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    Mark III, again

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    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Mark III, again

    Post by wildiowa on Tue Sep 04, 2012 9:06 am

    After a few weeks to regroup I am ready to dig into my Mark III rebuild again. So far I have replaced all caps on the board, the bias caps and the rectifier. I wanted to see if I could get it running without replacing the multi section cap (I know, this should be the first thing to go) and tried to fire it up to test the voltages, but the fuse popped every time so I put it aside until I could get the good higher voltage cap from Dynakitparts. It should be here today and will begin this project again.

    A couple of points...this is an old timer, maybe one of the oldest I have seen, perhaps from the 50's or certainly early 60's. Also when I first replaced the bias caps I had the polarity backward and flamed those rather quickly on start up, not sure if it did other lasting damage from that incident? They have now been replaced and are the correct polarity.

    So, today I will replace the multi cap. From what little I have provided do you guys think this will get me up and running, at least to start checking voltages? Any hints or thoughts before I take the plunge? As always, thanks for the help...


    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mark III, again

    Post by Bob Latino on Tue Sep 04, 2012 9:48 am

    The popped fuse could be caused by either the quad cap OR a bad rectifier tube. If the fuse pops after you replace the quad cap, try a new rectifier tube ..

    On those bias caps > The bias system runs a NEGATIVE DC voltage and the caps are (supposed to be) put in with the positive side of the cap connected to ground. This usually bothers people but that's the way they are supposed to be installed.

    Think of the bias system like this. Compare it to someone walking a baby carriage downhill. The baby carriage going smoothly downhill being held back by the mother is the amp running. The bias system is like the mother's arms pulling backwards to control the baby carriage's descent down the hill in a smooth and controllable manner. If the mother lets go (no bias!) - well only bad things can happen. If you lose the bias system (you blow the 15.6 ohm cathode resistor for that channel, blow the diode etc.), there is no CONTROL of the output tubes and you get a "runaway bias" situation, the two output tubes on that channel will draw a large amount of current, the plates of the tubes will turn red and the output tube(s) or rectifier tube may self destruct. Hopefully the fuse will blow before you actually do lose a tube ....

    Bob

    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Re: Mark III, again

    Post by wildiowa on Tue Sep 04, 2012 10:07 am

    Thanks Bob...you may recall I have a rectifier tube mod a guy did on my other Mark III where he took a old tube base and used a couple of diodes to make a solid state rectifier. You suggested getting rid of it and I put the tube back in but still have this thing laying around. Although you have discussed the dangers with these (instant-on, voltage spikes etc.) maybe with the larger cap from Dynakitparts in a pinch I can use that to test if the multi cap does not solve my problem. Thanks.

    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Re: Mark III, again

    Post by wildiowa on Thu Sep 06, 2012 12:47 pm

    A brief moment of success, then disappointment. Put the new multi cap in the Mark III today and wow it worked, biased up correctly sounded great. Turned it off for a couple of minutes to do some wiring on a speaker, turned it on again, worked great...then I thought I saw a blue flash on one of the 6550s and a loud "pop" on the AM radio I am listening to, sound started to fade and then nothing.....the tube that flashed is now burning a very bright red but I have no bias reading on the meter and no sound.

    I have replaced every capacitor and the rectifier but have been using some old 6550s I had laying around to fire it up and lord knows what condition they are in...but it did work great for a few minutes. Will get the tubes checked, am hesitant to put others in that are known to be OK in case I am frying them for some reason....any thoughts? Have to travel quite a ways to get to a tube tester so no immediate deal here. thanks.

    Bob Latino
    Admin

    Posts : 2377
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Mark III, again

    Post by Bob Latino on Thu Sep 06, 2012 12:54 pm

    Re: > "have been using some old 6550s I had laying around to fire it up and lord knows what condition they are in ..." You probably have a bad 6550. Put in FOUR NEW 6550's and try it again ...

    Another thing .... You may have damaged the GZ34/5AR4 tube rectifier you have been using and may now need a new rectifier also.

    Bob

    wildiowa

    Posts : 120
    Join date : 2012-03-19

    Re: Mark III, again

    Post by wildiowa on Thu Sep 06, 2012 5:02 pm

    Quick update....to test if the rectifier tube was OK I put in the solid state diode modification I had laying around and on power up it instantly blew the 3 amp slo blo. The fuse seemed to hold when I was using the actual GZ34 tube so the surge from this solid state replacement might have taken it out. I began with that and did not change the 6550s so the problem (hopefully) is there. Guess I'm going to have to take all the tubes in and see if that's the problem. I have some good 6550s here in another Mark III but hesitate to put them in if I just blow them out. Thx.

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