I send the unreduced schematic to what I reconstructed from your email description.

Thanks for the photos of the 6AU6 tubes.

You're welcome and thanks for the schematic. It is very legible even when enlarged several times.

I just had a thought. Could I gain a usable audio output that can be fed to my mixer if I put a switched jack in the 6BQ5 cathode circuit so that the switch is in parallel with the 1/4" jack and determines whether the capacitor is in the circuit or serves to pass an audio signal to the jack?

If there is any possibility of this working I'll try it when I replace the capacitors and see what I get.

Does a 1/4 track stereo reel to reel reproduce all the 1/2 track mono signal or does it miss some since it is a 1/4 track stereo head?

I was just looking at the schematic and noticed
that the 6AU6 in V1 position has the letter "T"
after it.Does anybody know what the letter T
stands for in that application?I was thinking
it has something to do with low noise but I
wanted to try to get an answer from someone
who worked on those decks back in the day
and maybe had to change that tube.
Regards,
Swanson

Could be that it was selected for low noise and or low microphonics.

Oops I did not realize that the 6AU6T's were already
discussed on page one.I somehow blew right over
the post.I have been interested in using 6AU6 tubes
in preamp applications because of the tremendous
gain you can get from them.I do have some 7543's
which are supposed to be low noise 6AU6 specifically
for audio applications.Have not had the chance to try
these in a high gain circuit yet.
Regards,
Swanson

I tried using the cathode bypass capacitor of the 6BQ5 to couple audio to my mixer and it works quite well.

The sound seems to have a bit much upper mids and treble, but that has to be due to the capacitors that have not been replaced yet.

Looking at the schematic I will need to add an 8 ohm load when I have the internal speaker disconnected to keep the amp happy. They have an 8 ohm load that is switched in when in record mode and the speaker is off. I assume the load keeps the amp from potentially oscillating or something or maybe it was done simply to make the VU meter operate properly as there is a 100 ohm resistor across the output transformer secondary.

Back then an AKAI would have most likely been one of the reel to reels worth having.

I just ordered the capacitors.

I will have about $40.00 in the reel to reel when all is said and done.

I checked the resistors and except for a few that read low (due to bad capacitors I assume) all checked within tolerance.

The plate of the 6BQ5 is the drive point for the record head, so when on record the
cathode bypass capacitor has got to be there.

C16, R16 and R17 should be good too or the head wont get the correct drive to mix with
bias through trimmer C18.

When recording I'll have the line out I made disconnected which will put the cathode bypass cap back in circuit.

My amp is different in the recording bias circuit than what the schematic shows.

R17 has a mica capacitor in parallel with it, R16 & C17 aren't present, R20 is 500K, there is no trimmer present and there are two mica capacitors in parallel with each other connected between the 6AR5 plate and the center conductor of the shielded cable.

Could mine be an early version of that model?

First of all I love your new find. You comment about recording and the speaker load is coinciental to my post just now. viewtopic.php?f=13&t=193006 I was wondering how they handle removing the speakers from the amp load during recording and if that's an issue. By the way, I have an M7.

This is another version of the early series. : Could you measure the resistance from
the cathode of the 6AR5 to ground with the power off and the record-play-stop switch in
the 'record' position.

(dwg. register 6)

I get a direct short when measuring from the cathode of the 6AR5 and ground with the record/play switch set to record.

I assume the different revisions were simply refinements to the circuit?

I see how the recording bias and erase head circuit works. The record/playback switch simply shorts the cathode of the 6AR5 to ground when the record/playback switch is set to record which causes the circuit to work. That said why is the 3 uF capacitor needed if it is simply shorted to ground in record mode? only problem there is if the 3uF capacitor ever decides to short or even partially short the circuit would be turned on and start erasing the tape until the reel to reel was stopped.

I'm happy to report that I have the new capacitors installed and the reel to reel works great.

I will have to abandon the idea of taking the line out signal from the 6BQ5 cathode circuit as it was lacking in midrange. I plugged in the cable to the speaker out jack and the audio through my mixer is very good.

Do I need to install an 8 ohm resistor to serve as a load since the internal speaker is disconnected when the cable is plugged into the speaker jack? If so I can go by my local Radio Shack and get an 8 ohm 20 watt non-inductive resistor.

