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Phono "motorboating" solved(?)

Steven Tate

Steven Tate

CEO Flat Forehead Soc.
I have had recurrent problems with low frequency "motorboating" on several 2230/2245/2270's after updating the phono boards. There has been a lot of discussion about it here, with no definite fix determined that I could find. The closest I have come was thinking that the KSC1845's often used to replace 2SC458's were not as good of a match as KSC1815. In most of my cases, when I had the problem, I had replaced the 2SC458's and the four 2SC1000's with KSC1845's. That muddied the water a little bit as to the source of the problem. Well, today I had a 2245 that I did a complete recap/restore on about two years ago and has been sitting on a shelf since then. I wanted to check it out from top to bottom. Everything was wonderful until I got to the phono. In my case, what usually happens is that everything sounds good when I put it in Phono mode. Then I hit the start switch on my Dual 1229 turntable. During automatic translation of the tonearm, the turntable shorts the leads from the cartridge to block out the noise caused by arm movement. As soon as those contacts short, loud, low frequency motorboating starts. It continues until the tonearm sits down, the shorting contacts open, and then the music sounds perfect. I found the turntable didn't matter as long as it was automatic and shorted the cartridge during tonearm movement. I tried my Marantz tt530 linear tracker and got the same noise while the arm was moving. If you removed the turntable, and just short the RCA cable, you get the same result. Well, this time, there was a difference. The right channel did not motorboat, and the left channel reliably made the sound. Also, it turned out I had only replaced the 2SC458's. The 2SC1000's were still in place, so it removed them as a variable. I loosened the board from the chassis, and as long as it was 1/4" away from the standoffs, the motorboating went away. But if I pushed it toward the standoffs, just before it touched, the left channel would start doing it. It was completely repeatable. I looked for wiring problems to no avail. If that board was near the standoffs, it was going to motorboat on the left side. So.....I removed the KSC1845 on the left channel and replaced it with a KSC1815 with little confidence that would solve the problem. But I fired it up, shorted the inputs and zero motorboating. I could move the board away from the chassis or push it against the standoffs, and silence. So, I screwed the board to the chassis, plugged the turntable back in, hit the start button, and the tonearm moved and set down with zero noise. I did it several times and it never motorboated again. Why did it do it just on the left side? I don't know. But I'm 99% sure that KSC1845 is the problem. I guess before I close it up, I'll change out the other 1845, even though it doesn't make the noise at this point. Hopefully, it won't screw something up to do it, but I figure both sides ought to have the same transistor. I know in the future, I will be using KSC1815 to replace 2SC458 in these phono boards.
 
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What's the difference between the KSC1815 and the KSC1845? Is it gain? (I am restoring 3 2245's at this moment - one done so far, with replacing only the 2SC458's with KSC1845's) and no issues so far (although I have yet to use it with a muting turntable.)
 
AlphaEcho35 said:
What's the difference between the KSC1815 and the KSC1845? Is it gain? (I am restoring 3 2245's at this moment - one done so far, with replacing only the 2SC458's with KSC1845's) and no issues so far (although I have yet to use it with a muting turntable.)
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I noticed last Spring when I was putting together my Transistor/Diode Substitution Guide, that 2SC458 had a Vce of 30V ad Ic of 0.1A. The KSC1845 that is so often used has a much higher Vce at 120V and half the max collector current at 50mV. The KSC1815, on the other hand, has a Vce of 50V and Ic of 0.15A. It is simply a more robust transistor. The 1845F has a gain of 300-600, while the 1815Y has a gain of 120-240. But right now, I have one channel with an 1815 and one with an 1845, and there is no audible difference in volume with balance centered. Another difference is that the 1815 has a Current Gain Bandwidth Product (fT) of minimum 80MHz and the 1845 has a minimum fT of 50MHz. Which spec is making the difference? I don't know, but when picking a sub, I always like to use one that has at least the same max collector current. I figure the designers picked that slightly more robust transistor for a reason. Bottom line -- I don't know. :dunno:

If I were you, I would test your board for the problem by shorting the inputs one at a time and see if you generate that terrible sound. I have had it on several units with that board.
 
The only major difference in the spec sheets that can affect performance (that I notice anyway) is that a KSC1845 saturation voltage is higher than the 1815 - if the collector voltage is close to the emitter voltage (like around 2 volts or so) the curves are weird for the ksc1845. There was one other thread where the ksc1815 was used to replace the ksc1845, because of distortion, where the input stage had a very low voltage E to C. It was a tone control, but because of all the bass gain, in a phono stage, I would guess it causes other problems. :dunno: YMMV
 
On my 2325 P400 board I used ksc1845 to replace H401- H406. Do you think I would benefit by changing them to ksc1815?
 
