Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mdeetz on August 28, 2018, 09:22:57 pm

Title: Jet City JCA-100HDM with a short?
Post by: mdeetz on August 28, 2018, 09:22:57 pm

Hey all!

I'm taking a look at a Jet City JCA-100HDM for a friend. It was in storage for a little while and now it won't turn on.

Fuse is blown, so I plugged it into my light bulb limiter and jumpered the fuse connection (takes one of those smaller sized fuses which I don't have on hand) to see if there is a short. Standby in the off position (voltage not passing to diodes), turning the amp on lights up the bulb and it stays bright. So, short somewhere?

Primary winding measures 3ohms, HT secondary measures 36 ohms (17 per side to ground where center tap is connected), not measuring any continuity between the two, and no continuity from the primary winding to ground. My meter shows an overload when trying to measure HT secondary AC voltage while the amp is on and light bulb is lit.

Bias power supply resistances seem normal with no unexpected shorts to ground. I've disconnected the 6.3 winding wires and the light is still on. Output tubes are out and the light bulb is still on.

Here is the schematic I've been working with:

http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/Manuals/JCA100HDM%20-%20Schematic.pdf

I think it has some wrong connections in the pre-amp, but the power supply seems to be what is in the amp. This amp also has a 120v/230v switch that seems to be the same as in this amp:

http://starin.info/Product%20Info/-%20-%20Archive/Jet%20City/Manuals/JCA22H%20-%20Schematic.pdf



Any advice on where I should look? Am I misinterpreting the light bulb limiter?

Title: Re: Jet City JCA-100HDM with a short?
Post by: mdeetz on August 30, 2018, 09:43:39 pm

Disconnected all secondary windings and it's still flashing the light. With my meter measuring AC on the 120v input from the iec, the 120v drops down to about 30v when it's turned on.

I've got the transformer out of the amp now and it's doing the same thing (dropping voltage) when feeding a 1VAC signal into the primary. Drops down to about .13v. Other transformers I have seem to drop it around .003v at most, so I guess this one is dead?

What kind of things can cause a short in a power transformer? Failing filter cap? Weird power surge? Tubes test fine, seems like the output transformer is fine, and there's no visible damage on the circuit board...

Title: Re: Jet City JCA-100HDM with a short?
Post by: PRR on September 03, 2018, 03:04:10 pm

If it lights a good-size incandescent lamp with all the secondaries disconnected, it sure seems to be a bad transformer. Internal short.