Yepers on the voltmeter... I lit this thing up the first time with a variac, slowly, while watching the to-ground potential of the chassis. At 117VAC in, I had about 10VAC chassis to ground. I then reversed the polarity of the mains cord and did the same, and was (not) shocked (pun intended!) to see about 115VAC to ground. I'll find it... there are plenty of places for insulation to have migrated: The pass-thru holes in the chassis top are quite small and there could be abrasion or flow of insulation at any one of three or four places.

I've actually got an ST-70, factory wired, waiting in the wings... Along with matching FM tuner and pre-amp. This SC console amp is pretty much literally practice... What can go wrong with an old amp, in- and out-of-circuit testing of components, tube handling, voltages of tube components, etc. Besides, its' fun!

Without detracting, or treading on anyone's toes, and in keeping with my previous posts, on this thread, is this a likely good candidate?

http://orlando.craigslist.org/ele/674125151.html

The first candidate for the differing behavior I saw WRT mains cord polarity was the capacitor C73 (power supply portion of the schematic below.) I lifted the ground leg of the cap and 1) immediately noticed an audible hum (DUH!) and b) the to-ground potential of the chassis reached up into scaryland: With the mains cord orientation formerly showing ~10VAC I'm now seeing 95VAC. The mains cord orientation formerly showing 115ish is now showing 190+. This immediately put C65 and the mains transformer into question. I'm going to repeat having "floated" the mains transformer on an insulator and measure it's potential.

Very strongly leaning toward C65 now... I lifted the transformer and its' potential to ground dropped (but not all the way to zero, interestingly) while the chassis stays hot. And, yes, I used the Variac to bring the overall voltage down from scarytown to something like reasonable if I slipped!

It can be fun to trouble-shoot.  That filter cap possibility is related to the question I had about B+ voltages.  Have you checked anodes against the schematic yet? 

What might be done to get rid of the shock hazard of this dangerous chassis?

If I may, I would like to suggest addressing the hot chassis problem before you do anything else.  You need a drain. 

Erik

"Without detracting, or treading on anyone's toes, and in keeping with my previous posts, on this thread, is this a likely good candidate?"

I had the same console about 10 years ago.  A shop teacher friend asked if I wanted it, and I thought it would make for a good project.  If you could see behind the face plate shown in the picture, you would likely change your mind.  They are very complex in terms of construction and the integration of the radio receiving/source sections and output stage (amplifier).   If the one I had gotten hadn't deteriorated so much, I might have worked harder on it, but there was lots of damage and oxidation.

Erik

Thanks for the response! I'll heed your warning in it!

"Thanks for the response! I'll heed your warning in it!"

I'm not necessarily saying it's not worth looking into more, but If your interest is in a DIY tube project, you might consider some other options as well.  As a collector item, someone might like it; as well as someone with a special interest in vintage Grundig products.  It's a well-known company.

Erik

" I lifted the transformer and its' potential to ground dropped (but not all the way to zero, interestingly) while the chassis stays hot"

I assume you're saying you physically isolated the power transformer from the chassis?  Go back and check the schematic in the area of the center tap on the power transformer.  You don't know the origin of this potentially lethal current draw.

Erik

I assume you are referring to The Red/Yellow center tap on the 300VAC output? The other two outputs don't have a center tap.

The one winding is 6.3 volts for the heaters, the yellow secondary winding feeds to the tuner socket. It may be a heater winding for the pre-amp/tuner.

The PS transformer has the two 300 VAC secondary taps, (red) the center tap, (red/yellow) and another secondary tap (red/black) the feeds to that tuner socket. It doesn't indicate the voltage, but I would guess that is a higher voltage.

Maybe check around that socket to see if the red/black wire might have some bare insulation, or if the socket is grounded to the chassis.....I dunno.....If it were me I would lift that extra secondary tap, and cover the lead with a wire nut.

Then maybe try to just use a fuse and a power switch on the line side of primary, hook the nuetral to the other primary, and bypass the tuner socket. You can use a three prong cord and chassis ground the green wire to a PS transfomer bolt.

 

 

mike stehr:

The one winding is 6.3 volts for the heaters, the yellow secondary winding feeds to the tuner socket. It may be a heater winding for the pre-amp/tuner.

