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The Sears Silvertone Twin Twelve 1484 Major Restoration Project

Discussion in 'Tube Audio' started by JRUBIN, Oct 22, 2018.

  1. JRUBIN

    JRUBIN Active Member

    Messages:
    115
    Location:
    Florida
    Chris called me up from Cornerstone music with an amplifier in such disrepair that he was giving it to me for free. I have a sneaking suspicion he took hard to see pictures in the text messages for fear I might not show up to pick up the amp. What I saw was horrible. I have a good feeling that this was out in the weather for a while or at best in a wet basement for a great many years exposed to the elements. I have decided that regardless of all of this, the Silvertone would be my yearly Shango66 resurrection challenge, along with full restoration.

    There will be as much electronics work as body work and vice versa. This thing is absolutely horrible. I feel disgusted to touch it. layers of crap are coating this thing.

    Tubes falling out. Wires cut, chewed up by rats, the inside just covered in god knows what. Oh boy. So ill just remove the tubes and get this amp out of the cabinet. Surprisingly the underside of the amp looks really nice except for the caps that seems to have formed a film upon them.

    After blowing out the cabinet, chassis cleaning starts, and that means scrubbing, because its disgusting. and aluminum does clean up ok. The challenge comes in starting to pull the rust out of the front face. Its a cheap chrome plating on metal with silkscreen. I need to go deep enough to remove the rust, but I also need to grain it to cover the imperfections. All of which needs to be balanced to not remove any paint. The metal work went really slow.

    Slowly work is being done on the amp cabinet to pill the dirt out of the grain as well, I keep going back and forth between the two as I start comprehensive polishing of the chassis. The electrolytic cap will not be used anymore so it will be left in place and shined. The reverb tank will have to be removed and restored separately.

    After the final cleaning of the chassis its brought outside for the final blowout also allowing for bench decontamination. This begins the next phase, starting with the replacement of the fuse holder for the amp. Following this, the Power transformer is removed from the chassis, remembering where everything was connected to. Also the covers of the power transformers, exposing more places to clean.

    I tool this time to check out the works cheapest reverb tank. and shine up the aluminum for it, i put it to the side so I could make a test for it later. What I really want to do it test that power transformer with the impedance bridge.

    Testing showed that I almost made it out of testing without issue, sure enough we have a serious problem with this power transformer, an open!!! and its the filament voltages.







    In part two of this series, we continue with an attempt to repair the lead coming off of the power transformer for the filament voltages which proved to be futile. Im not sure how I was reading this or what. but internally somewhere there is a short. Due to the cost of a replacement transformer, and since this is the outermost winding, I believe I should have no problem replacing this with insulated wire, allowing for multiple final taps to produce a perfect 6.3 at 120v, not 117. I will however forgo a center tap and replace it with a humdinger circuit if I pull this project off.

    Turing a coil by hand is less than fun....

    Finally with 6 taps left I can set up the transformer for no-load testing on the isolation transformer and determine their wiring order in the coil. For safety, only 60VAC is used for testing. At 120v it looks like the 7.2 volt lead would be used for actual in-amp testing.

    With this its time to clean and test all of the tubes on the Hickok 6000. Everything looked ok except for one weak 6FQ7, but we can use that for the oscillator for the Tremelo...

    Along with the cleaning of the tubes was the cleaning of the knobs, which was quite a job, also the old electrolytic was polished in place. This was followed by Deoxit on all pots and switches and tube sockets.

    On the cabinet I was able to safely remove the handle, from under by removing the nut from under by taping it. it was to rusted to remove from the top without damaging it. this allowed for more body work on the cabinet.

    The reverb tank was set up very quickly on the o-scope just to make sure it was working, and it was, so it can be cleaned up and put off to the side.

    A large destroyed section of cable, probably by rats was then completely redone, right down to the color code using photos provided from the group online. Followed by this All ports were tested on the Heathkit VTVM IT-11

    The mounting bracket for the reverb tank was repainted and the polished tank will be reinstalled back into the unit right after the electrolytic capacitor swap-out. After the individuals were done, the replacement of the can was done with 3 caps on a special stand-off.

    Finally, the covers for the power transformer will be cut so that they can be easily removed without looming the cables through. Following this the power transformer installation begins. On the top side I set up temporary heater connections to determine which tap will be closest to 6.3 volts under load.







