Help The Newbie

[SRMcGee]

Folks:

I'd appreciate a little advice. I am building a Gainclone using BrainGT's PCBs; this is the fourth BrianGT-based amp I've built. All four amps are identical in parts and construction. The first three went together perfectly, but this one has been causing me a lot of heartache.

I've been using Brian's Rev 2 boards. The assembled PCBs in this latest amp are identical to the others I've built. I'm using the Avel transformer that everyone seems to be using (30 VA, 25v + 25v).

Here is the problem: when I power up to test the rectifier boards (Hot to fuse to switch to transformer (Brown and Gray), Neutral to transformer (Blue and Violet)) and the secondaries to the rectifier boards (Black to AC1 N, Red to AC1 H, Orange to AC2 N, Yellow to AC2 H), the fuse blows. I've checked for shorts and can find none. I've moved wires, disassembled and reassembled everything, and still those 3 amp fuses blow.

When I swapped in other PCBs, the problem continued. It seems clear that the problem is not with the PCBs.

Could the transformer be at fault? I get a 1.4 ohm load across the primaries and about 0.4 ohms across the secondaries. Is this normal? The switch is an NKK illuminated toggle switch, and I cannot find any shorts or loose wires there.

Any and all suggestions would be appreciated.

Regards,
Scott

[GregGC]

Try running the transformer alone, not connected to the PS PCB. See if the problem continues. If not it maybe the way you connect the transformer to the PCB.

Good luck

Greg

[carlosfm]

Originally posted by SRMcGee
I'm using the Avel transformer that everyone seems to be using (30 VA, 25v + 25v).

Is this a typo or are you really using a 30VA trafo?

[SRMcGee]

Carlos:

Yes, that was a typo. The transformer is rated at 300 VA.

Regards,
Scott

[carlosfm]

Follow Greg's suggestion, measure AC at the secondaries of the trafo, not connected to the rectifier board.
On the rectifier board, check out if you didn't reverse the diodes.

[SheldonD]

Scott I just measured a 100VA primary dual winding for 110 vac I get 7.4 ohms. this with both primary windings in parallel and connected.

Is your figure with isolated transformer?
Then I think something wrong in primary of transformer.

I get similar secondary readings as you.

I realize I have smaller VA but the difference is too big.

[SRMcGee]

Carlos and Greg:

I'll disconnect the secondaries from the rectifier and check the voltage (assuming that the fuse doesn't blow). I assume that the voltage of each of the four secondary wires should be checked against ground; is there another test that I should do as well?

Carlos:

The diodes are all installed properly.

Sheldon:

I double-checked my readings on the primaries (in parallel), and it still reads 1.4 ohms.

It'll take me a day to post an update. Thanks for your thoughts so far. Please check back this time tomorrow.

Much appreciated,
Scott

[SRMcGee]

Folks:

I couldn't wait. Here's what I got once the secondaries were pulled from the rectifiers:

Red 23.4 vac
Orange 4 vac
Yellow 24.8 vac
Black 2.8 vac

I'm a little surprised by the results. But what does it mean?

Thanks,
Scott

[GregGC]

Originally posted by SRMcGee
Folks:

I couldn't wait. Here's what I got once the secondaries were pulled from the rectifiers:

Red 23.4 vac
Orange 4 vac
Yellow 24.8 vac
Black 2.8 vac

I'm a little surprised by the results. But what does it mean?

Thanks,
Scott

Scott,

First it means the transformer is fine, because the fuse didn't blow.

I'm not sure how you measured those voltages. when the transformer is not connected to the rectifiers PCB there are 4 wires (2 for each secondary). So you measure the voltage across each secondary. You should measure around 25VAC on each secondary. It looks like you have something wrong on the PCB or you are connecting the transformer wires to the PCB the wrong way. Make sure you know exactly which are the secondaries (pair of wires) and then connect them to the appropriate places. Also check as Carlos suggested the way the diodes are mounted.


Greg

[Nuuk]

Shouldn't that be a 6 amp fuse for a 300V 25 volt transformer?