Don T., I need your help again

[Holger]

It's me again, Holger and once again you're the one that seems to be the expert to ask. I'm still looking for the 1977 Coliseum Bass schematics.

I'd be well happy if I could find them on this site, as you wrote, but I don't really know under which name or abbreviation they are listed, in the schematics section I can't find any coliseum bass schematics unless they have a different title, maybe something with numbers and letters, I don't know. And if it should be sad but true that they're not on the site, again where would you recommend me to look for them? Or is the power amp section maybe the same like that of another one or at least the circuit board of the power amp?

Well, what do you  say?

In the meantime I have had success at least in tracking down the mistake to the poweramp circuit board. The problem is that the power supply for the circuit board is OK, the power arrives, but it gets lost or faulty somewhere on the way to the transistors.
The sinus line is missing half of it and the remaining half is not correct either, if you know what I mean.
Do you have any idea, any usual suspect for a problem like that.
I guess I will need the schematics anyway.
An amprepair shop from the States mailed me that they have the schematics ( they're $ 20 ) and they say they are from 1970, is that possible  ( I'm asking because it sounds like before Hartzell and mine is produced by Hartzell )

Thanx a lot in advance!!!
Holger

[Don T.]

Hello:
 I did a search also and did not find the Coliseum Bass schematic. Wow - I thought that base was covered but obviously not. I will see what I can do. I still have not thoroghly searched the entire archive that I received. I have found stuff like the Gen l and Gen ll powered mixer schematics so you would think the Coliseum would be there.
As for your problem. The power supply for that model is a funny one. It was built in the days of single polarity transistors. The amplifier is biased at 50% of the supply voltage. In other words, measuring voltages from signal ground will show you +65v and -65v however the supply is really 130v. I have seen filter caps that short only the upper voltage limiting supply voltage and causing distortion and hum. If one of the output transistors is shorted that side of the supply will collapse. If a predrive transistor is bad then it can turn on and hold the output transistors on and that too will collapse the supply.
Don T.