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

[joeqsmith]

I would say ALL the solder joints look about the same.  Wiring isn't that great.  Some wires were hit with the iron.  Most likely, I will leave all the construction as it was built

Checking the two 2.2k, one for the cathode bias, the other for the plate supply.  Both are outside of 10%.  However the other three cathode bias resistors are 5%, not 10%.   The others are all well within tolerance.   Certainly a problem but not sure if this is the cause of our lack of high frequency drive. 

***
Checking the two foil DC blocks, they could be twins.  ESR, capacitance, leakage all look very good.   Also checked the volume control and balance between the two channels.   The plate resistors for the two tubes are well within tolerance.   Time to insert some new resistors....

[KE5FX]

Yeah, I'd be a little concerned that a carbon-comp resistor that measures OK with a DMM might look different with several volts (or more) across it.  Something similar might be happening in the hybrid circuit, for that matter. 

IMO, a serious restoration effort calls for replacing old carbon resistors right after swapping out the Black Beauties.

[joeqsmith]

Plan is to keep the amplifier in original condition as much as possible, right down to the solder joints and wires that were melted with the soldering gun when it was originally built.  Just a mild cleaning, address some problems with the wood/finish, and replacing a few damaged parts. 

With the two bad resistors replaced,  the base and treble set to max, the two channels track all the way to 8KHz.  Even at 20kHz, there is only 1dB difference.   
This is with the amplifier back to the original wiring and injecting at the AUX ports (all sections in play).   

Not thinking I can hear 20kHz, let alone a 1dB difference. ..... Indeed, testing my hearing, 13.4kHz is about it. 

[joeqsmith]

Next step, see if it will put out the rated power.  They rate it for 50W, 25W per channel.   I've been using 25W Ohmite resistor for loads.   The Right channel starts to clip before is reaches the max power level.   

Once again, cutting the problem in half, I drive the tape input bypassing the front end.  Still clips.

[joeqsmith]

Swapping the two sets of final output tubes, corrects the problem with the Right channel.   Of course, the Left clips now.   I had bought a used tube to replace the gassy one and that one isn't the problem.  At least we have our answer.   From what I remember the other three are original to the amplifier and would be over 60 years old now.  They survived the 60s and 70s listing to hard rock.   

Going to be a shame to put Russian tubes in it....
https://www.tubesandmore.com/products/vacuum-tube-7868-electro-harmonix
Complete set on eBay.
https://www.ebay.com/itm/122912628281

[trobbins]

Imho it would be worth your while inserting a 10 ohm 1% in the cathode of each 7868, to confirm you at least have balanced idle dc currents for each channel, and to allow you to monitor those tubes every 6 months or so for onset of failure.

[joeqsmith]

High failure rate on the Russian tubes?

[trobbins]

It might not be a failure, but rather an imbalance in dc bias and/or ac gain.  Not all tubes ares created equal, even from the same manufacturer.  If you can measure the idle dc cathode currents, then there is some chance that a better dc balance can be achieved with the new tube, assuming the new tube is ok, by matching it with the other 2 tubes, and assuming it is not ac gain differences that are an issue (you have feedback around the output stage to minimise that aspect).

[schmitt trigger]

Restoring vintage tube equipment is a labor of love. It is not a project you will complete in a pair of months, let alone a pair of weeks.
But when you are done, it provides a satisfaction that is difficult to convey with words.

I cannot offer any further technical advice to what has already been provided. But I sincerely wish you can complete your project.
Patience is your greatest asset here.

[joeqsmith]

I can't imagine anyone pulling apart their tube amps every six months to check bias currents. I would guess this particular amp was in use for 40-50 years and  outside of mom taking me to get a new tube for it once, it worked fine. If the Russian tubes are so poor as not to be trusted, I may try to hunt down some NOS parts.   

This particular amplifier is slightly different than the schematic I posted. That was for some revision of the X-100-B. I wasn't able to locate the correct one but may end up marking up that schematic to match.   

Restoring vintage tube equipment is a labor of love.
...
But I sincerely wish you can complete your project.
Patience is your greatest asset here.

