Restoring Toshiba Kt-RS1 - A Naive Endeavor

I now have five of them. I believe that the KT-AS1 is similar but does not record.
Except for the one which turned out to be heavily corroded, they each play the Radio-Cassette ok, and one very well, so I'm assuming a recap for the audio is not in order, at least not on the best one. There are two caps on the motor governor circuit. I bought replacements from Mouser (US Electronic parts vendor) The original 22UF 4 volts are 5mm long. MOuser has same physical size but 6.3V. I bought some in case.

Of course the "belt" in each of them has turned to sticky goo. It is believed that a new belt cannot be installed without a lot of soldering. I was able to do it with no soldering by removing the screws for the PCB and stainless steel mechanism back plate. One of these screws is secured with thread-locker. After a lot of fooling around with the first KT-RS1, I just drilled it out with a reverse twist drill. Sounds violent but after the dimple made by the drill gets close to same diameter as head of screw, it just unscrews. You could file a slot in it so you can use it again, or buy screws online or get it from a parts machine.

Once the screws are removed, the wiring which is lightly adhered to both the back-plate and the PCB can be gently lifted off so that the back-plate can be removed to allow access to the capstan pulleys, idler and the motor. They will have to be removed to clean off the goo. Be very careful to recognize the 2mm washers on some of these shafts so as not to lose them. The washer placement on the two I disaasmbled that appeared not to have been previously service did not agree with the manual. The washers on the idler did agree. Motor will have to be reomved to clean the pulley and lubricate and ths will require de-soldering its two leads.

ONce it's spearated from the chassis, and after cleaning up the pullley and trying to squeeze some oil to the upper bearing, you can hook it up to 4 volt DC and get some idea of its condition. One of the 5 motors sounded quite smooth and it is the one I'm going to use. It's possible t3 of the others could smoothe out with some exercise, but the one ion the corroded unit does not at all sound good.

If all you are doing is cleaning it up and replacing the belt, I don't think there's any reason to unsolder all the other leads. If you want to do a recap, though you'll have to. Best thing here is to take really good photos of where they are with your cell-phone. There is enough difference in the colors of the various leads that if you are careful you'll get them back where they belong.

Next: The service manual. I bought a pdf of the service manual online which turned out to be missing pages. Supplier went mute so was unable to get money back. I bought an original on Ebay and it is a very nice manual. Well drawn schematics and exploded isometrics of the mechanism. Not all the parts are detailed though especially the reel spindles and the pinch roller assemblies. Everything you could possibly need to restore the radio cassette is included. Based on experience with my decks, I recommend buying an original manual if you can find one and it isn't toopricey. Sometimes Hi-Fi engine has what you need although his are uploads from users and can vary in completeness and quality of scans. But then they are free.

Which brings me to my problem. The pinch rollers are very hard. They measure 7.41x6x5x4.5x1.5. I doubt that they started out at 7.41. I'm assuming new ones should be 7.5mm OD. Does anyone reading this know for sure?

None of the best pinch roller, belt suppliers has this size, nor OEM Toshiba roller assemblies.

Right now I have two sets on order from China and hope that I can get two towork.

Any ideas?

 

First of all, have the reel gears and retainers been checked ?
It's a common problem on these. Recently seen a NOS RS1 on TechMoan's channel that seemed to work fine just with a new belt, but think that's the exception not the rule.
This problem, apart from having low takeup torque and potentially eating a tape, will create a wow when the cracks meshes with the drive gear.

As far as rollers go, you need to find the closest one.
As far as the height goes, a roller with smaller height can be fitted with some nylon washers added, if no other roller with correct height is available.
If new roller hub is a bit taller than needed, note the hub can be filed/sanded to get the correct dimension.

Regarding your measurement of 7.4 x 6 x 5 x 4.5 x 1.5 it's not clear what these dimensions represent.
We only need 3 dimensions for a roller: outer diameter, height (complete with hub) and pin diameter.

7.5 x 6 x 1.5mm is available here: https://www.ebay.com/itm/315320218864 which seems to be the one you need.
There are also smaller are larger ones available, nowadays you can find pretty much any roller in China.
7 x 6 x 1.5mm is available here: https://www.ebay.com/itm/355777766346
8 x 6 x 1.5mm is available here: https://www.ebay.com/itm/234978993898

From what I've measured before, the measured OD is almost always a bit smaller than the spec value, so that 7.41mm can be safely assumed to be 7.5mm.

