1950s? Newcomb "Pathfinder" 7027-A Monoblock Amp questions

[JP]

Thank you. I finally went and saved the schem and took a closer look.

To make matters even more complicated is that this amp uses a tube (6V6) regulated voltage doubler circuit for a boost to about 600 volts.

If worse comes to worse, could he use a smaller xformer to develop the 70 volts needed for the bias?

I'd think, but I'm too tired to process it right now. From the readings he was getting, I don't think the PT is shorted. That capacitor across the AC line is leaking badly or shorted, so that's my bet. At least, that needs to be eliminated before we can move onward.

 

[JP]

the 200 ohm section did actually work I guess I just wasn't using it right. and no it didn't climb up there slowly, it was just giving me random readings between 20-60 ohms and it wasn't even attempting to "charge" up it would just jump all over the place giving out random values and wouldn't stay on a single value or even try to climb up slowly to infinite, even reversed it did the same thing.

Okay. Leave it disconnected, get more fuses, and try again. Did you ever build that dim bulb tester?

I'm off.

 

[captainclock]

Okay. Leave it disconnected, get more fuses, and try again. Did you ever build that dim bulb tester?

I'm off.

No I never built a dimbulb tester yet, but I think I could easily pick up the parts for it on the cheap from the habitat for humanity restore.

 

[cademan]

I did use a 4 amp fast acting fuse in the amplifier before I took it apart completely and it blew that fuse right off the bat (but that's only because it was a bigger fuse than what was recommended and you never use a larger fuse than recommended in any situation or you could overload the circuit.)

There is no circuit. Just the power transformer and no, you won't overload the transformer. If it is shorted, it will still blow a 10 amp fuse with no overload to any circuit whatsover.

 

[cademan]

If the fuse blows with both ends of the primary disconnected, what does the PT have to do with it at this point?

All things considered, uprating fuses is rather dangerous advice.

Nono, not when it is in this kind of situation. If the transformer is shorted, it will blow a 10 amp fuse all by itself. I never recommend a higher rated fuse for any equipment that it is rated for. This situation is for the power transformer only with nothing else connected to it. It is simply a test only.

I have plugged in shorted transformers without fuses just to watch them bubble, smoke, and fry! This is just me so don't try this at home kids.

 

[cademan]

That is correct, I have also drawn a circuit diagram of what it actually physically looks like in there for your information.

I hope this helps you see what's going on with the neon indicator circuit.

That green capacitor is actually in series according to your drawing.

 

[cademan]

for the third time in this thread, could someone please answer this question?

One thing about this PT is that it has no center tap, and uses the 70 volt tap to develop the bias voltage. Would another PT from a PA amp with the CT , (not used, left disconnected) and the 70 volt tap, work in this case?

Sorry Artcurus, yes it would work but some tweaking would be in order to dial in the correct voltages. I do hope that you mean the 70 volt winding (tap) on the power transformer and not the 70 volt speaker output tap.

We want to get the power transformer issue out of the way first.

 

[Artcurus]

Sorry Artcurus, yes it would work but some tweaking would be in order to dial in the correct voltages. I do hope that you mean the 70 volt winding (tap) on the power transformer and not the 70 volt speaker output tap.

We want to get the power transformer issue out of the way first.

thank you for the clarification. I finally gave in and d/led the schematic for a closer look.

Okay, so the 70 volt tap for the OT is out. From what I understand, this xformer is not that common, splitting the voltages between two xformers might be our only/cheapest option.

 

[captainclock]

There is no circuit. Just the power transformer and no, you won't overload the transformer. If it is shorted, it will still blow a 10 amp fuse with no overload to any circuit whatsover.

Yeah but there would be a circuit though if the primary was hooked back up to the switch and the fuse block, so how do you suggest I test the transformer with a fuse if the only way to test it is to put it back into the circuit? by the way like I said the primary wires are currently disconnected from the circuit so I can't really test it with a fuse currently unless I put the primary wires back up to the switch and the fuseblock.

