Akai X-355 Initial Look At

[DMcMahon]

Very recently purchased the X-355 from Pamphonica (Jeremy) under the Sets, Parts and Service Information Offered.

It is a mid 1960's 4 track stereo reel to reel solid state recorder, 2 speed (3 3/4 & 7/1.2 ips) using 7" spools. There are kits to convert to 15 ips and 10.5" spools but not included. It has internal power amps and speakers.

It uses the cross-field system for better high frequency performance.

It is a seriously heavy unit and extra deep (depth). Overall condition looks pretty good, the case is in need of various stains cleaning off and it looks like there are plenty of cobwebs inside.

Tonight have been trying to get the guts out of the case but struggling at the moment to get the tape deck chassis out.

The metal Akai spool on the Take Up Reel Table looks a little strange. In the User manual it refers to 7" Automatic Reel, so I am wondering if it does automatic threading like my Apex 1163. Have not yet seen any reference to automatic threading in the manual though.

[DMcMahon]

After some head scratching worked out that the rear of the tape deck was secured via a metal tie bar to a large metal bracket across the rear of the case (not mentioned at all in the strip down instructions), after removing the tie bar, the tape deck came out relatively easy but heavy. Lots of cleaning required.

[Boulevardier]

Nice machine, David. It'll be interesting to hear your assessment of the X-field bias arrangement. I've often wondered how much improvement that gave to HF response, and how much of a hassle it is to set up and align the bias head.

Mike

[TIMTAPE]

X Field bias really did improve high frequency response but at a price. It exaggerated tape dropouts. Dust on the tape's oxide side pushed the tape further into the bias field, increasing bias and reducing the highs even more, a sort of runaway situation.

Whereas with the conventional biasing arrangement dust reduced bias, making it self correcting to a degree.

For X field bias to work well, tapes had to be good quality and very clean at the time of recording, with excellent tape to head contact, especially at slow tape speeds where dropouts are more of a problem anyway.

Akai and Tandberg gave up on X Field and reverted to the normal biasing arrangement.

The later purely electronic Dolby HX Pro constantly measured high frequency levels and whenever they got so powerful as to add to the bias signal, it reduced bias to compensate. It was a great system because there was no downside, and unlike X Field it could be used on cassettes.

X Field was a great idea but thwarted by practicalities perhaps not foreseen.

[Radio Wrangler]

Cross field bias was invented by Tandberg, and Akai licensed it from them.

David

[Ted Kendall]

Interesting, in that case, that Akais with crossfield bias (X-4, X-355), appeared about 1964-5, whilst the first crossfield Tandbergs (6X, 12X) came out in 1968. Nothing like having your licensee do the spadework!
The X-4 was underbiased anyway, to extend the measured frequency response, and sounded crumbly as a result.

[TIMTAPE]

At 3.75 and 7.5ips my X4 had a treble peak even on "nothing special" tapes. Increased bias on its own didnt seem to solve it. I read somewhere that a different NAB curve could have taken advantage of the increased high frequency energy with less tape hiss but then the machines would not have played back other tapes truly and dual playback curves for each speed would have made the machines more expensive to produce.

[DMcMahon]

Originally Posted by TIMTAPE

X Field bias really did improve high frequency response but at a price. It exaggerated tape dropouts.
on cassettes.

Yes had read that dropouts were a potential downside but also have read some cross-field owners reporting that they had not experienced drop outs.

David

[DMcMahon]

Mystery component, look like an axial capacitor but has 3 leads.

[Ted Kendall]

Originally Posted by TIMTAPE

At 3.75 and 7.5ips my X4 had a treble peak even on "nothing special" tapes. Increased bias on its own didnt seem to solve it. I read somewhere that a different NAB curve could have taken advantage of the increased high frequency energy with less tape hiss but then the machines would not have played back other tapes truly and dual playback curves for each speed would have made the machines more expensive to produce.

I think the mediocre quality of Japanese tape at the time had a lot to do with it - cassette machines and the advent of Dolby were the major drivers of improvement in their product.

[TIMTAPE]

Originally Posted by DMcMahon

Yes had read that dropouts were a potential downside but also have read some cross-field owners reporting that they had not experienced drop outs.