 

HI Valentin,
Thank you for your useful advice.

On the long pinch roller description, in order it is OD, Overall Height, Tire height, Hub flange OD, and ID.

PDF is drawing of what I think I need based on dimensions of present rollers. Obviously hub diameter is not critical, unless "crush" space is needed for deformation of tire.

I tried buying some of the 7.5x6x1.5's at Ebay and the vendor accepted the order then sent me a note saying because of the uncertainty of tariffs on de minimis shipments to US he'd like to cancel. I agreed. It looks like de minimis tariffs on anything other than a list which gets higher ones, its 54% for things shipped by Post and 30% for things shipped by DHL and its competitors.
I ordered a new set and so far it appears to be underway.

As to OD, becaue the RS-1 is reversible and has two capstans the feed side roller is held away from the capstan so as not to drag. This means that the diameter cannot be much more than what I have now at 7.41. I'm pretty sure that 7.5 will be ok. I'll find out.
There is a youtube video of a guy in Korea rebuilding one of these. He greased the gears. Do you think that's necessary?

Thank you again for your help with this.

 

Ok, understand now about the dimensions. Thanks for sharing the drawing !
Tire height and hub flange OD are only important in case only the tire is replaced. Something I don't recommend as the assembly will usually become eccentric after.
These rollers are cut in the lathe to make them perfectly round, this may be the reason why the measured dimension of OD is usually a bit smaller than the spec.
Wouldn't be surprised if that new roller will measure less than 7.5mm OD.

Yes, fact there needs to be clearance between the non-engaged roller and the capstan does limit the OD.
That clearance should be a bit higher than 0.05mm (0.1mm/2 - since it's only the radius of he circle), so even if real dimension is 7.5mm it should still work.
Then there's also the option to use a 7 x 6 x 1.5mm instead, if the original is indeed 7.4mm (which seems unlikely, but is possible).

As far as greasing the gears, no it's not necessary. If the reel gears are intact, no need to do anything else in that regard.

 

Hi Valentin,
It is difficult to measure clearance between roller and the feed side capstan but it llooks like 1.1 mm which doubled and added to my 7.4 present OD would get a roller with no clearance of 9.4mm, so even 8mm OD MIght work. I have some of them on their way so I will be able to try this next week.
I have a small lathe with which I can true them up or reduce the diameter a bit more. I'll need to make a mandrel to secure them, but that's not a big deal.

best regards,
John

PS ABOVE IS NOT RIGHT. It's true that the clearance on the feed side of the system might allow an 8mm pinch roller. The takeup side doesn't have this much clearnce so likely nothing bigger than 7.5mm OD will work. Takeup side i where the capstan which moves the tape past the head is.

5/24/25 PS2. I installed 8x6x1.5 pinch rollers and they work fine.

5/25/25 PS wrong again. Althought the 8x6's work ok in play, they are too big to allow clearnace needed for FR and RW, so now I wait for the 7.5's to show up

 

Being one of Techmoan's patreons I know that his went horribly wrong, and started chewing tapes. What you see in the video was the best it ever worked for him.

 

Lognman could you post a link to this video. I have 5 of these things and not one of them has any problem with the gears. problem after belt to goo, seems to be pinch rollers. all electrolyutics seem fine.

 

Thanks for pointing it out ! Was actually surprised when I saw the video, but thought maybe he was lucky. Now it becomes obvious his also has the cracked gears problem.

Have you actually checked the reel gears ? Ask because these are not even visible without a full teardown. I'm reffering to the small gears that are attached to the reel shafts, not the big gears (which are fine).
Unfortunately can't find any video showing the repair and don't have any thread showing that either. Intend to create a thread covering this repair at a later date.

 

I've checked the gears on two of them and they look ok. The one that I'm trying to fully revive first seems ok, but I didn't take it apart enough to look at the gears. It does have a sound if you hold the case up to your ear while it's running which seems to have about a one second period (also shows on the T100 W&F meters.) I suspect it is the larger gear. I need to figure out what its RPM is.

Are the gears about the size of a US Quarter (26mm +/-) which are on the reel shafts the ones known to crack?