 

[captainclock]

That green capacitor is actually in series according to your drawing.

Yes, and your point...?
I was only drawing what I saw in the cabinet I didn't claim in anyway to know what kind of wiring was being used in that circuit.

 

[captainclock]

OK So green capacitor (which is a .22 MFD 600 VDC capacitor) is completely out of the circuit I unhooked one leg of the 52k Ohm resistor and one leg of the neon bulb from the neon indicator circuit so that it was out of the circuit and I hooked the black primary leads from the transformer back up to the fuseblock and the switch so hopefully sometime this week when I have the time I can hop on over to radio shack and pick up some more 3 amp Slo-Blo fuses and test the circuit out again and see if it blows a fuse or not.

 

[cademan]

Yes, and your point...?
I was only drawing what I saw in the cabinet I didn't claim in anyway to know what kind of wiring was being used in that circuit.

If it's in series, it is not across the switch and cannot present a short situation.

 

[cademan]

Yeah but there would be a circuit though if the primary was hooked back up to the switch and the fuse block, so how do you suggest I test the transformer with a fuse if the only way to test it is to put it back into the circuit? by the way like I said the primary wires are currently disconnected from the circuit so I can't really test it with a fuse currently unless I put the primary wires back up to the switch and the fuseblock.

I suppose you misunderstand about when I say circuit. Lifting all secondaries and primaries from the power transformer removes the power transformer from the circuit entirely.

If you had the power transformer sitting on a table all by itself, it has been removed from the circuit (amplifier) completely.

Even if the power transformer were still attached (bolted) to the amplifier but nothing is connected, it is out of the circuit regardless if you have to use the exsisting fuse holder and switch. Make sense? That's the way it is!

I don't see if it was mentioned but when thew fuse blows, is it blackened or just opened? A blackened fuse indicates a critical short and if the transformer is doing this to fuses without it being attached to the amplifier circuit whatsoever, then it is bad.

The transformer seems to be a blocking point here but is not really that difficult to diagnose for shorts.

 

[JP]

If it's in series, it is not across the switch and cannot present a short situation.

Based on the drawing and inferring the missing connections using the schematic, the cap is across the AC line.

 

[cademan]

Even being across the AC line, this is the standard connection to prevent pops through the AC line to be picked up by other radios or stereos. Either way this capacitor (I believe) is not the problem here because if it failed shorted, it would have been blown apart just like his Magnavox stereo in another thread happened to do. If it failed open then absolutely nothing would happen and it would just sit there and do nothing.

Ever turn on a light switch and heard a loud pop through your radio or stereo?

This is that very cap that prevents that from happening. This is the so called death cap. Sometimes they short open and sometimes they short closed.

I have a double turntable set-up that I built but forgot to put that capacitor back on the AC line, and everytime I turned them on or off, I got an awful pop through the speakers and it would even sometimes send my amps into protection for a few seconds. This capacitor prevented that and now I enjoy quiet switching without pops.

 

[captainclock]

I suppose you misunderstand about when I say circuit. Lifting all secondaries and primaries from the power transformer removes the power transformer from the circuit entirely.

If you had the power transformer sitting on a table all by itself, it has been removed from the circuit (amplifier) completely.

Even if the power transformer were still attached (bolted) to the amplifier but nothing is connected, it is out of the circuit regardless if you have to use the exsisting fuse holder and switch. Make sense? That's the way it is!

I don't see if it was mentioned but when thew fuse blows, is it blackened or just opened? A blackened fuse indicates a critical short and if the transformer is doing this to fuses without it being attached to the amplifier circuit whatsoever, then it is bad.

The transformer seems to be a blocking point here but is not really that difficult to diagnose for shorts.

