Resistances off, issue? -- update: FIXED!

[SammyJ]

Hey guys,
So most of the my resistances are spot on, except for: the following

T - Expected - Actual
23 - 475K - 47.3k
31 - OL - *

I'm hesitant to plug in and start checking voltages until I figure out what's up with the resistances. Any ideas?

thanks,
S

[Paul Birkeland]

There's a resistor between 23 and 22 that sets that resistance.  My guess is that you have a 475K resistor across one of the RCA jacks instead of the 47K resistor that's supposed to be there (which is now between 22 and 23).

OL on 31 is an acceptable reading.

[SammyJ]

Yup, took a look again this morning and that was exactly it -- a 47k across 22-23, with the 475k across the B-side input.

I'll de/resolder tonight and see if that corrects.  Just to clarify, my T31 readings are jumping all over the place, usually right around 6M.  I'm guessing the right resistance across 22 should hopefully correct?

Thank you Paul!

[Paul Birkeland]

Yeah, I think you'll be OK.  There's an 18K resistor that feeds T31 and should land where the red power supply wire comes in.  If those connections are present, you'll be OK.  This is one of those terminals that just isn't easy for meters to read.

[SammyJ]

Hey guys,

OK, so I desoldered and resoldered.  All resistances were spot on now, with the exception of 31 which was varying, but that seems OK.  Flipped the chassis over, plugged in and flipped the switch.  Started watching the tubes to see if the filaments glowed.  They didn't, and after a couple of seconds, I noticed smoke coming up out of the vents.  I unplugged immediately.

Rechecked all of my connections, all looks perfectly fine.  I'm nervous to flip it on while upside down and see exactly where the smoke is coming from.  Any ideas on what to do?

Thanks!
S

[fullheadofnothing]

You most likely have a capacitor in backwards.

[SammyJ]

Yeah... I had the banded end of a 1N5818 going into 34L.  I rechecked the rest of the the polarities and everything else is fine.  I desoldered, flipped, resoldered.  Still having smoke come out of the A side vent.  Should I try to run with the chassis upside down to see what exact part is smoking?  I'm afraid of doing more damage than I've already done


Thanks again for all the help, I feel like quite a moron at the moment.

[Doc B.]

If you blew the diode you will have to replace it before you can power up again. You might be able to tell what part overheated with a visual inspection.

[SammyJ]

Yeah... the diode is blown.  Off to mouser. 

Visually, the rest of the system looks OK, but there was a decent amount of smoke.  Hopefully that's it.  Anyways, I'll spend my wait time rechecking, and rechecking my connections.  Learned the hard way that I shouldn't stay up too late doing these things.

Thanks again for the help guys, I'll post back once I get in the part.

[SammyJ]

One last question before I remove my newbieness for the evening -- any particular brand/part that I should be looking at or will just about any N5818 do?

Thanks folks,
S

[Grainger49]

With the diode blown you might have blown the fuse.  Also check that the power supply caps don't have ends that are bulged.  The ends are flat when everything is ok, but the top or bottom can bulge when they are blown.

[Paul Birkeland]

Any old 1N5815 will work.  Heck, a 1N5820 would probably work as well.

Some local electronics shops will stock a full line of NTE semiconductors, an NTE585 will work and is very likely available for purchase close by.

[SammyJ]

OK, took a good look today.  The power supply caps seem to be fine visually.  The fuse is actually OK too, but I'm ordering a few just in case for the future.  The one schottky is bulging in the middle, obviously blown.  The other (the one that was in correctly to begin with, seems fine visually, but I did notice smoke from that area, so I'll plan on replacing anyways.  I think my 3300uf is OK, doesn't look blown at least.

Thanks again guys, I really appreciate the help here.

best,
S

[SammyJ]

Hey guys,
So I got in the burnt out rectifier diodes.  Replaced both the one that goes from 34L to 39L and also the one from 36L to 41L.  Flipped her on and the tubes were glowing (and without the drama this time)!

So, I started checking voltages.  Flipped my MM to DC, clipped the black to the center ground post and started taking measurements.  Some were pretty far out of band:

3 - ~300mV
18 - 135V
19 - 135V
20 - 135V
25 - ~300mv
26 - 135V
27 - 200mv
28 - 300mv
29 - 160V
30 - 300mV
31 - 130V
46 - 300mV
47 - 300mV
50 - 170V (the rest seem OK, but slightly high)
51 - 170V
56 - 160V
57 - 160V
59 - 170V
60 - 170V


What could be causing the high voltages?

thanks!
S

[Paul Birkeland]

3 - ~300mV
18 - 135V
19 - 135V
20 - 135V
25 - ~300mv
26 - 135V
27 - 200mv
28 - 300mv
29 - 160V
30 - 300mV
31 - 130V
46 - 300mV
47 - 300mV
50 - 170V (the rest seem OK, but slightly high)
51 - 170V
56 - 160V
57 - 160V
59 - 170V
60 - 170V

3 has nothing connected to it.  0.3V is very close to 0V, and close enough to call 0 for these tests.  (So the same for 25, 27, 28, 30, 46, 47)

18 and 26 are wired together, do you see the LED at B3 illuminating? 
19/20 relates to 18/26

If you swap tubes between channels, do these elevated voltages follow a particular tube?