The PS transformer has the two 300 VAC secondary taps, (red) the center tap, (red/yellow) and another secondary tap (red/black) the feeds to that tuner socket. It doesn't indicate the voltage, but I would guess that is a higher voltage.

Maybe check around that socket to see if the red/black wire might have some bare insulation, or if the socket is grounded to the chassis.....I dunno.....If it were me I would lift that extra secondary tap, and cover the lead with a wire nut.

Then maybe try to just use a fuse and a power switch on the line side of primary, hook the nuetral to the other primary, and bypass the tuner socket. You can use a three prong cord and chassis ground the green wire to a PS transfomer bolt.

 

 

That's the direction I'm leaning... After verifing that there is no shorts, that the "problem" being perceived is not really a "problem." A polarized cord, with the correct polarity, and an earth ground should resolve the 50/50 chance of having this chassis live to unsafe levels.

"That's the direction I'm leaning... After verifing that there is no shorts, that the "problem" being perceived is not really a "problem." A polarized cord, with the correct polarity, and an earth ground should resolve the 50/50 chance of having this chassis live to unsafe levels."

I have to differ just a little -- with all due respect, of course.  The perceived problem is very correctly described as a problem.  You have high voltage on the chassis that as it is right now could kill someone or start a fire.  That you have not been able to find a short does not mean one doesn't exist, and could be hidden in a place that visually you can't access.  The 'drain' I mentioned you needed above was an earth ground connection for the chassis -- to send this voltage to ground through the chassis ground lead instead of through your body.  That earth ground connection is protection for the user in the event of a problem, and that problem has made itself known.

In order to find the problem, it may involve a complete overhaul and rebuild of the circuit, including power transformer replacement.  The amp is not safe until you locate and repair this issue.  Just as a remote possibility:  what if the amp were sold, along with its newly installed earth-grounded 3-prong plug, and the person who purchased it read somewhere that it is a perfectly safe practice to use cheater plugs to defeat safety grounds, and then one day decides to pick up the amp with both hands, while it's energized, in order to move it to a slightly different location?  What if this same person had his or her audio system in the basement, with a bare concrete floor, and he or she happened to be barefoot at the time?

If you can't find the source of the high voltage on the chassis, please consider taking or sending it to a professional technician who is very familiar with vacuum tube electronics.

Best of luck,

Erik

Oh... I have every intention of riding this one into the ground... I'm intrigued. And, I have access to technicians greater than myself (for example, a retired Northeastern University professor of Engineering that worked on tube transmitters for Raytheon, IIRC.) I'm riding the learning curve until I decide to call in the calvary. Too few lunchtime minutes, unfortunately. 

 

--Edit: Good point on the cheater plug, BTW. I have no intention of selling this thing intact, but who knows what the future holds. For the sake of our Dear Future Reader, a little plausible denial: I did not mean to infer "just slap a grounded cord on it and call it a day"  

 This might be something you can try if you want to. Just be really careful about it.

If that extra secondary to the tuner socket isn't the problem, maybe try unhooking the PS transformer from the amp chassis.

With just the PS transformer unhooked from the chassis, jimmy up a cord with a inline fuse if you want on the line side. You'll be checking it unloaded, so make sure to tape or use wire nuts on the secondary taps and heater taps, so they don't touch each other, or you. Same with the primary connections.

Hook up the line and nuetral to the primary wires of the PS xformer. Set your DMM to AC voltage at the highest setting.

Now I'm not really certain where you would use the ground of the DMM, I would guess the center tap of the PS transformer. (don't trust people on the internet) Then take the hot lead and affix it to one the PS transformer's end bells.

Plug it into a variac, and ramp it up to 120 volts AC. You want to have everything setup so you are checking for AC leakage from the PS transformer's chassis without touching anything, except that variac knob.

If there is any AC voltage leakage from the PS xformer to the chassis or from the CT, I would guess the DMM should detect the AC voltage on the PS xformer chassis itself. I would think if the transformer is shorting AC to the chassis, that it would take out the fuse...but I'm a forever n00b, so take it with a grain of salt.