CBS Volumax fet

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princetown

Active member
Joined
Nov 19, 2005
Messages
26
Location
WARRNAMBOOL AUSTRALIA
hi guys, was wondering where to obtain a part for a CBS volumax, the transistor is a fet made by Texas Instruments TI558 (grn) which means it was especially selected.
A google search has failed to locate it.
I have contacted TI but they can be unhelpful as it would be an obsolete part.
So I might have to experiment with some modern ones which will take time.
any clues?
cheers ,Princetown
 
> a part for a CBS volumax, the transistor is a fet

Is there an FET in the Volumax?

The Audimax had a tube, which may have been changed to an FET in later models.

Post a schematic so we are all on a same page.
 
TI stopped making discrete transistors _years_ ago. :sad:

Requests for old transistors normally come in hand written form, with an offer of a cheque or postal order to pay for it. :grin:

There's always a temptation to say "please visit out website", but that would be just cruel for a guy that writes a hand written letter :grin:
 
Here is what I found on the TIS58
http://www.datasheetarchive.com/semiconductors/specsheet.php?specsheet=2097469

Part Number = TIS58
Description = N-Channel JFET
Manufacturer = Various
V(BR)DSS (V) = 25
V(BR)GSS (V) = 25
Absolute Max. Power Diss. (W) = 360m
I(GSS) Max. (A) = 4.0m
V(GS)off Max. (V) = 5.0
I(DSS) Min. (A) = 8.0m
g(fs) Min. (S) Trans. conduct. = 1.3m
g(fs) Max, (S) Trans. conduct, = 4.0m
@V(DS) (V) (Test Condition) = 15
C(iss) Max. (F) = 6.0p
Package = TO-92
Military = N

This page had a pinout.
http://home.att.net/~theremin1/144/142transistors.gif
 
I have a volumax 441 stereo unit here that I´m working on as time permits. I was thinking about making this a DIY project, but there´s a can named frequency compensator that I don´t have a clue what´s inside... Probably a passive circuit. Anyone knows anything about this thing?
 
Note that adrianh's theremin link has the pinout of the TIS58 and the 2N5484. The '84 is the same family and pinout as the 5485 and 5486, so you can see the differences from that.

Most FET's are very nearly symmetrical with respect to interchange of the nominal source and drain. So the only serious mistake you can make is getting the gate wrong.
 
[quote author="jdiamantis"]IIRC, the FET is used as a buffer for the control voltage. Assuming Vds is not exceeded, just about any fet should do.[/quote]

What is the variable element then?

It would be nice to see a schematic.
 
[quote author="bcarso"][quote author="jdiamantis"]IIRC, the FET is used as a buffer for the control voltage. Assuming Vds is not exceeded, just about any fet should do.[/quote]

What is the variable element then?

It would be nice to see a schematic.[/quote]

The schematics are available on line (google it)

The variable element is a "variolosser"; forward biased diodes as the shunt part of a series/shunt attenuator. Changes in the bias (i.e. control voltage) change the dynamic resistance of the diodes, creating attenuation.

Thinking about it, the later Audimax's had FET's for control voltage buffering; the Volumax used a darlington connected pair of BJT's.
 
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