Got in a copicat for repair ,transformer had an open circuit primary . Got that sorted ,still didnt work ,everything powered up fine ,and clean signal passed through ok ,but still no echo , Bias oscillator ok ,everything seems fine in the tape path .
Well anyway eventually I found the fault ,no contact below about 5 megohms with the sweep contact on the echo output pot ,which also has the mains on/off switch attached .I pulled out the pot leaving the wiring attached ,opened the tabs and extracted the wafer and the wiper ,sure enough right at the spot where the wiper meets the ring contact the brass had a thick layer of corrosion . After quite a bit of scraping and re-buffing of the contacts ,everything was reassembled ,and bang she was back up and running again . The original transformer had an interwinding screen ,the one I installed seems to be concentricly wound and Im unsure if there is a screen ,should I just be looking for a pin that has a wire but no connection to any of the other windings ?
Noise performance is ok ,but once you wind up the gains the charachter of the noise floor is pretty grizzly sounding , I did replace the first filter capacitor which helped a lot ,but to be honest most of the electrlytics look way beyond there good days and need to be replaced .
In their standard form most of the copicats are noisy to the point of unusabillity , the original valve MKI and MKII have horrendous amounts 50hz both motor and transformer induced as well as residual noise from the halfwave ht rectifier arrangement , long unscreened conductors carrying audio to high z mixing resistors are another weak point in the design .
In the mk3 solid state the input gain from the playback heads is set in such a way that self oscillation is less likely , where the valve unit is much more easily provoked into 'howlround' , you can reach a point where its no longer musical or related harmonically to whats going into the machine ,but thats part of the fun too .The motor off switch on the mk2 is another thing thats missing from the later units ,because of the fly wheel it takes several seconds for the tape to come to a halt ,and this can be used to very bizarre effect on vocals and beat poetry .
A modernised fully valve tape delay based on the copicat would be a nice project , its really such a simple circuit , the bias oscillator/record side of the electronics is the most basic possible you could get away with. I wonder if full track 1/4inch heads could be fitted to the copicat , the original heads only record on less than half the diameter of the tape which seems to be a waste . The special high impedence heads for tube circuits are hard to comeby ,the main supplier of copicat spares John Beer no longer has original heads in stock . I guess in a pinch if cassette player mono heads could be found they could be used with 1/4 inch tape , but if someone could better explain how to compare tape heads in terms of dc resistance/impedence /inductance that would be great.I seem to notice low impedence heads used with transistors have dc resistances in the tens of ohms range ,while the high z types might be a 100 or more ohms.
Well anyway eventually I found the fault ,no contact below about 5 megohms with the sweep contact on the echo output pot ,which also has the mains on/off switch attached .I pulled out the pot leaving the wiring attached ,opened the tabs and extracted the wafer and the wiper ,sure enough right at the spot where the wiper meets the ring contact the brass had a thick layer of corrosion . After quite a bit of scraping and re-buffing of the contacts ,everything was reassembled ,and bang she was back up and running again . The original transformer had an interwinding screen ,the one I installed seems to be concentricly wound and Im unsure if there is a screen ,should I just be looking for a pin that has a wire but no connection to any of the other windings ?
Noise performance is ok ,but once you wind up the gains the charachter of the noise floor is pretty grizzly sounding , I did replace the first filter capacitor which helped a lot ,but to be honest most of the electrlytics look way beyond there good days and need to be replaced .
In their standard form most of the copicats are noisy to the point of unusabillity , the original valve MKI and MKII have horrendous amounts 50hz both motor and transformer induced as well as residual noise from the halfwave ht rectifier arrangement , long unscreened conductors carrying audio to high z mixing resistors are another weak point in the design .
In the mk3 solid state the input gain from the playback heads is set in such a way that self oscillation is less likely , where the valve unit is much more easily provoked into 'howlround' , you can reach a point where its no longer musical or related harmonically to whats going into the machine ,but thats part of the fun too .The motor off switch on the mk2 is another thing thats missing from the later units ,because of the fly wheel it takes several seconds for the tape to come to a halt ,and this can be used to very bizarre effect on vocals and beat poetry .
A modernised fully valve tape delay based on the copicat would be a nice project , its really such a simple circuit , the bias oscillator/record side of the electronics is the most basic possible you could get away with. I wonder if full track 1/4inch heads could be fitted to the copicat , the original heads only record on less than half the diameter of the tape which seems to be a waste . The special high impedence heads for tube circuits are hard to comeby ,the main supplier of copicat spares John Beer no longer has original heads in stock . I guess in a pinch if cassette player mono heads could be found they could be used with 1/4 inch tape , but if someone could better explain how to compare tape heads in terms of dc resistance/impedence /inductance that would be great.I seem to notice low impedence heads used with transistors have dc resistances in the tens of ohms range ,while the high z types might be a 100 or more ohms.
