Basic Transformer Testing

[nelsress]

I've got a line on 700B that blows a fuse whenever it's powered on. I would like to totally WOPL it anyway so I'm not concerned that it doesn't work properly but I a would like to make sure the power tranny is still okay. I don't have an o-scope, a sine wave generator, etc., just my trusty fluke so is there a test I can run that will give me a basic "dead or alive" indication? The guy's not asking a ton of money for it but if I would plop say $250 in it and the tranny is bad, is there any hope of getting another power transformer reasonably or at all? Thanks guys.

 

[Northwinds]

According to Joe and Lee, PL trannies rarely go bad so I would not be too worried about it

 

[nelsress]

Good to hear!

According to Joe and Lee, PL trannies rarely go bad so I would not be too worried about it

Thanks Northwinds, that's good news, I'll get back to the guy and see if we can strike a deal! I'm still wondering though, even though they "rarely go bad", on the off chance that it IS in fact bad, is there hope of a replacement, without having to buy another 700B, do you know? Thanks again.

PS What's in the glass NW?

 

[jbeckva]

I've got a line on 700B that blows a fuse whenever it's powered on. I would like to totally WOPL it anyway so I'm not concerned that it doesn't work properly but I a would like to make sure the power tranny is still okay. I don't have an o-scope, a sine wave generator, etc., just my trusty fluke so is there a test I can run that will give me a basic "dead or alive" indication? The guy's not asking a ton of money for it but if I would plop say $250 in it and the tranny is bad, is there any hope of getting another power transformer reasonably or at all? Thanks guys.

If you inspect the two +/- connections to the main caps, you should see that there are two wires coming forward from the bridge rectifier, and a few others bolted on using the screw terminals. Take off all others leaving the two from the bridge - that completely isolates the amp from the power supply. Apply power, but preferably using either a DBT or a Variac, then see if you have voltage on the two terminals you disconnected the extra wiring from.

BUT.. before you do this, I strongly recommend you have on hand a few wirewound power resistors, at least 100 ohms possibly more. You'll likely need these to drain the caps after power is taken away. With zero load, those caps retain a very high voltage (and conversely current) potential for a very long time. I use a pair of 50 ohm 10 watt resistors in series, then short the caps with a periodic "tap" on one of the screw terminals, repeatedly tapping until the voltage is drained off.

 

[jbeckva]

Resistor selection..

Or as far as the bleeder resistors, read this thread by Joe

http://forums.phxaudiotape.com/showthread.php/2356-Forming-Phase-Linear-Power-Supply-Bulk-Capacitors

I'll gladly defer to Joe on the "safest" way to make sure those caps drain quickly after power is removed.

 

[nelsress]

Excellent

Thanks jbeckva, good info to know, I appreciate it. So I just have to decide if I want to lay that much down on the hopes that it's okay as I don't have any wirewound resisters in my box that are that big, at least I don't think I do. I'll check to make sure but will probably have to order a couple. I suppose any of you would give a couple hundred and a half for one that's in pretty good cosmetic condition, right?

If you inspect the two +/- connections to the main caps, you should see that there are two wires coming forward from the bridge rectifier, and a few others bolted on using the screw terminals. Take off all others leaving the two from the bridge - that completely isolates the amp from the power supply. Apply power, but preferably using either a DBT or a Variac, then see if you have voltage on the two terminals you disconnected the extra wiring from.

BUT.. before you do this, I strongly recommend you have on hand a few wirewound power resistors, at least 100 ohms possibly more. You'll likely need these to drain the caps after power is taken away. With zero load, those caps retain a very high voltage (and conversely current) potential for a very long time. I use a pair of 50 ohm 10 watt resistors in series, then short the caps with a periodic "tap" on one of the screw terminals, repeatedly tapping until the voltage is drained off.

 

[jbeckva]

Thanks jbeckva, good info to know, I appreciate it. So I just have to decide if I want to lay that much down on the hopes that it's okay as I don't have any wirewound resisters in my box that are that big, at least I don't think I do. I'll check to make sure but will probably have to order a couple. I suppose any of you would give a couple hundred and a half for one that's in pretty good cosmetic condition, right?

Speaking for myself, yep pretty much LOL. Now "conversely" you can also check the rails for shorts .. but again disconnect the amp from the caps as that voltage will skew your readings (and quite possibly damage your meter). With the four rail fuses pulled (this accomplishes the decoupling from the PS), measure the amp side to the buss bar, and from where the rails hit BOTH channels (four measurements in total). Chances are you'll see a dead short on one of them, then go from there...

 

[laatsch55]

Nelson...I'd buy that in a heartbeat. Jers got ya lined out on transformer testing. When we say "rare" Ed Blackwood has only seen one in his 25 years of workin on em. Buy that thing before it gets away..

 

[nelsress]

Got it!

Nelson...I'd buy that in a heartbeat. Jers got ya lined out on transformer testing. When we say "rare" Ed Blackwood has only seen one in his 25 years of workin on em. Buy that thing before it gets away..

Thanks Lee, the more I thought about it, the more I was inclined to think the same as you, given that they're not easy to find at a reasonable price. So, after I got an e-mail back from the guy saying I could have it for $200 cash today, it took me about 10 minutes to get ready and start the 1 hour and 20 minute drive to his place. It looks pretty good and I'm psyched. I'll post some pics soon. Thanks again.

Nelson

 

[laatsch55]

Ya stole it for 200 Nelson...ya did good...

 

[nelsress]

Photos

Ya stole it for 200 Nelson...ya did good...

Thanks Lee, I was pretty happy with it. Check it out!

Here are some pics of my new PL 700B. It looks as if one of those transistors on the back is fried, could this cause the fuse to blow upon powering it up? There's also a small dent in the one level knob, I tried to get a close up but I'm not sure how visible it is.

 

[Gepetto]

Thanks Lee, I was pretty happy with it. Check it out!

Here are some pics of my new PL 700B. It looks as if one of those transistors on the back is fried, could this cause the fuse to blow upon powering it up? There's also a small dent in the one level knob, I tried to get a close up but I'm not sure how visible it is.

555's should not be in a PL700 Quasi comp amp

 

[laatsch55]

Aye....but they often are....

 

[Gepetto]

Aye....but they often are....

You can see they were replaced from the originals by some well meaning soul...

 

[laatsch55]

Yes indeed Joe.


Nelson....outwars appearance of a TO-3 does not indicate condition....usually. I wouldn't draw any conclusions on that one other than it looks bad. Got a 700 service manual?

 

[nelsress]

Quasi-comp?

Joe, are you referring to this as a quasi-comp 700B because someone worked on it and installed the wrong transistors? Next I need to open her up and see what's under the hood. I noticed that not everything that's offered for the 400 is also offered for the 700B.

Lee, no I do not have a 700B service manual, not yet anyway. I got my 400 service manual online, I'll go there again for the 700B manual too. I think it was Hi-Fi Engine. I don't have a lot of test equipment though so much of what's in those manuals isn't a whole lot of use to me. Still good to have though.

 

[laatsch55]

If you have a DMM , that's all you really need for basic troubleshooting...

 

[nelsress]

If you have a DMM , that's all you really need for basic troubleshooting...

Well, I've got one of those!

 

[laatsch55]

Then you can check for shorted outputs, which represent 90% of all fuse blowing problems....

 

[nelsress]

Then you can check for shorted outputs, which represent 90% of all fuse blowing problems....

Sounds like a good place to start, thanks Lee.