Steelphon - Fuzz Unit  [schematic]

Discussion regarding early stompbox technology: 1960-1975 Please keep discussion focused and contribute what info you have...
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RnFR
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Post by RnFR »

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Post by modman »

If the power caps are fitted afterwards, they may even be older than the circuit, and they say '1972'. This unit is maybe late 60s but probably early 70s. Not 1966
But Steelphone is an Italian brand for sure... they made amps, synths etc
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Post by analogguru »

IMHO the capacitors were fitted later to enable external transformer use replacing the battery.
And the circuit is nothing else than a Vox V828 Tonebender with a 6k8 instead of 8k2 at the collector of the second transistor and an output coupling cap of 10n instead of 3n3.

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Post by RnFR »

it's that WEM box that is the real prize!
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Post by LucifersTrip »

Was there ever a schematic posted? Since the board is only visible from the side, it looks difficult to trace:
http://stompboxes.co.uk/forum/viewtopic.php?f=6&t=1093

RnFR brought it up a while back:
https://www.freestompboxes.org/viewtopic.ph ... LPHON+FUZZ

What the PEP relationship? rebranding or variant?
https://www.youtube.com/watch?v=nJcYUatyQ5k


thanx

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Post by jrod »

I remember that thread from a while back. I don't think there was ever a schematic posted.

In the FSB thread, AG suggests that it is pretty much a Vox V828 Tonebender.

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Post by LucifersTrip »

jrod wrote:I remember that thread from a while back. I don't think there was ever a schematic posted.

In the FSB thread, AG suggests that it is pretty much a Vox V828 Tonebender.
Thanx...I actually skipped that in the thread somehow.
https://www.freestompboxes.org/viewtopic ... 426#p43443

Funny, I actually built that already.

thanx again

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Post by Solidhex »

When you look at the pics you basically see a Vox Tone Bender. It has a .22uf input cap, a 47K feedback resistor, .0033 output cap. A very hard to make out 820ohm resistor between lug 1 of the "Pep" pot and ground, Looks like a 25uf emitter bypass cap and a 6K8 resistor on Q2's collector instead of the Vox's 8K2. The transistors are arranged in the usual Vox Tone Bender/Fuzz Face arrangement. The weird thing is that while both the Steelphon and its closest Vox relative have 6 resistors the Steelphon doesn't have a 47K in parallel with the output cap. The 10K resistor in plain view you would assume would be the same 10K found in the Vox feeding the junction of Q1's collector and Q2's base but its not. Its actually obscuring that supply resistor (value unknown). The 10K in plain view actually appears to me to be connected between lug 3 and 2 of the "Pep" or fuzz pot. That is what I'm seeing. The problem with that in theory though is this will change the resistance between the emitter of Q2 and ground depending on where the knob is turned upsetting Q2's bias. When turned all the way up the 10K is shorted and the pot value strictly determines the bias. When turned all the way off the parallel value of the 10K and the pot value determine the bias. Doesn't really make sense.

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Post by bhill »

Had one of these back in '68 that I bought in Italy. It was the first fuzz unit I ever bought. Now, knowing it is a variant on the two tranny Tonebender, I think I will have to build one. The original was "given away" by Eastern Airlines along with a brand new Rickenbacher 6 string about 6 months later. :evil: I do remember it being a monster running into a Fender amp.

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Post by LucifersTrip »

Solidhex wrote:The 10K in plain view actually appears to me to be connected between lug 3 and 2 of the "Pep" or fuzz pot. That is what I'm seeing.
It is difficult to see and doesn't make sense yet....Isn't one side of the 10K hooked to the + side of the 25uF...ground?

Image

Image

this is what I used as a starting point:

Image

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Post by Solidhex »

If you look at the trackside pics you see the positive end of the 25uf cap is connected to ground at the outer edge of the board. Where the 10K meets is the other end of the cap. You can follow that trace to Lug 2 of the Pep pot. You can follow the other end of the 10k's trace to both the emitter of Q2 and around the pot to lug 3.

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Post by LucifersTrip »

Solidhex wrote:If you look at the trackside pics you see the positive end of the 25uf cap is connected to ground at the outer edge of the board. Where the 10K meets is the other end of the cap. You can follow that trace to Lug 2 of the Pep pot. You can follow the other end of the 10k's trace to both the emitter of Q2 and around the pot to lug 3.
man, it just really looked like that resistor was connected to that cap into that solder blob
Image

I guess I just missed the depth...one must be deeper, behind the other.

if you have the time, feel free to alter the schematic to what you see and re-post it....or draw on top of the trace side. on quick searches, I couldn't find any other pics yet

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Post by Solidhex »

You are confusing the positive and negative ends of the cap. The positive end is the one with the row of +++ on it. Not the striped end. The 10K does connect to the cap but at its negative end (also connected to lug 2 of the pep pot) the positive end is connected to the strip of ground surrounding that end of the pcb.

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Post by LucifersTrip »

Solidhex wrote:You are confusing the positive and negative ends of the cap. The positive end is the one with the row of +++ on it. Not the striped end. The 10K does connect to the cap but at its negative end (also connected to lug 2 of the pep pot) the positive end is connected to the strip of ground surrounding that end of the pcb.
haha...yep, don't know why I had it burned in my brain that the stripe was pos.

with that odd 10K placement, the unknown supply resistor and the lack of the 47K output, obviously it's not as close to the V828 Tonebender as noted before....if that Steelphon hasn't been altered.

we'll have to wait for more info/pics


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Post by Solidhex »

Dragging this one out of the trash bin. Calling Analogguru! He had mentioned in the other post about this pedal that it was a Vox828 with 6K8 instead of 8K2 and a .01uf output cap. Like I mentioned above the pics show 6 resistors like a Vox but the Vox's usual 47K in parallel is gone and the extra resistor seems to be a 10K placed across Q2's emitter and the middle lug of the fuzz pot. Can someone check out the pics and confirm what I'm seeing? I've had a pair of SFT352's just like the ones in the pic and would love to build one up.

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Post by modman »

A unit with a factory schematic turned up here: https://stompboxes.co.uk/forum/viewtopi ... 3&start=30
but no values
Solidhex wrote:resistors like a Vox but the Vox's usual 47K in parallel is gone and the extra resistor seems to be a 10K placed across Q2's emitter and the middle lug of the fuzz pot.
You are right about the 10k - but there is a resistor where the 47k is in the Vox
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