My valves arrived yesterday.. sent in an overnight package from Rosewood... excellent service!!!
Couldnt wait to plug them in... 2 of these are NOS ( amplifiers) the rectifier is new made in Russia
As with every other part of this machine... fixing one stage leads to a problem at the next.
Amp started off sounding good if a bit crackly ( but hey!! it works!!)... then gradually a high pitched tone starts off softly then goes WOLLOP!! and its deafening even with the ppotentiometer turned right down.
A clever bit of detective work on the CRO gave us the output signal with and without the projector plugged in...... its all over the place plugged in.
I had been avoiding taking this part apart, but it now needs to be done. Glad i had as its a delight to see this.
its a very early photocell made by CONTINENTAL part #918
Now i had always assumed that the connection to the amp from this was an input... wrong... its also an output. The photocell ( we think) requires a voltage around 200v across anode and cathode.
The AC comes down the coax to the photocell... this is then chopped up by the cell depending on the light input to this. This chopped up signal is fed back up the line as electrical pulses, through the amp and out the speaker as sound.
My cell looks damaged... you can see in this picture some staining on the collector ( cathode). Not sure how this happened, but my guess is that this plate needs to uniform throughout, peaks and stains will allow a collection of electrons to occur and send spirious signals to our amplifier.
A check with my high freq vacuum tester confirms this. There should be an almost perfect vacuum here ( around 10 X -3 mbar). A good vacuum at this range will give off a very light blue colour... anything OTHER than that indicates impurities... air shows as pink... here we have an almost purple colour... more than likely from the breakdown of the cathode
.
To confirm this a test across the connector is made with the CRO with and without the cell attached and we confirm that cell is faulty.
Now i managed to source my valves within Australia... i didnt have much luck with the new lamp for the projector and had to source from USA.. so i bought 5 of these. I did find a guy in QLD who apparently has everything youll ever need for 8mm, 16mm, 35mm projectors. I contacted him enquiring about my bulb... they dont exist and never have was his reply.. this is despite me having the thing in my hand as i spoke to him... DICKHEAD!
I have found a place in USA that has these photocells for $10 ea.. lets just hope he ships overseas as im also finding that USA sellers can be bloody awkward with non US customers!
