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Old 12th Oct 2017, 8:26 pm   #1
REDGUUZ
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Default REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

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
Attached Thumbnails
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Name:	Capstanboard  onderbemeten Q5 BC108B.jpg
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Old 12th Oct 2017, 9:17 pm   #2
paulsherwin
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Default Re: REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

The BC107 is exactly the same as the BC108 except for the higher working voltage so can be substituted everywhere. The BC109 is a low noise BC108, and again a BC107 can be substituted where the noise performance isn't critical.

The modern version of the BC107 is the BC547. This is a cheap and easily available transistor.

The BC337 is a different transistor but will probably also be OK. It has a higher collector current capacity than a BC547.

The BC639 has even higher voltage and current ratings than the BC337. It will probably be OK as well, but I have no personal experience with it. The BC639 has a bce leadout, unlike the cbe leadout of the other types mentioned here.
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Old 13th Oct 2017, 8:37 am   #3
ricard
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Default Re: REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

Quote:
Originally Posted by REDGUUZ View Post
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.
I would say it depends on whether the transistor fails because the voltage gets too high or because it gets too hot (i.e. the power rating is insufficient).

The BC140 is a bit of a halfway house between a small signal and power transistor. It's got a high Vceo but it's still in a standard (albeit slightly larger) metal can so it can't handle that much continuous power. I wonder if a BD139 (1.5A / 80V) wold have an even better margin here. Like other power transistors, it does have a low Hfe though.
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Old 13th Oct 2017, 9:51 am   #4
paulsherwin
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Default Re: REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

The point is that changing a BC108 to a BC107/BC547 has absolutely no electronic implications for the circuit. If you change to a BC337, BC639 or any other different type there may be implications, though this seems unlikely having quickly looked at the circuits.

I should state that I have no direct experience of working with these control circuits and am certainly not a Revox/Studer expert.
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Old 13th Oct 2017, 10:41 am   #5
REDGUUZ
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Default Re: REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

Paul & Ricard,

Thank you for your replies/suggestions,

Since the BC108B (s) at the output of the TCA561 (s) only fail (s) occasionally, I think it is due to a too high Voltage (Vce=20V is too meagre!) given the application.
It does not have to dissipate that much power because after all it is followed by a MJ411 or MJ413 power darlington, which control the reeling motors.

The low Hfe of the BC140 (or the even lower Hfe for the BD139) might be an issue (maybe not) and can only be determined by Trial and Error.

I think the upgrade to a BC337-140 (Vceo = 45 V, Icont = 500 mA, P= 625 mw, Hfe = 400) versus BC108B (Vceo = 20V Icont = 100 mA, P = 300 mW, Hfe = 450) should be sufficient. This was also suggested on the Dutch Forum [https://www.circuitsonline.net/forum/view/138222/1/a700.
Maybe the BC639, Vco =100v, I cont = 1000mA, P=830mW, but Hfe=150 is not so appropriate. It might be overkill for Volt, Current and Power ratings but does not work due to a too low Hfe (maybe it is not an issue in this application).

(I would simply like to swap these parts now that I am "recapping" the machine, especially the Tape Control and Capstan Boards.
Maybe it is not necessary after all (so far all tantalum caps which I already replaced were within specs, but if one shortcircuits it could possibly destroy an now unobtainable IC (e.g. SC10429, TCA561, TDA1000), hence my desire to do it now properly instead of "Do not fix it, if it ain't broken" attitude sofar.

If anybody on the Forum has further suggestions for upgrading the REVOX A700, I would very much welcome input!!

Best regards,

Martin
Attached Files
File Type: pdf BC 109.pdf (53.5 KB, 3 views)
File Type: pdf BC337.pdf (56.7 KB, 3 views)
File Type: pdf BC639.pdf (51.4 KB, 2 views)
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Old 13th Oct 2017, 10:54 am   #6
paulsherwin
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Default Re: REVOX A700 Transport Control and Capstanboard: Underdesigned transistors?

I doubt if the slightly lower hfe of the BC639 would be significant in this application. It isn't too far from a BC108B (200-450). However, it is always safer to make a substitution where this has already been done without problems. If others have switched to a BC337 then I would choose that - it gives a significant increase in voltage and current handling. The BC639 is probably overkill.
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