I took another look at the voltages while the motorboating was occurring. The biggest anomalies I found are fluctuating values (by as much as 0.5V DC) at the emitters of H403 and H404, as well as at the bases and collectors (by several volts) of H405 and H406. Suggestions for other spots to check that aren't marked in the schematic appreciated.
It's seeming that both sides are affected more or less equally, unlike earlier where I believed that one was upsetting the other. So I'm thinking of what's common to both: H409, R405, and C413. Both the diode and capacitor were replaced, and I pulled the diode once already to confirm that it was ok, but perhaps I should just put in another pairing of 1N1418s in series.
It's seeming that both sides are affected more or less equally, unlike earlier where I believed that one was upsetting the other. So I'm thinking of what's common to both: H409, R405, and C413. Both the diode and capacitor were replaced, and I pulled the diode once already to confirm that it was ok, but perhaps I should just put in another pairing of 1N1418s in series.
Just resoldered ground pin J402 (already did J407), but no change. I also stripped back the insulation on the ground wire to make sure there weren't any micro breaks from being flexed.
One new observation, fwiw. Once the motorboating starts, if I tilt the board out and it doesn't stop, I can get it to stop by moving the phono input cable away from the chassis and the other cables that it runs next to. Also, when I did it a minute ago, the motorboating stopped and I started picking up FM music (not the same station that the tuner was currently tuned to).
One new observation, fwiw. Once the motorboating starts, if I tilt the board out and it doesn't stop, I can get it to stop by moving the phono input cable away from the chassis and the other cables that it runs next to. Also, when I did it a minute ago, the motorboating stopped and I started picking up FM music (not the same station that the tuner was currently tuned to).
If it is the higher hFE that causes the issue after all, and if it causes a problem because the bias current of the previous stage drops too much, then reducing R428 and R429 to 33 kohm might also help.
By the way, your original transistors must be in really bad shape: H405 and H406 are the output stages and unless it is very, very bad, the noise of the output stage transistors hardly has any impact on the overall noise.
By the way, your original transistors must be in really bad shape: H405 and H406 are the output stages and unless it is very, very bad, the noise of the output stage transistors hardly has any impact on the overall noise.
With the 24k resistors in R428 and R429, I was able to put the 2SC1845s back in without any motorboating. That's having the two trimmers set approximately center. Of course, I've been down this road before in thinking the issue had been sorted, so time and repeatability will say for sure. Next step is to put the UKLs back in where the tantalums are now.
Hi Matt,
Just checking: at some moment you used a 100 MHz analogue scope to determine that there were no high-frequency oscillations. Were you measuring the internal nodes of the phono circuit or some recording output of the amplifier? In the latter case, is there an RF filter between the phono circuit and that output?
Regards,
Marcel
Just checking: at some moment you used a 100 MHz analogue scope to determine that there were no high-frequency oscillations. Were you measuring the internal nodes of the phono circuit or some recording output of the amplifier? In the latter case, is there an RF filter between the phono circuit and that output?
Regards,
Marcel
Unfortunately, I don't recall where I measured for HF oscillation.
I reinstalled the UKLs, and there's still no motorboating. FWIW, I've since come across a few other people who've had trouble with oscillations when installing 2SC1845s in place of the 2SC458s in these two spots, leading them to use 2SC1815s instead. So it's definitely an unheard of issue, though why some people have no problem using the 2SC1845s in the P400 board, while others, like myself, run into a problem is a mystery to me.
I reinstalled the UKLs, and there's still no motorboating. FWIW, I've since come across a few other people who've had trouble with oscillations when installing 2SC1845s in place of the 2SC458s in these two spots, leading them to use 2SC1815s instead. So it's definitely an unheard of issue, though why some people have no problem using the 2SC1845s in the P400 board, while others, like myself, run into a problem is a mystery to me.
Interesting. Is there any reason/need to add that 47 ohm resistor close to H405 and H406 now that the circuit seems to be behaving with the 2SC1845s and substitution of the 24k resistors at R428 and R429? Or for that matter to put 2SC1815s at H405 and H406 and revert back to the original resistor value for R428 and R429?
Correction to my previous post: I meant to type, "So it's definitely not an unheard of issue. . . ."
Correction to my previous post: I meant to type, "So it's definitely not an unheard of issue. . . ."
Not if it keeps on behaving properly and everything sounds and measures normally. If you get excessive distortion or motorboating or other forms of misbehaviour as soon as the temperature changes a degree or two or the mains voltage changes by a few percent, then adding base stoppers might be a next step.
Directly in series with the base, so between base and the node H407/R428 or H408/R429, and physically close to the transistor.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.