Tektronix 221 T633 repair question (revisited)

[5065AGuru]

Working on a Tek 221 miniscope. Works on battery but no charge when plugged in. Did some troubleshooting and did replace a bad DIAC but still no go. Finally removed T633 to test. (Windings Ohmmed out OK in circuit) Measure the inductance between pins 1 and 3 and got 6uH, several notes online indicate it should be around 5.7mH and if bad the ferrite core is probably cracked. Anyone have any idea of what kind of core to replace it with?

Cheers,

Corby

[squadchannel]

https://w140.com/tekwiki/images/9/9c/Transformers_120-0500-00_to_120-0998-01.pdf

P 18x11 size core
Does anyone know the types of ferrites?

[5065AGuru]

Thanks for the document!

I've decided for now since I'm keeping this one for my own use to just remove the batteries, and run the scope from a 12VDC wall wart. I found one in my junk box (a netgear 12V 500mA regulated unit) and wired it in. Works just fine.

Cheers,

Corby

[5065AGuru]

Did some more thinking of building a replacement.

The data and physical measurement show an 18x11mm core size which I see a lot of when I look.

The doc gives winding info.

But how to select core materiel and will there need to be a gap?

Cheers,

Corby

[zike]

It's been some time since I worked on these, though I now have 2 sick ones in the queue. I reckon T633 is the "easiest" of the 3 transformers to match, as T623 has to be tailored to regulate by variable DC saturation and T701 has to run all the CRT HV.

The drawing you posted indicates 3.8 to 4.2 mH for the primary, which is 100 turns, so about 400 nH /(turn)^2. Maybe start with that and your overall dimensions; I didn't do an exhaustive search but this gapped N48 pot core looks sort of in the ballpark? Unlike the other two transformers I don't think this one is very critical for, e.g., magnetic losses etc.

As I recall both my previous 221's had other issues due to battery leakage, and I ended up fitting a miniature 2 watt 120VAC - 12VDC switching power supply, plus a one-transistor current source to charge the battery. This eliminated T623, T633 and all associated circuitry, and easily fit in the well between the batteries under the CRT neck. I believe I also fited NiMH batteries in place of the original NiCd packs.

Edit: here's the thread on TekScopes from 2012, and the schematic.

[5065AGuru]

DigiKey has this one that is the correct size and ferrite for the frequency involved. I believe it is Not gapped. Would I really need a gapped core for this application and if so how much gap?

https://www.digikey.ca/en/products/detail/ferroxcube/P18-11-3C91/7041585

Cheers,

Corby

[zike]

I'm not an expert but I would think you'd need to match the target primary inductance. If I read the datasheet correctly that ungapped core has about 10x too much inductance per turn-squared. I'd be concerned this could lead to saturation, hysteresis, overheating etc.  There may be a variant with the appropriate preset gap. Alternatively you might introduce a gap yourself to "tune" it.

I have done this with ungapped pot cores by introducing plastic shims or layers of tape between the halves, but this was in a signal application, not power (though the very low power here is admittedly a grey area).

[5065AGuru]

Well, I've been using the 221 using a 12v wal-wart and no batteries with no problems. Bought a pot core and accessories from digi-key and rewound it per the Tektronix build notes. I used a 1779-1125-ND core set, a 1779-1287-ND clip, a 1779-1365-ND bobbin, and a 1779-1439-ND spring. Wound the core per the instructions and after trying several adhesive tapes found one that gave a spacing to provide the pin 1 to 3 inductance from the instructions. Installed it with a nylon screw for tension to test. That looked good so installed the clip and removed the nylon screw. Reinstalled and still looking good with the proper battery charge current. Now working on AC or battery! One aside, while working another 221 the low voltage regulator was dead. Ohm checked the transistors and they all looked good. Removed the 2N2222A and it Ohmed out OK. However plugging it into my transistor checker showed no HFE! Replaced with a new transistor and working now!