Hi guys.
Today I'm messing around with a Yamaha B100-115ii bass amp from the '80s. Actually it's only the power amp, the preamp does not exist anymore.
The heatsink on this amp gets very hot, 84 - 88 degree Celsius, you can fry eggs on it...
My question is: is this normal?
Today I'm messing around with a Yamaha B100-115ii bass amp from the '80s. Actually it's only the power amp, the preamp does not exist anymore.
The heatsink on this amp gets very hot, 84 - 88 degree Celsius, you can fry eggs on it...
My question is: is this normal?
On the heatsink are 2 transistors 2SC1586 (TO2) and an STV-3H-0 varistor that I replaced with 3x 1N4148 diodes in series.
I'm not familiar with this amp, but judging from SM page 4, bias current should be quite modest and the heat sinks shouldn't be hot.
Focus on the idle current adjustment procedure, as I suspect the bias current is excessive. Double check the three-diode substitution. My memory is that perhaps there should be only 2 series 1N4148 diodes.
Good luck!
Focus on the idle current adjustment procedure, as I suspect the bias current is excessive. Double check the three-diode substitution. My memory is that perhaps there should be only 2 series 1N4148 diodes.
Good luck!
I took out one 1N4148, leaving just two in place. With no input signal applied, the heatsink now is death cold, before it got hot within less than a minute. Bias is not stable and keeps rising over time, compared to before. And I cannot get the center voltage lower than 51.1 Volt. Beside that, it does kind of work... I used a very small 8 ohm speaker as a load.
Tomorrow I will try it with a proper preamp and a 15 inch speaker, to see how it behaves.
Tomorrow I will try it with a proper preamp and a 15 inch speaker, to see how it behaves.
Don't use a good speaker! There's still a defect present that must be located and repaired. I would troubleshoot with no load applied.
I'll study schematic and suggest further measurements.
I'll study schematic and suggest further measurements.
Would you report observed voltages, with all voltages measured with respect to "ground" pin 2:
Board pin 6 (B+)
Pin 1 (B-)
Pin 5 (Output)
Gate TR301
Gate TR302
Base TR303
Collector TR303
Base TR304
Base TR309
Base TR311
Emitter TR311
Base TR310
Base TR312 re B- (i.e. across 150R)
Emitter TR312 re B- (i.e. across 0.22R)
I may ask for further measurements, but this should get us started.
On closer review, three diodes at D304 makes a lot of sense, but for the present let's leave D304 at only two, since you were seeing excessive current and output bias was at +51.1V. Maybe we'll revise later when things look better.
Good luck, be safe.
I'm off to bed. 🙂 TTYL
Board pin 6 (B+)
Pin 1 (B-)
Pin 5 (Output)
Gate TR301
Gate TR302
Base TR303
Collector TR303
Base TR304
Base TR309
Base TR311
Emitter TR311
Base TR310
Base TR312 re B- (i.e. across 150R)
Emitter TR312 re B- (i.e. across 0.22R)
I may ask for further measurements, but this should get us started.
On closer review, three diodes at D304 makes a lot of sense, but for the present let's leave D304 at only two, since you were seeing excessive current and output bias was at +51.1V. Maybe we'll revise later when things look better.
Good luck, be safe.
I'm off to bed. 🙂 TTYL
Someone before me had already repaired this amp before and replaced a few components.
TR301 and TR302 were replaced from originally 2SK30A to K127
TR306 from 2SC1624 to TIP29C
TR309 from 2SC1624 to TIP31
TR310 from 2SA814 to TIP32A
TR311 and TR312 from 2SC1586 to 2SD873
Pin1 and pin2 are actually ground
Tomorrow I will take the measurements your asking for.
TR301 and TR302 were replaced from originally 2SK30A to K127
TR306 from 2SC1624 to TIP29C
TR309 from 2SC1624 to TIP31
TR310 from 2SA814 to TIP32A
TR311 and TR312 from 2SC1586 to 2SD873
Pin1 and pin2 are actually ground
Tomorrow I will take the measurements your asking for.
I swapped all electrolytic caps on the board with new ones except for the two 2200uf 160v that are hard wired to the DC-board. I had no suitable caps on hand and could not buy new ones jet.
I made the requested measurements:
No load applied
Board pin6 to pin1 ( ground / gnd): 108V (DC)
Pin 5 to gnd: 50V
TR301 gate to gnd: 541.2V
TR302 gate to gnd: 50.9V
TR303 base to gnd: 102V
TR303 collector to gnd: 52.1V
TR304 base to gnd: 103V
TR309 base to gnd: 52.9V
TR310 base to gnd: 50.6
TR311 base to gnd: 51.6V
TR311 emitter to gnd: 52.1V
TR312 base to gnd: 3.3mV
TR312 emitter to gnd: 565mV
I made the requested measurements:
No load applied
Board pin6 to pin1 ( ground / gnd): 108V (DC)
Pin 5 to gnd: 50V
TR301 gate to gnd: 541.2V
TR302 gate to gnd: 50.9V
TR303 base to gnd: 102V
TR303 collector to gnd: 52.1V
TR304 base to gnd: 103V
TR309 base to gnd: 52.9V
TR310 base to gnd: 50.6
TR311 base to gnd: 51.6V
TR311 emitter to gnd: 52.1V
TR312 base to gnd: 3.3mV
TR312 emitter to gnd: 565mV
Thanks, PhantomBox.
Many voltages make sense. Some raise questions and will prompt further measurements..
