Trouble shooting an original Model T

[JoeArthur]

measure the resistance from that R43-CR1 junction to ground (the amp chassis) = 40kohms

R43-CR1 do attach to the same terminal strip point.

Exchange the meter leads and remeasure - remember, CR1 is a diode and you have to measure in both polarity directions.

[brennansrevenge]

i appologize, my meter adjustment was off, here are the corrected readings:

measure the resistance from that R43-CR1 junction to ground (the amp chassis) = 50kohms (+) at junction to ground, and infinity (-) to ground from junction.

i remeasured all the resisters in this circuit and they come out right on.

[JoeArthur]

The infinity reading is good - verifies that CR1 is good.

The 50K reading is about 20K too high.  The reading here should be the series connection of R44 and R45, 10K + 18K = 28K.  

First validate the value of these resistors.  The 10K should be marked, probably stamped on the pot and the 18K should have a brown-gray-orange color code.

If you feel comfortable with having zeroed out the meter, make sure that it is on the correct scale.  I want you to confirm that we are reading a double digit "K" value... and not something like 5 or 50 ohms.

If all of that is valid... I can't explain the 50K reading especially since you say the value of the resistors check out.  But, the good part is 50K would not cause R43 to burn up.

Only a couple of things I can think of, and only you know the answer.

First, as part of your rewiring, did you replace R43 before it burned up?  The possibilities here are 1) using the wrong value or 2) unintentionally creating some sort of ground connection - like the lead of R43 touching something like the center ground lug of the terminal strip, the one that attaches to the chassis... before it got to the CR1 connection.

Second, when R43 burned... were you possibly making measurements and in the process, could you have accidently created a short to ground at the R43/CR1 junction.

Third... the transformer lead is incorrect.  We can verify this.  

Take the tubes out of the amp, make sure you have the meter set to read AC volts - and you might want to start with a high voltage scale at first.  Turn the amp on, leaving the standby on, and measure the AC voltage where the transformer lead connects to a terminal strip (this should be the one where the other lead of R43 is attached) and the chassis.

Be very careful - keep one hand in your pocket and don't touch the chassis with anything but the other meter lead.  If you get too low a reading, reduce the voltage scale.  If the correct lead is attached here, then the voltage will be less than 100 volts - probably around 40.

Remember to turn the amp off after taking the voltage reading.

[brennansrevenge]

First, as part of your rewiring, did you replace R43 before it burned up?
-Not sure because it was 11 years ago.  But after i got the replacement transformer, i did replace R43 with the spec'd 1/2w 330ohm.

 The possibilities here are 1) using the wrong value
-only if the 330ohm is wrong?

 or 2) unintentionally creating some sort of ground connection - like the lead of R43 touching something like the center ground lug of the terminal strip, the one that attaches to the chassis... before it got to the CR1 connection.
-all connections are isolated.  the particular post i'm using does not use any of the leads as a ground connection, the posts and mounted are separated by the insulation material.

Second, when R43 burned... were you possibly making measurements and in the process, could you have accidently created a short to ground at the R43/CR1 junction.
-not quite sure what you mean here, other than if i was using an alligator clip and connected more than what i was measuring?  i am very careful not to do that, so i'd have to say no.

Third... the transformer lead is incorrect
-i measured as directed and if i am reading my meter correctly (it's analog), it should be 60V.  i turned it off, then on again to double check the scale and blew the fuse on the amp.  i guess i should have expected that.

[Isaac]

With that inductor, the amp has to be a second generation.

Just what I was thinking. That means I can take some measurements on mine, if that would be helpful.

[JoeArthur]

11 years ago?  

The voltage on that tap at 60 is probably correct.  It is only half wave rectified.

Ok... well, since you have resistance at CR1 and the resistor values seem to check out... There isn't anything now that explains how R43 got smoked.

I'd replace R43 with another 330 ohm 1/2 watt.  With the amp still upside down and without tubes, I'd move R44 to the center of it's rotation.  If you have alligator clips that would be great - because I would then clip the meter leads to the center lug of R44 (put the negative lead here) and ground the positive lead to the chassis.  Make sure the alligator clip on R44 touches nothing but the center lug.  If this isn't possible, you might follow the wire leading to the bias test point and clip it there.  

