I've a Zenith TTL SLR camera with loads of lenses

Most brands of camera's do have own hot shoes for the flash light. That is because they al have a different protocol for starting and stopping a flash, when the camera should measure the amount of light coming back. (TTL or through the lens)
And perhaps, to keep costumers to their own brand?
I found my Nikon commander is capable to trigger my canon flash too, but only to start it not to stop. so the flashes accept commands from more than one brand.

None directly. The T90 was the last Canon camera to use the older FD lenses. All newer Canon SLRs, film and digital, use the EF mount. However, by using an adapter such as http://www.rakuten.com/prod/adorama-canon-fd-lens-to-eos-body-adapter-with-correction-glass/216466533.html?listingId=120668308, you can use the old lenses on any Canon digital SLR.

With the proper adapter, Contax-mount lenses may be used on several different makers' digital cameras, including Canon and Sony.

It sounds like the mechanical lever in your camera that normally couples the aperture to the camera to lock it to the widest opening for focusing is locked into the fully closed position, or that the electronics are not allowing the lens to open to fully open, depending on the type of lense being used.

Also, it could be an issue with the lense.

The simple way to test this will be to remove the lense, and to rotate the aperture ring while looking through the lense. If you see the aperture getting larger/smaller the lense aperture is fine. If it is not - the lense is damaged and needs to be replaced, or at best serviced. This can be done through Nikon, a local shop if you have one, or through online shops, I'd recommend KEH Camera in Atlanta if you have to go that way.

To test the camera side, if the lense appears to be working is also simple for the mechanical lever. With the lense removed, hold the camera in front of you looking into the lense mount. On the left side, you should see a small lever like piece. With your finger, see if this piece will move downward - it is spring loaded and should smoothly go down and pop back up. If it doesn't go down, or doesn't spring right back up, then the issue is on the camera mechanical side.

If both of those check fine, you may try a manual focus Nikon mount lense if you have access to one - to eliminate the camera electronics from being an issue, and the lense from being an issue as if this lense doesn't work, it's a mechanical issue. If you have access to another AF lense you know is working, and it doesn't work on your camera, you've narrowed it down to either the mechanical lever, or the electronics.

Most likely culprit is the lense itself.

On these cameras, if you're using ANY of the auto modes (P, A, S on the dial), the lense must be set to the lowest aperture setting (the highest number). Check to make sure this is the case. If it isn't, that's most likely your issue.

If you're using a non-CPU lense, this camera can only be used on M(anual) mode on your dial. If you haven't changed lenses this isn't likely an issue. If however you're using a new (to you) lense, this is a possibility.

According to the manual, the only other possibility is that your attached flash isn't set to the proper setting when using "P" mode. In this mode, the attached flash MUST be a Nikon dedicated flash unit set to TTL mode, otherwise fEE flashes with a little lightning bolt symbol. If you're not using a flash specifically designed for this model, remove it and try again. If you are, cycle the mode button on the flash until it reads "TTL" on the flash's LCD panel.

The manual makes no mention of fEE (error) and the battery indicator blinking, so these are the only things I can suggest with the information given.

The only place to get them is by obtaining other complete models as spares donors. Spare parts are long discontinued, and as most Zeniths are completely without resale value and there are so few remaining users nobody is able to make a viable business out of breaking them for spares.

You don't have to pay for a donor, almost every Zenith and Praktica I've ever owned has come from my local FreeCycle and Freegle groups and the rest cost almost nothing from boot sales and charity/thrift shops.

If you have Ricoh lenses that do not have electrical contacts, you can use the lenses on Pentax digital SLR cameras.
If you have Ricoh lenses with electrical contacts, you can not use them on Pentax digital cameras.

Quantaray appears to not be a company at all It is affiliated with Ritz Camera and neither Quantaray nor Ritz actually manufacture anything.

The Quantaray SLR lenses are made in the Sigma factory. But many feel that they are not equal to the Sigma branded lenses.

I haven't found the maker of the flash units, yet. I should learn more this evening. An interesting mystery.

In here, you can find the User Guide for Quantaray 900 for Nikon TTL Flash

Good luck.

You are able to use your old OM lenses with a new Olympus digital SLR camera. You will need to purchase an MF-OM 1 lens adapter to be able to connect these lenses to the digital SLR camera. Also when using these lenses you will only be able to use the camera in manual focus mode as these lenses are not capable to shhot with an auto focus mode.

Generally, the lenses used with the Olympus cameras will not be compatible with the Nikon cameras.

However, there are some converter adapters available in the market. You can look for the Olympus to Nikon converter adapters. These adapters will help you in using the Olympus lenses with the Nikon cameras.