Simple SE with KT88's

[nikolas812]

Motor run disconnected.

2.5a fuse.

5ar4 started to flicker blue then the fuse blew




Nick

[chrish]

Originally posted by nikolas812
Blew it.


the 12at and 5ar4 started to light up then 5ar4 start flickering with blue and the fuse blew...




Nick

As mentioned before, if you have a 5A fuse (BAD!!!!!) and blue arcing in the rectifier, you have a short.

I see you don't have the FREDs installed. That is good. You have a whole bunch of wires on that work bench. Plenty of opportunity for errors there. Check everything on and off the board. Make sure you have no shorting unused wires from the power transformer.

When you have done this, take a systematic approach. As there appears to be a high current draw downline of the rectifier, I would first remove the rectifier and make sure that all of the heaters are working on the power valves and input valve. They should be OK. Then start looking for shorts, damaged components and mis installed components...

[chrish]

Originally posted by korneluk

I suppose I could write it a different way, using Volt Amps

750volts .2A = 150 VA (Volt Amps)
6.3volts 6A = 37.8 VA
5volts 3A = 15VA

Total 202.8 VA

Input voltage 120 volts therefore 202.8 / 120 = 1.69 Amps drawn. 2.5 Slow Blow fuse required

[korneluk]

Originally posted by chrish


I suppose I could write it a different way, using Volt Amps

750volts .2A = 150 VA (Volt Amps)
6.3volts 6A = 37.8 VA
5volts 3A = 15VA

Total 202.8 VA

Input voltage 120 volts therefore 202.8 / 120 = 1.69 Amps drawn. 2.5 Slow Blow fuse required

Chrish,

My bad, I should have been more explicit why I made the recommendation I did.

The 6K7VG is rated for 117V
750V 150mA
6.3V 5A
5V 3A
for a combined total of 159VA.

At 159VA/117 = 1.36A.

Now, using the rule of thumb I learned from old HAM radio guys for troubleshooting equipment, take 3x the primary current or the total heater current draw (whichever is greater) to calculate the fuse rating.

If this is exceeded and the fuse blows, then there is a major fault. This is just a troubleshooting back of the envelope calculation, that is why I recommended a 5A fuse.

Again, my bad for not giving more details.

[nikolas812]

I've checked and re checked the power transformer wiring.

Everything seemd correct.

Except... The green and yellow wire. Its not hooked to anything.


On top of the transformer it says.. Sec2 6.3VCT, 5a Grn-Grn/yel-grn


Should the yellow green be hooked up to the two green wires somehow?


Nick

[korneluk]

No, the center tap of the filament winding is not connected to anything.

[nikolas812]

so now what?

take out the rectifier and see if the amp continues to run with out it?



Nick

[nikolas812]

I've checked and re checked the power transformer wiring.

It looks good to me.

went over evey solder joint on the board and they look good.

What didn't look good I cleaned and made look good.


Also all I have is fast blo fuses and I am quickly running out..


A slow blo wouldn't be problem would it??? I wouldn't think so but I don't know much about it.



Could I have a bad rectifier tube?


also. everythin is grounded. I mean everything. The rcas the binding posts each transformer choke and cap are all grounded to the IEC ground. Could I have them grounded wrong somehow?


I just don't know what esle to look for.


Should I put the freds in and try and run it without the recitfier?



Nick

[chrish]

Originally posted by korneluk


Chrish,

My bad, I should have been more explicit why I made the recommendation I did.

The 6K7VG is rated for 117V
750V 150mA
6.3V 5A
5V 3A
for a combined total of 159VA.

At 159VA/117 = 1.36A.

Now, using the rule of thumb I learned from old HAM radio guys for troubleshooting equipment, take 3x the primary current or the total heater current draw (whichever is greater) to calculate the fuse rating.

If this is exceeded and the fuse blows, then there is a major fault. This is just a troubleshooting back of the envelope calculation, that is why I recommended a 5A fuse.

Again, my bad for not giving more details.

No problem, just want to make sure we are all on the same page. You had me scratching my head making sure I was not giving incorrect information


As for the unused power transformer leads. Make sure that the ends of the wires are insulated (I use a little heat shrink here) and then use a cable tie to neatly hold them out of the way.

The motor run cap is not polarised, so connection should not matter as long as it is wired correctly. It looks like all of you electrolytics are installed with correct polarity...

It is a waste (of fuses and other parts) to just replace items without finding the problem. Re-check for any shorts across traces on the board. Check all installed components are correct value and no burnt out resistors etc. Re-check all off board wires. There are quite a lot here, plenty of scope for error. Any reason they are not screwed in to the terminals rather than soldered to the board?

Also, a previous short may have blown the rectifier. I have had this happen, but don't just replace without exhausting other fault-finding as you may just end up blowing another fuse and rectifier...

[nikolas812]

How do I check for shorts?






Nick