I'm assuming a typo, so that TR301 gate is 54.12V? Seems about perfect, as it's half the 108V B+.
The service manual pictorial on page 4 indicates center voltage should be 48V. I suspect it should be (ideally) equal to TR301 gate voltage.
Tell me about your DC voltmeter. Do you know its input resistance?
Your measurements indicate TR312 voltages as 3.3mV on base, 565mV. Might these be reversed? The emitter voltage should present between pins 4 and 1 of the 6 pin connector.
More later.
Many voltages make sense. Some raise questions and will prompt further measurements..
I'm assuming a typo, so that TR301 gate is 54.12V? Seems about perfect, as it's half the 108V B+.
The service manual pictorial on page 4 indicates center voltage should be 48V. I suspect it should be (ideally) equal to TR301 gate voltage.
Tell me about your DC voltmeter. Do you know its input resistance?
Your measurements indicate TR312 voltages as 3.3mV on base, 565mV. Might these be reversed? The emitter voltage should present between pins 4 and 1 of the 6 pin connector.
More later.
TR301 gate to ground is indeed a typo. Correct value is 51.2V, not 54.12
And TR311/TR312 are TO-2 casings. I always get confused with those, especially when they are upside down as it is the case with this board. Also, I did not pay attention to the last page of the manual: the actual arrangement of components on the board.
Anyway, thanks a lot for your involvement in this matter!
And TR311/TR312 are TO-2 casings. I always get confused with those, especially when they are upside down as it is the case with this board. Also, I did not pay attention to the last page of the manual: the actual arrangement of components on the board.
Anyway, thanks a lot for your involvement in this matter!
At the moment I have two DVMs. The 1st one is an older Taiwan made TMC-160, without continuity check, and 2 sections on the display have gone blank. Internal resistance is 0.8 ohm. I still use this one because it has no automatic range detection and gives a reading instantly.
The other one is a Chinese made Craftsman #82029 that a friend gave me (basically it is crap...) . Internal resistance is between 1.0 and 0.3 ohm. This one does have automatic range detection and continuity check with a "beep". Although it works, I don't really like it that much because it takes forever to give a reading.
The other one is a Chinese made Craftsman #82029 that a friend gave me (basically it is crap...) . Internal resistance is between 1.0 and 0.3 ohm. This one does have automatic range detection and continuity check with a "beep". Although it works, I don't really like it that much because it takes forever to give a reading.
I was trying to inquire about the voltmeter's load resistance when it's measuring voltage--- reason being that many of the amp's bias resistors have large resistance and the meter's load resistance can present a significant load on the circuit. But if your meter is "digital" rather than an analog movement, the load resistance is likely several Meg Ohm and can be neglected, for most purposes.
If the voltage between pins 4 and 1 is 3.3mV, that's a very encouraging sign, and the "center" voltage is about half of B+. The bias current procedure calls for only 5mV, so it would seem to be in the ballpark. Does the pin 4 voltage respond to adjustment of the bias current pot (470R)? Does the amp deliver sound? Do you have a scope and generator to pursue the SM adjustment procedure?
If the voltage between pins 4 and 1 is 3.3mV, that's a very encouraging sign, and the "center" voltage is about half of B+. The bias current procedure calls for only 5mV, so it would seem to be in the ballpark. Does the pin 4 voltage respond to adjustment of the bias current pot (470R)? Does the amp deliver sound? Do you have a scope and generator to pursue the SM adjustment procedure?
I set the bias current to 5 mV but it starts to rise to 10mV almost immediately, measured between pin 1 and pin 4.
The amp does deliver sound. Well, it makes noise when touching pin 3 input with my fingers, just like touching the tip of a guitar cable connector.
I don't have a scope, unfortunately.
I do have an old sound generator from a vintage console.
The amp does deliver sound. Well, it makes noise when touching pin 3 input with my fingers, just like touching the tip of a guitar cable connector.
I don't have a scope, unfortunately.
I do have an old sound generator from a vintage console.
Was the original D304 attached thermally to the heat sink?
You can probably couple any electronic music source into the amp for a listening test. We can make some educated bias measurements and probaby get reasonable performance.
You can probably couple any electronic music source into the amp for a listening test. We can make some educated bias measurements and probaby get reasonable performance.
Sound comes out of the amp when using my signal generator, confirmed. I can select between 30 / 100 / 700 / 3K / 10K / 15KHz. I am going into a 15 inch 8 ohm speaker.
The original D304 was indeed attached to the heat sink. And so does the now 2x 1N4148 (former 3x 1N4148) that I made.
Can you post a picture?
You were correct that there should be nominally three Vbe junctions to provide compensation. Can you add the third back in? But if the third diode is in place, is minimum pot setting already too much bias?
You were correct that there should be nominally three Vbe junctions to provide compensation. Can you add the third back in? But if the third diode is in place, is minimum pot setting already too much bias?
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Hm. I could set it at exactly 5 mV, if I remember correctly. But it gets really hot with 3 diodes. And stone cold with just 2 diodes, and with rising bias and center voltage at 51V, I cannot get it set up correctly.
There was a discussion a while ago about the correct replacement for the STV-3H-0 diode (now discontinued) and the closest someone got was 3x 1N4148. This is what I did. The original diode was working until I removed the board from the heatsink. It just fell apart...
There was a discussion a while ago about the correct replacement for the STV-3H-0 diode (now discontinued) and the closest someone got was 3x 1N4148. This is what I did. The original diode was working until I removed the board from the heatsink. It just fell apart...