1Khz square wave 350MV input level on AUX port Volume at "2"
1Khz square wave 350MV input level on AUX port Volume at "2" slower sweep rate.
1Khz square wave 350MV input level on AUX port Volume at "3.5" slower sweep rate. Rise deteriorating.
I got curious and decided to run some tests on the KX-100 with a signal generator and Oscilloscope. I inputted a 350mv signal to the AUX left and right channels and monitored the output on the speaker terminals with the scope with the speakers also attached. Inputting a sine wave the test results were pretty good with the amp showing pretty linear output up to 20Khz with slight falloff at the higher frequencies. I had the amp volume at “2” during the test. The left channel is the pink trace and the right is the yellow.
The square wave test at 1Khz is shown in the first photo of the traces I uploaded. At a volume level of 2 it wasn’t too bad (photo #2) but when I turned the volume up on the amp up to 3-3.5 the square wave really started to deteriorate on the rise side pretty quickly. (photo #3) It also deteriorated when I increased the frequency of the input signal. I have not replaced the power supply caps yet and am wondering if the power supply was an issue when I started to increase the volume level and/or the frequency?
I’ll repeat the tests after I install the new caps but was wondering if my thoughts make sense. Since both channels are affected equally, I am assuming that a common component is the issue (power supply). Or am I just expecting too much from a 60-year-old amplifier?
Thanks for any thoughts anyone wishes to share!
Yes I can.
Well while I was waiting for the replacement filter caps for the KX-100 I started working on the X-100B again. I replaced all of the signal path caps and installed the screen resistors and 10 ohm cathode monitoring resistors. I replaced one power supply cap C3C as the voltage across it was way low and returned to normal once it was replaced. I installed the tubes, hooked up speakers and gradually powered the unit up. The voltages looked good and I set the bias for 42V per the manual. Amazingly enough the unit played quite well. I started measuring across the 10 ohm cathode resistors and found that one tube was doing way less (.11) than the other three. I measured voltages on it's socket and found there was no Plate B+, in fact it read slightly negative to ground. Looks like one leg of the output transformer is open I'd imagine. I'm amazed it played as well as it did.
I looked at the Transcendar site but there is no place to contact them or place and order. Are they still in business? I found an email address and sent them a request for information but failing that is there another source for replacement output transformers for the X-100b/Kx-100? Looks like I'll be needing one. Now I know what the event was that damaged R27.......... Oh well........
I looked at the Transcendar site but there is no place to contact them or place and order. Are they still in business? I found an email address and sent them a request for information but failing that is there another source for replacement output transformers for the X-100b/Kx-100? Looks like I'll be needing one. Now I know what the event was that damaged R27.......... Oh well........
Thanks for your kind words. Too bad about your KX-200 but sure you will bring it back to life soon! I would like to get a hold of one myself but had better finish with the units I am working on now. I am really enjoying bringing these units bacl to life!
I exchanged emails with the Transender folks today and they can provide a replacement transformer for the X100B. I decided to order two and replace both output transformers and perform the modifications suggested in other threads to tweak things to best mimic the original Fisher output transformer characteristics. Figured the X100B has all new tubes and might as well give it new output transformers as well to maintain channel symmetry. It can be like a Frankenstein monster Fisher so to speak though I suspect it will still sound wonderful! I'll keep the good output transformer as a spare for the KX-100 or help out someone who needs a replacement as the original Fisher units are difficult to come by.
I received my Replacement output transformers last week. I was fortunate to get my order produced as Transendar's website later stated they weren't taking any more orders and now the website shows being down. Hoping this is a temporary situation. The new units look well made and were carefully packed to ensure they arrived undamaged. I will install them in the next week or so time permitting.
