[iamdom]

Hi folks, just joined this forum. I'm wondering if someone here is familiar with an old bipolar transistor mic amp that I have had among my collection for about 20 years. I purchased it second hand from an electronic vintage store here in Adelaide, South Australia [from "Robby's", off the bottom of Port Road]. It uses low power, low noise A.F. transistors in all its stages [Fairchild SE4010 2N3566, 2N4250 - 2N4360, RCA 40361 - 40362]. It runs on 3 X 9V batteries [27V]. There is a model number at the front-- TA 125, and another tag with the letters A.B.C [I presume the first letter is A for it is torn]. Followed by an etched number RE 3922. There is one other number that is written with a marker 33-140, however that may be a reference that was written by possible Robbie for book keeping?

It has a built in balanced microphone input transformer hard wired to a 4 pin Cannon plug input at the front. Next to it is a 3/4" mono "crystal phone" jack, a push button for battery test and signal level on a small VU meter, and on the left are 2 output posts for speaker connection.

I take it that it's quite likely to be a field instrument, for microphone amplification. It works well as a small portable amp. On the circuit board, there are 2 trim pots, one for Gain and the other has "Thresh" written on the solder track side which I presume is "'thresh hold", but for what? ... I'm not certain. I thought it may be for voice activation, but i'm sure that's not the case. Maybe AGC? I am an electronics hobbyist and not a professional technician by no means.

Any help would be appreciated. Please see photos.

all the best

[Jan Jongejan, PE1SBG]

iamdom wrote:


Hi folks, just joined this forum. I'm wondering if someone here is
familiar with an old bipolar transistor mic amp that I have had among my
collection for about 20 years. I purchased it second hand from an
electronic vintage store here in Adelaide, South Australia [from
"Robby's", off the bottom of Port Road]. It uses low power, low noise
A.F. transistors in all its stages [Fairchild SE4010 2N3566, 2N4250 -
2N4360, RCA 40361 - 40362]. It runs on 3 X 9V batteries [27V]. There is

This newsgroup is about tubes.

As seen by the pictures, it looks modified.
If you want to know how it works, you can draw the schematic from the pcb.
Even a hobbyist can do this.
Sorry, that's all I can help.

Regards,
Jan

[Phil Allison[_3_]]

"iamdom"

Hi folks, just joined this forum. I'm wondering if someone here is
familiar with an old bipolar transistor mic amp that I have had among my
collection for about 20 years. I purchased it second hand from an
electronic vintage store here in Adelaide, South Australia [from
"Robby's", off the bottom of Port Road]. It uses low power, low noise
A.F. transistors in all its stages [Fairchild SE4010 2N3566, 2N4250 -
2N4360, RCA 40361 - 40362]. It runs on 3 X 9V batteries [27V]. There is
a model number at the front-- TA 125, and another tag with the letters
A.B.C [I presume the first letter is A for it is torn]. Followed by an
etched number RE 3922. There is one other number that is written with a
marker 33-140, however that may be a reference that was written by
possible Robbie for book keeping?

It has a built in balanced microphone input transformer hard wired to a
4 pin Cannon plug input at the front. Next to it is a 3/4" mono "crystal
phone" jack, a push button for battery test and signal level on a small
VU meter, and on the left are 2 output posts for speaker connection.

I take it that it's quite likely to be a field instrument, for
microphone amplification. It works well as a small portable amp. On the
circuit board, there are 2 trim pots, one for Gain and the other has
"Thresh" written on the solder track side which I presume is "'thresh
hold", but for what? ... I'm not certain. I thought it may be for voice
activation, but i'm sure that's not the case. Maybe AGC? I am an
electronics hobbyist and not a professional technician by no means.

Any help would be appreciated. Please see photos.

all the best


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|Download: http://www.audiobanter.com/attachment.php?attachmentid=238|
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|Download: http://www.audiobanter.com/attachment.php?attachmentid=239|
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|Download: http://www.audiobanter.com/attachment.php?attachmentid=240|
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** Looks very much like something made specially for the ABC - probably
by AWA.

The 4 pin Canon mic input connector is an ABC standard.

AWA had a close business connection with RCA and the design is clearly by
RCA.


For the uninitiated:

AWA = Amalgamated Wireless Australia.

ABC = Australian Broadcasting Corporation.

Google is your friend.....



..... Phil

[iamdom]

Jan Jongejan wrote:

This newsgroup is about tubes.

As seen by the pictures, it looks modified.
If you want to know how it works, you can draw the schematic from the pcb.
Even a hobbyist can do this.
Sorry, that's all I can help.

Regards,
Jan

thanx for your reply. i noted this list is about tubes, however there was a listing among this list regarding vintage audio gear... the equipment is vintage be it bipolar transistor, so i thought it would be ok to post here, for interest sake at the least. My humble apologies if I got it wrong.

The amp is in original condition. It was not modified when I took the photo yesterday. However, I since replaced all the electrolytic caps, they were all leaking electrolyte. We know how electro caps especially in the audio area have improved since this little gem of an amp was made.

Yes, i agree, my next step is to draw out the circuit to see it's layout. but I wanted to know if anyone knew what it was used for exactly.

As per the other reply, I too concluded it was used by the ABC here in Adelaide, I'd say. I wondered if it was for commercial ABC TV or ABC radio? Anyway, when I draw the schematic, I will post it here, or at the appropriate list that you or others may guide me to.

In the meantime, if anyone does have a schematic or can contribute more, I would very much appreciate it.

Dom

[iamdom]

Originally Posted by iamdom

Hi folks, just joined this forum. I'm wondering if someone here is
familiar with an old bipolar transistor mic amp that I have had among my
collection for about 20 years.


Phil Allison[_3_] replied:

** Looks very much like something made specially for the ABC - probably
by AWA.

The 4 pin Canon mic input connector is an ABC standard.

AWA had a close business connection with RCA and the design is clearly by
RCA.

For the uninitiated:
AWA = Amalgamated Wireless Australia.
ABC = Australian Broadcasting Corporation.

Google is your friend.....
..... Phil

thanx for the reply, and especially the info, Phil.

i wasn't aware that ABC used 4 pin cannon plugs as a standard. it's wonderful to know and it helps to determine the ABC tag at the front is extremely likely made for, or, from the ABC [per as i thought].

Are you suggesting the design is RCA but the unit was built by AWA? i would have thought RCA designed and also made it? After all they had the facility, then. Could you clarify? please.

thanks,
Dom