Tascam 122mkIII

[A.N.T.]

I've got recently a couple of these decks, in a pretty decent condition, not overworked, heads like new. From the look of it this couple was sitting in somebody's rack switched on (as bulbs in the meters are dead) but not doing much work. These machines are a mix of things great (the heads, the meters, calibration facilities, balanced I/O) and not so great things at the same time - the mechanics barely meets the spec of 0.06% WRMS W&F (with most of the W&F coming from the DD motor), the electronics built around Dolby CXA1330S, 4 BU4066 chips in the signal path.

The frequency response after calibration is ruler flat from about 100 Hz to almost 20 kHz on Metal and Chrome tapes, but the low frequencies are cut so heavily that the deck does not meet the specs in this respect (-3dB cut is at 35 Hz but the specs says 25 Hz)

I've uploaded a small album on the Tascam 122mkIII (http://www.ant-audio.co.uk/Tape_Recording/Tascam/122mkIII/index.html), including the PB frequency response (measured with the ABEX TCC-162B tape, mostly nice and flat with some HF roll due to the low value of the PB head load - only 15K, and a noticeable LF roll too) and a REC/PB frequency response using Teac mdx metal tape.

Alex

[Skywavebe]

Hello ANT,
So besides the 4066 chips which I am sure you will change what other improvements could you suggest? I have a couple here too. The Capstan motor may have been spinning for some years if it does not power down which I don't think it does. This is also the famous motor that fails a lot and I tried recapping on without any good results. Well I don't work there so they will probably can the units. I have some new heads too.
Last time I worked on one the Idler tire cracked and I could not find any.
Your input for improvements from the Tech bench would be helpful.

[macster]

I have one of these that I sent in to have refurbish and it is a good little deck and will play nicely with tapes made on other machines. I like mine, its a a keeper. I'm curious to see what modifications you are going to do to it.

M~

[A.N.T.]

Hello ANT,
So besides the 4066 chips which I am sure you will change what other improvements could you suggest? I have a couple here too. The Capstan motor may have been spinning for some years if it does not power down which I don't think it does. This is also the famous motor that fails a lot and I tried recapping on without any good results. Well I don't work there so they will probably can the units. I have some new heads too.
Last time I worked on one the Idler tire cracked and I could not find any.
Your input for improvements from the Tech bench would be helpful.

The DD motor in these decks is actually spinning only if the tape is loaded and from the look of it on both decks there is very little evidence of wear on the motors. I need to look in details what can be done about W&F as this is my biggest disappointment with the Tascam 122mkIII . If I can not improve that I will just disassemble both decks for spares. There is no point in trying to improve the electronics if the mechanics is the weakest link. W&F on these decks can easily wary from 0.06% DIN wtd to 0.15% DIN wtd and that is not good. It would be a shame to take them apart as I like the appearance and potential capabilities of these decks. I'll keep you posted.

Cheers

Alex

[tapetech]

Are you sure you've eliminated the supply reel as a contributing factor to W&F? That's one of the reasons for dual-capstan design (to isolate the supply reel).

Try turning/rotating the bronze bearing and see if that lowers W&F.

Is the W&F periodic at all? If so, perhaps both decks have motors starting to go bad. As you mentioned, they have a high failure rate. One failure is a very periodic, very high level wow. See if freeze spray on the board effects W&F.

[A.N.T.]

Are you sure you've eliminated the supply reel as a contributing factor to W&F? That's one of the reasons for dual-capstan design (to isolate the supply reel).

Try turning/rotating the bronze bearing and see if that lowers W&F.

Is the W&F periodic at all? If so, perhaps both decks have motors starting to go bad. As you mentioned, they have a high failure rate. One failure is a very periodic, very high level wow. See if freeze spray on the board effects W&F.

I've already tried many things when I've serviced the deck and managed to lower the W&F somewhat. The mech looks almost new - the tension on the supply reel is steady and I've adjusted it for the best W&F. I've adjusted the bronse bearing for the lowest W&F too (its position changes the gap between the coils and the rotor in the DD motor). I've cleaned the idler tyre (it looks new but was slipping occasionally before cleaning). However the W&F is still too high for my liking - about 0.1% DIN wtd, up to 0.2% DIN not wtd. It is within the specs but I have some single capstan decks in my collection that have less than 0.06% DIN wtd W&F.

Alex

P.S. - the W&F is mostly periodic with the main component at the capstan frequency (5-6 Hz) and quite a lot of HF flutter. However the W&F do change in time without any apparent period - it may go from 0.05% DIN up to 0.12% DIN occasionally (usually not more than 0.1% thought) on my W&F tapes. On Rec/Pb the W&F may go up to 0.15% at some points, not often.

[Jonny Ramone]

Didn't realize they had an idler - I'll have to check mine. I was rather astonished that my old ho-made live recordings sound better on this machine then the deck they were recorded on. I think my capstan motor is shot, though. Pioneer owners rebuild motors all the time, couldn't these be rebuilt as well? The pitch control would be very handy to re-master some 3" reels taped on a R2R with atrocious spped variation - the reason I bought it.

[A.N.T.]

I think my capstan motor is shot, though. Pioneer owners rebuild motors all the time, couldn't these be rebuilt as well? The pitch control would be very handy to re-master some 3" reels taped on a R2R with atrocious spped variation - the reason I bought it.

I've retraced the circuit on the Tascam DD motor today and also found datasheets for the chips used. There is very little there that can not be fixed thought the Hall sensors may be difficult to obtain. The FG servo is rather crude on that motor and I suspect it is mainly responsible for rather poor W&F figures. I may try just for fun to put a PLL circuit there and see what happens. At the moment the speed difference between idle and loaded modes is about 0.3%, a PLL should cure that and also it could reduce W&F as the motor itself looks capable of much better performance. I will put the schematics of the DD motor on my site after I re-draw it.

Meanwhile I've discovered that if I disconnect the pitch control and engage the on-board pot, I have a noticeable reduction in the HF flutter, the weighted figures did not change much but the un-weighted is reduced from over 0.2% to about 0.12-0.14% . I suspect it is due to a pick-up on the pitch control wires as these directly connected to the saw-tooth generator in the FG servo. The DD motor also contains all bits required to run the deck at a double speed - 3 3/4 ips, just an external switch needed.

Alex

[A.N.T.]

I've done some work on the electronics in the Tascam 122mkIII, preparing it for opamps and 4066 chips replacements, and found a rather nasty original design fault. If the output level goes over 0dB on the meters and the output level knob is on full output ("10"), extended harmonic distortions occur with harmonics visible to the end of my FFT analyser scale - up to 47-th (!) for 1 kHz input . At +6dB DIN the distortion in the source monitoring mode reach 0.5% ! The source of these nasties - two muting transistors Q108/Q208. After I've put a couple of 1N4148 diodes in the circuit (a diode in series with each of R141 and R241) these additional distortions are gone completely and the linear range is extended at least by 10 dB, up to +12 dB DIN.

Two graphs show the THD v Level, with 100 mV on the X axis equal to 0dB on the Tascam meters, one of the original deck, another - after the diodes mod, in the Source Monitor mode.

Alex