Sound city overhaul

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Re: Sound city overhaul

Postby Mr. Foxen on Sat Jun 18, 2011 5:37 pm

Was looking for places not selling them specifically for 'tube amps' because that just seems to rack up the price. Are the ones on that site new? Getting old stock ones pretty much defeats the point.

Actually, might stick the build diary I'm doing in this thread too:

Bought these:
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I like them because they are made of good components on turrets, so they are easy to work on, and 6 EL34 is loads of power. The pre has a bad rep because it is much more complex than most valve amps, but it is actually very flexible, just a bit unfamiliar, and it can get pretty noisy with the amount of components that can drift in it. Hopefully that can be fixed with a good service. I'm not too techy on the electronics side of things, but I'm learning. I did just break my multimeter, which isn't ideal.

Step one, after selecting one that looked basically complete, was give it a wash. Damp scourer sponge and some washing up liquid for the tolex, and the top bit of the chassis. Nothing is gonna be passing current soon, so a bit of moist in there is going to do no harm.

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Top of chassis after a wipedown, this was pretty seriously grubby, bit dust/grease and corrosion, the scourer side was applied to that, care taken to not scour off the partridge stickers on the trannies, I have the screening cans for the pre valves put aside.

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Now for the actual electronic bits, here they are:

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Bit neater and much less f***ed with than the previous ones, snubbers on the diodes are installed in a much more fancy twisty way (smarty looking jobs).

Essential tools for this part:

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Keep water away from electrics, so alcohol is the way, meths for bits you can get at, servisol (aerosol alcohol) for the bits you can't, like inside pots. Don't use WD40 for this, WD40 is an oil in a solvent, the solvent part acts like servisol, dissolving grease, but the lubricant oil part sticks around making a pain of itself. Plus it dissolves/corrodes some things. Servisol is safe and nice. Cotton buds dipped in meths for cleaning the jack sockets and contacts, and the grease with dust stuck in it around valve bases, enough dust stuck together and you can get a conductive path that shorts out your valve, high voltages around there make this very much more likely. The dust is burnt to carbon, which is more more conductive, and makes things worse, you get more flashes until something properly explodes or catches on fire. Everyone should clean out their amp once in a while, even just the jacks and valve bases can make a fair bit of difference.

Also in that pic, you can see some of the main thing I was trying to achieve, a list of electrolytic caps (the ones that look like batteries, full of rolled up stuff and wet paste) I need to order. They degrade with age, and are a fairly major thing to be f***ing up in amps. I am just going to replace them all because modern ones are plain better, not point in messing with trying to keep things original at the cost of being noisy and not working right. You can mess with testing and reforming them, but I reckon it is pointless for a working amp. I decided to read all the values off the amp, because I suck at schematics, and these amps are a bit 'custom' and not necessarily consistent.

Some were easy to figure:
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Others less so:
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Had to get the soldering iron out to get at this, did a pretty bad job of it, as only have lead free solder hand and it totally sucks. Some lead solder has been ordered.

Some of the pots feel pretty dodge, stiff bits, but a good servisol spray and twiddle has freed up most of them. Won't be an update for a while as gotta wait for parts to arrive, and some money to come in so I can afford a set of valves, and a new multimeter.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby gldtp99 on Sun Jun 19, 2011 1:02 am

Mr. Foxen wrote:Was looking for places not selling them specifically for 'tube amps' because that just seems to rack up the price. Are the ones on that site new? Getting old stock ones pretty much defeats the point.


It is difficult to suggest parts vendors for people who have no location listed in their posts----many people will no longer bother because it's impossible to guess where on the planet they might live.
I linked to a vendor that has large pics on their website so one can easily see the parts----- one could buy these same parts from a vendor close to where they live---or from another vendor who offers a better price. Google is your friend.
The Sprauge Atom axial caps have gotten extremely expensive--- but they are the only one i know of that will properly fit the spring clip found in many SC 120 amps----these particular 30uF/500V axial caps are 1 inch in diameter---most modern similar caps are much smaller in diameter (and MUCH less expensive) and therefore won't fit the stock Sound City spring clip without wrapping with tape, plastic, etc--- or using another mounting method to replace the single 32uF/450V axial cap------as clearly seen on the schematic.
I suggested these particular parts because you wrote that cosmetic issues were important to you----these F&T and Sprauge Atom capacitors (yes, they're new parts) are high quality and available from many sources---and they will fit the stock mounting clamps/spring clip in a SC 120.
Use whatever parts you want to and good luck with your amp projects...................gldtp99
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Re: Sound city overhaul