 

Managed to find some pictures of an older repair of KT-AS1, which may be enough to post a thread later on.
You can see in picture attached the gears I am reffering to circled in red. Even the crack is visible on the white gear circled in yellow.
It's also the reel retainers that crack, see second picture.

 

You were right Valentin, The small gears on at least the reel hubs on one of my RS-1's are cracked. Hard to see, though.
What can I do? Glue them with IsoCyanurate glue?

I think I found a source which is selling them for Toshiba KT-VS1 to wit:
https://www.ebay.com/itm/316578741510

I downloaded the VS1 manual and it looks like at least this part of the mechanism is the same.

I don't need new cap-locks (part that holds plastic spindle in place, but new gears would be good. It does look like these gears are sued only for FF anf FR, so If I don't need to do that, I can leave them in place. the 8x6x1.5 pinch rollers are showng up today, sio If they firt, I'll be able to complete the first machine assuming no problem with the reel-spindle gears.

I thinkkI'm going to order a pair for the gears from China just in case.

 

The only way of repairing is enlarging the hole with a drillbit so it's no longer a press fit, then glue the gear to the shaft while keeping it clamped.
It's important to glue it at the right height, otherwise the reel assembly can sit too low or too high causing problems with autoreverse or FF/REW.

Same goes for the retainers, but those are even more tricky to repair since the hexagon is a bit eccentric and you must keep them in the exact position they are initially installed.
Also, the reel itself must be able to slide up/down that hexagon. Adding too much glue can seize it for example.

The best course of action is to replace with new gears, didn't know they were available.
And unfortunately no the gears are not just for FF/REW, but also for playback. There's a small hexagon that meshes with the upper drive gear which drives them in PLAY.
If these gears are no longer a press fit, they will slip on the shaft causing low takeup torque in PLAY and eating the tapes.

Also pretty sure both of them are cracked, despite on the black one it's less visible. Same goes for the retainers, they may look good at first glance, but they're more than likely cracked as well.

 

Hi Valentin,
At least my hex cap-parts are not cracked.

 

The 8x6.15 rollers arrived, installed and work fine, nothing tricky about clearances. Plays very well in one direction, not good in the other, could be cracked gear.

PS 5/25/25 They played ok but were too big for clearnace needed for FF andf RW, so now wait for 7.5x6x1.5 to show up.

 

HI Valentin, The cracked gear repair: Do I understand this correctly?

you remove the smaller gear from the shaft, clamp it together to close the crack, drill it a bit larger, probably same diameter as the major spline diameter on the shaft, then checking first to make sure it slides onto the splines you leave it clamped, slide it over the splines to which you've already applied suitable glue being careful not to get any glue where it will cause problems.

When the glue sets, remove the clamp, slide the larger gear on and make sure it moves smootely on and off the small gear, replace the spring and the c-washer.

Is that it?

 

Yes, drill it so it's same diameter as the major spline diameter on the shaft, can even be a bit larger since the glue will fill the extra space.
Idea here is to make it so when clamped to the shaft, the gap created by the crack goes away and distance between any 2 adjacent teeth is the same.

Glue I used for this repair is liquid super glue, but other types of glue may also work. Would avoid things like 2 component epoxy though, since it's too thick.

After the glue cures, clamp can be removed and top drive gear, spring and washer can be installed back.

 

Valentin,
You apparenlty have found that the small parts with the hex which mount on top of the spindles also crack. Mine seem ok. It looks like their purpose is to drive the outer spindle frm the shaft with a somewhat flexible connection to absorb any lateral movement in the cassette hub. I'm getting two new ones with the new gears just in case.

 

Please add a link to the new hex reatiners on the thread, for reference.

The up/down movement of the reel itself (the part that engages with the cassette hub) is present on any cassette machine.
This movement is there to allow the cassette to be inserted even if one or both reels are not aligned with the hubs.

The lateral play these reels have doesn't serve any purpose, it's actually a problem when repairing these hex retainers because that play can introduce eccentricity in the reel if not aligned properly.

 

Valentin, here's the link to the toshiba KT-RS1 (among others) gears:
https://www.ebay.com/itm/316578741510

PS: is there any way to change the title of this thread?

 

My mistake I didn't pay attention, now I realise there are 2 gears and 2 retainers in the kit. So this is all that's needed for the repair.

Title can be edited using the Thread Tools button at the top of the thread (see picture attached).