Bogen CHB20A Blowing fuses after Mod----Can't Quite Figure it out---HELP!

[Platefire]

The amp has been running fine. Last night I decided to do a few minor mods that included installing a standby switch, installing 8Ohm output jack and installing a ground leg on input jack switch to gound it when unplugged.

Got it all completed and fired it up for a test and it has been blowing fuse after fuse. This is blowing my mind because I can't figure out what I did wrong? I'm pretty sure the problem is in my Standby switch connections/changes and I've tried everything I know to do without success. I'm not too familiar with SS diodes so I may have took a wrong turn there. I haven't lifted the PT Red secondary leg yet but I don't think the PT has gone bad. Please take and look at the attached pixs and schematic and I hope you can help me figure it out!!

Here is what I did on the Stanby:

1-Lifted the output diode leg/lead out of it's connection with first filter 40Mf fileter cap, output trasformer power tap and following power resistors/Fileter caps and installed the output leg of diode on a terminal strip. Routed the diode output through a stanby switch back to the terminal strip connection that the diode output leg was previously connected to(filter caps/power resistors).

2-I removed the old paper 40Mf 250V filter cap(first filter cap) from it's metal mounting ring and installed a 40uF/500V SPRAGUE ATOM to replace the paper.

3-I think the problem is the DCV output it shorted to ground because I'm getting continuity between chassis and DCV output???? but I can't figure out where it's shorting to gound? Help Please!!

I've attached a schematic for your reference.

Thanks Platefire

[Willabe]

Hi Platefire,

3-I think the problem is the DCV output it shorted to ground because I'm getting continuity between chassis and DCV output???? but I can't figure out where it's shorting to gound?

Don't turn that amp on untill you find that short!

From the pic. it looks like your taping the diode from the wrong point. Standby switch goes after where the 2 diodes meet together, looks like your taking the tap from only 1 diode.

Also your using a lamp limiter to test new mods/builds, right?


           Brad           

[Willabe]

Oh,  You did'nt have the schemo attached yet when I first saw this. It's not a FW supply.

Find the short, its got to be the problem.

I can't see it very clearly, but have you got those red and red/yellow cloth covered wirers crossed at the term. strip?


             Brad

[sluckey]

It's unclear from the pics, so, where is the negative lead of that Atom connected? It should connect to a positive lug on that cap can. And the red/yel PT wire should be connected to that same lug. Nothing else should be connected there.

[RicharD]

My money is on the standby switch current rushed the diodes and blew them.  Check em, if shorted, replace with a series pair instead of individuals.

[Platefire]

Thanks for your response!

To clarify location point of stanby on the previous schematic is between the in series diode and the 40uf cap to ground--which I think is the correct location it should be? The 40uf filter cap + end is connected to terminal strip connection where the 2.7K/3W resistor goes to can capand  the OT transformer primary DC. This is where the output DC leg of the in series diode was removed and routed through the standby switch back to the same location where all B+ begins to be filtered, dropped and routed.

I'm beginning to suspect the filter cap can. The PT ground (Red/yellow) is connected to a ground lug on the multi cap can and I noticed it was loosing continuity with chassis when I was moving that term. Makes me wonder if it's not shorted out inside. I think I may relocate that PT Ground to a PT mounting nut with a new ground lug connector. Then with standby switch on should remove any short that may be occuring in the can with DCV cut off to that location.

The tubes are pulled so there is no load on the amp. Is there any easy way to check the diodes or 40uf filter cap to see if they are bad with a MM. Platefire

[Willabe]

I'm beginning to suspect the filter cap can. The PT ground (Red/yellow) is connected to a ground lug on the multi cap can

The schemo shows R/Y trany wire goes to where neg. end of 40uf cap (C18) connects to pos. end of 1 section of cap can (C19A) not directly to ground, but standing on 1 section of cap can. It's not directly going to ground but goes through C19A to ground.

Also looks like there's a droping resistor at the C18/C19A connection. Schmo does not show this.  


                Brad      

[sluckey]

The PT ground (Red/yellow) is connected to a ground lug on the multi cap can and I noticed it was loosing continuity with chassis when I was moving that term. Makes me wonder if it's not shorted out inside. I think I may relocate that PT Ground to a PT mounting nut with a new ground lug connector.

NO! This is a voltage doubler circuit. The PT red/yel wire does not connect to ground. It SHOULD connect to a positive lug on that cap can. Along with the negative lead of that Atom you installed.

Is it connected right?

[DummyLoad]

if possible, go back to square one - undo your mod.

from past experience of mis-wiring doubler ckts.; if you blew a fuse then you likely blew both diodes as well.

use, or build and use a lamp limiter.

lastly, make SURE that the hacked cap clamp does not/is not cutting into the insulation on the ATOM cap you installed - the body under the insulation is also the (-) terminal.

--DL

[plexi50]

If the diodes are bad and shorted you will get a infinite tone from your DVM from both ends of the diodes

There cheap enough and best to replace in either case. A lamp limiter has alerted me immediatley of problems like this

Make a 2 amp  fuse breaker with leads and alligator clips to clip to the fuse holder to save on them expensive fuses

[Platefire]

Last move was a stupid move. Moved the red/yellow to a PT bolt--as sluckey says--that needs to go right back where it was on the can cap with the negative end of the 40uf cap also attached. I will put that back like it was . So Sluckey to answer your question, yes it was hooked up right originally and it will be re-installed as it should be.

On checking the diode with my MM continuity check, I'm getting continuity/tone through the diode to ground but not getting continuity/tone through the in series one before standby switch. Does that mean the diode to ground it blown?