Postby Mr. Foxen on Sun Jun 19, 2011 10:10 am

It is only the ones poking into the chassis I fussed about, the inside one is inside, and I can get a smaller clip easy enough. 90% of stuff I've bought for this has come from Hong Kong, so the fact I live in Bristol UK doesn't seem to make that much odds.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Mon Jun 20, 2011 5:57 pm

Got solder. Also found leftover caps from previous SC120 I worked on. Interestingly the red plastic job, that was hidden under other stuff in earlier pics, was a 1uf but on the previous amp it was a 4uf. I checked the schematic and that says 4, so getting a 4 in there. Took a ton of photos for this bit, because it involved disconnecting things do give me space to solder other things. Will try and pick out informative ones. Nutpains here were: The screws in the clamps were really corroded, so couldn't loosen the clamps much, and someone has wrapped one can in tape and stuck some sort of foam to the other to lodge them in, so they didn't come easily, ended up WD40 around them, and pushing the out from the inside with a blunt thing. Then I realised I had to do the exact same thing because the clamps wouldn't tighten either, I used some stick on gasket stuff, like weather strip for doors, but a bit finer, came with hifi drivers, 4 bits around the can made it a snug fit.

The objective: underneath the rectifier diodes (the smarty caps attached are snubbing caps to suppress noises from the diodes switching) Wrapped in black electrical tape.

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Replaced bias cap on the left, tape wrapped smoothing cap, off shot the diodes from the rectifier have been lifted off the turrets along the bottom:
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The other one, with resistors lifted out of the way, looks like someone was crushing the turrets to hold the components legs in place, bit of a ballpain.

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New ones installed and bits stuck back down, lumpy soldering where the crushed turrets made a pain of themselves, they should have a nice slot on the top which makes stuff really easy:

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The new caps have two false legs for securing them to PCBs, since they are chunky items.

Also, small electro I replaced on the preamp board, I used some stripped off sleeve to insulate the legs to avoid shorting fun with the inappropriate radial cap in an axial's place.

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New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Mon Jun 27, 2011 1:12 pm

The 33uf electros arrived today. Due to my refusal to use overpriced made for audio caps, these are totally the wrong size for the clips and clamps holding in the originals (we're talking £8 for one 32-32 450v cap in a big can, or £7 for ten single radial ones).

Here's the ones in there, and the new ones in the background:
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Took a ton of pics in case I mess something up, and labelled the wires before I took anything off. A cluster of three caps looks about right to fit in the clamp where one was before, the originals are doubles, so the third one can replace the cap in the clip, making things much neater.

Here they are assembled, and everything labelled, floating cap and clip removed. Paused because I can't find my spanner to undo the clamp:

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New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Mon Jun 27, 2011 9:24 pm

This one involves loads of really similar pictures, where I hope anyone might notice stuff I may have cocked up. Turns out labeling things 'positive' and 'negative' relative to the lugs from which I detached them isn't great when the positives and negatives are sometimes connected together.

Old cap pushed out, new ones pushed in, wrapped in electrical tape, classy, but appropriate to the originals (stick on foam held it in the oversize clamp). Old wired trimmed of melted insulation from desoldering, and the ends curled round need nose pliers to secure to legs, since there aren't proper lugs, looks a lot like a paperclip sculpture due to nothing being trimmed:

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Same again, with solder and poking out ends trimmed, smooth shiny soldering contrasting the horrible bodgery it is on (cleaning everything with a meths dipped cotton bu seems to help a lot). Insulation slipped over the last leg that is the cap that was in the clip so needs connecting to the board:
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Connection to the board, nice trailing wires, faithful to the existing style or willy nilly cable routing. I don't know enough about lead dress to know if I can do this better.

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And from the top, on the left, original cap, wedged in with foam you can see at the edge, middle my work with the tape, and on the right, nicely fitted big modern cap I put in earlier:

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Replacing the last one should be much less trouble, gonna go for similar style, but with just two caps, and figure wedging them in the clamp.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Sat Jul 02, 2011 9:07 pm

I just cautiously turned it on with no valves in it to see if it seemed about right. Haven't done any poking in it with a meter because I wanna make sure someone reliable is around in the house, my lodger is here, but she is actually too stupid for me to place the slightest risk to my life in her hands. Plus I would rather do it with my mate at MammothSounds available online so I can pester him. Thanks also to the tech at Heron Music for the fuses. It is fine so far, no exploding.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Sun Jul 03, 2011 1:51 pm

Moment of truth. Valves in, the tape is where I had them all tested and wrote a note on them, RCD in place.

