~ Selmer TruVoice TV8/T ~ Selmer Mercury 5 ~

So finally after more than a year I get to look at the Selmer amplifier starting with finding a schematic for it ~ Searching the internet and asking a few friends revealed that there is a schematic out there for the Selmer Mercury 5 which was originally the TV8 but with a magic eye tremolo indicator added and is not the clearest of schematics

Another task I set myself now that I had the time was to move my schematics and PCB layouts drawn with MicroSim 8 to a new package and as OrCad took over MicroSim and had translators for such that would be easy ~ As usual not the case and I spent a lot of time trying to recreate the style of my many symbols made over a 20 year period

OrCad was not as advanced as I expected and little had improved in 20 years ~ one thing that was worse than MicroSim was the ability to create good looking pictographic circuit symbols because OrCad insists on using fixed grids and line styles ~ Although initially frustrating to use for drawing I eventually created some custom symbols

One symbol library that OrCad completely screwed up was my valve amp and vintage transistor symbol set ~ I would now find it difficult to quickly draw some of the clear schematics that I had made in the past and it would be extremely difficult trying to draw something like the QUAD MPX Schematic again

However I accepted the challenge of making OrCad look as clear as my old MicroSim did at the turn of the century and one of the first new schematics I drew was for the Selmer TV8 Mercury 5 ~ The reason I spend time recreating schematics is they can then be quickly edited to show modifications and they look good in web pages and documents

This is not a valve amp modification article it is simply a step by step record of how the Selemer T.V.8/T was repaired with a few changes to the original layout and some modern components where they were needed and where they would not change the characteristic of the amplifier

With the paper coupling capacitors removed and some new valves fitted the d.c. voltages around the valves were better but not correct according a schematic I found online and calculations I made

I guess a problem was found in production where the EL84 anode connection fed back signal to the adjacent ECC83 and caused oscillation ~ Even with the OPT bracket firmly connected to chassis the rivet the screen connects to is not a good ground so I provided a formal connection from the wire screen and speaker output to chassis ~ Swapping the OPT primary connections would probably be as good and not require a screened wire

The other 2 sections C1 and C12 were replaced with separate 33µF 450V capacitors mounted on the main tag strip as shown above with their ground connections wired to a new chassis ground tag rather than rely on the rivets ~ With most components removed it is also possible to solder the original tags to the chassis if you have a powerful enough soldering iron and use a liquid flux to clean under rivets

The original mounting of R18 was across the EZ80 base from cathode pin 3 to a spare pin 8 ~ This mounting method is not good especially for an amplifier that will be carried around and thrown in the back of a van ~ The picture above shows R18 mounted away from the EZ80 base using an insulated standoff so it does not flap around

This filter has a steep response of 18dB per octave at about 120Hz so very little Bass gets passed to the small output stage transformer

Capacitor C13 across the OPT primary acts as a low pass filter and with the high pass gives the Selmer TV8-T a peaked response around 3–4kHz