FT-901DM blowing capacitors

[kb3ouk]

This originally happened 2 saturdays ago, walked ino the shack and wondered what that smell was, looked and noticed it was off but the switches were still on. Bad sign. took it apart and found one exploded cap and many leaky ones. fast forward to tonight. Got replacement caps and installed them tonight. put all the boards back in (btw they were in the rectifier a and c units) and fired it up. It also blew the fuse when the caps blew, so i put a one amp, 250 volt fuse (which i figured should handle 2 amps at 117 volts) in it. Vaporized it as soon as I hit the power switch. OK, should have taken that as an omen. Took the fuse out of my FT-101 which was a 10 amp 125 volt fuse and put it in the 901, figuring i would catch it in time if anything decided to crap out. hit the switch again. heard a hissing noise that i figure was in the fan, shut it down and hit it again, this time without turning on the filament switch, which also controls the fan. hissing sound started to quiet down, then the thing lets loose this huge puff of white smoke from the rectifier compartment. hit the switch off before anything burned up or blew the fuse, tore it apart again and noticed one of the 350 volt 22 uf caps i had replaced in the rectifier c unit had bulged at the top ad the one resistor, R1809 i think, had a scorch mark on the side that i had noticed earlier but didn't think anything about since I could clealy see that the caps were most of the problem. So now I believe that the resistor is shot. The resistor is R1809, i just looked and put my multimeter on it in the 2000k ohm position and the numbers kept climbing till it past 2000k ohms. The bulging capis C1801 and the rectifier c unit is board PB-1717. Anyone have any idea what caused this to happen?
Shelby KB3OUK

[kb3ouk]

http://foxtango.org/FT901/FT-901-902%20Web%20Page.htm

Thought i had seen about this before, now maybe I have found my own answer or am on the right track.

[Pete, WA2CWA]

Generally, if a part blows in a power supply section, board, cage, etc. it's probably a good idea to check all the parts, even the ones that still look OK (diodes, resistors, regulators, transistors, etc.). Parts can go bad (burn up, blow up, leak out, etc.) on their own, but sometimes they have help from other parts of the circuitry.

[kb3ouk]

It said on that website that in those rig the caps and resistors in on that board, especially the ones I said about, do have a habit of failing, one guy even had close to he same probblem i have with R1809 burning up, probably since it follows all of the caps i replaced.

[WQ9E]

With caps failing again, also make sure you don't have a shorted rectifier.  With the suggested updates from FT it should be a reliable rig.  I also have an FT-901DM/FV-901 station along with its close relative the FT-101ZD and I did replace the caps in both rigs.  Not too bad a job and since then no problems.  It was definitely an easier job than replacing the relays in my FT-102.

Depending upon the age of your rig, you may also have the failure prone digital display.  If it fails there is a JA ham who sells a replacement option that is simple to install and reasonably priced.  I had to do this to an otherwise mint FT-107 station.

[kb3ouk]

The more i look at that bord, it looks like there is also a burned out diode on that board too. If i understand right,thats the low voltage  board for the solid state stuff and the other one is the screen supply fo the tubes an handles the bias also. no, haven't had display problems yet, but i wish this had the self calibrating  counter like they put in the later models.

[WQ9E]

Shelby,

The C board (18XX series components) also supplies screen voltage to the finals and B+ for the driver in addition to LV for the solid state components.  D-1801 is the rectifier for the driver plate supply.  Are you sure it was C-1801 that was bulging?  C-1801 is a smaller cap and not one of the electrolytic filter caps.  C-1802,03 and C-1804,05 are the 47uf @ 250V filter caps associated with this circuit.

Rodger WQ9E

[kb3ouk]

no, this was one of the two 22 uf 350 volt caps on that board (C1808 and C1809). The bulging one's positive side is hooked to R1809, the burned up resistor. C1801 isn'teven an electrolytic, so i guess i was wrong. too much stuuf leaked out of these things to see sme of the numbers. C1808 must be the bulging one. i discovered  something else lurking inside this thing. When i'd move it i  heard something rattling around inside. turned it upside down, took the bottom cover off and opened up the bottom of the final cage to find a loose screw from somewhere sitting on this big toroid in the final cage. wonder if that could have caused problems if it had fallen across something it could short out.

[kb3ouk]

I figure that while i have the rig on the bench does anyone know of any mods that can be done to  the transmitter and receiver to improve use on am?