I finished installing the new output transformers this week and also replaced all the filter Caps with Hayseed Hamfest units and also the power supply diodes. I also performed the Zener diode mod to replace the screen Cathode resistor and the individual bias modifications and installed inrush inhibitors. At this point it is practically a new amp! Since the unit came without tubes originally I decided to install new EH 7868's in the output stage. It took a bit of finesse to install them given the larger pins but it was not too difficult. I installed six JJ ECC83 12AX7 tubes in the other positions and gradually powered up the unit. I connected a digital audio source (Ipad) to the aux input on the amplifier to see how it sounded. All I can say is WOW!!!!!! The amp sounds amazing even before allowing everything to burn in for a while. I dialed in the bias on the EH 7868's and let the amp play for several hours with all types of music and I was very pleased with the sound. It has plenty of punch and is crystal clear. After a few hours I substituted a vintage pair of Sylvania 12AX7's in the tone control section to see if I could hear a difference. The change was subtle but very noticeable particularly on vocals. The vintage tubes definitely added something to the sound of the amp. It was quite a lot of work to get this unit back in working order but in the end the results were very gratifying. I am definitely considering replacing the Vintage 7868's in my KX-100 with the EH tubes since they work so well in the X100B though I suspect the new output transformers may be contributing to the improvement also.
This is an older post of mine but am posting something I observed this evening. I was playing the KX-100 referred to in this thread on my bench. No reason ,just decided to play it for a while. It has all the mods described including the zener mod to the cathode bias configuration. I had installed a 100MF 100V Nichon capacitor as the cathode bypass capacitor during my original restoration and the amp played well. On the second amp, X-101b I mentioned in this thread I had installed a 200MF 150V cap as the cathode bypass cap. That amp seemed to play stronger than my original KX-100. I attributed that to the Fender output transformers I had installed along with mods to the negative feedback circuit to accommodate the different turns ratio of the Fender transformers.
Tonight I removed the 100MF/100V cathode bypass cap in the original KX-100 and installed a 200MF 150V in it's place. When I turned the KX-100 on and played it for a while I was surprised and VERY pleased with how much better it sounded with the new capacitor. It almost sounded like listening to a KX-200 vs a KX-100, more solid bass and overall stronger sound, like it was producing more power at a given volume setting. I was wondering if anyone else had a similar experience with modifying the cathode bypass cap in a cathode-biased Fisher?
Tonight I removed the 100MF/100V cathode bypass cap in the original KX-100 and installed a 200MF 150V in it's place. When I turned the KX-100 on and played it for a while I was surprised and VERY pleased with how much better it sounded with the new capacitor. It almost sounded like listening to a KX-200 vs a KX-100, more solid bass and overall stronger sound, like it was producing more power at a given volume setting. I was wondering if anyone else had a similar experience with modifying the cathode bypass cap in a cathode-biased Fisher?
I tested the 100MF cap I removed from the KX-100 and it tested good. I thought it would make sense to check the bias on the 7868 outputs as I have the IBAM mod installed on the KX-100. The bias was a little bit high (43-44ma per tube) so I dialed it back to 41-42 ma. I did not expect this to have an impact on the sound and it did not. Just for fun, I took a picture of the old and new capacttors together. My theory is "bigger is better" as I do not have another explanation for my observation about the sound improvement.
"Auditory Dementia". 
(I'm not far behind!)
(I'm not far behind!)
Larry, I know that my state of mind definitely affects how things sound to me. Sometimes you hear what you want to hear I think but still the change was significant. I think I will put the 100MF cap back in and see if the sound deteriorates or clip them in somehow and switch back and forth, hopefully without getting zapped!
Earlier this evening I performed my experiment with the KX-100, switching the cathode cap from 100 to 200 and even to 300MF. This time I used the same pieces of music instead of the randomly selected ones I did the first time out. How a song is recorded and produced of course has a large impact on how it sounds when played. (duh!)
The results of my experiment is that I could hear little or no difference when the values of the cap were increased or decreased listening to the same selections each time. I must have mistaken a difference in a given recording for an improvement caused by the cap change. (is my face red?) LOL. Oh well, it was an interesting experiment, I ended up leaving in the 200MF capacitor so the two KX-100's I have are consistent in that regard. The sound of the KX-100 does improve noticeably when I am sipping my second Jack Daniels..... perhaps this phnomenon requires further study............................
The results of my experiment is that I could hear little or no difference when the values of the cap were increased or decreased listening to the same selections each time. I must have mistaken a difference in a given recording for an improvement caused by the cap change. (is my face red?) LOL. Oh well, it was an interesting experiment, I ended up leaving in the 200MF capacitor so the two KX-100's I have are consistent in that regard. The sound of the KX-100 does improve noticeably when I am sipping my second Jack Daniels..... perhaps this phnomenon requires further study............................