Dear fellow Tape Enthusiasts,
I'm overhauling a
REVOX A700 Tapedeck (1975), which I own since 1999.
It has by now failed three times:
2006: Blown FRAKO Capacitor ad rectifying bridge in +12V power supply;
2013: Started smoking heavily during play back but worked All Right next day and no more smoke (no action from my part: insufficient time); It turned out later (2017) to be an exploded 0.1uF RIFA cap across the mains.
2017: most recent failure: Inability to rewind: (another) blown RIFA Cap and burnt safety resistor on the Motor Relay Board and associated Fuse F2: Rewind motor AC supply).
I decided now to do things properly and fully overhaul the deck, which is still occasionally used.
BTW. I was fully unaware that the above failures are well known; (I had not done any Internet investigation at all).
By now, I have already replaced all 10 RIFA Noise suppressor caps in the deck (these tend to shortcircuit and smoke) and all the Gold FRAKO ElCO's (these tend to dry out and shortcircuit). Most of them had only 1/3 of their nominal value.
Tantalum caps will still be replaced by WIMA MKS2 (film) 50V film capacitors ( < 10 uF) or new tantalum caps with a higher voltage 35V (except the large (> 10 uF) coupling caps in the audio circuits, for which I will use Nichicon FG electrolytic caps).
By now, I also have done a detailed study of the diagrams of this deck (and its sisters: STUDER A67 and B67 Mk1) and have studied various Internet Fora (
www.Tapeheads.Net,
www.audiokarma.org,
www.studerundrevox.de), so I am now more aware of the various well known problem areas of these decks.
However, I have noticed that the following issues have been barely addressed:
I noticed that in the A67 and A700 STUDER was very skimpy with the specification of the operating voltage of the FRAKO caps and certain Bridge rectifiers in the Power supplies and motor driver boards (partly rectified in newer REVOX A700 models and the STUDER B67Mk1). I have therefore installed higher voltage (and current) replacements , just to be sure.
But I have still several questions;
1) In the Tapecontrol board (Attachment 1) there are 2 Motor Drive IC's, (TCA561), each followed by a transistor
(BC108B) which each drive a Power Darlington (MJ411) and a reel motor (wind/rewind).
These BC108B transistors sometimes fail (destroying the (by now) unobtainable TCA561 driver IC); Source : "TINMAN": STUDER expert on the Tapehead.net forum.
Note: in the STUDER B67 a heavier Duty BC140-10 (Higer Vco 60V, higher I 800mA) has been applied.
My question : Could I not better replace the
BC108 (Vceo=20V), Q17 transistor by a repacement with a
higher Vceo (say 50-60V) ?
Ideally I would also specify a higher current and power rating: what transistor would you recommend? The BC140-10 used in the B67 has a lower Hfe, but maybe this not that important in this application.
(EDIT: on the DIYAUDIO.com site I got the recommendation] of the
BC337 (NPN). . If I look at the site of my local parts shop the
BC 639 (NPN) could be used as well, in my opinion. Would you agree??
2) Same question for
Q5 (BC108) in the Capstanboard (attachment 2)
3) I noticed that the STUDER B67Mk1 uses
BFR18 (Vceo = 60V) transistors in exactly the same locations of the Tape control IC (SC10429) whereas in the REVOX A700
BC107 's (Vce = 45V) are used.
(compare Attachmen3 with Attachment 4).
Is the BFR18 overkill?? In my opinion a BC107 would be adequate for the voltage (24V) employed.
4) I noticed that in the A700 a
BC109C (Vceo=20V ), Q1 is used for the optical shut off circuitry, whereas the STUDER B67 uses a BC107 (Vceo = 45V), which, again, would be OK for the voltage (24V) employed.
The BC109C seems to be underdesigned.
Can I simply exchange this
BC109C by a
BC337 as well?
5) The A700 Deck employs a lot of BC107 transistors, so I do not understand why at certain places BC108B 's were used (were they much cheaper at the time (1975??).
All advice would be welcome!
Thanks for the help!
Martin