Dentron DTR-1200L repair help!
I inherited a Dentron DTR-1200L and when I power it up and attempt to transmit several things happen. first my rig shows swr meter is pegged to the full scale! I am driving the amp with a JST-245, and because of the swr, the power foldback circuit engages, reducing my drive to less than 20 watts. Also when I was working with it the other day the toroid on the input tune pcb was very hot to the touch. On the advice of a local ham I removed the plate caps from the pair of 572b tubes and the swr in transmit is now at 1.2 to 1. Of course, as soon as I reconnect them and attempt to transmit, we are back to full-scale swr reflected! I am speculating that the "doorknob" cap from the plate caps to the output tank circuit may be open? or their are two .01 ceramics that isolate the filaments from the input in parallel that may be shorted or open.....? I am really at a loss where to look. the B+ is at 2200 volts just like it should be the filaments are good. I swapped tubes with two brand new Taylors and got the same results. I also notice that the tuning of the outputs is a bit flakey (not as responsive as I would expect), but that could just be a result of the low output. I also did have an issue with the changeover relay humming, not chattering just humming about 3 out of 4 times it engaged. I think I fixed that, cuz it does not happen any longer (burnished the contacts) Anyway I have given you all (I hope) the pertinent info, can anyone out there help? PS If I get this amp working I am also trying to track down the information about the 10M add-on kit. Thanks hope I can get some help. Bill KA9GNG my email is firstname.lastname@example.org
Okay, first thing to do is put the amplifier on a dummy load. The input of the DTR-1200L uses tuned circuits for matching. You need to check all the bands and see if the problem is one band or all bands. If it is all bands then check and clean the band switch. Remember you are opening up the amplifier and that will expose you to voltages that can be fatal. So make sure you unplug the amplifier from the power source and wait about 30 minutes before opening the case. Then take a long insulated rod and tie a metal rod to the end of it. Make sure you have at least 6 inches of insulation between the rod and where you will be holding it. Now clip a jumper wire to the chassis, make sure you have a good ground connection. Clip the other end of the jumper wire to the isolated metal rod. Now using the isulated end, that is away from the metal rod, touch the metal rod to the capacitors, plate choke, plate caps and tank coil. Just about everything in there should be contacted to make sure you have no stray voltages present. After you have made sure all voltages have been discharged take the jumper that is connected to the metal rod and remove it from the rod. Place the jumper on the plate choke or plate cap connections. Leave it there while you clean the switch and check other parts. While your in there check the continuity of the T/R relay. Make sure it is making good contact. After you have completed your repairs take the jumper off and put the cover back on. Now you can power up the amplifier and see if that made any difference.
The 10 meter add on kit is a rare animal and just about unobtainium. You may have to develop the circuit yourself. It could be fun, maybe.
Hope this helps
Last edited by KO6WB; 04-03-2012 at 06:57 AM.