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Need advice on issues with my Knight Kit

Discussion in 'Tube Audio' started by spliteye, Nov 8, 2018.

  1. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    Hello everyone,

    I'm currently in the process of restoring my old Knight Kit 32W stereo integrated amplifier, model 83 YU 933. I found this amp with a cut power cord and no tubes, so I know it must have had some issues that someone either couldn't figure out or didn't feel it was worth it.

    I started by replacing all the capacitors (except for two ceramic disks that I don't have on hand). I had purchased the 12AX7 tubes and a single 7025 from a member of a different forum a while back, which were supposedly all tested and good. I purchased a Mullard 5AR4 reissue and four Sovtek EL84M outputs, matched, to complete the tube compliment. The amp originally came with 7189 outputs. I did find a single 1W 33 ohm resistor that had burned and was in two pieces, which I of course replaced. This went to heater pin 4 on the 7025.

    Upon initial test, the amp sounded good for about two minutes (apart from a small hissing sound), and then my issue occurred. The right channel began to pop, emit a high pitched whine, and then lose volume.

    I checked resistance on the cc resistors under chassis and found that 70% of the ones I tested were out of tolerance, so I replaced all under chassis except for one odd ball size that I don't have, but will order. Found one bad solder joint on the right channel phase inverter.

    The hiss is now gone, thanks to tossing the original cc resistors, however the problem still persists. I noticed that one of the right channel EL84M output tubes began to flash brightly this time I retested. I of course cut power immediately. Sounds almost perfect for two minutes or so.

    I have not made any voltage measurements yet, I wanted to ask you guys for advice before I start chasing things around. If you have any suggestions on which direction I should head next, feel free to chime in!

    Thank you,
    Dan
     

     

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  2. cademan

    cademan Addicted Member

    Messages:
    9,027
    Sounds like it might be a bad EL84M tube. Did you try swapping it to the other side to see if the other channel shows the same symptoms?
     
  3. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,349
    Location:
    Melbourne, FL
    Do you have a schematic? (Perhaps you do since you talked about checking voltages?)

    I'd get the tubes tested, then clean and retension the sockets and check the voltages. As others have said you can switch the affected tube to the other side and see if the problem follows the tube or not. That will tell you something.

    Did you go through all the resistors or just some? Note that you can try to test the resistors "in circuit" first. If they test right on or high then it was ok to test them "in circuit". If they test low then you should lift one leg (desolder) and retest. This is a good afternoon project. Did you de-oxit all of the controls and switches? Check for leads on resistors and capacitors touching each other where they should not? Make sure that leads on any components are not in position to touch the case when the case is re-assembled.
     
    Last edited: Nov 9, 2018
  4. linuxslate

    linuxslate Member

    Messages:
    87
    Location:
    Central Florida, USA
    Things like burned power resistors are always a hint. They seldom fail on their own. They are stressed by something downstream.

    In this case, the resistor probably died due to a failed capacitor that you already replaced, but it still shows the history of what was wrong with the amp.

    If, for example, the resistor is dedicated to the channel than now has the problem, and there is an identical resistor in the other channel that is pristine, that would be a significant clue.

    You really need a schematic.

    Another just general comment: I rarely find "check all the voltages" to be helpful. I've never had the attention span to make nice tables of voltage readings, and I would end up with a mess of numbers, and no real pass/fail criteria. You can learn a lot by just checking the voltage at a small subset of points.

    Check the B+ at an appropriate point. (OPT center taps in most PP amps.)
    Check negative voltage at the bias supply (for grounded cathode circuits.)
    Check the voltage at the control grids of the power amp tubes. (Should be near 0 for cathode bias amps, should be near bias voltage (-) for grounded cathode)

    All of these should be close on both channels. All of the measurements are taken (carefully!!!) with amp on, tubes in, speakers connected, inputs connected, but no music playing.

    The amp also has to be basically stable. I would not do the above by the erie blue light of an arching tube.
     
  5. triode17

    triode17 Super Member

    Messages:
    1,319
    Sounds like loose socket pins to me. the clue was "two minutes". If the socket pins expand a little faster than the tube pins, this can happen. I would tighten up the pins for now, but ultimately, change the sockets. It's a pain but worth it.
     
  6. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

    Messages:
    2,733
    Location:
    Terra, 21st century CE
    This is when a SS rectifier and a variac is very helpful. With that you can run your amp voltage up slowly and monitor the voltages and not risk your tubes or transformers. This can help pinpoint the problem area. Since you suspect the one el84 socket you can do measurements comparing the good section to the suspect area. At around 70% of full voltage all the tubes except maybe the phono section should be operating enough to pass a signal. At this level none of the components should be warm or hot, any resistor or electrolytic cap is suspect. Any DCV on the grids of the el84 means that the coupling caps are leaking and need to be replaced.
     

