Another Kenwood KA-7100 recap

roger2 said:
lee - I am curious about the heatsinks you added to those two transistors in the power supply.

are those mouser parts? are they mounted with a special screw/nut? it appears that there may be some type of thread lock compound applied to them, is there?
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Yes, they are Mouser parts, the number is : 532-577304B00.

I just used a small nut and bolt I found in my spares box, nothing special, you should be able to find something at your local hardware store.

As for the "thread lock", it was just some blue touch-up paint I had lying around. Put a smear of heatsink compound between the transistor and the heatsink.

Hope this helps,

Lee.
 
EchoWars said:
Good work.

Only criticism: Should have replaced those tantalum caps (C25, C26, first pic, 0.33µf 25V, use a stacked film cap here), and popped in a new protect relay.
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Nice work...I also agree...Any recap that I work on...Those Tantalum's get ripped out.

Maybe for the Next 7100...Use ELNA Silmic II for the Power Area.

Keep us posted.
 
Lee, thanks for the list, it's been very helpful for my 7100 recap project. I just finished the main board, but am wondering about these:

Resistors,,,,,55
65,220,,replace with 2w,71-CPF2-F-220-E3,56
66,220,,replace with 2w,71-CPF2-F-220-E3,57

These seem to be 1.8K ohm resistors if I recall from measuring them yesterday. R65-R68 are not listed in the SM parts list, but I did not measure any of these as 220. Looking at the SM, I see Re89/90 are 220 ohm, but I don't see any other 220 ohm resistors. What am I missing?

This maybe should go on a separate thread, but I am seeing the DC offset reading (center voltage) jumping around quite a bit when trying to set it on both channels. This is true for the bias as well. This was also the situation before the recap. I replaced the trimmers. Any thoughts?
 
They're in the service manual under "control", listed as Ri65 and Ri66. You'll actually find them on the board behind the front panel with the volume, bass ,treble etc. controls, sitting next to a pair of 330uF caps.

Hope this helps,

Lee.
 
Hi Lee.
Impressive work on the 8100. A friend gave me a dead 30 yr old Kenwood KA 501 stereo amp. Noted the voltage selector jammed between the 110/120V and 220/240V positions. Removed, cleaned it and set to 220/240V. Used dim bulb for 5 mins. All ok. Plugged into wall power. Still ok. After a few mins, connected MP3 into AUX. Connected a pair of 8 ohm speakers. When vol reaches 4 on the front panel, amp cuts out but reconnects 3-5 secs after vol is decreased. After 30 mins, there was shhhhh noise in left channel. 10 mins later there were gunshot sounds from left channel. Shut down amp. All electrolytic caps on pre-amp, tone and power boards are ELNAs (original) and match the schematic. Some dried glue seen on the power board around the 2 main filter caps. What's causing the problem? I have repaired small transistor radios and boom boxes from the 1980's but this is my first attempt on a power amp. Any help is greatly welcome. Robert.
 
Soundphile said:
Hi Lee.
Impressive work on the 8100. A friend gave me a dead 30 yr old Kenwood KA 501 stereo amp. Noted the voltage selector jammed between the 110/120V and 220/240V positions. Removed, cleaned it and set to 220/240V. Used dim bulb for 5 mins. All ok. Plugged into wall power. Still ok. After a few mins, connected MP3 into AUX. Connected a pair of 8 ohm speakers. When vol reaches 4 on the front panel, amp cuts out but reconnects 3-5 secs after vol is decreased. After 30 mins, there was shhhhh noise in left channel. 10 mins later there were gunshot sounds from left channel. Shut down amp. All electrolytic caps on pre-amp, tone and power boards are ELNAs (original) and match the schematic. Some dried glue seen on the power board around the 2 main filter caps. What's causing the problem? I have repaired small transistor radios and boom boxes from the 1980's but this is my first attempt on a power amp. Any help is greatly welcome. Robert.
Click to expand...

Thank you!

You should start a new thread on your amp, then we can all post to it. Some pictures would help, with close-ups of anything that looks burnt. Sounds like you have some bad caps in there.

Lee.
 
Kenwood KA 501 Left channel problem

I'm afraid, I'm new to AK. I did start a new thread some two weeks ago but there have been no replies. Am wondering if that thread was posted correctly and can be viewed by the AK experts. Meanwhile I have traced the immediate problem to a short in the left 2SA 1094 output transistor. Also believe the caps are faulty. Hope you can find the thread I started. Thanks.
 