Question??? If the diode connected to ground is blown, is that the reason I'm getting continuity to ground on the terminal strip where the bias circuit it connected a red PT secondary attches. On the other part of the terminal strip where the other good diode connects through the stanby switch, I'm not getting continuity/tone with ground.



So have we determined that the diode to ground is bad and causing the problem? I have one new In4007 diode, will that work?

Here is another overhead view of that section with the 40uf + side disconnected, that I will have to re-connect. The diode to ground is  hard to see and is directly under the red wire to 40uf cap. The other diode is directly under red wire to stanby switch.

[gldtp99]

Yes, this is a Voltage Doubler PT------ the PT HT leads, the two rectifier diodes, and the two 40uF caps must be connected as shown in the schematic-----you put the Standby Switch in a bad spot, also---- you could put the Standby later in the circuit then where you had it---- put it on either side of the 2.75k/3W dropping resistor that's downstream of the series 40uF cap pair (on the top rail on schematic)---i'd replace both rectifier diodes with 1N4007's---- also a good idea to check the orig cap can---- a JJ 40/20/20/20@500V is an inexpensive option but you'll need to enlarge the chassis hole.
So--- rebuild the power supply with new diodes (observe proper polatity), new multi-section cap can (keep 40uF Atom)---wire it properly according to schematic---install Standby Switch downstream of Voltage Doubler/Rectifier section----- and hope that the PT wasn't damaged...................gldtp99

[Platefire]

If I located the standby switch downstream to 2.7K resistor with he present hookup, the power tube plates would still be getting DCV and the preamps would be cut off on standby. I've been thinking it over
and I think the question below is a better solution but let me know if I'm thinking right.

Question: Regarding location, how about I relocate the 40uF + lead from the larger terminal strip where  it's presently connected to the 2.7K dropping resistor and OT voltage connect and move the 40uf + lead to the tiny terminal strip that I moved the output end of the diode to. Wouldn't that make the doubling circuit complete except for stepping down voltages for the downstream PI and preamps? Platefire

 

[sluckey]

Question: Regarding location, how about I relocate the 40uF + lead from the larger terminal strip where  it's presently connected to the 2.7K dropping resistor and OT voltage connect and move the 40uf + lead to the tiny terminal strip that I moved the output end of the diode to. Wouldn't that make the doubling circuit complete except for stepping down voltages for the downstream PI and preamps? Platefire

Yes, that'll work. I was fixing a couple pics to show just that...

[Platefire]

OK, Thanks! I think you have got me on the right track now. Appreciate it!

On the 40uF Cap, is there any way to test that to make sure it's still good. It was slightly used but is suppose to still be good but I would like to make sure before I re-arrange things. Platefire

[sluckey]

Just use your ohm meter. A short circuit or low resistance is bad. A good cap will read a high resistance and the resistance should increase with time. After about a minute the reading should settle to a high reading. An analog meter is very good for this because you can see the cap charging. A DMM 'may' not give very good results.

[Platefire]

All I have is a DMM but I tried it and on a 200K setting it increased steady from -20.0 until it got up to 200K and then jumps to 1   . and stopped on that reading. So I thinking from that it's good! Thanks, Platefire

[Platefire]

Well this is the most trouble I had with any of my amps in a couple of years!!! but thankfully the amp is happy again and I am too. I did all the changes last instructed by sluckey and first fired it up on standby and I was sure happy to see that pilot light keep burning this time. I removed the metal band previouly around the 40uf cap because after I fired it up the first time seen fire fly from the band to the body of the cap---and that band is grounded to chassis. So the cap is floating in mid air supported by the leads but is still pretty solid.

I went ahead and installed the tubes and hooked my little test speaker up and it's running nice and quiet with plenty of punch. I will test it a little more, check voltages and make sure it is stable before I stick it back into the bogen housing.

It's a shame the shape I get in when one of my amps have problems that I can't seem to solve. I was taken my wife to the Dentist yesterday and we were riding down the road and she said, "Why are you not your normal cheerful self, your so quiet"---"your thinking about the amp aren't you?" ----I have to say she nailed it on the head!!!! Thanks guys for your support! Platefire  

[Geezer]

thankfully the amp is happy again and I am too

Woo Hoo! Way to hang with it, Brother! 

[sluckey]

I removed the metal band previouly around the 40uf cap because after I fired it up the first time seen fire fly from the band to the body of the cap---and that band is grounded to chassis.

Ah ha! That was likely the whole problem. Glad you got it working.

[Platefire]

Just to sum it up---Thanks all for your support on this and especially thanks sluckey for the corrected schemaic and edited picture you provided for standby mod correction.  

Don't know if it's a senior moment or my taste in things are changing but I like this old bogen circuit as is and even like the old rusty crusty Bogen case in its present state--think the wear, rust and tear looks cool. I'm telling you some of you that modded this thing to death need to take a second listen to this original circuit--I'll say no more about that and leave it alone! Just noting a few things I did do:

1-As shown on attached front view the input jack for the mic channel is in the chassis only, so the front of the bogen housing had to be drilled out so all types/sizes of 1/4" guitar plugs would fit through the hole and into the jack.

2-Same situation for the standby switch toggel, had to be drilled out to all switch toggel motion.

3-Rear is all original bogen with the exception of the added unlabled 1/4" speaker jack. Screw on mic connector and aux rca jacks are in place but are disconnected and abandoned in place.

4-It got a fender type handle and new rubber feet from parts left over from previous projects.

5-The #12 bi-pin pilot light was hard to find. Fortunatly my bass player/hospital maint man friend had a whole box of them.

Thanks Platefire