I was also thinking about it and I have a spare Edcor 15K balanced/unbalanced to balanced/unbalanced transformer. Since the reel to reel must be located a distance from the mixer (speakers near mixer) would it be wise to use the transformer to provide a balanced signal so that there is less chance of noise pickup over the cable run?

Playing a tape that someone had recorded years ago of some 45's on a mono reel to reel at 7 1/2 IPS and I honestly believe the tape sounds better on the Terecorder than it does on my AKAI GX-255 reel to reel which is more than likely due to the fact that the Terecorder is 1/2 track mono like the machine that made the recording was and the GX-255 is 1/4 track stereo. I'm even hearing more of the dirt in the grooves of some of the records.

I am only assuming the tape was recorded many years ago due to the music contained thereon and the fact it is 1/2 track mono.

I plan on using this to transfer all my 1/2 track mono tapes to my computer.

I will eventually need to replace the counter belt as sometimes the counter stops.

My goal is to one day take all my home recorded reel to reel tapes, get the songs and make new tapes with better audio quality while keeping the originals intact. Will be easy with the three or four I have that contain music from records, but the four recorded from a radio station would be hard to properly remake.

The print I am looking at shows the erase head grounded at both ends during playback and stop.

The 3 uFd capacitor is connected to cathode and the erase head in record. The cathode DC current is the
erase current and the 3 uF cap blocks the bias component. There is another issue. Depending on
how fast the levers are operated, and circuits are switched mechanically, recorder makers want no
transients from the bias oscillator slamming down and dumping all the energy from their oscillator
tank into the play head, magnetizing it. That 3 uF may address that.

DC ERASE? Well, it DOES erase. But it magnetizes the erase head and unless the head is mechanically removed from the tape path it CONTINUES erasing during play. I haven't seen DC erase since my 1954 Wilcox-Gay.

Concerning the smaller plug how am I to know which small pin of the plug connects to the small pins in the jack?

Is it like this which shows the cathode is grounded in record with the erase head receiving AC voltage?



Or like this which shows C25 being grounded in record mode while the erase head is in parallel with the 3 uF capacitor?

There is the issue of the exact version of the machine. Akai was quite nimble with the Terecorder
branding. From the changes in the bias circuit, and especially the exact bias oscillator transformer, that
is used, they might have been trying different heads and circuits while dancing around foreign patents..

An interesting study would be on patents on oscillator circuits licensed for sound reproducers
for commercial/non commercial application and the head designs.

To be sure which pin is which, just locate the R/P head wires. These will lead to the input of
the preamp on play. The other is the erase head.

DC erase is not as good as AC erase, and for an old half track machine, of that era, I dont
think it would make any difference. They got away with DC bias and DC erase for millions
of cassette recorders, after manufacturers discovered few people actually used their ghetto-
blasters for record, especially because the ECM was in the case of the machine and picked up
all the mechanical noise of the transport, which masked any hiss.


Look at the 6AR5 circuit. This version can allow AC erase.



Lots of fairly decent machines used a magnet for erase that was moved to the tape mechanically
on record. One of the first recorders I owned, Utah Magitape, had a little Alnico horseshoe magnet
on a lever.

Ok then the first picture is correct where the cathode of the 6AR5 is grounded in record mode and the erase head is capacitively coupled for AC erase.

I'll look at the actual amplifier this afternoon when I remove the line output mod. That way I will know for sure if I have AC erase or DC erase.

Does your machine have automatic stop for end of tape ? Could you photograph the Deck to
amplifier plugs and the name plate with voltage (if it is present)

Mine doesn't have automatic stop unfortunately.

I can take some pictures of the tape transport as I have to remove it when I pull the amp to remove a mod I did and troubleshoot a feedback issue when the volume control is turned up past 6 or the tone control is turned CCW.



Removed the mod and the feedback went away. I have noticed that with these AKAI amps they are pretty touchy concerning the wiring as even adding a shielded cable can sometimes upset these amps.

I noticed the amp is virtually hum free with the tape transport in play and the volume control turned full CW so long as the amp is sitting above the tape transport, but as I start to put the amp in the case the hum level increases somewhat. What would cause that and is there any way to fix it? I did notice that the botom and rear metal access plates in the cabinet are grounded.