Tube amp leaky power transformer?

[Sleeper]

I'm working on the tube amp from on a wurlitzer 145 electric piano.
I've done everything you can imagine but it's still buzzing.
I put on a ground lift adaptor and reversed polarity and now the amp is totally quiet except the power indicator lamp is ON with the amp switched off and vice versa.

This quote from PRR gets me exactly to where I am now.

As Dave says, the idiots at H-K (and many other older gear) put a capacitor from line to chassis. Usually they told you to reverse the power plug in the outlet for least buzz. Usually this left the chassis capacitor connected to the Neutral power wire, which should be a very low voltage to ground.

But sometimes it ends up the other way, cap to Hot.

The cap hold the maximum leakage current to less than one milliAmp. In the 1950s, this was considered "not dangerous". It usually won't kill a healthy person. I think the acceptable leakage is much less today.

As an "improved" (NOT) version, one such mixer I worked with had TWO caps, one to each side of the line. Mostly it never buzzed, and it sure made no difference which way the plug was. But it ALWAYS had 60V from chassis to power ground!

I was running PA for jazz. My system was properly grounded, because the bassist also signed my paychecks and I didn't want him dying before payday. A film crew came in with this mixer and lashed their mikes to my mikes. The musicians complained. Of course it was "my fault", since they knew they were brushing my mikes (and didn't notice that they had also brushed the film crew mikes set a bit back).

I confirmed the 60VAC leakage with a meter, but the film crew didn't want to hear it. They went out for dinner; I whipped the cover off the mixer and confirmed the two-cap connection. I nearly cut the caps but I hate to mess with another man's gear. Instead I noticed an unused Line Out on an RCA jack riveted to the chassis. I got my best RCA cable and ran their line out to a spare hole on my mixer. I told them I wanted to add their mix to my PA feed. In fact I just wanted a hard-ground between their leaky beast and my solidly-grounded system.

I sure was glad I'd done that. We got Eubie Blake in. He was approaching 100 years old, and obviously this was going to be one of his Last Performances. The film crew was there, of course, with their leaky mixer. His mind and fingers were sharp, but boy did his skin and bones look thin, and there were a lot of miles on his heart. I pictured a headline: "Eubie Blake Killed By Soundman" and made double-sure that wouldn't happen.

Find the chassis cap, cut it out. Then test for AC voltage on the chassis to wall-outlet ground, with a 100K resistor in parallel with your AC voltmeter. It should read very low, less than 10 or 20 volts. Any more suggests you have another chassis cap or a leaky power transformer. If you can't get the leakage low, be very careful with this beast! In any case, put on a proper 3-pin plug, green to chassis, and always use a grounded outlet.

I removed the chassis cap and I'm reading 42 volts.
Leaky power transformer?

It's the stock unit and the secondary voltages are about 20volts lower than spec'd on the schematic... I changed the value of the first RC filter resistor to get the voltages up a bit.

I'd like to pull it and test it before buying a new one. How do I tell if it is too far gone?
Thanks
Sleeper

 

[Sleeper]

heres the schemo:

OK so while I'm waiting I've been looking for a new power transformer.
Does this sound right

max output of 6ca4 is 150 ma according to that datasheet, so I'm thinking 150ma should be adequate for the power trafo.

1 6k11 max plate voltage 20ma
2 7868 max cathode current 90ma but there are different specs for plate and grid current- grid current totals in most of the push pull configurations listed are well within the 150ma range,

plate current is much higher though... but that's amplified current right? so should only pertain to output transformer specs right?
http://groupdiy.twin-x.com/displayimage.php?album=lastup&cat=10032&pos=1
http://groupdiy.twin-x.com/displayimage.php?album=lastup&cat=10032&pos=2

Thanks again.
Kelly

 

[adamasd]

The power light is a neon bulb run off of the 120 volt AC line voltage, you have something on P-1 wired wrong, or possible S-1. Stare at it and the schematic for abit, you should figure it out.

adam

 

[Sleeper]

Hi Adam, I've been staring for days... I upgraded the harness to the frontpanel as this is a notorious source of noise on this piano. some people like to use DC for this and I might just do this If it's safe to operate with the potentially leaky power transformer...
my power lamp has it's own seperate ground and AC feed comes directly off the switch.
When the AC polarity is reversed then I'm breaking the neutral instead of the hot with the switch but the neon is grounded- so it has a neutral and then it's getting voltage off of the chassis. when I switch the amp on this gets shorted by the switch (now carrying neutral) and the lamp goes off.