There is a 7300 in my area. I'd buy it but I'm not knowledgeable in electronics. I like old receivers but the fact that they will die like old cars scares me off.
 
Just picked one of these up today. Just wanted to thank OP, and everyone else, what a post! Looks like I should be able to re-build it 80% of the way without a question. Part numbers and everything, Hooray!
 
I am in process of reviving a 7100 as well for a friend. So far I have found -27 volts on Power amp board was gone due to ?100? ohm resistor open, fixed that =- the big electrolytics tested good so I left them alone for now. Then found on control board that one of the 220 ohm resistors (in the 20V supply circuit) got so hot it melted the solder joints! The 3300 uf/25V caps were ok but the 18 volt zener diode was SHORTED! I;ve never seen a shorted diode before-- I figured it would burn up but it didn't! SO on order are some new 3300 /25v caps and a few 18v zeners. Once I get the amp working so that I can ''baseline' it then I will figure out where to go with cap replacement/transistor -relay work, etc etc.
 
leesonic said:
Thanks.

Only thing hard to get at is a single cap under the RCA connectors. Desoldering it is easy. To replace it, cut the leads down on your replacement cap, and carefully insert it using a long pair of tweezers or needle nose pliers. It should only take a couple of attempts, make sure your holes are clear of solder.

As for the wirewraps, or anything else to remove, I cut all the wires to the 5-pin DIN connector at the back to remove the rear board. I didn't bother to put them back this time, who uses DIN connectors any more? There's a single ground wire soldered onto a screw high up on the back panel that needs to be cut to work on the board, and re-soldered when you put everything back together.



Thanks also.

No need, those caps are for the subsonic filter. I've never ever found myself using one in all the years I've been listening to vinyl. Old relay was working fine.

Lee.
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I just want to say this is an awesome thread. :beerchug:

I just finished the main board and preamp board on my KA7100 (purchased new ca. 1979). Still have to do the Control board (tomorrow), but for now a couple of notes:

1. It's possible to work on the preamp board without fully removing it or cutting any wires. It's a bit fiddly, but very doable.

attachment.php


2. Yeah, that "single cap under the RCA connectors" is Cd6. Easy enough to get out. I mounted the replacement on the bottom side of the board, which is way easier than trying to put it back in its "proper" place and will make it easier to replace it again if it needs it. Call me lazy, but I'm not entering this thing in the Pebble Beach Concours d'Elegance, and there's plenty of space between the board and that metal shield that space isn't an issue.

attachment.php


3. There is no 3. Onward! :)
 

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Nice photos! I was able to work on mine without cutting any wires, as well. I was able to get Cd6 mounted by holding one of the leads with a thin pair of needle nose pliers and guiding it into place.
 
They're getting close to 40 years old.

Mine sounded anemic. The recap plus Deoxit on the controls brought it back to life. That said, all the caps I removed tested OK (for value; didn't check ESR)...
 
Drummerboy75 said:
Are these known for needing recapped?
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Any amp this old would almost certainly benefit from new caps. ESR is the most notable parameter that changes (goes higher) as caps age.

KA-7100 is known for speaker protection relay problems because the components that operate the relay fail: one transistor and a few caps. I did a KA-7100 refurb and (two of) those components were grossly out of spec.
 
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I have one of these,it's in a really nice Walnut case.It sounds fantastic with my AR 5's,very punchy...but I have to warm it up for 15 minutes before I use it ,or 1 channel won't play,unless I fiddle with the Tone defeat.Any easy remidies? Ive tried de-oxit,but some pots and switches hard to access.Should I remove the faceplate?
 

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Tone Defeat Switch on Kenwood KA-7100's is Public Enemy #1.

Check out my post [thread=353462]here[/thread] on what you're up against.

There are also several good ideas on how to deal with this switch, although the suggestion of spraying through the hole near the word "Japan" misses the area of the toggle mechanism. It's in the upper third of the switch.

I guess the best method is to set the unit on it's hind legs, flood the switch with the contact cleaner of your choice while vigorously manipulating the switch, and then shooting some type of protectant after the cleaner dries. Even then, the problem has been known to reappear. It is after all a switch that probably cost $0.25 and is nearly 40 years old.

Good Luck!
 
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