My Dad's Vintage Hi-Fi Fisher Amplifier KX-100 / KX60 Tuner

[joeqsmith]

I had saved my dad's old Hi-Fi.  Sure it has sat around ever since.   Noticed my wife had decided to move the box to the garage, which is just one step closer to the dumpster.  So I brought it back into my lab where it is safe for now.    The amplifier is a Fisher KX-100.   

I remember going with my dad to buy it.   I've spent countless hours listening to it.  Like my wife's grandmothers old radio that her mom remembers listening to as a child,  I just didn't have the heart to let it go to the dump.   

The amplifier has not been powered for several decades, so first things first, check to see if it has any damage.   First thing I noticed was one of the finals appears to have started to leak.  I remember being around 11 or so YR and one of the tubes failed.  I had my mom take me to the local store (tubes were common then and they even had testers for customers to use).   It turns out, this is the tube that appears to be failing.  Note the others are still marked with Fisher. 

Opening the chassis up (zero concern about any sort of shock, as 30 years of sitting, everything should have discharged. lol).   Started going through all the caps and sure enough, one was shorted.  Outside of that, none of the smaller caps appear bad. 

I then started to reform all the electrolytics.   Nothing appears to have a high leakage.  I'll let her sit and cook.   

The case is made from cheap plywood.  Similar to how I have made cases for some of my home projects.   The wood had split where the grill slides in and it had a couple of deep scratches I wanted to smooth out.  After repairs, mixed up some stain to match the original colors.   I have no plans to use poly on it like I have with some of my projects.  Thinking just some oil and I'll call it good. 

Maybe tomorrow I can turn on the chassis, less tubes, assuming all the caps check out after reforming. 

[rsjsouza]

Joe, great job keeping this relic out of the dumpster, especially with such sentimental value attached to it. I don't see any wax bombs, fortunately.

I have my grandpa's Philips radio restored and it is a family heirloom.

Have fun!

[TimFox]

The cap in your first photo:
Capacitors of that type and vintage often absorb moisture.
I replaced each of the range caps in my Tektronix L,C meter, and they all tested bad out of circuit.
I would replace each such cap with a modern polypropylene unit.

[floobydust]

It looks like a fine amplifier.
I would advise caution, I've never seen those electrolytics (cans and orange Cornell-Dubilier) at that age that has not failed. They don't have safety vents either.
As long as the output tubes have proper bias, or else they will roast. I can't tell if the design is self-bias or not here, then the cathode cap is critical.

[joeqsmith]

Typically I won't shotgun and most likely I will leave everything alone,  only addressing the problems I identify.  The small 150V 100uF orange,  below 0.03*100*150 or about 450uA it's fine.
https://www.mouser.com/datasheet/2/231/161587867495-2585920.pdf

At 159V (over the rating) it's around 220uA.   At 130V,  its below 60uA.   BK RLC meter,  ESR measures 0.65.  Capacitance also checks good.     

The 200uF 250V cap measured about 40uA at 260V.   ESR was under 1.5 ohms.  Capacitance was a bit low but nothing of concern.   The other 200uF was also alright but it's shared 40uF section isn't looking so good.   During the cycle, the leakage was about double what it should have been below the spec'ed voltage,  I cranked up the current to about 1.5mA.  It made it to about 470V and broke down.   No idea yet about that last can.     

I couldn't let the finish go and put a light coat of poly on. Well maybe one more tomorrow.  lol.   Scratches came out fine.   Guessing those came from us kids. 

[Jeff eelcr]

Carefull running without tubes some output transformers may arc over internally.
Jeff

[Johnny10]

I love restoring old tube amps. Right now I have 6 Stromberg Carlson Tube Amps from the late 60's
They Drive a set of high Efficiency Klipsch Cornwall speakers.

Without recapping they tend to last 3-6 months before they start having problems.

Power Supply caps and signal caps tend to go. I believe from higher present day voltages and age of course.
They sound as good or better after recapping.
Also, I always up the voltage rating some of those 450 VDC caps they often see 550 VDC at start-up.

[joeqsmith]

Carefull running without tubes some output transformers may arc over internally.
Jeff

I won't bring it up until everything has been repaired.  All the high voltage testing I am performing is with each part isolated and an external power supply.  The small supply will only put out about 700uA.  To test that 40uF leaky cap, I had to use my large supply.   I've gone ahead and cut that cap open.  I plan to insert the new caps inside the old can.

[joeqsmith]

Without recapping they tend to last 3-6 months before they start having problems.

When I repaired my wife's grandmas old radio, I think I changed two caps.  But had to change the design to use a different tube.  Many years later, I found a tube for it a the Dayton show in a bushel basket.  Bought two and tipped the guy.  Changed the design back to stock.  Maybe 10 years or so later I ended up having to pull some of the other caps.    That has those old wax foil caps in it.   Some now around 80 years old.

The other can checks good so I plan to order some parts tonight.  Maybe in a week or so I can fire it up.

[andy3055]

In my experience, you can power up minus the tubes because the output transformer primary will be open circuited without the output tube. But, you cannot power it up with the output tubes inserted if you don't connect the speakers. That will kill the output transformers.

[joeqsmith]

Most likely I will not be testing with speakers but instead the network analyzer and resistive loads.    New parts are on order. 