    Here in part 3 we continue with preparation with turn-up by installing the pot needed for the hum balance which replaces the filament center tap. After this a dry run without tubes is conducted just to be sure everything is ok. If the tap cant meet 6.3 without tubes it cant be the right tap and i have to switch to the next one. Given the one at around 7 volts its time to add the tubes to the amp.
    With all of the tubes in the amp we come to find that the final filament voltage is 5.9. It looks like another turn or two will be needed out of that transformer. A retest with 2.5 wraps shows 6.2 volts at 120vac.

    Following this The temperature gun will be used to measure the heat off of the transformer after 30 minutes or so. No heat issues were found. To celebrate, the transformer covers were stripped and painted, as well as the rewiring of the reverb tank.

    I break out the impedance bridge to test the cabinet and confirm the impedance at 8 ohms.

    I found an extra speaker jack that was inappropriately added to this cabinet that I could remove to place the hum pot into. After that, its time for the first full smoke test.

    I wish this amp had an adjustable bias..... also I discover at this time that the standby circuit is garbage. I end up having to use several tubes from Cornerstone music to find something that operates within safe parameters.

    This brings the project into the noise isolation phase. and there is no shortage of noise in this amp; all sorts of different types of noise from different stages.

    This starts from the finals backwards to the phase inverters. Where things are already looking strange.

     
    Last edited: Nov 19, 2018
    I LIKE MUSIC and labios like this.

     

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  2. JRUBIN

    JRUBIN Active Member

    Messages:
    115
    Location:
    Florida
    With this the fourth part in the series , we continue the troubleshooting with the 6FQ7 phase inverter section. A method employed was simply disconnecting the phase inverter section from the finals physically and ruling out that section. With this the final section was quickly ruled out as the culprit. Interesting point, given the different output and no tube in the socket, a passible component must be producing the issue!!!!

    I quickly learn that we have some considerably STRANGE resistor issues throughout!. Resistors out of spec are simply replaced, thats a no brainer. and i got lucky too because that fixed one of the first issues in the amp. Though its improving, there are so many artifacts, thee are other things too.

    At this point I bust out the T4 Heathkit signal tracer. With this I can hear the clicking, and the hum and the his and everything, But its noise mode that really finds things like this. Turns out its a noisy resistor causing this one. Even removed we can hear the resistor cracking in the T4 in noise more. One less noise in the amp. but still work to do, but the phase inverter is finally cleared.

    Working back to the pre-amp section, there is no shortage of problems either. Though, voltage measurements show that there is more resistor changes to be done, identifying complete failures in circuit.

    Even with the voltages finally within reason, I continue to find noisy resistors, so I continue swaps just for that reason alone.

    An added bonus is a tube socket pin repair I did, as I managed to break one off. Luckily I keep spare parts for this occasion.


    A function test is done to include reverb, tremolo and also.... AM radio.... Yes.... AM radio

    Another round of voltage re-measurements are conducted and shown to be good, except for a few minor changes to do.

    We get the amp sitting straight on the bench and begin some signal testing one issue I found was an oscillation in the reverb section i could not initially identify that was tied into the treble somehow. I get it back in test position to validate the feedback. Turns out, I needed a smaller test speaker when the amp is out of cabinet. No trouble found. The Crapola mini test speaker validated this. Including my no frills Peter Framption impression.

    A last walkthrough is done with the T4, and the output coupled to a dummyload.

    A last artifact lead to an interesting event, finding a bad connection during more resistor testing, turns out it wasn't the resistor, but its solder to the phase inverter. Test with the T4 shows no hum at the 6AU7, and a test on the speaker sounds good.

    Not a single artifact is heard through the speaker on a shorted input with the volume turned up. That completes this portion of the project.


     
    Last edited: Dec 3, 2018
  3. JRUBIN

    JRUBIN Active Member

    Messages:
    115
    Location:
    Florida
    Part 3 has been added to the original post
     
  4. JRUBIN

    JRUBIN Active Member

    Messages:
    115
    Location:
    Florida
    Part 4 has been added to the thread
     
  5. Dandy

    Dandy Super Member

    Messages:
    4,531
    Location:
    North West, UK
    Is there one more video to come? I'd be interested to follow the last part of this job visually.
     
  6. JRUBIN

    JRUBIN Active Member

    Messages:
    115
    Location:
    Florida
    There was supposed to be 5, but due to some issues found during testing there will be 6 parts, and hopefully a live demo by my official restoration amp tester down at Cornerstone music
     
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