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8008guy
June 3rd, 2018, 07:56 AM
Might anyone know anything about the little pulse transformer on the Cromemco 8K bytesaver? Who made the transformer or specs?

It's used in the step up inverter circuit for the programming voltage. It's listed as an XT8K on the parts list. I google and find no information. I'm repairing a Bytesaver that is missing the transformer.

Cheers,

len

8008guy
June 4th, 2018, 04:48 AM
Does anyone have a Bytesaver, any model, that they can look at the XT8K transformer and tell me who the manufacturer might have been?

Dwight Elvey
June 4th, 2018, 05:23 AM
Most such coils were wound to a specification. There would be a turns ratio, the core saturation level, inductance of the coil and wire size. I have a 8K board but it is in storage. I doubt you'll find it today. My guess is that it was a 1:1 transformer, used in a flyback mode to get the higher voltage.
Dwight

8008guy
June 4th, 2018, 11:58 AM
Thanks Dwight,

I'll start with a 1:1 and prototype up the circuit and see what I get out of the convertor.

Thanks.

len


Most such coils were wound to a specification. There would be a turns ratio, the core saturation level, inductance of the coil and wire size. I have a 8K board but it is in storage. I doubt you'll find it today. My guess is that it was a 1:1 transformer, used in a flyback mode to get the higher voltage.
Dwight

Dwight Elvey
June 4th, 2018, 02:05 PM
Thanks Dwight,

I'll start with a 1:1 and prototype up the circuit and see what I get out of the convertor.

Thanks.

len

I was looking a the schematic. It looks to have at least one separate winding and one center tap.
It might be easier to make a three winding 1:1:1 work. One might consider winding ones own on a core.
Dwight

deramp5113
June 4th, 2018, 03:27 PM
I looked at my Bytesaver II and the coil is labeled “CROMEMCO XT8K A”. So yes, as Dwight mentioned, looks like it was a custom wind for Cromemco.

Mike

8008guy
June 4th, 2018, 07:24 PM
Thanks guys. I have a bunch of cores around from when I was designing a switcher, although they are a bit large. In any case i should be able to wrap up something for a test.

Chuck(G)
June 4th, 2018, 08:12 PM
If you get nowhere on the transformer, you might consider substituting one of the little step-up adjustable switchers. They're tiny and can deliver up to 35VDC--and tiny and cheap.

Dwight Elvey
June 5th, 2018, 07:12 AM
Looking at the schematic, I'd say 1 & 4 are the feedback. 3 is the bottom of the output coil, 2 is the center ( may be offset towards 3 ) and 5 is the top of the coil.
3 and 4 could be swapped but it makes little difference.
Dwight

Terry Kennedy
June 5th, 2018, 07:13 AM
If you get nowhere on the transformer, you might consider substituting one of the little step-up adjustable switchers. They're tiny and can deliver up to 35VDC--and tiny and cheap.
Back in the day, I had a Solid State Music programmer. I was running dozens of EPROMs through the UV eraser and the programmer each week to support our Pro-Comp systems out in the field, as well as new orders. At some point, the programmer died. It had to go back to SSM for repair. Fortunately, I had enough already-programmed parts to hold me over until it came back. There was some TI switcher-on-a-DIP chip used to generate the programming voltage. Things worked fine for a while, until the same thing happened. I looked at the chip and it had melted - apparently I had a bad 2716 EPROM (out of hundreds). I wasn't about to scrap every EPROM, so I hooked up a nice Lambda 50A rackmount adjustable power supply (which conveniently fit in the rack below the TEI chassis that held my programmer) to the appropriate place on the SSM card. Programming went fine until a couple months later, when I tried programming an EPROM that came back and there was a loud "Blam!" as the glass erasure window blew off the part and went flying across the room. I'd found the defective EPROM. :eek:

Chuck(G)
June 5th, 2018, 07:23 AM
Um, isn't that what fuses are for? ;)

Dwight Elvey
June 5th, 2018, 09:11 AM
Looking at the schematic, I'd say 1 & 4 are the feedback. 3 is the bottom of the output coil, 2 is the center ( may be offset towards 3 ) and 5 is the top of the coil.
3 and 4 could be swapped but it makes little difference.
Dwight

Giving some more thought, the feedback winding might be from 1 to 2. The power winding from 2 to 3 and the output from 4 to 5.
That sounds better. The additional winding 4 to 5 would be to keep the flyback voltage low, on the transistor. The feedback is capacitor
isolated so pin 2 is essentially ground through bypass caps.
Dwight

Terry Kennedy
June 6th, 2018, 01:18 PM
Um, isn't that what fuses are for? ;)
You know the saying "A Transistor Protected By A Fast Acting Fuse Will Protect The Fuse By Blowing First"? That goes double for the TI switcher IC on that board. My solution prevented further disruptions to EPROM production and located the defective component, so I'll call it a success.