hifi engine

RT-909 Capstan Speed Drops

AlexGomez's picture

Hello everybody, I have been a happy owner to a nice looking exaple of the well known RT 909 for the last 13 years. It has performed flawlessly over all those years (after replacing the belt, pinch rollers and the infamous Q214 and its sidekick). Even though control board B showed the signs of extensive work, it gave me no trouble... until last month. Capstan motor started to slow down every now and then... until one fine day it slowed down to zero and wouldn't come back up. I went into it and found a toasted R29 on the capstan motor board. Replaced it. Worked for a couple hours. At thah moment I figured R29 was only a victim, and after some research, I replaced Q214 and its partner. it started running, but only on reverse mode. Found a video on youtube which pointed to Q210>Q213 which I suppose do the switching. Replaced all 4. Worked, but a little unstable in fwd mode. I ckecked voltage output on posts 24 and 25 on both modes. in fwd mode, I get 24v from 25 and 10v or so on 24. In rev mode, output voltage readings get inverted. I noted that when in fwd mode, voltage in 24 fluctuated around 12- 15% up and down, randomly. Replaced Q205 and 208. got a little improvement. After working for several minutes, it was running perfectly... Checked voltages...24v on post 25 and 10v on p24... let it run for 30 min and checked...24v on p25 and 6v on p24. one hour later speed dropped. I got 24v on p25 and 2v on p24 and this inverts perfectly when i put rev mode. No toast resistors or zener diodes. Only 3 or 4 caps that looked suspicious were replaced. All 4 drivers get hot after a while of being in usebut no heatsink so I'm not sure how hot they're supposed to get. Any help would be greatly appreciated.



AlexGomez's picture

Re: RT-909 Capstan Speed Drops

Played a couple tapes... capstan motor got hot and speed dropped a couple times. This morning wow and flutter appeared. Took capstan motor apart a couple hours back, cleaned up the commutator and lubed the bearings. I was putting it back together when I noticed that the solderings holding the coils and commutator together were chipped. Re-soldered all and I'm testing it right now. Both motor and control board run much cooler now. I'll run it a little bit more today and tomorrow after work and see how she does.

Re: RT-909 CAPSTAN SPEED DROPS

Did you check/replace the electrolytic capacitors on that board?

AlexGomez's picture

Re: RT-909 CAPSTAN SPEED DROPS

Just replaced ALL ELECTROLYTHIC CAPS on both control board b and capstan control board. most of them looked funny and kind of moist on the bottom. Voltage now looks nice and steady. I'll put a 3 hr tape in it and see how it goes :)

AlexGomez's picture

Re: RT-909 CAPSTAN SPEED DROPS

Thanks a lot, Johnnysan! I so far I have replaced 7 of them. 3 or 4 looked kind of moist on the bottom. one of them is a 100uF that is linked directly to the two little C1740s controlling the direction drivers. I replaced those two as well... felt like a good idea. Ran a short test and the 10v on p24 are now steady. I'll run a short test and replace the remaining ones later after I get them :-)

AlexGomez's picture

Re: RT-909 CAPSTAN SPEED DROPS

I replaced a couple 13 years back. I guess it's time to replace them all... I already started to replace some of them. I'll keep you posted.