     

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  7. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    I have not tried that yet, however I did consider it. I wanted to check with you folks first. I began restoring radios years ago, however this is my first amplifier restoration and I hadn't encountered this issue until now. Thank you for the advice, I will most likely do this tomorrow, time permitting as usual, of course. Thank you!

    Yes, I actually do have a schematic. I managed to locate a complete, original assembly manual off ebay a little while back. This not only has a schematic, but detailed assembly diagrams and parts lists that I used while I was out in the shop late and feeling a little lazy.

    I went through some of the resistors, however I decided to replace them all, being around 70% of the ones I tested were out of spec. At that rate, I figured it would be justified to just replace all of them and not worry about them in the future. I have not cleaned the controls yet, I will do that tomorrow, before I do any more work. I looked for any crossed connections and so far have been unable to locate any, along with any other cold solder joints.

    I have a simple emissions tester, however it's a little "testy", no pun intended. I'll see if I can't do a test on the outputs tomorrow. I didn't consider testing them, as they were supposedly tested by the vendor, however something could have happened to that tube during shipping. I'll also spend a little more time inspecting that socket. Thanks for the help!

    Yeah, this is why I was concerned about doing a voltage test while it's in it's current state. Haven't blown the new resistor yet, so maybe that's something! Thank you! Hopefully I can get it a bit more stable before probing around under chassis.

    Fortunately, I have a couple spares that I could swap in if needed, and those output tubes would be the easiest out of all of them. I'll inspect that socket carefully tomorrow and see if I can't tighten the pins a little, as well as swap that tube to the left channel. If the tube swap doesn't kill the left channel, I'll know it needs a socket. Thanks!

    I should have mentioned that I do have a variac and an old Weston ammeter that seems to be accurate ( as long as my toaster really pulls 8 amps like it says it does, that is *humor intended*). My apologies for not mentioning that, I was double checking things under chassis while I made the thread. I did do the first test with the variac, ammeter, and dummy loads on the bench, however this was more of a smoke test. I have not checked voltages. I did see the ammeter swing a small amount while that tube was flashing, however that makes sense. If the other tests don't work out, I'll try this next. Thank you!

    I will try to do whatever I can tomorrow and I will let you guys know what/if any progress has been made.

    Thank you so much,
    Dan
     
  8. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,349
    Location:
    Melbourne, FL
    So in looking around the model 83 YU 933 appears to be a Knight KA-40 Amplifier. A cursory look didn't find a schematic on the web, but there is info out there for them.

    The output tube flashing sounds suspiciously like screen grid arc'ing. I'll defer to experts here, but you might want to ask if adding 100 ohm screen stability resistors would block arc'ing. I can't speak to the particuar circuit in this amp though as I am not an expert and I haven't seen the circuit. I'd get that solved or at least protected; and I would focus on testing that one tube for damage - in addition to looking at the cause of course.
     
    Last edited: Nov 20, 2018 at 5:36 AM
  9. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

    Messages:
    2,733
    Location:
    Terra, 21st century CE
    Or the tube is weak and/or has been subjected to an arc incident before and susceptible to arcing again..
     
  10. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    Hello everyone,

    just to give an update....

    I went ahead and cleaned all the controls and sockets with D5, as well as swapped that tube with one from the left channel. I intended on testing the outputs, when I found that my old NRI (Tripplett) tube tester doesn't have a 9 pin socket. I used this for mainly pre-war radios, so I rarely ran into 9 pin tubes, pentode, dual triode, etc.I went ahead and powered it up, and noticed the same right channel tube was brighter than the rest after a solid warm up period. I went ahead and shut if off while I felt I was ahead. This is the tube, V-7, that the filament voltage comes into the string. I suppose I could understand the filament heating up faster than the rest, however not three times as bright. I proceeded to install a new socket, as it wasn't a huge task on this amp. Once the install was finished, I powered it up once more, only to experience the beginnings of the same issue. Lots of popping, volume control nonexistent to control the crackles and pops.....my girlfriend is asleep as I should be, but I'm a bit obsessive when it comes to finishing a project. However, I'm not going to wake her up trying to continue diagnostics and end up with another task.....don't take that the wrong way, I'd earn a chew out with midnight amplifier noise....haha.

    I've had this amp for years. I put it off years ago because I was a teenager who didn't really have the funds to properly restore it, however I can now. It's been on my mind for close to a decade. I'm almost positive I've asked questions about it years ago on this forum. It took me years to find a schematic for it.

    I live in northern Ca, we have a large fire up north where my folks live. It's too smokey to work outside. If I don't end up helping them evacuate, I'll be back on the bench, as I don't really need to be outside. Other than pulling tubes and measuring unimpeded voltages, if you have any suggestions, I'm all ears. This amplifier had been worked on before, and it was stripped for a reason, so I'm sure there may be something tricky afoot. Unless its a bad transformer, which only makes partial sense to me, I'm starting to think a ground issue. This was a kit built unit, and it's complete chaos under the chassis, from my point of view. That being said, I haven't found anything that isn't correct from the manual's point of view. The builder followed Allied's instructions fairly well.

