Author Topic: Zenith x930 20XT20 Chassis  (Read 645 times)

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Zenith x930 20XT20 Chassis
« on: October 24, 2016, 02:17:45 PM »
Ran into some troubles with this the other day.  After I put in new electrolytics, replaced a few out of spec resistors, and it sounded better than before and had slightly more response.  I did notice however that the dual volume pot /fader still had to be tweaked about 1/8th of a turn to sound 'balanced'.  I also noticed that there seemed to be a slow tick noise in the right channel, throughout all sources, that did not change in volume.  Friday I turned it on and had no response out of the right side at all, in either mono or stereo.  The stereo indicator light did flash a couple times throughout the dial but still no noise in the right channel. 
I am working off of the Sams photofact in the Downloads section.

I do not know much about transistors, and I suppose this is going to be somewhat of a multifaceted question/thread.

-The bass and treble did not seem to affect anything until almost maxed out. Is this normal?

-Would the volume/fade setup being slightly 'off' indicate a failing/drifting part somewhere or is that part of having the adjustment.

-I have some confusion on transistor cross-reference. Zenith 121-418 crosses to NTE179, Sam's lists a Zenith cross to GE-3 and that crosses to NTE121.  Also the 1st Amp(121-433), 2nd Amp(121-430) and Driver(121-442) all cross to GE-10 on the Sam's which crosses to a NTE123AP.  How reliable is the NTE Cross-reference?

-Germanium transistors are prone to failure?

-I have not touched the rectifiers or the ceramic caps with them.  I'm assuming that replacing the ceramics (C83, C84) with X2 Safety caps is correct? Rectifiers are reliable enough to leave alone?

I do not plan on shotgun replacing the transistors but would like to learn more, as I feel a little blind working on this compared to tube gear.
 

Motorola Minion

  • Sr. Member
  • ****
  • Posts: 499
  • Southern Pennsylvania
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #1 on: October 27, 2016, 10:11:53 AM »
I have recently picked up two Zenith SS consoles a year older and newer. Hopefully, you replaced 300 mfd caps that couple output to speakers. The popping may have been a driver transistor heating up and failing. See photo below, where a heat sink is installed on the tiny driver Q12.

 As for the bass-treble controls not seeming to affect much change in sound, I have noticed that in the 1968 Y942 I have, using a very similar chassis 26YT20. Those being the tone control issue, stereo light does not glow, and the FM reception is OK but not what a Zenith should be. Sometimes walking nearby causes multipath and makes the volume drop or come back, seemingly at random ??? This was not noticed until after I put a dozen screws in to hold the back cover on. I restored a correct 2g record changer for it and THAT part sounds great.

Fortunately I have two new SK3006 transistors but have yet to start plugging into the tuner's RF amp and mixer socket. Im not sure I want to chase down the indicator problem as that may require another rare Ge. transistor

I can answer the parts numbering question best. NTE is one of the few making discrete transistors for replacement that provides a cross reference to the 121 prefix original part number. cross the Zenith number directly to NTE. AFAIK only the SK-numbers (RCA) may also be trusted from the sams.

Remember the sams publication is 50 years old and may have part number cross mistakes from the day it was printed.

Now, for the germanium question: It has been proposed on other forums that Germanium transistors "wear out" or lose emission like tubes do. Making matters worse, few parts sources  offer Germanium replacements. Replacing with silicon requires resistance changes to re-establish correct biasing.

I have wasted time trying to fix lesser-brand transistor consoles and prefer tubes as a rule. But the average Zenith RCA or Magnavox SS console probably needs just a few key components replaced.

Tubes - Magical - Tubes

Dave

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #2 on: October 27, 2016, 11:33:34 AM »
I did replace the 300mfd caps in my initial cap replacement with Nichicons (although they were 330mfd, but I don't foresee that being an issue).  Only 1 electrolytic remains (C5) which is coming off of the 1st AM IF coil.  To me, this seems like 2 problems, since the stereo indicator did not light anywhere on the FM band for but a few flashes that I could not duplicate, and losing the right channel entirely.  I have not been able to look at anything since my original post due to other commitments.

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #3 on: October 29, 2016, 07:33:01 PM »
I noticed today that the Stereo Indicator light was lit throughout the entire AM band, and FM band but still only have left channel regardless of the stereo/mono switch setting

Motorola Minion

  • Sr. Member
  • ****
  • Posts: 499
  • Southern Pennsylvania
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #4 on: October 31, 2016, 10:26:29 AM »
Q12 and Q13 are NPN silicon type subbing by a NTE-123AP is probably correct.
The output transistors are Germanium and those rarely cause trouble. How do the voltages compare to the schematic?
Tubes - Magical - Tubes

Dave

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #5 on: November 01, 2016, 07:45:58 AM »
Last night I replaced Q17 and Q12 and now have both channels back, and the stereo indicator light is functioning correctly.  Q17 looks like it had been replaced previously, as it had a Motorola logo and numbers 636 and 121430 on it.  I only replaced Q12, which looked original, as to make sure that both channels were identical.  I do however, still have a problem with the volume/fader control.  I can see where the markings are to be centered, but it has to be tweaked about 20 degrees rotation for the sound to be centered.

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #6 on: November 16, 2016, 10:51:08 AM »
While I had the chassis out and checking the transistors, I checked a few of the resistors in the amp section, only to find that a few of them in one channel were way out of spec.  I started replacing any that were out of spec and then replaced the corresponding resistor in the other channel as well.  I also replaced the resistors that were attached to the pots for bass/treble adjustment, but they were borderline on needing to be replaced. Now I have a lot more bass and treble adjustment and a much better overall sound. 

Another question- The tape recorder power supply terminals in the chassis obviously mean that such a unit was available.  Does anyone know what the model # of the recorder was?  Or if it was available for this model console, as dropping in a reel to reel would mean giving up the record storage bin.

B-Rad

  • Newbie
  • *
  • Posts: 11
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #7 on: January 17, 2017, 10:27:09 AM »
heats00 glad you figured it out. While waiting on my membership to get approved last night I was going to tell you to check the internal/external speaker switch if yours has it because that is what caused similar symptoms on my Zenith X940 (26XT20). I would always also suggest putting some DeoxIT through the pots and switches to clean off any corrosion from the unit sitting. Anyhow glad you fixed it.
- Brad

heats00

  • Newbie
  • *
  • Posts: 25
    • View Profile
Re: Zenith x930 20XT20 Chassis
« Reply #8 on: January 17, 2017, 03:26:57 PM »
I checked the speaker switch as well, and cleaned it for the sake of cleaning it since it was apart.