1568a power tranny

[1568A]

I need some help or advice. I was using my 1568A power amp when a fuse blew. I replaced the fuse and it blew again. I took a DMM to the power tranny leads and there was no resistance.

Does this mean my PT is blown? If so, where can I get a replacement? Do any of you know what specs I need for a new replacement PT from someone like Hammond?

Any and all advice is GREATLY appreciated!

Thanks,
1568A lover

[jmarkwart]

1568A Lover,
The 1568 A schematic shows the power trans (Peerless 6288) has a secondary center tapped HV secondary and two filament windings. The HV sec needs to produce 390V after 5U4 rectification.The HV sec also has a single tap used to supply output grid bias voltage through a selenium rectifier, but its voltage output is not labeled. After rectification it needs to supply -37V to the grids. Altec listed the power supply at 125W.
Jeff

[airboss]

Mr 1568A:
The primary DC impedence, (that's what your DMM measures) of any power transformer is going to be quite low, as will the secondary, though it will be somewhat higher in most cases. The first step in trouble shooting your problem is to disconnect the secondary of the transformer from the components downstream, like the choke and the filter capacitors. Then plug it in and see if it pops the fuse. Do not under any circumstances increase the fuse size beyond the manufacturers recomendation. Sometimes a slo blow fuse will hold when a single element doesn't. If your unit still pops fuses after disconnecting the filter, then it is likely that either the primary or the secondary is shorted. I have repaired transformers (even the potted ones) by carefully removing the end bells (and/or cooking out the goop)and searching out the short. If this problem has newly surfaced, and it sounds like it has, I strongly suspect a shorted filter cap, not a blown transformer. Such transformer failures are exceedingly rare. Good luck.

[1568A]

Jeff and Dan,

Thank you so much for the replies. That makes me feel better knowing that the PT may not be blown.

I will have to check the filter caps. What is the best way to check them?

As you can tell, I'm a complete rookie at this but I'm trying to learn.

Thanks again,
1568A lover

[airboss]

Well, the first thing that a rookie needs to remember is that there are voltages in there that WILL KILL YOU!!!! These voltages can be present in a fully loaded filter capacitor with a burned out bleeder resistor EVEN WHEN THE UNIT HAS BEEN UNPLUGGED FOR SOME TIME. I have seen this on two occasions, once it hurt for a moment (about 350VDC), and the other I used a shorting line first. This was in a hugh power supply for a commercial radio transmitter with 3200VDC accross the cap. I may not have survived that one. BE CAUTIOUS IN THE EXTREME. My advice, honestly, is to either set this unit aside for awhile and get some education, or send it out to a shop that specializes in tube gear. If neither one of these alternatives is paletable just keep one hand in your pocket at all times......this will keep the jolt from crossing your heart and stopping it.
BTW, does this unit have SS rectifiers or tube?? If it's a tube rectifier, (usually a 5-something) pull it out, plug the amp in and see if you pop a fuse. If the fuse doesn't pop, it's the filter caps or a shorted choke. Send it out and get it fixed. If it has SS rectifiers, learn how to recognize them, disconnect them and test as above.

Any advice beyond this would be beyond the scope of this forum.

[Jim D]

From my experience with the 1568-69 amplifiers, I would guess you might have a bad rectifier tube (5U4). This will blow the fuse----right now. Airboss is right if you pull the recifiers and the fuse doesn't blow this may be it. Also a shorted output tube (6CA7) will blow the fuse, but it is often slower. These amps were amazing, the 1569s (80W) would put out near full power on two of the four output tubes. The 68 only has two. I am still using a 68 for my bench amp. Jim

[1568A]

You guys are the greatest!

I pulled the rec. tube and the fuse didn't blow. Put the RT back in and the fuse blew.

This is great news. I ordered a new 5U4 and will install it as soon as it arrives. I will let you all know if that does the trick.

Thank you again so much. Everyone elsewhere told me I needed to purchase a power tranny. I sure am glad I found this board!

1568A Lover

[Todd W. White]

You might also want to use a 5AR4 - they ramp up more slowly, and will give longer life for your power cap's and transformers....

Todd W. White, Owner & Webmaster
Altec Lansing's (unofficial) Homepage

[1568A]

Thanks for the tip! I'll check on a 5AR4 immediately.

Gary