Bogen C100 PA Amp

[DJ951]

Looking for a parts list for a Bogen C100 amp. Newer style with D718 and B688 output transistors.

This one is throwing me for a loop. All the transistors check ok, but using a 60 Watt pass through lamp on the ac inputs shows a short and the amp produces a loud buzz.

I have the schematic, but no parts list.

Thanks,
Dennis Jones

 

[PRR]

Divide and conquer.

Cut between amp and main filter caps. Now no short? Bad amp.

Still short? Cut rectifiers from main caps. If that's a fix, the caps are shorted.

My 2 cents is on the rectifiers. _I_ might even start by snipping the PT from the rectifiers. If still short, look for stray ends on the PT or fuse or switch rubbing the wrong way.

 

[wg_ski]

If the outputs (and drivers, and bias stack) all check out ok then perhaps there is an issue with the power supply. Like a shorted rectifier or short between turns in the transformer. A shorted transformer will make a lot of racket even when using a dim bulb limiter since the current in that one shorted turn will be very high.

If by “loud buzz” you mean coming out of the speaker and not the amp itself - what happens if you disconnect the speaker? If an amp puts DC into a speaker VC, it will light the bulb and you’ll hear the power supply ripple - which will be a loud hum.

 

[DJ951]

Thanks for the replies.
Pretty sure its not a power supply issue, that was one of the first things I checked. All diodes have been removed and tested OK. Filter caps tested OK also. One key to this is in the bias section I believe as I can remove a small pnp transistor and this allows the AC bulb in series to glow for a few seconds then go out and no loud buzz/hum from the output. The output transformer is direct coupled to the center of the push pull output transistors so removing it I believe would compromise the output altogether.

I don't think posting the schematic publicly is a good idea as far as copyright rules go and am not sure if this forum has Private Messages so I can send it via that.




Thanks,


DJ

 

[PRR]

...I don't think posting the schematic publicly...

Bogen's plans are fairly common knowledge. There must be variations over the years as devices go out of production, but the core design is solid and troubleshooting will be much the same.
Download Schematics For Free here at makearadio.com!
http://makearadio.com/schematics/images/bogen-c35-5.jpg

You say "a small pnp transistor". Only Q6 qnd Q9 are PNP. Q9 is a larger device. So I would *guess* you are pulling Q6. Pulling transistors in a powered-up power amp is VERY Risky. Most of them, you pull them, the amp "falls over" to one rail or the other. Because this amp is cap (and transformer) to the speaker, it won't burn the speaker; it still can burn other stuff that wasn't sick before the pulling started.

With Q6 pulled we expect R64 to pull the output center-point from mid-way down to zero. That sure will stop some forms of smoke but is not near a working condition. It does suggest that Q101 Q103 are not melted-short, though they could be burned-open. Q102 Q104 unknown.

 

[DJ951]

Ok, I would never remove or insert a component under power.

You are referring to the wrong schematic.

Attached is the schematic.

DJ

 

[bskoros]

Just found this forum and wanted to say thanks to DJ951,

I have been looking for the correct schematic for a Classic Series C-100 for a week and yours seems to be the correct one with the exception of the transistors being labeled G# and not Q# but that's minor compared to the schematic in post #5.

I might actually be able to repair the one at work now! (has a few burnt and unreadable resisters)

Thanks again!

 

[Michael Chua]

The output transformer is direct coupled to the center of the push pull output transistors so removing it I believe would compromise the output altogether.

According to the schematic, the amp is not connected to the output transformer. The output bypasses the transformer at 4 ohms.

To use the transformer, jump a wire from 4 ohms terminal to LINK.

I suggest you check the 4 ohms terminal for DC.

If there's high DC, one or more power transistors are blown.