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SX1250 hum and switch issue

Discussion in 'Pioneer Audio' started by sendust, Aug 5, 2018.

  1. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    Hi guys, i earlier posted issues with popping sound when using switches while using phono. I read through the post form premiumplus that seemed to have the same issue. In reading some of the responses i noted that dirty switches were the possible fault. Since then i cleaned all switches again using Detoxit, standing the unit on its back, face off and connecting nuts removed i sprayed the red into all openings, i worked all switches about 40 times each, then let the unit sit for a few hours. After which i sprayed detox with lube and reworked switches. I left the unit over night and reassembled the next morning. No change but i get a bit of a hum from speakers at 45% plus volume increasing with volume, this was there before but i notice the hum doubles if i connect an outside ground to the chasis, i was wondering if the two problems are connected.Lastly i notice when using the unit the right side gets warmer than the left.
     

     

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  2. Watthour

    Watthour Electron Rancher - JS3600

    Is it the right heat sink which is warmer than the left, or the chassis/case in general? The PS stabilizer board is also at the right side, and will dissipate heat.

    Regarding the hum, does this occur in PHONO mode only, and not in AUX?
     
  3. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    The hum is only in phono but if i switch to aux and increase volume i get a tic tic tic sound.
     
  4. Watthour

    Watthour Electron Rancher - JS3600

    If you connect an oscilloscope to the ±65V and ±70V supplies, can you observe an associated spike or dip in raster mode?
     
  5. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    will get back to you on this
     
  6. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    I happen to own an oscilloscope but i have no clue on how to use it to measure those voltages, i will need to wait until i can get one of my friends that know this equipment to take measurements.
     

     

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  7. kevzep

    kevzep Its all about the Music

    Normal for the right heatsink to run warmer, the one I have here does that, its just all the heat from the stabiliser board dissipating through that heat sink....
     
    Last edited: Aug 5, 2018
  8. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    At least thats one thing i dont need to worry about, thanks
     
  9. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    I notice that when i start up the unit on phono and lp playing, the right side starts immediately after the relay, the left takes 15-30 seconds and gradually fades in. Then no issues. I tried on am/fm and there is no lag instant. Am i right in thinking this is probably a transistor on the 021 board? , if so which one do you think i should focus on, and lastly could this be the same issue as the popping and thunder sound i hear when switching sources. thanks pete
     
  10. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    Strange one, the best way to figure it out, is to scope the area in question, it is hard to say exactly which comp is the culprit.
    read the owner manual for the scope, that is what it is for, to learn how to use your new scope.
    You have to understand the channel amp function, the horizontal sweep function, the trigger function as the basics. would help if you stated what scope model/mfg that you own :) let me take a wild guess, a tek 2230 :)
     
  11. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    Actually its not new, i purchased it from a retiring audio specialist. Its a Heathkit 10-18 and i do have the manual.
     

     

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  12. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    That is a very old scope, but it will do to trace signals.
    Connect the probe to the cal signal and see if you can get a stable square wave as a starter.
    popping sound can be leaky caps, thundering could be caps or bjts.
    Isolate the faults to pre-amp section or power amp sections. Some times these problems are hard to find.
    I know poor connections in switches can cause these hum problems. They act like little antenna for AC line hum pickup.
     
  13. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    I turned on the unit today and noticed the right channel (right speaker ) emitted a sound like dead air on a tv without an lp playing. Also if i connect the speakers to A or C there is no sound at all just on B . Any thoughts?
     
  14. kevzep

    kevzep Its all about the Music

    Have you replaced the speaker relay?
    Have you dismantled and cleaned all the push switches and level switches?
    Have you replaced the known "bad egg" transistors in the pre-amp and power-amp?
    If you remove the pre-main jumper, does the noise go away.
    Does the noise go away of you bypass the tone controls.

    Just a few things to try...
     
  15. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    I replaced the relay on the protection board but not the relay beside the large caps, i couldn't find a replacement
    I didn't completely dismantle the switches but pulled out boardd and made sure i got detox in everywhere and pushed switches at least 50 times
    Ive changed all caps but only a few transistors, im not sure which ones are bad eggs but i have many different types on hand, im sure there must be a post with these references but would appreciate if you know which ones i should target.
    When i remove the jumpers the unit doesn't run or at least produce sound
    I have not tried to bypass tone but i will after i read your responce.
    When i received the unit right channel didnt work at all, left was intermittent,, i got it to run both channels and was afraid to mess with transistors but will do whatts needed to resolve the issues.
     
  16. rcs16

    rcs16 Super Member

    Messages:
    3,893
    Location:
    Baldwin, Ontario, Canada
    Assuming that you have speakers installed and turn on, that would mean that the amp section is quiet and not the source of the hum.

    You can verify this by taking the 1250 pre-amp outs and using them to drive another power/speakers to determine that the hum is coming from the 1250 pre-amp section.
    Hum is also checked, that the inputs, such as auxilary inputs are shorted to ground, selected and volume control is at max. You can monitor the noise/hum at the pre-amp o/p's using a scope and/or a good AC voltmeter, spectrum analyzer/sound card etc.

    is the hum coming from both channels? equally?
    sometimes the hum is due to the circuits, sometimes it is a broken ground wire, esp if you have been working the pcb's and flexing/tugging on the wires.
    I wish I could easily point to one source.
     

     

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  17. sendust

    sendust Music feeds the soul Subscriber

    Messages:
    736
    Im updating this to maybe help others like me who like to recap, clean or service our own units but have limited knowledge and are just learning valuable skills. It turns out although i was feeling good that i got my unit running, it is also my fault that i had new issues. After help form akers i was able to repair the stabilizer board and phono section, so i decided to recapp the unit. While installing the large filter caps and being lazy i used a small flat head screwdriver to tighten the new screws, unfortunately i did not tighten one of them completely, this in turn caused a faulty connection and issues while using switches and listening to music. After moving the unit to try and work on the issue the loose connection turned and came in contact with the chasis, this heated the main resister and blew the thermal fuse. I replaced the fuse and made sure all connections where tight , tried unit with dim bulb and found no problem now unit runs , no pops or issues with switches except the A speaker connection. I see the bulb is out and wonder if this could cause an open circuit. I played the unit for about an hour and once heard a loud pop, nothing has changed and unit plays perfect. Im not sure if it takes a while for new caps to work in or if there is some other issue but i can find no problems with Am FM or the phonos at this time. I have learned that time has to be taken working on these units and never take a short cut, check everything many times over before moving forward. Im sure you techs already know this and hope this helps someone else.
     
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