    I'll do another test tomorrow. I just wanted to give an update to you folks who have taken the time to help me out with this.

    Thank you again,
    Dan
     
  11. trainbuftony

    trainbuftony Electron Herder Subscriber

    Sorry to hear your family is affected by the fire. If you are in the area, i have a variety of tube testers, and i have rebuilt a few tube amps before. I would be happy to take a look at it and see if my eye spots anything which yours didnt.
     
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  12. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,349
    Location:
    Melbourne, FL

    Spliteye,

    Seems like help is arriving or trying to anyway. If you post your location in your profile you might get even more assistance. Also if you could post the schematic you'll get even more help.

    So I think you have the heater voltage to focus on in the near-term. I don't know if you have any series strings. If you do you'll need the tubes in while you look at the heater voltage problems. Do you have a variac?
     
    Last edited: Nov 12, 2018
  13. triode17

    triode17 Super Member

    Messages:
    1,319
    Split-Some tubes are simply brighter than others. I assume you are using the same brand for all. try swapping (for example) a 12AX7 for another 12AX7 or whatever is in there, as long as its the same tube number. Then see if the brightness follows the tube.
    I am working on a tube phono pre. design and I have to run 6v tubes in series from 12v. When I light them up, one always glows more than the other. So i swapped them and it followed the tube. Can you post a schematic for us to see?
     
  14. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    Hey everybody, sorry for the late reply. Had a busy last couple days. My family is ok so far, the fire hasn't traveled further in their direction, but they've told them to be on alert. Thank you very much for the concern.

    I'll update my profile with my location after this post. I live in Lake County. Not in an area where we want to be, but hey, gotta start somewhere.

    I'll try to post a schematic tomorrow evening. The next day I can get back into the shop is Thursday.

    Thank you everyone,
    Dan
     
  15. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    Hello everyone,

    so, I got a chance to work on the amp today. I went and looked for cold solder joints, shorts, etc, however I could not find any. However, I believe I may have found where the issue lies. The output transformer, where the primary connects to said power tube with the issue, measures open on a resistance test.

    This weekend I will do my best to look into this further, inspect the transformer closely, and maybe attempt to find a break in connection. However, at this point, I'm assuming that the amp will need new output transformer(s).

    The schematic is too large to scan, I will attempt to take a picture of it an upload it for you guys and for anyone who happens to be looking for the schematic of this amplifier. It took me a while to find it myself.

    Any advice is still welcomed and much appreciated.

    Thank you,
    Dan
     
  16. cademan

    cademan Addicted Member

    Messages:
    9,027
    You can always fold the schematic to make 2 or 3 individual scans. I can read them forward, backward, upside down etc.
     

     

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  17. spliteye

    spliteye New Member

    Messages:
    17
    Location:
    Lake County, Ca.
    Hopefully these scans comes through. I don't know if anyone is still reading, however this is a slow project and I probably will not be able to replace the OPTs until early next year. I appreciate all of you taking the time to help and being patient with me and my slow responses. I have an 11 hour shift and I commute 2.5 hours a day. The days I have off, I spend taking care of the house, partner, etc. I do my best to spend as much time as I can in my shop and I would spend every day in there if I could, there's nothing quite like it.

    Unfortunately, money is an issue at the moment, however I'm determined to make this amplifier work correctly. This amp has a limited ceiling, literally. Anything beyond 3" tall from mounting is pushing it, assuming I still want to be able to put it back in it's enclosure.

    For the time being, I plan to remove the bad OPT this weekend and see if I can't find the broken wire in the first part of the winding. I remember someone taking apart an OPT from a similar amp to mine and finding the break in an easy to repair spot. I appreciate any additional help and opinions that you can offer.

    Thank you,
    Dan
     

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  18. s-petersen

    s-petersen Scott Subscriber

    Messages:
    4,584
    Location:
    Middletown,New York
    The balance pot is interesting, it is a single stack pot that shunts the input of each channel to ground, it is certainly thrifty, I wonder if there is any crosstalk because of this?
     
  19. stanko

    stanko Hoarder Subscriber

    Messages:
    790
    Location:
    Oshkosh, WI
    I put a link on about rewinding a transformer. Search my history of postings, Scott 130 transformer.
     
  20. Tim D

    Tim D AK Subscriber Subscriber

    Messages:
    1,349
    Location:
    Melbourne, FL
    I repaired an output transformer in my Fisher KX-100 where the break was on the beginning of one leg of the primary right after the leads coming in joined the primary winding. I was able to overlap the two ends of the break and solder it back together. A 1 or 2mm overlap is sufficient. It wasn't as easy as it sounds as the wires were hair-fine. Really challenging to work on something that small. Not everyone is going to be that lucky.
     
    Last edited: Nov 20, 2018 at 5:48 AM

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