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Bogen DB212 Hot Chassis!

Johnny_Law

Johnny_Law

o.O
Subscriber
My first tube amp. :banana: Very excited.

Problem: chassis is hot. Intermittently. If I hold my finger on the faceplate while it's plugged in, I'll get a nice sizzle up my arm from time to time but not constantly.

Has anyone seen this before? Know what to check? I have the circuit diagram I'll study, but I don't believe it's supposed to be like this.

Some pics on the bench just so we know what we're working with:

IMG_20160812_201457184.jpg

IMG_20160812_201441075.jpg

Thanks a million,
Ryan
 
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It's not really a hot chassis as what a hot chassis really is but rather you are getting some voltage on the chassis which is probably due to a bad capacitor also known as the death capacitor. Does this amp have a capacitor connected to the chassis and the mains somewhere near the power plug or power switch?

Maybe show us the schematic or a better bottom pic near the power transformer so we can see if it does in fact have one of these capacitors.

Also while messing around with it before doing the mandatory recap, try turning the plug around in the socket when you happen to feel a little voltage on your finger. Stay safe and keep one hand in your pocket and don't be barefoot on a cement floor when fooling around with it. :yikes:
 
Agree, probably faulty cap on the mains. I'll repeat my usual mantra of removing those caps and installing proper 3-wire mains cords with safety grounds. If you really need caps on the mains to shunt noise to ground, use properly-rated safety caps.
 
Great, thank you - see that one green cap? That's the one:

IMG_20160813_074019491.jpg
Here it is in the circuit:

DB212DeathCap.jpg

Was hoping it'd work right off the bat ... but it's been in storage since 1961! Will start to build the re-cap order list.
 
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Removing it and installing a 3-wire cord is best, but if you replace it, use a smaller value. Modern safety standards allow no more the 0.7 mA ground leakage current, and .047 will pass three time that. Use .01 or 015 uF, SAFETY rated part (Y-cap). You can tell these because they have a half-dozen symbols from the safety agencies printed on them.
 
gadget73 said:
Find a computer or other electronic device that someone threw out and sacrifice the cord. Tons of power cords in the world that will cost you $0 if you have a pair of cutters.
Click to expand...
Yes, or find any old piece of electronics that has a removeable IEC cord ( computer, cdp, etc.) and take it apart to get both a three wire socket and cord.
 
Another question - in conjunction with ordering parts for the recap. I am wondering how to read the schematic in terms of capacitance values. The schematic lists some values as "mmf" (e.g., C12 is "27m mf 10%) and some values with no units (e.g., C28 is ".1"). I am assuming that I should interpret them as 27 pico farads and .1 micro farads, respectfully. Is that correct?
 
yep, you got it. Units are generally assumed to be microfarad unless otherwise marked. It usually says that somewhere on the schematic.
 
Ok now I'm trying to figure out which caps to replace. I gather that the small ceramic caps are OK to leave in place. I have made a spreadsheet with all the capacitors from the schematic and cross-referenced it visually with the caps and found most (all) of those small picofarad values to be little orange film caps.

The other caps look either paper or wax (?). You can see in the underbelly picture that the paper ones are mostly orange or brown and the smaller wax-looking ones are white. Are these OK to replace with electrolytics or is the preferred option to use film/poly (e.g., https://www.tubesandmore.com/products/capacitor-600v-716p-series-polypropylene)?

The list of caps that aren't little orange ceramics are (I believe this is the list of caps-to-be-replaced ... but the list on reddit is a little different so I'm unsure):
  • .047 600V (green) - death cap
  • 10uF 50V (electrolytic black - maybe a previous replacement)
  • 10MFD 25 WVDC (orange)
  • 0.1MFD 400VDC (small white) x 4
  • 100MFD 6WV (brown)
  • 5MFD 200 WVDC (orange)
  • 50MFD 50WVDC (orange)
  • 20uF 350VDC (light orange - maybe a previous replacement)
  • 5MFD 200WVDC (orange) x 2
  • .022MFD 200VDC (small white) x 4
 
Things under 1 microfarad are almost definitely paper. You'd want a film capacitor to replace those.

The rest all look to be electrolytic cap values.

Confirm by counting the parts in your unit. Its not uncommon with this old gear to find that it changed during production, and the schematic may not reflect the unit in front of you.
 
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