destroy it yourself part 2

[zipzap]

I just sort of killed my old Tapedeck. It´s a three head Technics RS-BX646.
I had replaced the trimmer+resistor that controlls the motorspeed with a pot. It was making great delay and flanging sounds! Wonderful.
Unfortunately i started looking for a voltage i could use to boild some vc-circuit (for the tapespeed). Touching one resistor near the regulators gave me a little lightning.
Now the deck is still running, motors and controlls ok, but i don´t hear anything.
There is a cable coming from the pb-head going to a section on the pcb saying Playback EQ amp.
My scope detects nothing around there. Also if i pull the cable and run the tape i mesure nothing. The VU-meter is halfway-light and not moving.
Whatever i´ve killed, do you think it´s possible to use some new selfbuilt circuit to record and playback the tape? I don´t think i´ve killed the head.
Those low speed dark dub echos were so nice.

[blue hell]

Ouch ...

Playback might be doable, but recording is tricky as you'll need some high frequency bias along with the signal.

The pb-head should have some measurable signal when you measure it with the probe on one end of the coll and the earth clip om the other (both conections are isolated from the metal case usually, so grounding the scope the usual way will not show any signal). I'd expect some 10 .. 50 mV, but its a long time ago I did such experiments.

It's pretty unlikely I'd think you blew the head as it should have a pretty high resistance (lots of turns of very thin wire).

I remember that in the 80ties when casette tapes were used for computer data storage there were some circuit diagrams floating around and one could buy casette mechanisms without electroncs - probably optimized though for "digital" signals.

[zipzap]

Think i blew part of the the PSU.
I can meassure a signal from the tape the way you described. When set to record i can see a strong HF-Sine going to the rec. head. Bias.
On the Board there are some points labled Vcc, Vee, etc
I measure:
Vcc 8,2v
Vee 0,6v
Vcc_PB 1,25v
Vee_PB 0,7v
5v 4,8v

The voltages are generated by 3 B1357 NPNs and 3 D2037 PNPs
One of the Pins is lables Emitter ,the others i don´t know.
I measure: Emitter, Middle Pin, outer pin:
D2037
-9,5 -16 -10
0,6 -15,8 0,05
0,6 -15,8 0,05

B1357:
5,6 10 6,2
8,4 11 9
8,5 13 9

So there seems to be something wrong there, right?

[blue hell]

This sort of suggest the negative supply rail is not working, which could cause the level meter to have a bias.

It would be helpful to have schematics of the power supply, maybe from a service manual, but if everything fails maybe some tedious reverse engineering on the supply ... maybe http://www.manualscenter.com/allProd.php?page_id=54 ? ($14.95 though)

[zipzap]

It was one little resistor hanging in the air, going to ground. Sort of a fuse. I noticed it had 0 resistance, after cutting it out everything is working again. lucky me. So now i can move on to the diy tapedely thread.

[blue hell]

[Yorky]

Phew !!