I couldn't resist laying down one more coat of poly.  The plywood is looking good.  I'll let it dry a few days and put the final finish on it.   

[andy3055]

That is OK.  As long as you do not leave the output without a load, it will be safe.

[TimFox]

The reason why you should not run a tube amplifier unloaded is that the load presented to the plates of the output tubes is then merely the magnetizing inductance of the primary, which is a very high impedance compared with the normal load seen with a terminated secondary.
Starting from the normal DC bias current, when the tube is driven OFF, the result from that change in current into the high inductance can be a strong voltage spike.

[joeqsmith]

There is a note in the operators manual warning about running it with a single speaker and adding the load resistor.    The amp has internal loading resistors if you want to run it without speakers. 

[joeqsmith]

Most of the parts arrived.  After replacing all the bad parts, I put power to it and checked all the supply voltages.  Everything seemed reasonable so I put some audio into and plugged in my headset.  Been listening to it for a few hours now.  Photo of the 4 finals glowing in the dark.  Smells and sounds great. 

Poly has been drying the last few days.  Hope to buff it all out this weekend.  I had sanded out two deep scratches in the photo shown. It will look better once buffed. 

[andy3055]

 

[joeqsmith]

My hearing isn't near good enough to tell if there is a problem with the amp or not but my test equipment is.   Ran it though a series of tests on one channel and it seems decent enough I guess.  Filters all appear to work correctly.   Eventually I moved onto using the external load resistors and it appears the second channel can not achieve the same flatness.   They state +/-1dB 20Hz to 20kHz.   Sweeping 10Hz to 25kHz, the good channel is flat to about 1dB, where the bad channel droops about 4dB as you get beyond 10kHz (yeah, I can't hear that...)

Swapping the entire set of tubes between the two channels had no effect.   Amplifier has a tape recorder input that injects much further downstream.   Like the tube swap, cuts the problem in half again.  No effect.  There are two more mylar caps used in the loudness contour circuit that I had not checked.  You can change the state of that function which shorts them out.   It has no effect.     

They have these hybrid 7-pin printed circuit filters that I assume were custom made.  They are a coated package similar to an old disc cap.   I wonder if these have changed over time.   

[TimFox]

Did you post your graph for the channel down by 4 dB at high frequencies?
The lower one you posted is down by roughly 1 dB at high frequencies.

[joeqsmith]

They spec +/-1dB.   +/-0.5, good enough.  +/-2dB, not so good. 

I did not save the data for the bad channel but it has a similar shape.  Once it gets past 10kHz, it runs out of steam.  You can increase the treble to increase the level beyond 10kHz.  Of course it raises the mid range as well and will still not meet the flatness spec.   I was going to attach the HV probe to the network analyzer to track it down.   

If it is the hybrid, I doubt I will try to hunt one down or make a replacement. 

[joeqsmith]

Comparing the two channels with the high filter off, loudness contour off.   Doesn't really matter as I am just wanting to show the difference between the two channels.    The bass and treble were both then set to the max, min and mid.  Signal level was adjusted not to overload the network analyzer.   

First, you can see the problem is not the base.  In all cases, both channels track well to about 1.5ish kHz.  With the the treble set the minimum, both channels track well to 25kHz.  As we increase the treble, the left channel starts to roll off.  The more we increase the treble, the greater the difference.   

With the tape input, I can inject directly into this base/treble circuit, isolating everything before it.  And again, also changing the loudness to short the two mylar caps you basically have a tube and a blocking cap and that hybrid.  I had swapped the tubes with no effect. 

It's possible the hybrid filter is bad on the left channel, or the final stage is just rolling off. 

[TimFox]

Can you put your probe to a point just ahead of the tone controls?
An error that large does look like a problem in the tone control circuit.

[joeqsmith]

Easier to split the problem in half again.  Its the final, driver or it's the tone control.   To answer this, the two drivers were tied to the common working tone output.  Doing this they track perfectly.  So, yes it is the tone circuit.  Not much to it, except that stupid hybrid RC network....   To split the problem in half again, I would use the good hybrid to drive the next stage.   This will determine if the tubes (which were swapped), DC block and basically two resistors are fine. 

[joeqsmith]

Adding a couple of resistors to T off to the two tubes, and then driving them off the suspected good hybrid.   We can see the Left channel is still low.  So, not the hybrid.

[joeqsmith]

Not that I wouldn't have swapped the tubes last time, but I have been known to make mistakes.  Several a day actually.   I went ahead and swapped them back to their original locations and the left still droops.    There's not much left and I suspect we are seeing the RC of the block and volume control pot.   It's the only thing left. 

[joeqsmith]

If you want to follow along, so far in attempt to track the low gain at the higher frequencies I have swapped all 5 tubes from L channel to R.  No effect.   

Inject signal to tape input (1) to isolate the earlier stages.  No effect.  Problem is after this stage.

Connect grid of L channel driver to R channel(2) to isolate the amplifier stage.  Left channel now tracks Right.  Problem is not driver or power output stage.

Connect grid of V4 pin7 Left channel to grid of right channel to isolate the 7-pin hybrid.  No effect.   Hybrid is good.

With switch set to short C21, both C19 and C21 are removed from the circuit.    What is a bit odd is that the cathode voltage is low and for some odd reason the resistor used is also a different style than the other three used.  Also oddly enough, the capacitor that was shorted C13, drives that hybrid.