 0.539V
+e to -b = OL

These look OK to me - should I replace them or just put them back in ?

5) Removed D1 and confirmed it is shorted
6) Several solders look to have signs of shadowy halo

I am feeling confident that this amp will come back to life. I reckon the issue was a shorted diode, and a few cracked solders. Nothing else
I have a question concerning the best choice of caps on the control board :

There are numerous orange Elna caps which I believe were low leakage types (8ea 1uf 50V & 10ea 10uf 25V). All others are blue Elna caps.
Regarding the low leakage caps I read the best low leakage caps available are Nichicon UKL, but read that these can be fairly clinical sounding.

What 's are the ideal caps to replace these orange caps with ? UKL or some audio grade caps such as UFG or UKA types ? Definately don't wan't the amp to be too bright sounding. For the 1uf caps I will put Wima films in there.

Then for the blue Elna's again what would people recommend ?: Pansaonic FR's, FC, Nichicon UKL, or something like Nichicon UPW ?

Any advice welcome

 

[HWKI]

There are numerous orange Elna caps

depends on location. Low leakage nich. Ukl, KA are a fine choice.
Power supply caps, protection and supply rail decoupling caps PW low esr caps to keep ripple down.

for caps less equal to 4.7uF i tend to use a poly film.

Your power supply was definitely bad which was causing issues in your preamp. Good to hear you have now found issues in your power amp section.

Get your power supply solid. . But jumping around from board to board is fine. However, it can lead to chasing your tail.

I’m all for replacing pass devices in power supplies, diodes, zeners and caps. If you have the skills its your unit do as you wish. I personally like to go in once and never return.

like i said if you have a 24 or 27v zener handy . Temporarily Drop it in and see if this clears up a majority of issues. Get unit working before you start editing a load of boards is usually the best practice. Sometimes units are too far gone for this to work. But yours doesn’t sound to be in that category .

 

[claneg]

Low leakage capacitor are used as decoupling, that is follow the schematics signal travel (the bold line) and all the capacitor in series with the signal has to be low leakage type. Elna Silmic, Nichi UFG or even Nichicon BP are a good choice. Or you could even use polyester type, till 10uF.

For others caps, Panasonic FM are fine.

 

[Dom123]

Thanks Guys,

No I reckon this amp is in nice condition. Definately not a basket case, just a failed component(s) on the control board. The board is very clean looking
It sounded pretty good fed by the pre-amp from my Sansui AU-7900

Don't have the appropriate zener so I will wait for the proper 25V to arrive and drop it in.
In theory I could just change this, put all original components back in, including the original pass transitors, re-flow some of the cold or suspicious looking solders, and see if everything is working before recapping. On the otherhand the recapping looks a very simple task, especially as the board is out now & nicely marked with the component number & position on the solder side.

I don't have a load or experience but have recapped and repaired a few non working Sanusi & Pioneer amps. I always use Panasonic FM in the power supply, and FC's elsewhere and wima film below 3.3uf or so. Apart from a Marantz 1060 I had, I have struggled to hear any sonic improvement from recapping amps.

Then just recently recapped a Sansui AU-5500 using many UFF's and I must say the amp sounds really wonderful to my ears for such a simple design.

I checked for UFG at Mouser, who say they are discontinued, although they have 10uf 50V in place of 25V . I will get these - they are still smaller than the original cans. Also Nichicon seem to have discontinued many of their audio caps ra. Same for Panasonic with lothe FM series dicontinued many type of Nichicon audio caps, although the ubiquitous UKL's and Panasoni FC

 

[Dom123]

Sorry pressed the wrong key, mean't to say, difficult to get some of the traditional choices.
Anyway Nichichon FG's for the 10uf caps, Wima for the 1uf, and then either FM's or UKL's for the remaining caps
Here is a pic of the board

 

[Dom123]

Got the goodies today from Mouser. I have decided to put off recapping the control board off until I am sure the amp is working.

I changed out D1 & D2 with the 1N5253B. 25V. 1/2W, re-installed the old 2SC1419 & 2SA755 transistors which I had determined were OK, and changed just the 2ea 330uf caps (Nichicon HPW caps) adjacent to the mini heat sink with the SC1419 & SA755 transistors. Then re-flowed several ill looking solders, including some definate cracked solders.

Good news the amp seems to work . The DBT (60W) bulb glows fairly dimly. Sound is clean on both L & R channels and all tone, filters, loudness switches etc working normally. There was a brief concern with no sound on the right channel, but this seemed to be connected to the low 40Hz switch. Activated there was sound & deactivated no sound !. Anyway some rapid repetitive actionning soon got it working normally.

I want to go ahead with recapping the control board now, but I have couple of concerns :

1) The mini heat sink with the SC1419 & SA755 rapidly gets really hot ! Too hot to touch after only 5 minutes. Is this normal ? Admittedly the board in this area has seen alot of heat judging by the charred colour . Would fitting the KSC2073/KSA940 transitors I bought from mouser help here ? Is there anything that can be done ?

2) The 1N5253B 25V zeners are really dinky & tiny compared to the old zeners. These are 0.5W zeners. Should I have looked for zeners with a higher wattage ?

 

[HWKI]

Congrats !

1) sounds like you need to give all the switches and pots a good good ole fashion cleaning. Show them whose boss.

2)pass devices for power supplies tend to get toasty. replacing the T0220 devices is not going to hurt. A better heat sink would dissipate heat better. But im afraid without a modification such as moving off board to a large isolated service area . Keeping things as is methinks it will always be warm to hot. New thermal grease and isolate collector if you’re feeling frisky. But it will still be warm. There are some nice finned heat sinks on the market.

3). The 1/2 W zener is fine you could of used a 1Watter but in this application. Its fine.

4). Im all in on the cap change in Hifi audio equipment. Its 40-50 yrs old. Caps in audio equip are not like wine. There are a few winos out there that would beg to differ. I say bottoms up!

Prost!

 

[Dom123]

Thanks HWKI for the great advice !

I guess the heat sink is really small for obvious reasons, but I was wondering why Kenwood chose steel and not aluminium alloy for the heatsink.
It would be easy to make a similar mini heatsink from some aluminium profile, possible increasing it in size a little here & there. Wonder if this would help or not.

 

[Semanon]

1) The mini heat sink with the SC1419 & SA755 rapidly gets really hot ! Too hot to touch after only 5 minutes. Is this normal ? Admittedly the board in this area has seen alot of heat judging by the charred colour . Would fitting the KSC2073/KSA940 transitors I bought from mouser help here ? Is there anything that can be done

They get very hot. But they have been getting very hot for 40 years and have not failed yes. I have a KA-8150 with the same issue. I have considered moving one of the voltage regulator transistors to the bigger, main heat sink, or adding a micro fan, but have not got around to it yet. They just keep chugging along hot or not.

 

[Dom123]

Good to know. I kind of came to the same conclusion.
Been running the amp everyday for the last 10 days at some fairly unreasonable volumes ! Happy to say it sounds & works perfectly !

Bass is natural, less pronounced than on my KA-6100, but clean & deep. Also the amp is detailed, but definately not tiring or fatiging - I can listen for hours.
I do like the way it projects cymbals & some percussion instruments really well.

A nice classy looking amp.