# Dead Bogen CHB50 - warning- Idiot (me) worked on it



## whywhyzed (Jan 28, 2008)

stock PA CHB50 never modded. I think I knocked it on its side gently on the carpet, but still it seems that killed it.

symptom - immediately popping power fuse at turn-on
I remove all tubes, same deal
with a variac, the big green resistor gets hot and smokes when I get to about 30V

here's were it might get dumb.... I thought I would replace a capacitor (thanks internet), so I actually opened (clipped 1 wire to) what appears to be the first cap, and I repowered the amp and no heat, no fuse blow (but probably because I basically opened the circuit) so that's where I suspected the cap 
So, have I eliminated the transformer? - probably not completely eh? no load on it
I bought a Fender cap with roughly the same specs as the original, and replaced the first cap that runs parallel to the resistor.

same symptoms exactly

So my question now is, should I keep going, or just take it to a pro. Recommendations in east GTA/Ptbo area.... for diagnosis, maybe mod it with 1 gain stage

All the reading seems to be over my head.... or oversimplified- like they say "you can check caps with a multimeter"...yeah sure...but not in the circuit? no?
I guess I have to desolder each one to do that or my meter is going to find continuity through the rest of the circuitry...


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## whywhyzed (Jan 28, 2008)

I wish I hadn't started on it, but here's what I did - replaced this cap


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## Furtz (Nov 27, 2010)

My guess is that your power transformer is OK. 
Did you observe the polarity on the cap you replaced? Negative goes to ground.
If so, it could be another power supply filter cap. They should all be replaced anyway if they are the originals.
Could also be a bad power tube.
Here the schematic. http://makearadio.com/schematics/images/bogen-chb50-6.jpg


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## Furtz (Nov 27, 2010)

Actually it's probably not a power tube seeing the fuse popped with the tubes removed.


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## whywhyzed (Jan 28, 2008)

thanks! I think I got the polarity the same - I pointed the "negative arrow" towards where the original cap was marked negative. I hope that's right. 
I have another filter cap ordered to replace the black cardboardy looking one. 
Funny that one was aluminum and the other was cardboard wrap, when they are both 300V, 100microF

Caps are originals, but the amp had zero hum.

I think this amp was new when I got it a few years ago. Original tubes. 

appreciate the schematic!

I see a couple of real easy things I can eliminate..... like clipping that 70V tap off the transformer to make sure it isn't shorted to ground in one of the 70V speaker plugs or something- I won't be using that voltage


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## whywhyzed (Jan 28, 2008)

a-ha... the schematic actually shows both 100uF caps with the same part number...the number that's on the cardboard one
so somebody has been in here before...


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## whywhyzed (Jan 28, 2008)

follow up years later... finally had time to look at this amp again. I found a bad diode (red circle). I wonder if it might be the only issue. 
It reads .011 or so (hunts around) in fwd or rev bias. Excited to change it out and see what happens.


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## jb welder (Sep 14, 2010)

So you had previously replaced both those 100uF 300V caps? One of them being bad would be why that resistor was smoking.
Triple check that they are installed as shown in the schematic.
If you are going to replace the diode, I'd suggest doing the other one too. It may have been stressed by the fault.
You might want to build a "lamp limiter" to power it up without damaging other parts if there is still a fault.
Edit: you mentioned a variac, so ignore the lamp limiter comment.


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## whywhyzed (Jan 28, 2008)

I replaced both 100uF 300V caps yes - Last night I tested around some other components and I think one of my .1uF is shorted. I'm going to replace all the old caps, and both diodes. 

It's tricky trying to figure out what can be tested without removal. Takes a lot of tracing to see that for some tings, simply pulling the tube will allow bench testing. Other components need to be detached to bench test so ohm, cap test or diode check on the meter doesn't measure a parallel path.


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## whywhyzed (Jan 28, 2008)

replaced the two diodes and it's working great. The other caps were all fine. I wish my other amps were this quiet at idle. 

Now - to read up on how to do some mods. It actually sounds "ok" with my Tonelab SE in front of it. I might take it to next practice and see how it sounds in a band setting. It's very tight and punchy in the high/mids which is not great for basement noodling tone.

flippin' loud.


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## whywhyzed (Jan 28, 2008)

well that was easy - I installed one 47 microfarad cap parallel to the 470 ohm resistor off the V2B half of the 12ax7 and it's a completely new amp tone wise. Still not gainy, but I use pedals in front of it anyhow. 
That cap took all the flab out. 
Not sure I can reason why that cap rendered the Mic1 pot almost useless, but I never touched that volume anyways. I just use the master. 
Pretty fun amp now.


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