HI,
I'm restoring a REVOX A700 Taprecorder (1975) and have already replaced all RIFA Noise suppressor caps (these tend to shortcircuit and smoke) and all the FRAKO ElCO's (these tend to dry out and shortcircuit).
Tantalum caps will be replaced by WIMA MKS2 (film) or new ones (except coupling caps in the audio, for which I will use Nichicon FG electrolytic caps).
I noticed that STUDER was very skimpy with the specification of the voltage of the FRAKO's and certain Bridge rectifiers in the Power supplies and motor drivers (partly rectified in newer models). I have installed higher voltage (and current) replacements , just to be sure.
I have now 2 questions;
1) In the Tapecontrol board (attachment 1) there are 2 Motor Drive IC's ,(TCA561), each followed by a transistor (BC108) which drives a Power Darlington (MJ411) and a reel motor (wind/rewind).
These transistors sometimes fail (destroying the unobtainable TCA561 driver IC).
My question : Could I not better replace the BC108 (Vce=25V), Q17 transistor by a repacement with a higher Vce (say 50-60V) ?
Ideally I would also specify a higher current and power rating: what transistor would you recommend?
2) Same question for Q5 (BC108) in the Capstanboard (attachment 2)
Thanks for the help!
Martin
I'm restoring a REVOX A700 Taprecorder (1975) and have already replaced all RIFA Noise suppressor caps (these tend to shortcircuit and smoke) and all the FRAKO ElCO's (these tend to dry out and shortcircuit).
Tantalum caps will be replaced by WIMA MKS2 (film) or new ones (except coupling caps in the audio, for which I will use Nichicon FG electrolytic caps).
I noticed that STUDER was very skimpy with the specification of the voltage of the FRAKO's and certain Bridge rectifiers in the Power supplies and motor drivers (partly rectified in newer models). I have installed higher voltage (and current) replacements , just to be sure.
I have now 2 questions;
1) In the Tapecontrol board (attachment 1) there are 2 Motor Drive IC's ,(TCA561), each followed by a transistor (BC108) which drives a Power Darlington (MJ411) and a reel motor (wind/rewind).
These transistors sometimes fail (destroying the unobtainable TCA561 driver IC).
My question : Could I not better replace the BC108 (Vce=25V), Q17 transistor by a repacement with a higher Vce (say 50-60V) ?
Ideally I would also specify a higher current and power rating: what transistor would you recommend?
2) Same question for Q5 (BC108) in the Capstanboard (attachment 2)
Thanks for the help!
Martin
Attachments
There is less than 30Volts available so no requirement to select a different component.
I recommend BC107 (Vceo 45V) and BC108 (Vceo 25V) respectively. And there is no worry regarding exceeding Ic 200mA.
I recommend BC107 (Vceo 45V) and BC108 (Vceo 25V) respectively. And there is no worry regarding exceeding Ic 200mA.
These TO-18 devices are obsolete since a long time. Their direct succedessors are TO-92 BC548 (NPN) and BC558 (PNP). If you feel more comfortable in using higher VCE devices, use BC547 or BC546 and BC557 or BC556, resp. Higher Ic devices also are available in TO-92 packages: BC337 (NPN) and BC327 (PNP). Chose at least the -25, even better the -40 hFE designations.
Bes regards!
Bes regards!
Hmmm... my ancient T D Towers book shows all BC108 varieties coming in with a 20 Vceo rating and 100ma max Ic.
On-line data sheets seem to mention 25 volts and 200ma.
I'd go with the ancient 😉 and fit something a bit higher specced. BC546B comes to mind as being easily available.
On-line data sheets seem to mention 25 volts and 200ma.
I'd go with the ancient 😉 and fit something a bit higher specced. BC546B comes to mind as being easily available.
Solved; BC545 selected io BC108B/
Thanks everybody for the fast reply.
A little bit background: The BC108 B on the Tapedriver board and on the capstanboard have been reported to fail occasionally (TINMAN professional REVOX/STUDER repairman on the Tapeheads forum, USA)
Also Q5 BC108B has been reported to fail on 2 instances reported on the WEB (e.g zelfouwforum.nl)
This A700 machine uses almost exclusively BC107, BC108 transistors (apart from some BC140-10); I do not understand why STUDER did not use a BC107 instead of the BC108 on these spots (was the BC107 slightly more expensive at the time?).
Hence, I already upfront decided to upgrade because for instance, Q17 is marginal to inadequate for its Vceo.
The BC546 (NPN) / BC 556 (PNP) , with a Vceo of 60 V should do.
Therefore, Molly, I happily accept your advice!!
Many thanks,
Cheers,
Martin
Thanks everybody for the fast reply.
A little bit background: The BC108 B on the Tapedriver board and on the capstanboard have been reported to fail occasionally (TINMAN professional REVOX/STUDER repairman on the Tapeheads forum, USA)
Also Q5 BC108B has been reported to fail on 2 instances reported on the WEB (e.g zelfouwforum.nl)
This A700 machine uses almost exclusively BC107, BC108 transistors (apart from some BC140-10); I do not understand why STUDER did not use a BC107 instead of the BC108 on these spots (was the BC107 slightly more expensive at the time?).
Hence, I already upfront decided to upgrade because for instance, Q17 is marginal to inadequate for its Vceo.
The BC546 (NPN) / BC 556 (PNP) , with a Vceo of 60 V should do.
Therefore, Molly, I happily accept your advice!!
Many thanks,
Cheers,
Martin
A small update;
I replaced the BC108B,s after the TCA561 motor IC's by BC141-16 's and the deck works fine.
The BC141-16 's are also used in the STUDER B67Mk1. It looks like the print was already designed for them (were the BC108B used to reduce costs, who knows).
Regards,
Martin
I replaced the BC108B,s after the TCA561 motor IC's by BC141-16 's and the deck works fine.
The BC141-16 's are also used in the STUDER B67Mk1. It looks like the print was already designed for them (were the BC108B used to reduce costs, who knows).
Regards,
Martin
Attachments
Capstan Board Service Note
Hi all,
I scanned the service note from Revox from Nov 1987. It looks like it was a design error on the Capstan control board with missing resistor, which may lead to a malfunction or damage to TC 561 IC.
Attached recommendation came to me with the NOS TCA 561 which I ordered recently. It says modification is recommended in case a replacement of TCA 561 becomes necessary. I don’t know if the same is applied as a preventative modification to avoid need for TCA 561 to become necessary to replace.
I have also mentioned that there is no transistors repeater on the Capstan board between the IC TCA 561 output and the MJ411/RCA411 power transistor. So in case of any issue with 411 or on the line to it - it will go and kill directly the TCA 561. I don’t know if anyone considered adding a simple repeater based on BC141-16 at the output of this board, same way as it is done on tape drive control board towards the spooling motors. In my view it should possible.
Hi all,
I scanned the service note from Revox from Nov 1987. It looks like it was a design error on the Capstan control board with missing resistor, which may lead to a malfunction or damage to TC 561 IC.
Attached recommendation came to me with the NOS TCA 561 which I ordered recently. It says modification is recommended in case a replacement of TCA 561 becomes necessary. I don’t know if the same is applied as a preventative modification to avoid need for TCA 561 to become necessary to replace.
I have also mentioned that there is no transistors repeater on the Capstan board between the IC TCA 561 output and the MJ411/RCA411 power transistor. So in case of any issue with 411 or on the line to it - it will go and kill directly the TCA 561. I don’t know if anyone considered adding a simple repeater based on BC141-16 at the output of this board, same way as it is done on tape drive control board towards the spooling motors. In my view it should possible.
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