Baldwin C1 issues

[Bishop]

Hey guys, I'm checking out a friends old Baldwin C1.

Channel 2 works OK, but Channel 1 has weak signal and no reverb or tremolo. (they look like they would be labeled opposite on the schem)

From scoping it appears that the signal is losing steam right where the tremolo kicks in. At the 2uF cap on the base side of the NPN there. It's not the 2uF or the transistor, so I thought if I pulled one end of the 500uF 6V cap that it would omit the tremolo circuit, but it actually just kills ALL my signal.

Checking out the tremolo circuit I noticed on the base of the first transistor is basically < -1VDC, (opposite the 120K resistor there is the proper -30V.) The 120K resistor measures about 14K in circuit. Does that seem odd?

The main problem I have is that the way this amp is built makes it extremely difficult to swap components without making a mess (that 120K for example). So I'm just looking for any advice that could make troubleshooting less stressful.

A way to bypass the tremolo would be great to rule that in or out, but short of pulling that 500uF cap I'm not sure the best way to go about that.

Sorry it's kind of a jumble of a post, any advice is appreciated. Sometimes I have to post first and then it comes to me.

[nevetslab]

This is the Baldwin combo amp that has that 5-button 'Supersound' tone circuit, with both channels on the top of an aluminum panel that spans the width of the amp, right? I've still got the one my band bought back in the late 60's, that got put aside for better sounding amps. I've never seen the schematic for it until now, and have actually never worked on one. Just looking at the Ch 1 circuit....the preamp signal has a lot further to go before it gets to the summing node to join with Ch 2, and feed the power amp section.

I too would have guessed lifting the 500uF/6V cap out of circuit would bypass the tremolo circuit for the time being, and should have gotten the signal to pass on thru the voltage amp, then on thru the emitter follower portion that feeds the reverb output driver stage that finally combines with Ch 2's signal path. I'd spend a bit more time at that voltage amp where the Tremolo circuit ties in. As for the Reverb, I'd pull the tank and see if you don't have a broken wire on either the drive xfmr or the return xfmr from the tank springs.

You could also try injecting signal into the base of the emitter follower that drives the reverb tank (tank fed from the collector). I only vaguely recall how the amp was built....I seem to recall terminal strip construction, not making it easy to identify everything. Is it Silicon or Germanium semi's?

[52 Bill]

A way to bypass the tremolo would be great to rule that in or out, but short of pulling that 500uF cap I'm not sure the best way to go about that.

You are looking at this in the wrong way, that transistor is biased on by the negative voltage through the 120K resistor, which grounds the one end of the 500uF cap. That sets the ac gain of the stage. The transistor is turned on and working as you found that the signal dies when the cap is no longer grounded.

Have you tested the value of the 500uF cap?

[J M Fahey]

Please measure and post DC voltages at points shown in red.
Generally transistor collector voltages, we check healthy DC before worrying about AC.
When I ask about supply voltages, such as various "+22V", etc. , I want them measured at the resistor or transistor leg they are supposed to reach, checking they are fine at the supply is not enough here.

[soundmasterg]

I've got one of these amps at home that needs a rebuild. I got it for free and I figured it would be a good learning repair, but I haven't been able to get to it yet. Very late 60's look and design....no idea about how they sound though I suppose it could make a good keyboard amp or something similar. I guessed it was using germanium BJT's from the era, but I haven't had a look at it yet in that detail.

Greg

[Bishop]

Thanks so much for the replies, I took a few days away from my bench...

So I reinstalled the jumpers to test this thing again and the reverb and a good amount of signal came back. I suspect a bad connection there.

BUT... It's still much weaker than the other channel and the tremolo still doesn't work.

I haven't really taken the time to get back into it, but I thought I should update.

I did take voltages that J M Fahey recommended except the ones in the reverb chassis.

Rounded to nearest whole number

1. 6VDC
2. 11VDC
3. 12VDC
4.
5.
6.
7. 14VDC
8. 23VDC
9. 23VDC
10. 45 VDC
11. 21.4VDC (I think this one was oscillating)
12. 23VDC

I'm away for a few more days, but I will scope it again and see if I can figure it out.