David

Actually in the X-4 manual Akai mentioned the narrow tape tracks (quarter track) increasing the chance of dropouts, which is true and so the need to use good quality tape, but I suspect they also knew the X-Field bias was also a potential contributor to dropout. Perhaps they hoped that the warning to use quality tapes might be enough to avert the problem without people discovering the new X-Field bias was also a contributor at least potentially.

I dont know about other X-field models but the X-4's bias adjustment was basic. The bias head's coils were wired in series so there was no way to adjust bias for individual left and right tracks. You had to strike a compromise.

[ms660]

Originally Posted by DMcMahon

Mystery component, look like an axial capacitor but has 3 leads.

No values given on the schematic, it's three RC snubbers in one, connected across the repeat/reverse switch contacts on the schematic I'm looking at.

Lawrence.

[DMcMahon]

Underside of the tape deck now looks a lot cleaner after removing probably more than 50 years of dust.

Will clean topside (not really dirty) and the heads once the deck is back in the case, cannot easily really put the deck sitting horizontal due to the delicate motor impeller sticking out the back, to be able to clean the heads more easily.

Have cleaned the inside of case and tried to clean up really grubby mains cable but the cable is proving stubborn to get really clean so will probably renew the cable. The cable is hardwired into the power supply and to get the power supply out of the case there are 4 securing nuts that are locked solid with blue Threadlock, so need to find something that will dissolve the Threadlock.

On the right securing screw for the motor transformer on the power supply, is signs of something that has leaked, not sure if it is from the transformer itself.

Something has leaked all over the impedance roller, with hard brown deposits. There are a bunch of components part of the Relay Assembly (including electrolytic capacitors) directly above that area, but impossible to see if anything has leaked from them, later will check out it more, will probably have to do a strip down of the Relay assembly to better see/access the components.

At the end of the Capstan Flywheel shaft there is a ball bearing that interfaces to a conical brass piece, I would have thought the ball bearing would be roughly in the middle of the brass piece but it sits close to outside edge, there appears to be no adjustments. Will look at this more closely later and at same time clean and regrease.

[DMcMahon]

Originally Posted by ms660

No values given on the schematic, it's three RC snubbers in one, connected across the repeat/reverse switch contacts on the schematic I'm looking at.

Lawrence.

Well found Lawrence, I will look out for that on the schematic.

David

[DMcMahon]

Used Cellulose Thinners to try and free up the Threadlocked securing nuts for the power supply. It freed up 2 of them but the other 2 still stayed locked solid. While trying to loosen the 2 remaining nuts by trying to rotate the nuts and and hold the Philips heads stationary just resulted in the Philips heads getting chewed up. Had to use mini mole grips to hold the domed screw heads stationary and very slowly managed to loosen the nuts.

With power supply removed replaced the mains cable and checked out all the components including the three 500uF/50 volt electrolytics. Noticed that the schematic symbol for electrolytic capacitors show the -ve side as a narrow line and the +ve side as a broad black line, not sure if this is typical of Japanese schematics.

Powered up the power supply off load and checked all transformer windings AC outputs all OK. Checked the 3 DC output supplies, the 2 bridge rectifier circuits (-36 volt & 44 volt) were good but zero volts on the -20 volt output.

The -20 volt supply is supplied by a 2 diode full wave Metal rectifier (maybe Selenium) identified as Origin 12C-2. Initially thought the rectifier had failed but it measured OK both on DMM diode checks and forward/reverse resistance Avo checks.

The input to the rectifier is 60 volts from a multi tapped secondary winding of the mains transformer and looking at the schematic it has a zero volt centre tap which was effectively floating as not connected externally (external connector not connected). The schematic showed that the centre tap was connected to earth when the external connector is connected. So retested with the centre tap grounded and now get -44 volts for the -20V output which I think will reduce down when load connected.

All 3 DC supplies have very low AC mains ripple.

[DMcMahon]

Originally Posted by ms660

No values given on the schematic, it's three RC snubbers in one, connected across the repeat/reverse switch contacts on the schematic I'm looking at.

Lawrence.

A closer look showed that there are 4 leads, one common (all 3 snubbers connected together) and the other 3, the other ends of the individual snubbers.

Also now have seen them on the schematic. All 3 measured the same, a nominal 100nF with 90 Ohms ESR.

David

[Radio Wrangler]

Those values seem OK.