I have a picture of the original fuse that was in this thing which was blown and then I have a picture of the new fuses that I had bought at radio shack to test the circuit with. As you will very well see from the pictures the original fuse was blackened at the bottom (and that was when whatever failed in the circuit failed to the point of blowing the fuse failed), and the three new fuses that I had bought from radio shack as you will very well see isn't blackened and was just opened, and that was with the transformer out of the circuit, except for the primary side of the transformer on two of them and the and the other one was from when I removed the primary leads from the circuit and tested and still ended up blowing the fuse, but it wasn't blackened either. So you try explaining that.

The first picture is the original fuse that was in the amplifier when I got it, and the second picture is the 3 fuses that I had bought from radio shack that were from when I tested this thing with the power transformer out of the circuit. as you can see the original fuse was blackened but the new ones weren't when they blew.

 

[Artcurus]

Even being across the AC line, this is the standard connection to prevent pops through the AC line to be picked up by other radios or stereos. Either way this capacitor (I believe) is not the problem here because if it failed shorted, it would have been blown apart just like his Magnavox stereo in another thread happened to do. If it failed open then absolutely nothing would happen and it would just sit there and do nothing.

Ever turn on a light switch and heard a loud pop through your radio or stereo?

This is that very cap that prevents that from happening. This is the so called death cap. Sometimes they short open and sometimes they short closed.

I have a double turntable set-up that I built but forgot to put that capacitor back on the AC line, and everytime I turned them on or off, I got an awful pop through the speakers and it would even sometimes send my amps into protection for a few seconds. This capacitor prevented that and now I enjoy quiet switching without pops.

That's pretty sweet Cademan

 

[cademan]

Well now I do believe that the power transformer is bad because even though the other two slo-blos don't look blackened, they are still showing great stress and melted insides which looks like a short to me.

An opened fuse will look like a break in the middle and sometimes on either end and this scenario happens when the fuse rating is just simply too low to handle the inrush current when first being turned on.

 

[captainclock]

Even being across the AC line, this is the standard connection to prevent pops through the AC line to be picked up by other radios or stereos. Either way this capacitor (I believe) is not the problem here because if it failed shorted, it would have been blown apart just like his Magnavox stereo in another thread happened to do. If it failed open then absolutely nothing would happen and it would just sit there and do nothing.

Ever turn on a light switch and heard a loud pop through your radio or stereo?

This is that very cap that prevents that from happening. This is the so called death cap. Sometimes they short open and sometimes they short closed.

I have a double turntable set-up that I built but forgot to put that capacitor back on the AC line, and everytime I turned them on or off, I got an awful pop through the speakers and it would even sometimes send my amps into protection for a few seconds. This capacitor prevented that and now I enjoy quiet switching without pops.

That made absolutely no sense at all, the "death cap" that was in that magnavox that I had worked on for that guy who I'm now trying to fix an old Zenith two piece console for blew apart because it suffered a direct hit from a lightning strike and didn't have a fuse in circuit to protect the circuit and this amplifier did have a fuse to protect it and the amplifier DIDN'T suffer a direct hit from from lightning like that old Magnavox console I worked on did, so of course that capacitor isn't going to be blown open like the death cap in that magnavox I worked on. It more than likely failed shorted due to old age which is going to blow the fuse, and if a capacitor fails due to old age then its not going to show up as a blown and burned capacitor! You said so yourself!

 

[JP]

A death cap is connected between the AC line and the chassis, and if shorted can energize the chassis, and as the term started (AFAIK) due to guitar amps, the guitar and player. Hence the term death cap. This one is connected across the AC line - no chassis connection.

The amp is blowing fuses with the PT primary - again, primary - disconnected. Something is shorting BEFORE the PT. This cap is across the AC line after the fuse, so the fuse could be blowing before the cap blows apart. Now do you understand why I said putting a 10A fuse in was unsafe?

Captainclock said the cap measured between 20R and 60R, but did not climb. Apparently his meter was jumping around. At any rate, this leads me to believe the cap is leaking excessively, which could indeed cause the fuse to blow.

With the information at hand and considering the fuse is blowing with the PT primary disconnected, what's the likely cause?