Make sure the meter is set to read DC Volts, on a scale greater than 60 volts...

Then watching the meter closely, apply power to the amp.  If all is fine, the meter should register somewhere around 50 volts.  Keep a close eye on R43... but don't touch.

[brennansrevenge]

okay, tried the dc volt reading at R44 and just blew the fuse twice, once trying to take a reading, no reading, then without the meter.

i am out of fuses and will get more tomorrow for more tests, but this worries me because i think now that this could mean i blew the transformer.  this is just my hunch, but i will retake the previous test of the transformer lead for the 60 volts, but if it blows a fuse, then i know i'm in trouble.

[JoeArthur]

Hold on a second... what fuse are you blowing?  Doesn't your Model-T have a circuit breaker?

If you're blowing fuses without tubes in the amp then you have a short somewhere in the power supply and probably on the B+ side of things.  Don't plug it in again.

[brennansrevenge]

i have a glass fuse in the back, 7A.

i agree, it must be a short, but why do you say on the B+ side?  

i guess i'm asking if it's in the transformer?  also, from what i read it seems i can not test resistance in the transformer to check for shorts, also checking for bad windings is a long shot.

my next approach was to isolate the t-former to just check each output.  and, I'll check the B+ loop.

what do you think?

[JoeArthur]

You're blowing a 7 amp fuse?  Heck, the circuit breaker is only 4.95 amp.  7 amps across 120 volts - that's 840 watts!!

The bias circuit can't be pulling this much, not with the resistance figures you measured.  Neither can the heater circuit - especially with no tubes in the amp - 7milliamps based on a worse case setting of the balance control.

That only leaves the B+ side.  Each side of the transformer is pulling somewhere between 360-385 volts (an estimate on my part).  And if you left the amp in standby mode, there are only two components to deal with - CR2 and CR3.  If either one of these is bad or wired into the circuit incorrectly, then there will be in excess of 720 volts across the other one.

Disconnect both of the red transformer wires - one each going to CR2 and CR3.  Measure resistance, in both directions across these components individually.  If they are good, you should get next to nothing in one direction and infinity in the other.

Then measure resistance between these two - put your probes where the red transformer wires were connected.  Measure in both directions - you should get infinity in both.

Make sure the amp is in standby mode (and disconnected from the wall outlet) when you take these measurements - the power switch and standby switch should both be "down" - OFF.

If these check out, then you may have a shorted transformer.

[brennansrevenge]

i believe that is how i determined the bad t-former the first time around.  

well, i have to pick up fuses and i will do this test

if i blow one taking the first test of the 60v on the bias ciruit without R43, then i know i shorted it, because it was okay during the measurement, it just blew when i turned it off and on again during the same measurement.  i don't know why that would have happened.  

the caps were discharged the first time, then i took the measurement, then turned it off, then reclamped my meter, then turned it on again.

on another note, on the B+ side, Caps C19 & C20 where not replaced, but tested for resistance and charging.  i have the resistance and build of charge in both.

i'll let you know what happens, but if it is the t-former, i'm not going to get an original, they are just too questionable.

[brennansrevenge]

okay, well i unsoldered the secondary of the transformer and CR2 and CR3, finding one of them bad.  this would be a recent break, because they were good the other day.

but i think the good news is i was able to measure the power transformer voltage directly, do these make sense?
Red/blk = 60v ac
Red = 440v ac each
grn = 1v ac each.  although the drawings say 6.3v

thanks!

[brennansrevenge]

well, we have the bias loop fixed now, i had the wrong type of diode in there, a switching one rather than a rectifier diode.

so i put in all the tubes and it stays idle on standby, so then was the big test, switch off of standby.....

  fuzzle sounds?  i didn't leave standby off for long, afraid i would fry something immediately.

any ideas on how i can check my output transformer?  it would be nice to test components without power, but i don't know what i'm supposed to get out of it?  the schematic is based on power 'on' and with a sine wave.

[Gtr_tech]

If you hear "fizzle sounds", its more than likely a capacitor that's had its voltage rating exceeded, or is installed reverse polarity. What voltage do you have at the bias filter cap? And this cap is wired in with the "+" lead to gnd right? You have a negatative voltage there and the filter cap must be wired "backwards".

You need to get your voltages normal before going any further.