An Akai deck on a cast chassis with three motors and relay logic for tape movement. Just missing the direct-drive variable speed capstan.... Nice, though.

David

[DMcMahon]

After cleaning up outside of case (dirty lid still to be done) refitted the tape deck into the case. Cleaned up top of tape deck including heads and tape path.

Powered up OK. Without tape the tape transport all looks functional.

As originally reported by Pamphonica the left channel VU meter goes hard over when powered up and stays there. Found this only occurs when switched to Playback, if the left channel volume is turned up a little there is also a very loud horrible pulsing sound.

Swapped over the left and right channel Playback boards, this swapped over the fault to the right channel, although not quite identical, the bad board when in the right channel is a different frequency pulsing sound.

Swapped over the Playback boards several times and each time the bad board produced the loud pulsing sound in either the left or right channel. Found by touching/tapping the bad board could make the fault come and go. Eventually while doing this the fault currently has gone away.

A few minor faults noticed:-

1. Left channel volume control very scratchy at min end, will try switch cleaner.
2. The Reverse, Repeat and Shut-off switches are very reluctant to release when pressed after previously pressed to select. Not sure if just sticky or not mechanically unlatching properly. Looks like they are fitted to a small PCB, will have to remove the tape deck to check this out further.
3. Record lamp not illuminating when Record selected.
4. When speakers switched off, get a very loud horrible noise (even at minimum volume), would probably expect to get some sound, but this is very loud indeed.

Will try tape operation tomorrow.

David

[DMcMahon]

Originally Posted by Radio Wrangler

Those values seem OK.


David

I think the 90 Ohm ESR values for the 3 snubbers are most likely 100 ohm resistors.

I know from previous ESR measurements of serial CR snubbers that the ESR reported value is always lower than the actual stated snubber resistor value (when actual value is known).

I did a test yesterday measuring a new 100nF capacitor which measured as 100nF + 0.2 Ohm ESR. Then with a new 100 Ohm resistor connected in series, the series CR combination measured 100nF + 90 Ohms ESR.

David

[DMcMahon]

Briefly checked tape operation, overall all basically functioned but many things to recheck/improve.

First tried the special Akai Take Up spool Auto Threading, it did sort of work but not very well, not like my Ampex 1163 where Auto threading works very easily/well. For the Akai it may work better depending upon method used. The User manual shows the automatic spool on its front cover and lists the "Automatic spool" as a standard accessory but strangely there is no other mention of it in the manual or how to use it.

Auto threading is a misnomer really as one still has to manually thread the tape around the entire tape path, it is only the auto fixing of the end of the tape to the teeth on the spool hub that is automatic.

Playback worked on both speeds, had to waste time finding suitable tapes that had 3 3/4 and 7 1/2 ips recordings. As I said in a much earlier Thread I really need to make some reference test tapes covering 2 and 4 track, mono/stereo at the various tape speeds .

Even though Playback worked, the sound was somewhat disappointing, it did not sound quite right, a muddy sound to use a non technical term, at times the speed also did not sound quite correct plus now and again was getting noticeable bursts of crackle noise especially from the left channel. Need to do some test recordings on it and recheck. No more motor boating of the left VU meter/pulsing noise seen/heard (but I am sure it will return sooner or later).

Have not yet checked out reverse playback and the various repeat/reverse options.

Capstan shaft rotation is quite noisy especially at 7 1/2 ips.

Fast rewind and fast forward at times struggled (very slow) to get up to speed, but once up to speed the fast speed was very impressive, one of the fastest I have seen. After doing lots of tests noticed some small amounts of tape debris on tape guides/heads I think the final tape I was using maybe is a bit sticky, this may explain the fast wind symptoms. Will retest with known good tape.

Fast rewind is pretty noisy at times, noise coming from the Supply spool area, very difficult to judge if a noisy motor, reel table or something else. Fast forward also has the same noise but at a much lower level but once again from the Supply spool area. With no tape loaded fast wind operations are pretty quiet.

When Fast rewind or Fast forward is stopped the impedance roller takes a long time to spin down to a stop (normal) during this spin down it is quite noisy, manually spinning the impedance roller by hand it is quiet and smooth.

The track selector knob is very stiff to turn and I think that the mechanism is not working well for the 3 positions.