How to modify a 2 head Cassette Deck into a 3 head unit? Tech Advice Appreciated!

How to modify a 2 head Cassette Deck into a 3 head unit? Tech Advice Appreciated!

As an old school amateur/diy guy with outdated knowledge except the basic principles that I learnt from time of vacuum tubes and germanium trsanys, I still have some moments hands on inexpensive ideas. Here is one:

I want to modify my "not so great" Tascam 112 MKII into a 3 head deck. In fact I had a chance to pick up a defective 122 MKII, but I missed it by (maybe) couple of minutes, so led me into this project. My 112 MKII is still untouched except open up for close study. I don't want to destroy it unless I am pretty sure I can give it a new life.

I found it a Yamaha REC/PB sanwich head. I was thinking to simply jam the PB and REC signals into the main board between the headphone AMP and the last leg output AMP, with some mod's on the monitor and PB/REC SWs, bridging in the Dolby encoder/decoder etc. etc. Then I found one each the PB and REC boards of 122 MKII. I thought adding these discrete boards should make it sound better. But now I am confused by the Mainboard schematics, especially lack of info on design concepts like that on the 112 first model.

(1) There were a number of spots where the L/R of Inputs, Outputs and Monitors were merged into one single line. I compared the schematics with that of 122 MK III which is very similar, also had both L/R merged. Can someone kindly explain?
(2) The PB board needs +7.5V and - 7.5V but 112 MK II MB only has +-7.0Vs, measured +-7.3Vs. Is 0.5V short still good to run? The REC board is fine, it uses +- 7.0Vs.

I know there are more areas to address, like setting/aligning the new head right, re-adjusting the bias etc. etc, but these are the next steps.

Thanks for reading.

ps 1:
(a) or can some one point out where of the 112 MK2 circuit I can cut-in connecting the PB board OUT and REC board IN?
(b) after studying the PB REC boards, they each has a Sony DB chip, so I would think these boards have the decoder / encoder. The 112 MK2 uses a DB chip with a "X" designation, also found on other 2-head machines, that can handle both decoding and encoding.

ps 2: unrelated question.
I have an Akai timer/clock. I use it to turn on radio/lights when I'm not around, but this thing doesn't hold memory/program once powered down. I want to add a battery backup to the ROM MN6076's VSS VDD. I like a transistor to let batter to stop battery to pass when power is up. which transistor is best? PNP or NPN?

(ps 3: I'm short of internet data, so I can show only when I have access to public wifi).
 
Last edited:

tornislv

Member
2010-09-23 10:15 pm
Riga
Been there, done that. Not worth it, as long as we are speaking about having a decent 3head deck. Just buy one. I can't remember how the NR circuit is configured in 112, did it have separate encoder-decoder parts in the schematics? The most simple way would be just to add a separate PB amp and wire it to the PB part of combo head and then to output jack.
 

U230421

Disabled Account
2017-02-07 1:54 pm
Having a 112B, I consider it undoable. There's simply no space left for an extra head. The mechanics won't allow either. You'll render it in the next defective 122 you almost picked up.
However, as suggested by #2, you may copy the PB circuit (or steal it from the defective) and wire it up separately. The supply will allow this extra small load without drawbacks.
And again referencing #2, better buy a proper 3-head instead. Keep the 112 decent in its own. You can sell it undefective anyhow.