Epiphone Pacemaker 50T PT short

[angelodp]

Hi, buddy brought by an Epiphone Pacemaker 50T, it quit. Pulled all the tubes and put it on the limiter. It had blown a fuse so replaced that. With no tubes on the variac it lights up the bulb limiter as I bring it up. Saw that the bias cap was not looking good, so changed all the electrolytics. Still with no tubes, brought up the amp on limiter with variac and it shows a short. This seems to be pointing to the PT being shorted?

[maxkracht]

Could be the PT, could be a short in the heater or death cap or power cord itself. Have you measured resistances on the transformer? Can also disconnect secondaries one by one and see what turns the lights off. If you think the PT is bad, disconnecting wires one by one and turning it on every step won't add too much time.

[angelodp]

Heaters show 35 ohms, primaries are at 7 ohms , Plates are at 350 ohms.

[Stevem]

Clip out the death cap and unsolder the other grounds off of the PT, if the bulb still goes brite when you power it then yes, your trans is shorted.

[cdemike]

Clip out the death cap and unsolder the other grounds off of the PT, if the bulb still goes brite when you power it then yes, your trans is shorted.

Agree with this. If you find your PT is fine, I'd suggest disconnecting everything from the B+ rail and testing after connecting them one at a time. So if your PT is fine, then you'd want to test it feeding into the rectifier, then the OT center tap, then connect the screens, and so on.

[angelodp]

Ok, so they had these grounds bunched up together. The Mains ground, the cap ground and then the secondary ground and the heater grounds. I leave the mains and cap grounds on but pull the other two. The schematic shows a Transformer chassis ground I think, I don't see that.

Ok so with just the secondary and heater grounds off, I do get the operating light to come on and no short on the limiter with the Variac.

[Stevem]

If you go back up to near the start of this tech section you will find something of great help, it’s called “ fuse blowing / debugging.

[angelodp]

With the red and yellow wire added no short

With the white wire added no short

with both of those wires added yes I get a short. ??

[angelodp]

The white wire (which was grounded originally) shows continuity to the red yellow wire (was also grounded). I see a version of this amp on offset guitars and those wires are correct when grounded, but they are causing a short when they both are in circuit. I do not get continuity on that white wire to anything else in the amp?

Also, known 6X4 in there does not light up and the PT is vibrating so is the 6X4.

I should be able to have both the white and yellow/red wire grounded. Here is an example

[maxkracht]

Not sure what the white wire is, but the schematic you posted shows an internal shield, maybe that's what it is and it is shorting to a ht wire? If the white wire has continuity with the red/yellow (ht center tap) it should have continuity with both of the red wires. The resistance between each red wire and the red/yellow wire should be about the same.

[angelodp]

Ok with Red1 being the Left of pic and Red2 right

R1 -YR - 720K ohms
R2 - YR - 145 ohms
R1 - R2 - 720k ohms
YR - W - 165 ohms
R1 - W - 720k ohms
R2- W - 300 ohms

Not what you would expect

[maxkracht]

Think it's fried.

[angelodp]

Yup, it's also vibrating strangely, not a hum. Now to figure out what caused it. I tried to measure the Heater voltage and that lit up the limiter??

Any thoughts on replacement PT?? Hammond?

A

[maxkracht]

Combination of failed tube(s) and failed caps probably killed it. The bumblebee in there needs to go along with any electrolytics. Might have been an underrated transformer to begin with. Gotta look through transformer specs to find one with the same mounting hole spacing, around the same voltage, enough ht current, and enough heater current. Schematic says pretty low voltage so maybe something meant for el84s? Maybe google will just tell you? Hammond probably has something, weber has a lot of sizes with multiple ht taps.

[Stevem]

It’s done, so scrap it!

Electrical wise a Hammond 269BX will work fine for a replacement, but I would check how the dimensions match up.

For added protection and so this failure does not take place again I would add a in line fuse in the center tap of the heater winding and load it with .225 amp slow blow fuse.

I would do the same with the center tap of the B+ winding, but use a .100 amp fast blow fuse.