Hey, thanks for the tip! I just checked the vertical ceramic load resistor and it is open, just like yours. Last night I decided to just replace all the electrolytic capacitors under the chassis (and the diodes as well). Before clipping out the old parts, I took voltage measurements so I'd have a baseline to test against after replacing the capacitors. The HV seemed unusually high at 854 vdc. The open load resistor probably explains that very HV.
Thanks, Tom On 12/12/06, Donnie Garrett <[EMAIL PROTECTED]> wrote:
Tom: I have one of the early AC3 supplies that used a HV choke. Garey K4OAH and I had a lot of discussions about its design some time back after I discovered the ceramic 25W load resistor was open. You better check yours to see if its OK. It will still operate but the no load HV will float very high with an open load resistor. I was never able to find an exact replacement at the time I needed it so I installed a 25W wire-wound resistor in a vertical fashion with a small bolt running through it. Somewhat similar to the original setup. I choose a 25K 25W instead of a 15K to reduce the heat dissipation produced by the resistor. The 25K still loaded down the HV and was near the same as was prior with the original 15K 25W resistor which seems to regularly fail due to running it at or near it's maxium wattage dissipation. Also one last thing, If I recall the HV winding on this early choke version AC3 was higher than the later AC3 and AC4's that didn't use the HV choke. Seems I temporally jumped across this choke just to see what effect it had on the HV and if I recall correctly the HV jumped up in voltage considerably, and was then out of speck and was near the max voltage ratings of the 2 HV caps. (No Good) I would appreciate your findings on the HV secondary windings if you don't mind. Too put it quite frankly I don't care for the early model AC3 choke type supply due to these issues. The only nice thing about this model is that all the caps mount on the underside of its chassis. This AC3 supply now serves as my test bench supply and is not used in my station. Hope this helps, Don / WA9TGT On 12/11/06, Tom Taylor <[EMAIL PROTECTED]> wrote: > I have a number of Drake A, B, and C twin combos. I decided to upgrade all the power supplies with AC-4R boards. Since most of the caps are now 30 to 40 years old, I don't want to risk a bias supply going out and ruining the finals or some short ruining the transformers. I order a bunch of blank boards since I already had some of the parts. The rest of the parts I ordered from DigiKey. I assembled all the boards at the same time and then started upgrading the AC-4s, one by one. > > So far, all of the power supplies are different in one way or another and all of them have been modified by previous owners. Here are some examples: > > #1) A previous owner replace the mid-voltage supplies' capacitors with a bundle of three caps mounted to the upper end the supply. > > #2) A previous owner replaced the sand resistor below the chassis with two higher wattage resistors mounted to the upper end of the supply. > > #3) A previous owner replaced one of the diodes in the HV supply. The transformer in this supply has transformer bolts inserted the wrong direction to mount the AC-4R board. Unfortunately, it's not as simple as removing the bolts and inserting them in the other direction because the bolts are just long enough to reach, but not long enough to add the board spacers. > > #4) This is an AC-3, serial #101. I've never seen a Drake power supply like this. There's a large choke mounted adjacent to the transformer. All of the other parts, including the capacitors, are radial lead caps that are packed underneath the very low chassis. Has anyone else seen an early AC-3 like this? I'm planning to remove the choke and rebuilding the supply just like all of the others since all of the circuitry is now on the AC-4R board. > > Thanks, > Tom N7TM > -- WA9TGT / Don Garrett / Muncie, IN "Unique Radio Parts" www.wa9tgt.com
