4 blown fuses later i still don't know...

[kwijabo]

i have no clue whats wrong.

i followed the first-start-up procedure. when disconnected from the circuit, the PT HV's give their rated output. the 6.3V's are good attached to the lamp. but before even connecting the HT's, there is a hum coming from the lamp/PT area.

so i connected the HT's and flipped it on. within a second the fuse went.
#1. dang. so i recheck everything. nothing wrong that i can tell. so i change the fuse and turn it on again. but again it blows the fuse #2.

this time as i'm triple checking everything i notice that the standby switch now has no connectivity. NOTE: it was in the off position for all the above. its ready 4.6kohms no matter what position. so i change it and try again. blown fuse #3.

now, the PT is hammond 270EX. i was using the gray/white for 115. during my troubleshooting here i noticed that black/white if you have higher than normal (125) mains. i check them and i sure as heck-fire have 125V mains. so i switch to the black/white and check the circuit again. and try to power up again. blown fuse #4.

heater wires are continuous, R < 1ohm
HT's are not-continous, R = 100 ohm.

what in the sam hill? i have checked the resistance between powers and ground.



schematic is the AX84 P1x. pictures and it are attached.

[plexitone]

...

[kwijabo]

yes, it is. that is how the schematic has it. i think because the 270EX doesn't have the ground connection on the heaters, they recommend putting one of them to the cathode to raises its reference to suppresses some of the line hum.

UPDATE: Disconnected EVERYTHING from the circuit except:
- transformer
- rectifier diodes
- first power supply cap (47uf reservoir)
- bleeder resistor (220k, 2W)

blown fuse #5.

that means its one of those, right? At this point i'm leaning towards the diodes because the transformer works out of the circuit. the cap measures 80uf with the multimeter but i'm not sure how acccurate it is, and the resistor measures to spec.

now, how to verify the diodes are busted? they are not shorted from cathode to anode, so what next?

[kwijabo]

found out there is a diode setting on the multimeter so i used that and am getting 0.5V drop, so that means the diodes good, yeah?

soooooooo.....transformer?!

[surfsup]

You have a gray wire coming out of the PT that is stripped and not connected to anything. What is that wire? EDIT It looks like that is the 115V wire.

You have a wht and grn/yel wire going to the mains switch. On the hammond site, the GRN/YEL is the ground for the 6.3V heaters.

Where is the black wire for the 125V primary?
Is that green/yellow the 6.3V grounds hooked to the mains?
I don't see a 100W pot or dual 100R heater ground artificial centertap anywhere.

[labb]

Take the lamp out of the circuit and see if the problem goes away...By the way, would be a good time to build yourself a light bulb limiter.

http://www.geofex.com/Article_Folders/SPO_Test.htm

or any other version of this.

[briane]

It looks to me like the transformer wires are not hooked up correctly. Its hard to see all the details in the pics. I would say the outlet and power switch are wired incorrectly, if not more.

typically:
green/yellow - to ground
green- heaters
red/yellow - ground
blacks - to main (1 to outlet, 1 to power switch) - if their not black check the transformer code (I suspect grey and white)
red - HV
yellow - 5v

IMPORTANT: Dont plug that in as is without electrical tape on the exposed transformer wires and a light bulb limiter.

[ToneMerc]

It looks to me like the transformer wires are not hooked up correctly. Its hard to see all the details in the pics. I would say the outlet and power switch are wired incorrectly, if not more.

typically:
green/yellow - to ground
green- heaters
red/yellow - ground
blacks - to main (1 to outlet, 1 to power switch) - if their not black check the transformer code (I suspect grey and white)
red - HV
yellow - 5v

IMPORTANT: Dont plug that in as is without electrical tape on the exposed transformer wires and a light bulb limiter.

Yep, that's the size of it. The filament CT is wired to the mains switch line side output.


TM

[M Fowler]

Here is how the Hammond 270EX is wired.

If there is yellow/black wire that is a 5v CT shrink tube it and stow away.

The AX84 layout shows how to wire this amp!

[kwijabo]

i had been using the 115V connections, the gray/white primary. after the second fuse blow i realized the 270EX had a black/white primary for higher AC mains, ~125V. I measured and thats exactly what my house had.

Unfortunately i had cut the black wire nearly back to the transformer before this, so it wouldn't reach the switch. The only other heavy duty wire i had beside me at the time was the green/yellow chassis-ground wire. thats my extension of the black primary wire.

also, i thought maybe i had a bad 270EX cause looking at your diagrams, mine certainly does not have a yellow/green center tap for the filaments. looking around the web, i found the 270EX doesn't have it. you can see from the pictures here.

http://angela.com/hammondpowertransformer270ex.aspx

i'm heading out for the weekend so i won't be doing anything this weekend, but i'm thinking next week i'll try to find a different transformer.

[Tillydog]

Take the lamp out of the circuit and see if the problem goes away...By the way, would be a good time to build yourself a light bulb limiter.

+1

It looks like one of the lamp connections is grounded to the metalwork.

[kwijabo]

as i mentioned, i've taken everything except the rectifier and reservoir cap out of the circuit. see attached.

with the standby switch OPEN, as soon as i close the power switch (turn it on) the fuse blows. i'm not even letting anything get to the cap. and, two standby switches have broken as a result - they are now effectively open no matter what position the switch is in.

does this sound like a transformer issue?

[John_P_WI]

What size fuse are you using? Are they tempered or slow blow?

[rp]

Note Mark's post:

If there is yellow/black wire that is a 5v CT shrink tube it and stow away.

FYI: https://tubeamparchive.com/viewtopic.ph ... hlight=5c1

[surfsup]

lift a leg on that resevoir cap and check it. test the bleeder R. Are your filaments grounded anywhere in the amp? Since you don't have a center tap on the 6.3, did you put a humdinger in or two 100Rs to ground?