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Totally works, woo. Issues are that the power switch doesn't turn off (I was turning it on from the wall, in case of exploding), and there is still some noise from the pots an hiss, occasional crackle, but loads quieter than the last one I had. Some new resistors in the pre should sort that. New switch is fairly important too.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Sat Jul 09, 2011 4:06 pm

Having a little play with this now the switch is replaced, an some resistors replaced to clean up some hiss. Noticed a new problem: the speakers are moving in and out really slow, too slow to make noise, like 5hz or something. Any clues?
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby gldtp99 on Sun Jul 10, 2011 12:14 am

This is called LFO or Low Frequency Oscillation------ exactly what is causing it or how to specifically fix it is something i'm not sure of.
Quirks like this are why i gave up on trying to fix problem SC 120 Mk4's several yrs ago--- even when i got some of them working properly there were better sounding, simpler, more reliable amp circuits out there----- this is why i converted several SC 120's to Hiwatt circuits. I'm not buying up SC 120's any longer so i just have one SC 120 in need of a rebuild left---not sure what I'll do with it so it sits until i get time for it----I have two SC 120 Tour Series amps that don't have problems so i can get the SC 120 clean tone from them if i need it.
I wish you loads of patience and buckets of good luck with your SC 120 Mk4 amps..................gldtp99
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Re: Sound city overhaul

Postby satamax on Sun Jul 10, 2011 8:40 am

For the ocasional crackle, tap the burgundy resistors if there's any in there with a chopstick. Well, checked your pics, and no burgundy resistors. So tap pretty much everything in there. Carbon resistors become sometimes microphonic. And for the LFO, you might try moving the wires around PI and cathode follower, the ones that lay on the chassis, some yellow, some green and some white. IIRC there was capacitive coupling around there.
Huh?
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Re: Sound city overhaul

Postby Mr. Foxen on Sun Jul 10, 2011 10:26 am

The occasional crackles have been fixed by swapping out some resistors, I'll do some more when they arrive, because Maplins only kept 2 in stock, and suggested I go under the power rating I requested. It seemed to lose some hiss. Actually,t here is still some noise from teh volume pot, but sure that is a bad bit of track, it has a bad patch in the rotation.

Do you mean literally stir those wires up some? I had a go at neatening them up, maybe I stirred them into the wrong place, I didn't notice the LFO before, but I might just not have noticed it.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Mon Jul 18, 2011 6:06 pm

Opened it up, replaced some more resistors in the preamp bit because they had arrived, sprayed the volume pot with the crackle some more, the spray came out brown, so must be doing something, and stirred the wires round the phase splitter some. Fired it up again, no more moving cones, played a little, still fine, then I turned some knobs and post a crackly bit, it started again. It changed speed whilst I was fiddling with the mid knob, but I can't replicate that. The really loud crackle from the volume pot I know is duff really set it off. This makes me think it is a similar thing to what was making a Selmer T&B go into oscillation when pushed.

Edit:

Swapped in a new volume pot, seems to be a lot more volume, a little abrupt, but I think it is just lots more headroom, rather than the glitchy jump, there is still crackle, as the new pot turns, so I think that is an issue, dc on the pots or something. Also, if you look at the right angle, through the little slots in the plate, there is a blue glow inside that pulses along with the oscillation. Might be normal, not had that much of my face up against the inside of an amp before.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Tue Jul 19, 2011 10:26 pm

Current state of play: Ordered bunches of caps, but are from hong kong, will be sorted for the next one, but I picked some up in maplins, and others from the spares box, to replace some on the pre board, along with replacing various resistors (carbon comps can apparently get noisy whilst still measuring fine, so putting all new ones in a few at a time between firing it up to check other stuff, if it cures all the noise, I'll stop, but looking like they'll all go). The only caps of the right value and high enough voltage Maplins had were Polypropylene box ones, so had to get a bit creative, some axials are on order.

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The pink one on the end of the board fixed the DC on the volume pot issue, no more horrible noise from turning it. Still have oscillation though.
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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Re: Sound city overhaul

Postby Mr. Foxen on Fri Jul 22, 2011 11:56 am

New 0.047uf caps arrived, and the oscillation is fixed. Woo, wanna put it in the box and call it one, but some more resistors are coming and I wanna put them in, see if it seriously fixed the last of the hiss (which isn't very bad at all now, but I don't run a tweeter).
New music: https://warriorpope.bandcamp.com/album/anchorite
Gear tinkering blog: http://ampstack.wordpress.com/
Made a Facebook page for my ampguts stuff. Feel free to come post amp guts and rig shots: https://www.facebook.com/pages/Ampstack/351966881575839
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