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KA-7100 refurb

rickl

rickl

I'm just a bass player
Subscriber
picked up a dirty KA-7100.

Checked the restore thread sticky for 7100 threads to see what I got my self into.

I probably wont document this in the details so if you are here to get your 7100 running, this might help. If you are here to get the best out of it, I recommend the fine links in the sticky.

Here are a few pics

front-before.jpg
amp-board.jpg
rear-before.jpg
top.jpg
 
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I cleaned up the fuses and verified they were the correct value and not blow. tested good and 4A. I've come across unknown gear with 20 amp fuses or blown fuses before so this is a good start.

I connected to a DBT and the bulb quickly dimmed. speaker relay kicked in. Watt meter looks good. Tried a signal in the AUX port and got signal out of one channel. back tracked and got signal into the volume pot but not out (pin 1 on the amp board).

Looks like i need to clean the volume pot along with everything else.

I've picked up some dirty amps before but this might be one of the worst. Got it for a good price so I'm hoping a good cleaning, replace a few choice caps, add a triac to the power switch (seems to work fine with a DBT) and maybe a new speaker relay and it will be good.

I can't get the volume knob off. I did remove both set screws but still one come off. Might look at that again tomorrow.
 
One of my favorite amps. Good luck with the project. I just finished one and put it up for sale here in AK. Pretty extensive rebuild. Great sounding amp, the KA-7100!

I had a stuck volume knob like you mention. Drove me crazy! I learned the hard way what to do - after I broke the shaft. The secret is to de-solder the control from the pcb, and leave it attached to the face plate. Then you can get to everything to clean, replace, resolder, etc. A good tip to know.

Keep up the good work!
 
Nice, I would not be to tough on the volume knob. The last thing you want to do is damage it or the actual potentiometer. a drop of liquid wrench or light machine oil left for a couple of days might help.
 
tusntuk2 said:
The secret is to de-solder the control from the pcb, and leave it attached to the face plate. Then you can get to everything to clean, replace, resolder, etc.
Click to expand...

Thanks for the tip.

The pot may be broken. It doesn't feel good (like bits of the carbon are scraping), cleaning it doesn't help and i can turn the volume 360. Still trying to get it off but I may be looking for a replacement.

A scope shows signal going into the pot but nothing coming out.
 
Hmmm, that doesn't sound good. The 360 turn suggests a problem in the detente rotor, (the back section), which really never happens on its own. That in turn suggests some serious impact could have physically broken the control. I once found a treble control broken that way, not exactly the same control but the same principal. It takes some force to alter the inside parts. The detente stops can also be broken with excessive force while turning the volume knob - but please don't ask how I know that!

You seem to be on the right track with your tests.
 
I got the pre board separated from the volume pot. I pulled the volume apart and found the ball bearing in the bottom of the pot. The stop was bent. I rebent the stop, got the bearing back in, re-straightened the fingers but still can't get the right side to work.

kind of upset about this. I really don't want to make this a power amp.

A few pics
 

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Finally got the knob off. I should have taken a few pics but I added some penetrating oil to the shaft, put a wrench on the flats of the shaft (need to remove the left channel wiper) and twisted it carefully until it came off.

You can look at the pic above titled vol-bearing to get an idea.

An AK member is sending me a broken pot and hopefully i can put two together to get one working.

I've been reading many 7100 restore threads and compare with mine. The PS must run hot. I'm planning to add a heat sinks to both to-220 regulator transistors, replace R75/R76/R77/R78 with 1w parts and recap. Not sure who suggested this (leesonic or roger2) but these resistors in my amp are cooked.

I was more focused on the volume pot for any component updates but now that I'm getting closer with this, I'm looking at the PS.
 
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I got a broken volume pot from @leesonic (what a great guy!) and mixed parts and got a working volume control.

Thanks again Lee!

Added heat sinks to the PS regulators and a few fresh caps. if you look closely, the 100R resistors (Re77/78)are a bit cooked. I just got new 1 watt replacements and need to install them.

20191008_220517.jpg

little signal in and big signal out (sorry about the messy workbench). Adjusted bias and dialed in DC offset.

20191008_214919.jpg

This is a nice sounding amp. I didn't think I was going to keep it (I have a lot of amps!) but now I'm not sure.
 
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rickl said:
This is a nice sounding amp. I didn't think I was going to keep it (I have a lot of amps!) but now I'm not sure.
Click to expand...

This amp received rave reviews in the HiFi press back in the day. The notoriously difficult to please British magazine "HiFi Choice" gave it a "Recommended" review. They would probably have described the Sistine Chapel is "a nice piece of art". Though rated at 60W into 8 ohms, and 80W into 4 ohms, they tested it and got 71W and 95W into 8 and 4 ohm loads. As you probably know, this amp has separate transformer windings and power supplies for the left and right channels. Replace those old Elna main caps with some Nichicon KWs or FWs. You might ever trying going up a little on the capacitance, maybe to a 7200uF? I've never tried it myself, space would be the issue.
 
I just finished a check out and clean up of a 7100. The main filter capacitors are the old 3-pin type with no real drop-in replacements that I know of.

Craig
 
here is an odd thing about my 7100. 6 out of the 8 speaker screws are counter clockwise (anti-clockwise in the Queen's English) to tighten. of the 2 that tighten CW, one is R+ (A channel), the other is R- (B channel). Plus if you look close, they are not threaded all the way down so you kind of need to ball up the speaker wire to tighten.
speaker-screws.jpg
And these are different than KA-5700 speaker screws.

I've not see this before.
 
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It looks like someone swapped thumbscrews from a different model, such as a KA-9100. As I recall. the 7100 thumbscrews are staked in?

The 7100/8100/9100 terminals all look the same at a glance, but the 9100 are different and are much nicer.
 
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