hohner ca-12r
Still working on the ca-12r. Need help. I found one of the Marlboro m18-320 outputs open the other one might be ok. I can not find these anywhere. Tried several different npn t-03 outputs but only get bad distortion. Not familiar with this circuit common in the late 60's and earily 70's. This one is a 1974. The bias on the second transistor sounds best when turned to .7 v pot's max on the bass. Can't find a schematic or these t03's anywhere. Please help.
Still working on the ca-12r. Need help. I found one of the Marlboro m18-320 outputs open the other one might be ok. I can not find these anywhere. Tried several different npn t-03 outputs but only get bad distortion. Not familiar with this circuit common in the late 60's and earily 70's. This one is a 1974. The bias on the second transistor sounds best when turned to .7 v pot's max on the bass. Can't find a schematic or these t03's anywhere. Please help.
Hi platefire,
Well, you can't guess at part numbers at random. Have you considered that the outputs may have been germanium transistors? Measure the emitter-base drop with your DVM on diode check. Let us know what reading you got please.
As John suggested above, trace the circuit out. You could learn a lot from that! Also, you may have 1 bias control and 1 voltage center control for the output if it has just B+ and an output coupling capacitor. You set the DC center point to be just lower than the no-load B+ voltage. Once you have it running, run a sine wave at just before clipping and adjust the DC center pot for equal clipping. Raise the input level a little bit until you have it adjusted to the point of minimum "buzzing", or clipping sound. An oscilloscope makes that job a lot easier than trying to do this by ear (possible). Needless to say, you have the amplifier playing into an 8 ohm load while setting DC center.
-Chris
Well, you can't guess at part numbers at random. Have you considered that the outputs may have been germanium transistors? Measure the emitter-base drop with your DVM on diode check. Let us know what reading you got please.
As John suggested above, trace the circuit out. You could learn a lot from that! Also, you may have 1 bias control and 1 voltage center control for the output if it has just B+ and an output coupling capacitor. You set the DC center point to be just lower than the no-load B+ voltage. Once you have it running, run a sine wave at just before clipping and adjust the DC center pot for equal clipping. Raise the input level a little bit until you have it adjusted to the point of minimum "buzzing", or clipping sound. An oscilloscope makes that job a lot easier than trying to do this by ear (possible). Needless to say, you have the amplifier playing into an 8 ohm load while setting DC center.
-Chris
Another vote for John Ellis' suggestion that you may still have other problems.
And even if "parts" are right, you may still havedifferent problems, including but not limited to bad/cracked solder, cracked tracks, corroded connections, etc.
Remember it's a 30/40 y.o. amp.
So first trace that circuit, don't worry about "prettyness" , a picture of a pencil drawn schematic is fine if sharp and readable.
EDIT: just found http://us.playhohner.com/fileadmin/...nuals_historic/schaltplan_marlboro_g_40_r.pdf
If the same, having 2 bias pots is fine, and actually what *all* should have.
Inject 100mV 1kHz audio at the input and scope the output, the "distortion" word by itself is quite vague, we'd like to see what's really going on there.
And not sure about *how* are you adjusting bias.
Not a task to be accomplished "by ear".
And even if "parts" are right, you may still havedifferent problems, including but not limited to bad/cracked solder, cracked tracks, corroded connections, etc.
Remember it's a 30/40 y.o. amp.
So first trace that circuit, don't worry about "prettyness" , a picture of a pencil drawn schematic is fine if sharp and readable.
EDIT: just found http://us.playhohner.com/fileadmin/...nuals_historic/schaltplan_marlboro_g_40_r.pdf
If the same, having 2 bias pots is fine, and actually what *all* should have.
Inject 100mV 1kHz audio at the input and scope the output, the "distortion" word by itself is quite vague, we'd like to see what's really going on there.
And not sure about *how* are you adjusting bias.
Not a task to be accomplished "by ear".
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Not so fast - if they were germanium they would almost certainly be PNP. Most of the time when you see that totem pole output stage with the split secondary driver transformer it's with PNP Ge's. PNP Ge's were common and cheap in those days - NPNs not so much. NPN Si came later, with the same topology. In those days, complementary designs often had an NPN Si with a PNP Ge in the very first attempts to get rid of the transformer. PNP Si were nonexistent (at those power levels) and NPN Ge were rare and expensive. Go figure.
If you used Ge, 0.2V at the low end would probably be enough bias already, and 0.6V would be a crazy amount of overbias. You'd likely have the opposite problem.
I might be inclined to rework the bias circuit around a transistor that you can get easily, like the 3055. Possibly including a diode for temp comp - I didn't see one in the schematic. Maybe one of the resistors is a thermistor? Thermistors are also very delicate, and aren't the best for long term stability.
I *am* assuming they are the same.
Checked some 30-40 Hohner schematics, they boil down to 4 or 5 basic circuits with different labels and *minimal* differences, so I can safely assume they are the same.
And the schematc shows NPN outputs.
As in:
* Collector positive respect to emitter.
* base positive respect to emitter.
As of 2N3714 ... they are Silicon NPN .
I suggest you print the Marlboro 40 schematic (or Marlboro 50, almost the same but slightly higher rails and 2 visible speakers on schematic *and* individual bias trimmers) and check part by part, board to schematic, in fact pencil the actual PCB value on the schematic, then post it here.
IF parts have part numbers, (such as R1 , Q3, etc.) also write the correct ones on the schematic.
That way we can tell you "measure R52" instead of "the one almost halfway between xxx and yyy, a little above zzz"
Checked some 30-40 Hohner schematics, they boil down to 4 or 5 basic circuits with different labels and *minimal* differences, so I can safely assume they are the same.
And the schematc shows NPN outputs.
As in:
* Collector positive respect to emitter.
* base positive respect to emitter.
As of 2N3714 ... they are Silicon NPN .
I suggest you print the Marlboro 40 schematic (or Marlboro 50, almost the same but slightly higher rails and 2 visible speakers on schematic *and* individual bias trimmers) and check part by part, board to schematic, in fact pencil the actual PCB value on the schematic, then post it here.
IF parts have part numbers, (such as R1 , Q3, etc.) also write the correct ones on the schematic.
That way we can tell you "measure R52" instead of "the one almost halfway between xxx and yyy, a little above zzz"