Thanks.  The amplifier is basically working now outside of having a weak tube.  Sounds like the problem is going to be sourcing a good set.

I've cleaned up the mechanical bits as much as I plan to.   No polishing the parts.  It's old and I want it to look and smell its age.     

The wooden case is a different story.  Waiting for the coating to cure, more wet sanding, more coats....  I thought I would have time this weekend to finish it up but ended up adding another coat.   A few more days of dry time and I will have another look at it.   

I think I posted about restoring my dad's old Garrard turntable.  Now that thing took some time.  I was very lucky in that case as someone had given me the exact turntable when I was a kid and for some reason I had saved it all these years.  It was missing parts but was in good enough shape it made a very good donor.   It supports LPs, 78s and 45s.  You can auto change all record sizes.  There is an adapter for the 45s.  As a kid, we would stack 10 or so 45s on it.   It will automatically place the needle and pick it up and does this for the record type selected.  It's all mechanical and sorting that out took me a few days.   

Once the amplifier is done, I still have the tuner to go through. 

[trobbins]

Many vintage amps have voltage sense terminals, or jack sockets, to maintenance check the cathode currents.  The other common method is to use an insert bias checking adaptor that either comes with a meter or uses a separate meter.  Others extend a custom maintenance socket to make it relatively easy to check during operation.

Not everyone has the $ to afford what is being asked for NOS brand name matched-set valves, and not everyone has a pleasant experience installing a new set of matched valves, and not every amp survives an output stage valve going bad without collatoral damage.  Also quite a few find that a higher mains voltage, as often encountered nowadays, has caused them grief.  Just saying  

[joeqsmith]

Fisher saw no reason to install monitoring terminals.  After 60+ years, I have no plans to start making any changes to the amplifier beyond some basic repairs.   

At some point I will look at the output spectrum, maybe do a two tone test or something.   For now, single tone time domain is good enough.   

[joeqsmith]

Looking at the 2nd & 3rd harmonics of the amplifier, I ended up joining another forum to see if anyone could verify the levels I am measuring.  I ran the same test  on both channels with the gain set to about 25% to keep the weak channel well within it's linear range.   The levels match for both channels.   

Also have a lead on a set of output tubes.  A member here offered to send me a few unknown ones as well, which I want to again say thank you.  The last time I purchased a new tube was in the 90s to repair my old Drake ham radio.  Guessing that audio amps are far more common and with it being 30 years later, tubes are getting very hard to come by.   

https://audiokarma.org/forums/index.php?threads/question-on-fisher-kx-100-2nd-and-3rd-harmonic-levels.1041163/

[joeqsmith]

Replacing the weak tube, both channels clip symmetrically at the same level.   5 ohm loads, so say 20W.   Maybe with a better match it would run 24W.   That, or the manual may be suggesting that with higher THD and only running one channel at a time you can reach 24.     

[joeqsmith]

Testing it with a single channel, just starts to clip at 27W.   At 29.5W, it's  clipping pretty bad.    Power output stage looks good.   

Time for the debates of contact cleaners....  Pots had some drag and noise.  I put some lite oil on a toothpick and applied a VERY small amount on each rotating shaft and let it sit overnight with the shafts pointed upward.  Audio guys are cringing.  lol.   I then flushed the wipers with ProClean a few times while I worked the pots.   They have a nice feel to them now and the noise has greatly improved.   Running it tonight, when adjusting the power levels, it seems clean.    ProClean is not a contact cleaner.  I use it for PCB work.  I like it because it doesn't leave any residue. 

[IanB]

As a kid, we would stack 10 or so 45s on it.

I grew up in Britain where the 45s had the same small spindle hole as 33s and 78s. I remember the auto-change feature where you could stack records on the spindle where they would be supported on a small metal pivot and held steady by a control arm placed on top. When each record finished playing, the tone arm would move to the side, the pivot would retract and drop the next record down on the the platter, and the tone arm would move back and drop down to start playing.

