Sanyo VM-4509 Monitor - Horizontal Coil (variable inductor) - Need help identifying replacement

[booboo]

I salvaged a non-working Sanyo VM-4509 9" monochrome monitor, very nice for the Apple II line. It came in parts, fully disassembled. It's working(ish) in that it has a good power circuit, tube, case, etc. However I get really bad video that is not adjustable with the horizontal adjustments.
After investigating, it turns out that the horizontal coil (variable inductor) is physically damaged. The metal square can (10mm on sides) is missing, as is the screw. The ferrite is broken and missing about 1/3 of the body. The ferrite turns and I can get it to display an image, but it's terrible with triplicate content.
The signal coming out post inductor is way off, showing 19.9Khz on what I would expect close to 15.6Khz.

*THE ASK*

If someone has the same monitor, I need markings and (wishful thinking) some measurements of the H between posts. I can measure the broken one but the readings are unlikely to be correct.

Here's a picture of the current inductor, PCB location is L601. The documentation shows it as Sanyo replacement number 4273106170, which I cannot locate.

Any guidance greatly appreciated.

Oh, and the inductor is a 5 post kind, 3 posts on one side, 2 on the other.

 

[Hugo Holden]

Usually with this sort of part an exact replacement is only found in a parted out set of the same model. But that is not the end of the world. There are some ways around it.

The original design had a ferrite threaded inverted cup like arrangement to screw over the central ferrite core. This gives a wide range of inductance variation because as it screws down the magnetic gap starts to close. Sometimes a new cap/cup can be extracted from a generic part of the same physical type/size, on ebay. Ideally it needs to be adjustable to set the H osc frequency correctly. 10mm is a fairly standard size, so the ferrite cup from a generic 455kHz transistor radio IF transformer would likely fit, these are on ebay. You would need to remove the remaining fragment. This sort of thing, ideally a Japanese part not an American part as the ferrite cup is more likely to have the same diameter & thread, but you might not know for sure until you get it:

https://www.ebay.com/itm/273543884278?

One other method is to remove it, trace out the schematic if you cannot get it, and look at the capacitor value which tunes the coil. Likely it will have a tap on it, possibly an auxiliary winding. If so a dignostic test with the scope and a signal generator can determine the ratios, and it can be worked out what the original inductance was supposed to be by looking at its tuning capacitance. And an new one can be made by using and rewinding a donor coil assembbly from a number of generic parts.

One other possible option is to glue a small block of ferrite or a slug to the top of it to see in you can get the inductance high enough to get the frequency down to 15.75kHz or similar. It won't be adjustable, but then you could trim that by adjusting the associated tuning capacitance.

 

[booboo]

Thank you @Hugo Holden. Indeed, I'm leaning toward option 1 and see if can find a similar size ferrite. Option 2 I'm afraid is beyond my understanding (I have the tools just not the knowledge...). If there's no additional feedback from the community, then it's time to start ordering parts. I agree when looking at eBay I found lots of 10mm coils from FM radios.

 

[retrogear]

I've attached the service manual (hopefully) which shows L601 as a horizontal oscillator coil and it has a center tap. That limits what can replace it.
The Sanyo part# is 4-2731-06170. The best would be take one out of a parts unit or maybe someone has done that already. Umm the file is too large
to attach. Anyone got a place to put this ?

Larry G

 

[booboo]

Thanks @retrogear. I have the manual, and I mentioned the Sanyo part in the original post; but that part number goes nowhere in the internet. Yes, the metal can and the center tap 1/2 is broken off.
A parts units will cost >$100 shipped so not going that route; hence why I'm asking here to see if someone has the monitor and could meter the coil out.

 

[booboo]

Well for those playing along at home, here's a picture of the victim (coil). I'm taken it out and removed the ferrite. However, it was more fragile than expected and how has completely failed. So a ferrite swap would not have worked and need to find a replacement (donor).
Or data from a working one!

 

[Hugo Holden]

As I mentioned in my post. The coil would likely have a Tap. Otherwisw if it was a plain inductor, its more difficult to configure it as an oscillator without having a tapped tuning capacitor arrangement.

Completely failed ? It looks like the wires are still there or has one of them sheared of where you cannot get to it ?

The main parameter is the where the tap is on the coil, because this sets the feedback ratio to suit the particular circuit. If I couldn't fix that coil I'd unwind it count the turns and count how many turns from one end that tap is.

Then any suitable replacement is easy to make. The DC resistance is not critical as long as it is not excessively higher than the original. As I mentioned you can work out the required inductance from the associated tuning capacitor. Then you can rewind any number of adjustable coils taken other equipment used as doners to get the correct total inductance and you also then would know where to put the tap.

This sort of thing is not an uncommon problem in vintage electronics with items like proprietary transformers, you have to be prepared to study the broken ones and make substitutes where required, its standard old school repairs.

 

[booboo]

There's only one wire in the winding connected, the other two broke off during desoldering from the board. The one still attached to a leg is the tap. The other two are just hanging off the winding now. Would measuing resistance help here? I don't think I get any H with what I have now.

 

[Hugo Holden]

If the two wires are hanging off just re-solder them to the pins around the pins base near the former. Normally, when one of these transformers is desoldered from a single sided pcb, the wires simply stay soldered to the pins because there are no forces to pull them off, which you might get with a double sided pcb. There was no indication to remove the transformer from the pcb while you where waiting for a new ferrite slug. The DC resistance values are not a critical factor, as long as a replacement part does not significantly exceed the resistance of the original. Measuring its inductance in its current state won't help much because the slug is missing and it will be low. It is better to calculate the required value. But, it could be estimated. You know from before, the oscillator was running at around 19.9kHz, and that that point some of the original slug remained. So you can re-fit the coil, re-measure the frequency it will probably be over 20kHz now with less ferrite there, and a calculation can be performed about how much extra inductance is required to get the frequency down to 15.7kHz