inductor help needed - a 220uH inductor for use at 10MHz to repair an HP10811 - Page 1
Forum section: electronics / repair

author: jpb
author_rank: Super Contributor
timestamp: May 18, 2019, 07:08:09 pm
content: After much fiddling around I've deduced that my HP10811 I'm trying to repair has an inductor (L7 for those who have the manual) that has gone open circuit.It is 220uH :L7 9100-2280 5 INDUCTORRF-CH-MLD 220uH 10% .10SDX.26LG 28480 9100.2280Simple I thought, I'll order a few from digikey.Unfortunately almost all the through hole inductors of 220uH on digikey have a self-resonant frequency below 10MHz and I'm guessing that rules them out.The only one I could find with a higher self-resonant frequency was too physically big. (The lead spacing on the original is 10mm approx with the body length of 6mm approx).The original part doesn't look particularly special so I'm surprised it appears hard to find.

author: bob91343
author_rank: Super Contributor
timestamp: May 18, 2019, 07:19:25 pm
content: You could put two 5 mH inductors in series. They each have higher self resonant frequency.I don't know what your open inductor looks like but have you tried to repair it?Wrap the inductor in a wire and measure the capacitance between that and each end of the inductor to deicde if the break is nearer one end or the other. Most breaks occur at the ends so that gives you a start where to look.You could also wind a new coil. No reason that wouldn't be just as good as original, if not as pretty.

author: bob91343
author_rank: Super Contributor
timestamp: May 18, 2019, 07:27:24 pm
content: I just looked at the diagram and it appears the part isn't critical at all. Even if self resonance bothers you, try the part anyway. There is a resistor in series that prevents its impedance from screwing up the circuit.

author: jpb
author_rank: Super Contributor
timestamp: May 18, 2019, 09:46:03 pm
content: Thankyou very much for your help.I don't have any 220uH inductors at present so I need to order one from digikey but I'll just go for one that will physically fit. Since there is a big overhead (minimum order size) with digikey I might try a selection and see if I can physically fit two but I think space may be an issue (the HP10811 is quite tightly packed).The existing inductor just looks like a through-hole resistor and is quite small, I don't think there is any hope of trying to repair it.The other thing I thought of doing was to reduce the inductance and adjust the parallel capacitance for the same resonance. C17 is 10 pF but there is also the C23 bypass capacitor which is 120pF not to mention C24 with sundry resistors so it gets a little complicated.As I'm ordering stuff I will experiment with a 100uH inductor and a 22pF capacitor for C17 making the assumption that L7 and C17 are the key components.

author: bob91343
author_rank: Super Contributor
timestamp: May 18, 2019, 10:45:28 pm
content: If that inductor is so small, it may have a high self resonant frequency but I doubt it is that much inductance. Perhaps it has a magnetic core.You could probably wind a suitable part using a piece of ferrite for a core.However this turns out, it will be some kind of education and for sure a lot of fun.

author: jpb
author_rank: Super Contributor
timestamp: May 19, 2019, 01:06:55 pm
content: I realised over night that my plan of using a smaller inductance and more capacitance would mess up the impedance at 10MHz (the resonance seems to be set for ~20MHz).I think probably I'm worrying too much as almost all the inductors seem to be 220uH so presumably it wasn't particularly tailored for that part of the circuit (though I presume the 10pF capacitance value was picked to match it).I now have another expensive digikey order to go - in order to get a few cheap inductors I have to spend £33 ($50) + 20%VAT or else I get charged £15 + VAT postage!But it is educational and fun to fix things - I'll feel very pleased if I can get my HP10811 producing a nice sine wave again.

author: KhronX
author_rank: Frequent Contributor
timestamp: May 19, 2019, 08:43:52 pm
content: Have you considered Mouser?Yes, they're from the US as well, but they seem to have some "branch" in France, so ex.VAT purchases ARE possible, if you have a company / VAT numberAlthough they do also have the 50eu threshold for free shipping (but there might not be any customs concerns either way..?)... Or there's Poland's TME, who have a flat (and quite affordable) shipping fee.Quote from: jpb on May 19, 2019, 01:06:55 pm

author: jpb
author_rank: Super Contributor
timestamp: May 19, 2019, 09:12:07 pm
content: Thanks, but yes I have bought (and do buy) from Mouser and TME and from Farnell and CPC and RS and mini circuits, we are lucky in the UK to have a lot of choice now compared with say ten years ago, but as I'm not a company (it is hobby electronics) I have to pay the VAT. Digikey tend to be the cheapest out of these and have the most extensive range together with free next-day delivery as long as I reach the threshold - the thing to be careful of is ebay as components on there are often much more expensive than say Farnell - in fact various rather dodgy sellers seem to sell Farnell stuff at a 40% mark up on ebay, I'm not sure how they get away with it.

author: bob91343
author_rank: Super Contributor
timestamp: May 19, 2019, 09:59:31 pm
content: It seems to me that the biggest ripoff is Amazon. In every case I can find the same stuff cheaper elsewhere. At this point I do not consider Amazon as a potential vendor for anything.Having said that, they do have an almost limitless variety of merchandise, and sometimes I will look there just to get an idea of what may be available. If I needed something and price didn't matter, maybe I'd buy.

author: jpb
author_rank: Super Contributor
timestamp: May 22, 2019, 08:43:10 pm
content: The new inductor arrived today and I fitted it this evening (together with a replacement capacitor but this wasn't needed really) and adjusted the AGC and now have something resembling a sine wave of the right amplitude - very pleased even if I have spent a bit much on digikey orders. It is a pity it is only the 106111 model.Photos attached.