But I recall 45s in the USA had a big wide hole in the middle. How did the auto-change feature work with these? Did you have to clip in plastic adapters with the small spindle hole?

[joeqsmith]

You are correct, they had a much larger hole.   There were three different spindles the the records would fit onto.   A short and tall thin one for manual and auto loading LPs.  Then the large diameter tall one for the 45's.  There were a sets of tangs that would move inward and outward to drop the record.   These mechanical tangs were controlled though the inside of that spindle down inside the turntable.   

If you check out this site,  the spindle looks like what they show for the Garrard RC-80 and RC-90 (top right side of page).   

https://midimagic.sgc-hosting.com/changerx.htm

I don't have a single 45 anymore or I would make a demo video for you of it running.  It is quite the mechanical wonder seeing it work.   Searching YT, I found one very similar to my dads.  It appears to be from the 70's but the changer seems similar.



I believe this is the one my dad had. 

[IanB]

The first video you linked with the auto-change on the LPs worked exactly the same with 45 singles without having the middle knocked out. We never had the large center hole or fat spindle in the UK, and so I always wondered why 45s had that bit in the middle that you could in theory remove. As in the second video, no one would ever actually remove it as it would "damage" the record. There was just one spindle that would equally work with 33s, 45s and 78s. I never thought about the spindle being interchangeable. The cartridge would also flip over with a lever, one side for 78s and the other for singles and LPs.

Those turntables/record players honestly seemed like mechanical marvels.

[joeqsmith]

I attached a few photos of it for you with the three spindles and a spare needle.   

In this ebay ad, there are some photos of the mechanical wonders.   I think I spent a day or so just studying the thing before I attempted to work on it.   

https://www.ebay.com/itm/295657399428

[IanB]

You are correct, they had a much larger hole.

This is what a single looked like when manufactured in the UK. Coincidentally, this exact record was in my father's record collection. I played the heck out of it as a child.

[joeqsmith]

I have never seen a 45 like that.

I'm guessing that the large hole was to support the auto loaders found on jukeboxes.   Pure guess.   Did you have these industrial record machines where you lived during the 50-70s?

****
Diamond needle + dog hairs. 

[IanB]

Wow. Those Garrard Type As are "modern" (early sixties). Even in the fifties they made the then equivalent of a "boom box", which was a record player built into a carrying case with a locking lid and a handle that you could pick up and take with you to parties. (You still had to plug them in, though.) But even those one-box wonders sometimes had automatic play and record change features--because parties, of course.

I'm guessing that the large hole was to support the auto loaders found on jukeboxes.   Pure guess.   Did you have these industrial record machines where you lived during the 50-70s?

Yes, there were jukeboxes, but I don't have any knowledge of the mechanicals. The working mechanism was all hidden from view. At a guess, they might have worked in a similar way to more modern CD multichangers.

[joeqsmith]

I had a portable record player as a kid that my sister gave me that was inside a suitcase with cloth covering.  It had the amp and speaker but no auto changer.   

We had a jukebox during this time that was open so you could see the mechanics in operation.  The records were stored vertically, and played vertically.  The need came in from the side.   I had seen and used records by that time and remember thinking, how does that record play like that.  It was pure magic. 

[joeqsmith]

They spec hum and noise to be 88dB with the volume set to its minimum.   They don't specify any other details about the setup but the low end is where the noise is. 

Tried sweeping from 10Hz to 500Hz and we can clearly see the 60Hz signal just below 55dB.   Plugging in a set of headphone, terminating the two inputs to ground, turning off all the crap in the room and turning up the volume way past what I would ever use, I can hear that 60Hz.   It's faint.    Case is not yet buttoned up.   

I also had a look at how much coupling there was between the two channels by terminating one input to ground and measuring its output while driving the other input.   It's similar to looking at the harmonics, there's no spec for it and without something to compare it against hard to say.  Coupling increases with frequency and at 20kHz, its about 20dB down.   For all I know these harmonic levels and coupling may give the amp some unique coveted sound...

[joeqsmith]

Worked on the low freq noise tonight. It still doesn't meet their 88dB spec but it's close.