Sound City B120 MKIV restoration

[Mr. Foxen]

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.

Counting from the left with the amp facing you, including the one on its own, it is the third in, I worked back from the phase splitter, pulling each at a time (much wincing at horrible noise, as it was live when I did it), I pulled two valves previous, phase splitter and the next one in, and the oscillation stopped each time.

[3below]

[quote name='Mr. Foxen' post='1308465' date='Jul 18 2011, 09:45 PM']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.

Counting from the left with the amp facing you, including the one on its own, it is the third in, I worked back from the phase splitter, pulling each at a time (much wincing at horrible noise, as it was live when I did it), I pulled two valves previous, phase splitter and the next one in, and the oscillation stopped each time.[/quote]

Your description above suggests dc on the pots, its capacitors again (and / or design), I need to go and look in Merlin's book Designing Tube Pre amps for guitar and bass again - have mislaid in house so will not be tonight. He covers these issues.

I will have to look at my sc120 to visualise which preamp valve it is, more to follow tomorrow.

You are winning, and I must get started on mine.

[Mr. Foxen]

I'm eying the non mustard next to the replaced one here:



Mustards are supposed to be wicked reliable. Not sure what value it is though, not reading the markings.

[Mr. Foxen]

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.





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.

[3below]

[quote name='Mr. Foxen' post='1309782' date='Jul 19 2011, 10:49 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.





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.[/quote]

Good progress, by pink one do you mean the rectangular Wima 0.022 ? Can't see clearly which terminal on volume pot connection made to, where is the cap on the 4.7k potential divider resistors between normal and brilliant volume, one end or in between the pair.

Looking through the schematic I am trying to work out a) how the d.c. is getting to volume pot (was it one or both) and where it is coming from, decoupling again. If you look at V1 there is a potential route back to volume pots through either the (upper) 0.02 uF cap or the (lower) 1000pF cap. You could replace the two capacitors or investigate further. Disconnect your 'fix' at one end, and disconnect the wire that emerges from between the two 4.7K resistors. Clip voltmeter to ground and somewhere between cap and volume pot. You now need to work live so take real care !!!!. Switch on, hands well out. Check if any d.c. voltage present. Switch off, repeat same with other capacitor, care as electrolytic smoothing caps will be charged.

Finally connect one end of voltmeter to the disconnected wire from 4.7K pair. Check for any d.c. here. These tests should let you know where d.c. is originating from, in theory


Very nice 'new' noise free SC120 available soon courtesy of Mr. Foxen ??

[Mr. Foxen]

The pink WIMA box on the right in the top pic is the replaced one that sorted the dc on the volume.

I suspect it should be a good deal less nosiy than a stock one, with old components, but there'll still be some noise, because there is tones of unshielded wires running back and forth in there.

Umph is also working on one, he is a bit more experienced with amp tinkering than me. His should be even quieter, considering how much he rags on other amp's lead dress.

Edited July 20, 2011 by Mr. Foxen

[Mr. Foxen]

I wish I'd found [url="http://www.hobby-hour.com/electronics/resistorcalculator.php"]this resistor stripe translator[/url] before I worked them all out on paper.

[Mr. Foxen]

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).

Edit: Balls, still does it, just not pushing it as hard so didn't set it off.

Edited July 22, 2011 by Mr. Foxen

[Dad3353]

Fascinating read, Mr. Foxen. Nearly there, but, as my ol' dad would say: it's the last mile that's the longest. Makes me glad I have a Hiwatt, though.

OT, but whilst I'm here...
Later, when I can spare the ressources (early next year..?), we'll have a little chat about Hiwatt cabs, perhaps...

[Mr. Foxen]

If I still have Hiwatt cabs, happy to talk about them, if I don't it will be nostalgia for when I did, and we can talk that stuff now.

[Mr. Foxen]

Bunch more resistors arrived, which I loved due to my hangover, especially with multiple parcel delivery people turning up, followed by the builder and my Dad arriving seperately. Put them in, one removed seems to be open circuit, but is in a position where there is another path for current, I think that one might have been the problem. No more oscillation, even more volume, but also more hiss, or possibly both of those might be down to the hangover. A pile of ECC81 also arrived, which I might swap in, lower gain than the 12AX7 so might help, at the cost of reduced EQ sweep (which is still huge compared to passive stacks).

[Mr. Foxen]

A lot of cocking around with valves, and this now all works, and isn't stupidly noisy, still quite noisy though, gonna investigate the noise reduction mods, starting with the least invasive. Working on another one now. Further updates on other amps will be on this blog I started for the purpose, will be posts about other gear work I do and whatever I get my hands on, as well as stuff I'm selling: [url="http://ampstack.wordpress.com/"]http://ampstack.wordpress.com/[/url]

Eventually I'll be arsed to put appropriate decorations on here.

[3below]

An excellent job well done !