OK I did look more closely at my post.
now that I cut off the chassis cap, the lamp doesn't light at all- with polarity reversed. :roll:

I'm tempted to ignore PRR's warning and just let er rip.
Still pretty curious if there is some further test to be done to see if or how leaky that transformer really is.

Kelly

 

[adamasd]

It has nothing to do with the transformer, the bulb runs off the wall voltage, 120 VAC, it is before the transformer. Something is wired wrong in the vacinity of the switch or p-1. I Tried to look at the schematic but you shrunk it so now it is very hard to read to read.

Transformers do not leak, they just transform the voltage from oneside into a new voltage. If you have voltage on the secondary when it is off it is because it still have some voltage on the primary for some reason. The 42 volts you read is probable from a filter cap holding a voltage, they can store voltages for quite some time.

adam

 

[Flatpicker]

[quote author="adamasd"]Tried to look at the schematic but you shrunk it so now it is very hard to read to read...[/quote]
Yes - can you enlarge it or maybe just post a link to it's full size?

 

[Boswell]

The schematic you posted appears to show the power indicator as a low-voltage filament lamp running off the heater winding. However, the behaviour you have described for your indicator lamp is that of a neon wired to the switched mains. The two scenarios are different. Is it the correct schematic for your model?

 

[adamasd]

The schematic you posted appears to show the power indicator as a low-voltage filament lamp running off the heater winding.

I could have sworn that it came off of the AC last night. My eyes must be changing things around on me.

 

[Sleeper]

http://groupdiy.twin-x.com/displayimage.php?album=lastup&cat=10032&pos=0
Its a pretty big image soe 2000+ pixels wide, I'm not sure why it's been ensmallend.



my changes are AS described, NOT as on schematic.

I upgraded the harness to the frontpanel as this is a notorious source of noise on this piano.

my power lamp has it's own seperate ground and AC feed comes directly off the switch.

I'd still like to get back to my real question which is about transformer leak.
are there any further tests beyond what PRR mentions in the quote at the head of the post. mine seems to have more than 40 volts leakage.
Any additional tests?

Find the chassis cap, cut it out. Then test for AC voltage on the chassis to wall-outlet ground, with a 100K resistor in parallel with your AC voltmeter. It should read very low, less than 10 or 20 volts. Any more suggests you have another chassis cap or a leaky power transformer. If you can't get the leakage low, be very careful with this beast! In any case, put on a proper 3-pin plug, green to chassis, and always use a grounded outlet.

Should I return the chassis cap to circuit and live with it.
Is It Safe
Kelly

 

[Boswell]

No, it's not safe, and neither is a neon indicator wired in the way you intimated.

Remove the capacitor from mains to chassis. Wire the chassis to mains earth and fit a 3-core mains lead with 3-pin plug. If you need a power indicator, revert to using the filament bulb running off the heater supplies.

I don't believe your transformer is leaky. I think your odd measurements were a consequence of having the neon and/or the capacitor in circuit.

 

[Sleeper]

Thanky Boswell.
I just freaked when this happenned. This is one of those projects that you work on and then set aside for months and months, then take another go at it. Sonically I was thinking Yessss! I have it nailed, and then these lamp issues hit. At any rate, I'm very close. Going to continue to rethink the front panel as this is obviosly a major problem on this beast.

Think about ait, a big bundle of wires carrying 120VAC, 6.3VAC as well as volume and trem pot returnpassing just along the side of the 180v polarized reed bar. and these were originally not even shielded!
sounds like a bad idea to me... Lovely sounding piano though.

Kelly