KA-7100 Fixem Up

Short story -- reviving a 7100 for a friend, it sat for 20+ years and he tried to use it, but weak signal/crackling etc

I'm a modest electrical guy but have a decent scope DVM signal gen etc. I can read a schematic and spent a good deal of my youth building/fixing tube gear.

Anywho I got the service manual from hifiengine and started to troubleshoot, after blew dust/cobwebs/dirt etc out and did the ole contact cleaner gig.

First thing I checked was 27 and -27 V on the power amp board. No 27V, the 100 ohm resistor was burned up. Replaced it, checked the big elec caps, seemed OK so powered on, regulators seem OK and now I have 27 and -27. But still no gain in the amp to speak of, volume control works but even at full blast its clipped and sounds like a transisor radio volume-and-fidelity wise.

Protect relay working like it should and little DC offset on the output. More on that later

Then went to control board and found the 18 volt zener diode shorted! Hint here was when tone control bypass was engaged, no sound at all (not even shot noise!) Dead shorted diode. 220 ohm resistor apparently got hot enough to flow its solder joints as it was loose on the board! Never in my life seen a dead short in a diode-- I woulda thought it would burn up and go open --

anywho ordered a few 18v zeners and the 3300 uf/35 v caps and couple new 220 ohm metal film resistors. Once I get the control board power working I'll report back.

What I thought I would do is bring it back to life changing as little as possible to get a baseline, then go about the recap and relay driver xistor swaparooney, and maybe the driver xistors too.

Hints appreciated and helpful to me for this. I'm getting older and my visual acuity is not so great anymore but giving it a decent go!

In my 7100, which I hope to be posting about soon, I replaced those 100 ohm power supply resistors, as well as the 1k ohm, with the same 2-watt, 1% metal film that Leesonic used. Also used them on the control board.

On mine, the 100 ohm resistors were still intact and measured OK out of circuit. But one of them was looking very distressed.

As for the 18v zeners on the control board, I have decided to go back in and replace them based on your experience.

When you do caps, definitely replace all of the ones in the relay circuit because I found one, C41, that was pretty much toast. I measure all old caps that I remove and this one was by far the worst I have seen out of 5 recap projects.

OK done and working fine -- now try to baseline it and then dig into a recap if my buddy wants me to do it. Bias set perfectly and balance as well. Manual helpful ; the bias pot needed some exercise to settle down but no problem.

roger if you need the 18v zeners send me a PM and your address and I will send to you. I mistakenly bought 4 at something like 30 cents each and broke the bank!!!

That is a generous offer, thank you. But I already changed those 18v zeners from some I had on hand.

Glad to hear that your project is going well and hopefully you will get the green light to proceed with a recap. That amp is well worth the effort and a small investment in caps.

Based on what I found in my KA-7100 I would suggest that even if you do not do a full recap/refurb, it would be smart to at least replace Ce41 and Qe34 in the relay control circuit. KA-7100 is known for developing relay issues, and in my amp these two components were in really bad shape.

@goatster

I am getting some higher than expected voltage readings across the 18.0v zeners I installed at Di1, Di2 on the control board.

If you still have that KA-7100 on your bench, would you mind checking to see what the actual voltage is across those zeners in your amp please?

I don't have it ; gave it to my buddy he wanted to listen to it for a while and then figure out what he wants to do. BUT I did take readings at the +/- 20V on the schema (left side control board) and the measured voltage was... you guessed it, 18.3 or so for both +/-. I could not figure out how they arrived at +/- 20 when it was regulated by the zener (or at least that's how I understand it).

I still have two new ones, 2 Watt 18v zeners if you want them.

Thanks for the information. That is how I understand it too. It looks like a simple zener regulated supply to me, but I am certainly not an expert and could easily be missing something. I thought that I understood this type of circuit though.

So to me the +20/-20 markings on the schematic look like a mistake.

But in my amp, there also appears to be something else going on. I am seeing -19.1v/+19.2v at Di1/Di2. I installed new Vishay BZX55B18 2% tolerance zeners but was also seeing similar voltages with the original zeners in place.

So I am at a loss. Unless 3 DMMs could all be wrong by the same amount...(two of them are cheap)

I would not worry about that, personally. 19 or 18 or 20 V... does it play well? I don't fret too much about 10% voltage variance......

The amp plays fine and bias and DC offset dialed in nicely.

I would just like to understand what is going on. I am about ready to close this amp up after working on it for a few months on and off. I hate to close it and move on with an unsolved mystery.

The Vishay zeners I used are supposed to be 2% tolerance. 18v plus 2% should mean a max of 18.36 zener voltage. How could they both be that far off?

@ goatster

You say you used 2-watt 18v zeners? Do you know the part number? I would like to look at the spec sheet for those...

Sorry Brain fade- 1W 18V zener, 1N4746 as I recall
sorry for the confusion!

OK thanks goatster. I looked up the spec sheets and your 1w zener had a test current of 14ma. The .5w zener I used had a 5ma test current.

Perhaps the lower wattage zener is more likely to have increased Vz at whatever current is actually flowing in this circuit? At least that is the only plausible explanation I can come up with right now.

I think I am going to include some 1.3w 18v zeners with my next parts order just to see how much difference that might make.

Thanks for your help :thmbsp: And thanks again for the offer to mail me those parts, but you better keep them, another KA-7100 might come your way...

I will never see another 7100 in my life I will bet on it! I chose the 1W over the half watter due to (what I thought) was the max inrush current and the observation that the 220 ohmer got so hot it flowed its solder joints (or so it seemed). I think the manual calls out for 1/2 watter job but not 100% certain. I'm an old school tube guy so voltages only need to be 'reasonably close' for me!!! Cheers