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Re: Capacitors in DB4332B and DB4333B
开云体育Alex:Thank you for these pictures. This is very helpful information. I have a Johanson catalog and tried to match the capacitors at least in physical size.? It did occur to me the capacitor values might be 0.8 to 10 pf. because I think that is the same value used in the DB4076 duplexers.? The capacitor in the 850 MHz LPF are the physical same size, but that is all I can say. It looks like DB broke off the little lug on the end since they soldered to the top of the capacitors. Your construction is very well done. The wire in the UHF LP is insulated magnet wire and is 1 mil in width and I figured that was 18 AWG.? The wire in the 850 LPF is just bare wire, but also 18 AWG. The coil in the UHF LP had been slightly pushed down in the UHF LP case and it made me wonder if that was done to change the value or to help the return loss.? But the wire looks a little long and they pushed it down to get it away from the lid.? Your neat work suggests that deformation? is not necessary.? If the capacitors in the 850 LPF are not the same capacitor values as the UHF, I did see some of the values you mentioned? on eBay.? I have two of those 850 Mhz LPF's and one looked fine on the spectrum analyzer, but the other did not. This makes me think some of the capacitors in the one with the bad curve are broken.? So I may be buying capacitors from the listing you suggested anyway. I will know if the capacitors are the same when I put the coil in the good LPF. The only difference I had was that I tried to measure the coil diameter and got it roughly at 1/4"instead of 7/32". I have some 18 AWG magnet wire here at my office and was going to wind it using a drill bit as a form.?? Thank you Jeff for your suggestion of using a single piece of wire. That sounds like a very good idea and I am going to try that.? I was not going to disturb the connections from the input to the first capacitor or the output to the last capacitor, but may go ahead and wind it all as a single piece as you suggest.? The coils for the 850 LPF are a single piece of wire zigzagged across the four capacitors. Thanks again.? Micheal Salem N5MS On 4/4/2025 11:28 AM, Alex wrote:
About 3 years back I had one DB4332-B UHF low pass filer but needed two, so decided to reverse engineer it and build a clone. The variable piston capacitors are Johanson 0.8 - 10Pf rated at 150V. Back then I found several sellers on ebay offering them, and there was one selling them in 4 packs for $25 plus some shipping. The trimmer cap model I used was PC25J100. The model is stamped onto the top solder side of the cap, so usually not possible to see in the original LPF. Just checked on ebay but original listing is gone, however looks like listing #256220267439 is the same guy still offering those trimmer caps in packs of 4 still at the same price. The 3 coils are made of #18 wire, one side being 2T, the middle one 3.5T and the last one again 2T. Coil diameter is 7/32 or about 5.55m, stretched out same as the original coils. I also managed to find on ebay someone selling the identical metal boxes (Pomona model 2413) for $10-15 new old stock in the original bag and with connectors on each end, but these instead had one SO-239 and one PL259, which both where very easy to replace with NM/NF. I then also had to drill the four holes to mount the trimmer caps and assemble everything. The clone LPF was then checked on a VNA and compared to the original DB4332 and it had basically identical characteristics as the original one, and of course also tuned up just fine. About a year later after that I needed two additional UHF LPFs and did build two additional clones that also worked perfectly. The attached picture shows the original LPF at the top, and one of my clones in the blue box at the bottom. |
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