SEX 2.1

[allanp]

Hi,  last week while testing voltage with multimeter I apparently caused a short, the fuse blew and I saw a spark I think from fuse area..could be wrong. I bought replacement fuses from radio shack however the ends were not concave rs fuses are rated same but flat ends.
Tonight I rechecked and reheated solder joints including grounds. All resistances tested good. When I went to test voltages the tubes did not glow and I had no readings from 1st test point. So not powering on.  Any suggestions?

Allan

[Doc B.]

Did you check the new fuse?

[allanp]

Yes I did, has continuity. 

[Doc B.]

Also be sure fuse is in correct part of fuse holder - clip part, not square tube part.

The next step would be to make sure your power switch is working correctly. Check resistance of the contacts. Put the test probes on the switch terminals and see if it reads close to zero with the switch in the on position and open with the switch in the off position.

When all that is checked you can move on to measuring the power transformer voltages.

[allanp]

Hi Doc,
Checked all, fuse in properly.
Using r/s small digital multimeter when white dot pushed in reading varies from 0.5 to 0.1 with switch other way sometimes reads 0 and other times meter doesn't move just says 0.F
Using analog mm  when flipping switch back and forth meter moves to zero with switch dot in but not reading unless I push hard on probe with switch other way.
Allan

[allanp]

Hi, how do I measure the power transformer voltages? appreciate your help
Allan

[Doc B.]

OK, I'm not really sure what your readings are. I think the switch is working OK based on your description, if 0.F means open. Next you need to see if voltages are correct on the power transformer terminals. Set your meter to AC volts.

Plug in amp, power switch on, test the voltage across power transformer terminals 1 and 2 (across meaning black test lead on one terminal, red test lead on other terminal). That should be the same as your AC mains voltage, around 120VAC (assuming this is a 120V kit).

Next measure across power trans terminals 4 and 5. You should see 6.3VAC. Next measure across power transformer terminals 6 and 7. You should see about 150VAC.  If you don't see these voltages, power down the amp and we will have you test the resistance of the transformer windings.

[allanp]

Hi Doc,
Terminals 1+2 read 119.6v
4+5 read. 1.6v
And 6+7 read .024v

[Doc B.]

OK, you will need to measure the resistance of the primary winding to see if it is open. Unplug the amp. Measure the resistance across power transformer terminals 1 and 2 (red lead to 1, black lead to 2). It should be around 10 ohms. If it reads open or very high it will need to be replaced. Also check from 4 to 5 which should be around an ohm or less. Then check at 6 and 7, which should be around 40 ohms. My hunch is they will be OK and the primary is what may be damaged.

[allanp]

Hi Doc,
Power terminals:
1+2= 9.3
4+5= 0.3
6+7=40.8

[allanp]

Hi Doc,

Reflowed power terminals 1+2  no difference on 4+5 (1.44v) or 6+7   (0.25v)
what's next?
Allan

[Doc B.]

OK, you will have to disconnect the wires attached to power trans terminals 4&5 and 6&7 and check voltages at those power transformer terminals again, to see if the voltage comes up. If it does come up to normal (6.3VAC at 4/5 and about 150VAC at 6/7) that would indicate a short somewhere downstream of the power transformer. If they don't come up and you have 120VAC at 1&2 there is still some issue with the power trans.

[allanp]

Hi Doc,
Did as you said, disconnected 4+5 and 6+7 on power transformer no voltage at either one, have 120vac at 1+2
I noticed that terminals especially 6+7 were loose as a tooth when I was disconnecting wires. Gently pushed down as they cooled down.
Allan

[Doc B.]

It's odd that you get the correct resistance readings on those windings, but no voltage. You might try unplugging the amp and checking resistance from power transformer terminal 1 to power transformer terminal 6. It should read open, but perhaps there is a short between the windings.

[allanp]

Hi Doc,
New batteries, new probes.  Tested resistance at pt 1 to 6 reads open.
  retested 1 to 2 I get 120 or very close (119.9).  4 to5 I get no read but now when I test 6 to 7 I get 34v
Allan