Selmer Truvoice Echo Units
Echo 200, Echo 300 and Echo 400
The Echo 200 drawings below are from a strip down of a maroon and cream (blood and custard) box and the Echo 300 details were very kindly supplied by John Beer at amp-fix.com
So far, no sign of an Echo 400 to play with.
1. Acknowledgements
These are the three main sites to visit in connection with these units.
http://www.vintagehofner.co.uk/ THE site to go to for any Selmer information. Created by Steve Russell & Tim Fletcher.
http://www.amp-fix.com/ John doesn't accept repairs anymore but has years of experience with echos and everything else you can think of.
http://www.chambonino.com/ Lots of interesting repair write ups and photographs by John Chambers.
To hear Bert Weedon (the man who got me interested in guitars, bless him) demonstrating an Echo 400 go to
https://youtu.be/_9mdysaDSsg and https://youtu.be/av4y23marGw posted by Tony Rundle.
CAUTION
LETHAL VOLTAGES PRESENT THROUGHOUT THE CHASSIS ON ALL MODELS
1. Selmer Truvoice Echo 200
I don't know what happened to the Echo 100, but the Echo 200 seems to be the first Selmer unit that pops up regularly. The four way Effects selector switch provides 'Off' plus three echo options; 'head1', 'head2', 'head 1+2+3'. Not as flexible as a Copicat, but perfectly usable. You can use EITHER the Mic OR the Instrument channel, a limitation that was addressed in the later 300.
The drawings are taken from a maroon and cream (blood and custard) unit that came as 'spares or repair'. Still in very nice condition though and well worth putting back together.
PRINTING
Wix only provides webp images at the moment. To make a hard copy of a drawing :
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Left click when the magnifying glass icon appears and wait for the picture to clear.
(On a touch screen, tap on the image) -
Right click and 'save image as' to somewhere as a webp file.
(On a touch screen, touch and hold down to view the menu) -
Open the saved file with 'Photos' (Windows 10) or whatever app works for you.
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Use the print menu to print to A4 plain paper.
Selmer Truvoice Echo 200 Schematic.
DC voltages were measured with 240v 50Hz input on the 245v setting. With a silicon replacement for the u/s selenium bridge BR1, the voltage on C20 went up to 323v dc (not able to test with the original). A 560R series resistor brought it down to the 295v marked on the original hand drawn schematic from the internet.
Selmer Truvoice Echo 200 Component Layout.
Selmer Truvoice Echo 200 Power Distribution Layout.
Echo 200 Odds and Ends.
This unit had a replacement silicon bridge rectifier (instead of the original selenium one), a couple of mods, half a dozen wires/components snipped and (surprise surprise) no valves. The main problem turned out to be the collection of completely knackered filter caps and 100volts of ripple on the dc supply.
Right: Production engineering being what it was in the Sixties, the factory managed to force the filter can into the non existent space next to the mains transformer. At least it was non existent before somebody made a big dent in the windings.
Left: The replacement caps would actually fit inside the old can, but it's virtually impossible to put back together. The base and twist tabs form part of the grounding system though, so the new components are fitted through it. Not pretty, but it works. (Similarly, one of the bridge rectifier mounting screws is a bus grounding point and the old case is best left in.)
New caps from Mouser:
2 x Chemi-Con EKXJ351ETE330MK20S: 33uF; 350v; 10,000hours; -40 to +105degC; 20%; Ripple 330mA
1 x Chemi-Con EKXJ351ETD220MJ20S: 22uF; 350v; 10,000hours; -40 to +105degC; 20%; Ripple 200mA
Bias Oscillator:
Below: Bias oscillator waveforms as found. The frequency was 35.45kHz, but it may have been altered. Waveforms were measured at the heads. Although most DVMs won't read frequencies that high, they'll still show whether the oscillator's running or not. The old schematic was a bit confusing where it showed 6v dc across the erase head. That's about right for the DVM dc range (probably an AVO in those days).
The tuning cap C11 is fine and the coil tunes slightly either way, so it looks as though 35/36kHz is the where it wants to be. It does seem a bit low, but without finding another unit to test there's no way of knowing.
[EDIT: See notes below]. The Contact Us page is working now by the way, so if anybody wants to join in feel free.
Tape Heads:
These are the head properties as measured:
The resistance is at DC and the inductance at 1kHz. The two coils in each head are wired in series to give the total values shown below.
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ERASE: 790R 750mH (single coil) (BSR ???) (Not working very well at the moment)
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RECORD: 334R 490mH (across both coils in series) (BSR ???)
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PLAYBACK: 950R 1650mH (across both coils in series) (BSR MNI 55 ?)
These are the details from the BSR data sheet:
Recording / playback head type MNI 55
Track width 2.3mm
Gap width .005mm
Inductance at 1kHz 0.6H
Resistance at 50kHz 180kΩ
DC resistance 350Ω
Bias current 220µA
Current consumption (audio) 46µA (26µA for 10dB below maximum value)
Notes:
Still working on it, so things might change a bit. (The erase head needs looking at). More (relatively) soon. Meanwhile, on this particular unit there are a few observations:
It's very difficult to know what a sixty year old, budget echo unit sounded like when it was new, but the Echo 200 (or Small Echo) survived as blood and custard, crocodile and blue/grey versions, so someone must have liked it. This one was a bit of a head scratcher. Was the disappointing performance down to a hidden fault, old age or poor design? It seems, surprisingly, that some of it was down to the design:
a) There's way too much drive from V1A into the grid of V3A. The signal (monitored at the replay heads) is distorted with as little as 55mV rms into the Instrument input and 30mV into the Mic. That's what you get for being tight and not fitting a gain control. Replay distortion improves a lot if R10 is changed from 220K to 1M2.
b) The bias trap is just a low pass filter, not a notch tuned to the oscillator frequency, and there's a lot of bias leakage through it. Changing C18 to 100pF (which also rolls off the replay top end slightly) seemed to help. The rest of the HF pickup is probably in the wiring.
c) The most noticeable improvement came from changing C11 from 5000pF to 2700pF and bumping the bias frequency up to about 50kHz. With sensible settings on the level and sustain controls, it's possible to get an almost useable sound, even with modern Copicat tape stock.
2. Selmer Truvoice Echo 300 MkI
Many thanks to John Beer for coming up with the original circuit details of the MkI and for help with the other Watkins schematics on the site. This is a re-draw of Drawing No. 2160/5. Part No. D62. Left click to enlarge the image.
The 300 now has four push button head selectors instead of the 200's rotary switch and the three inputs have individual volume controls. Very useful.
3. Selmer Truvoice Echo 300 MkII
This is from a hand drawn diagram marked "Drg No. 2229" "Part No. D69". It shows the bias oscillator as an EL84 (odd) whereas the earlier versions used an ECF82.
See printing instructions above.
4. Selmer Truvoice Echo 400
I don't have a schematic for the 400, but I do have a copy of Bert Weedon's demonstration record with a picture of one on the label. Bert hasn't shrunk; the400 pic's been blown up a bit. You can hear both sides on YouTube:
https://youtu.be/_9mdysaDSsg (side 1)
https://youtu.be/av4y23marGw (side 2)
More details as they become available.
I'm too old to see it properly, but the guitar looks like a Hofner Colorama or 172.
(EDIT: I'm told it's a V2, the UK version of the 172. Thanks John)
It's surprising how many of these records are still about. They were dealer sales aids and, as it says on side 2, "Not For Sale To The Public".
The commentator, David Gell (pronounced Jell), was a well known radio presenter on Luxembourg and the BBC.