The Sears Silvertone Twin Twelve 1484 Major Restoration Project - Page 1
Forum section: electronics / repair

author: jcrubin
author_rank: Frequent Contributor
timestamp: October 22, 2018, 12:56:32 pm
content: 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-11The 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.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 radioAnother 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. Tune in for the next video in the series.We continue into part five with some of the cosmetic work, beginning with the prep and painting of the handle ends so that they can be re-installed into the cabinet. This gives pause and time to reflect on how far things have come since the beginning of the project. With that, everything is placed back into the cabinet and screwed back in. The top caps for the handle are snapped back in and everything receives and inspection.This starts the disassembly of the speaker cabinet for repairs and restoration. This will need a lot of work. Much water damage, pieces missing that will require research as well. Chunks of corners are gone and a deep cleaning will be required before anything else.Having cut back a section of inside material, repairs begin throughout the amp from easiest to hardest. Most sections roughed up upon completion to match the patina around the affected area. Corners were built back up with putty first.On the inside there will need to be repairs to the bottom that has been rotted out by water damage. A clear glue will be soaked into the wood and let to settle overnight to turn it back into plywood and then knocked down flat again and filled with wood putty. The second sanding with the putty preps it for stain and polyurethane.The cabinet handle end prep work is done the same way as the one for the amp cabinet. Once completed the handle is re-introduced to the speaker cabinet.We did a visit to Hobby Lobby to find material for the sides and ended up with a 3in elastic belt to do the job and it came out perfect. Having been given a picture of the original setup I was able to replicate it.One piece of speaker cable is chewed up in the 1st two feet so it was simply removed as to not cause issues later. With that the back of the cabinet was replaced and the Tolex received a final treatment.The remaining belt material is glued to the backstops to protect the amp, the Amplifier then reinserted into the cabinet to see how it fits. Everything looks really nice. Cable management wasn't too bad either.At this point I turn the Amp over to the wife for testing. sadly there were issues. Though the Amp itself ran great, troubleshooting shows that the speakers within the cabinet have issues. This proved to be conclusive and we proceeded with a demo of the amp using a different speaker and relegated the cabinet speaker repair to the next unscheduled video, part 6.We arrive at part six of this series, an unexpected installment due to the distortion that was found to be coming from the cabinet from the last video. This starts with the troubleshooting to determine if this is cause by one or both speakers, as well as a retesting of impedance due to an earlier mistake. Turns out, as expected, 8 ohm speakers with a combined 4 ohm impedance.Individual testing of each speaker determines that both speakers have distortion, one worse than the other, so both will have to be removed and inspected to see if they can be repaired. Upon removal, It is seen that substantial dust has collected on the lower portion, which is easily removed, and sections around the perimeter of the cone have been eaten away.These sections are repaired and reinserted into the cabinet and found to have corrected the issue so the process is repeated for the next speaker. As each speaker is tested its evident that the noise is removed so they are rewired together for a final testing.Final testing shows that the issue is resolved, completing this project. The only thing left is getting this amp down to Cornerstone music for a critique.I was able to get some time with Pete at Cornerstone music to test and critique the Silvertone 1484. Not only do my restorations generally get critiques here, but it was from this very place that I picked up the amp when it was first dropped off.

author: schmitt trigger
author_rank: Super Contributor
timestamp: October 22, 2018, 01:36:59 pm
content: These restorations are LABORS OF LOVE. Pure and simple.So I tell you my friend, Godspeed to you, and sincerely hope that you are successful.

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 05, 2018, 01:23:45 pm
content: Part 2 of this series has been added to the original post

author: jcrubin
author_rank: Frequent Contributor
timestamp: November 19, 2018, 01:06:41 pm
content: Part 3 of this series has been added to the original post

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 03, 2018, 01:32:00 pm
content: Part 4 of this series has been added to the original post

author: texaspyro
author_rank: Super Contributor
timestamp: December 04, 2018, 03:27:39 am
content: Neat trick for that flat-top solder sucker shown in the last video... place a coin on the yellow top, hold the sucker vertical, and trigger it. 95+% of the time the coin will land on the ground in the same way it was placed on the sucker.

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 05, 2018, 03:22:15 am
content: See, Now I have to try that.....

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 17, 2018, 01:51:08 pm
content: I have added part 5 to the series. Note that due to discoveries, this is now a 6 part series it seems......

author: jcrubin
author_rank: Frequent Contributor
timestamp: December 31, 2018, 02:21:48 pm
content: Part 6, the final part of this series has been added to the original post

author: bsfeechannel
author_rank: Super Contributor
timestamp: January 02, 2019, 09:36:13 pm
content: Excellent job. Kudos for giving a decent new life to that old soldier.

author: jcrubin
author_rank: Frequent Contributor
timestamp: January 03, 2019, 01:32:57 pm
content: There is a demo video scheduled shortly!!!

author: jcrubin
author_rank: Frequent Contributor
timestamp: January 14, 2019, 01:46:27 pm
content: The original post has been updated to include the demo video. This concludes this project for now unless something unforeseen happens...

author: jcrubin
author_rank: Frequent Contributor
timestamp: June 24, 2019, 12:28:59 pm
content: This is the compilation video of the six part restoration series of the Sears Silvertone Twin-Twelve model 1474. This is more of a non technical artistic condensed progression of this project which was. about 3.5 edited hours into about 14 minutes. Enjoy!