H701 and H702 only have about a volt drop across them, so those shouldn't be KSC1845. II don't see any other problems, but...-
 
Steven Tate said:
I noticed last Spring when I was putting together my Transistor/Diode Substitution Guide, that 2SC458 had a Vce of 30V ad Ic of 0.1A. The KSC1845 that is so often used has a much higher Vce at 120V and half the max collector current at 50mV. The KSC1815, on the other hand, has a Vce of 50V and Ic of 0.15A. It is simply a more robust transistor. The 1845F has a gain of 300-600, while the 1815Y has a gain of 120-240. But right now, I have one channel with an 1815 and one with an 1845, and there is no audible difference in volume with balance centered. Another difference is that the 1815 has a Current Gain Bandwidth Product (fT) of minimum 80MHz and the 1845 has a minimum fT of 50MHz. Which spec is making the difference? I don't know, but when picking a sub, I always like to use one that has at least the same max collector current. I figure the designers picked that slightly more robust transistor for a reason. Bottom line -- I don't know. :dunno:

If I were you, I would test your board for the problem by shorting the inputs one at a time and see if you generate that terrible sound. I have had it on several units with that board.
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Thanks for your highly detailed report on this issue!
 
catrafter said:
These are my thoughts on the whole deal. Under the 'right' conditions the phono will motorboat even with 2SC458 transistors.

Tom
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This episode raises so many questions. Why did only one channel have the problem when both sides had identical components? What was changing as the board got further away from the chassis? All I know is, it went from a repeatable problem to non-existent by changing one component.:dunno:
 
I think I have pretty much confirmed my theory on the 2SC458 subs on early 22xx phono boards. I remembered that I had a 2230 sitting on the shelf completely restored, but set aside because of the motorboating sound while the turntable tonearm was translating either way. At the time, I had no idea what was going on. But I tested it with my Dual 1229, and sure enough, it immediately started the motorboating when the tonearm lifted. As soon as it set down, and the shorting switches opened, it played the record normally. But that sound is not one you can live with for any length of time. So, I pulled the bottom and prayed that I found KSC1845's at those two spots. Sure enough, I did. Removed them and replaced with KSC1815, put it back together and fired up the turntable, and voila! dead silence while the tonearm was moving. This has been a burr under my saddle for a while because I was rebuilding those P400 boards according to several threads I had seen, and, at least with my turntable, I was getting the motorboating result almost every time. The KSC1815 seems to be an automatic fix, so it will be my go to in the future at that spot. :banana:
 
Steven,
Did you experience that issue with or without any device connected to the tape jacks?
I've had that issue with three 2270's until I removed the cables from the tape inputs (i think, it's been a while).
Didn't have any issues with receivers that only have one TT input.
 
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I was keeping quiet while watching this thread, because I think I have two units I've rebuilt that have this issue.
I need to follow up on my own equip but there's no doubt I've had this motorboating (I called it oscillation) and wasn't sure if it was my test turntable or the unit.
Thanks for this info.
 
Osibisa said:
Steven,
Did you experience that issue with or without any device connected to the tape jacks?
I've had that issue with three 2270's until I removed the cables from the tape inputs (i think, it's been a while).
Didn't have any issues with receivers that only have one TT input.
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Nothing in the tape inputs. I have had this on 2230, 2245 and 2270. And I've tried different turntables. It basically is a problem if the input is shorted. That shouldn't happen. I have a 1200 integrated that came with shorting plugs on one set of phono inputs. But in both of these units, changing the 1845's to 1815's immediately stopped the problem.
 
Dingman said:
I was keeping quiet while watching this thread, because I think I have two units I've rebuilt that have this issue.
I need to follow up on my own equip but there's no doubt I've had this motorboating (I called it oscillation) and wasn't sure if it was my test turntable or the unit.
Thanks for this info.
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If it does it all the time, it might be something else. But if it only does it when the inputs are shorted (like when an automatic tonearm is moving), this is a good possibility.
 
I'll give it a try as soon as I finish my current project.
Interesting to note that the issue occurred before and after changing the 458's, and then only when there was a connection to the tape out.
Same Dual as you, also happens with a 1219.
 
Osibisa said:
I'll give it a try as soon as I finish my current project.
Interesting to note that the issue occurred before and after changing the 458's, and then only when there was a connection to the tape out.
Same Dual as you, also happens with a 1219.
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I'm not sure I ever tested the phono input before working on these. So can't say if it was a problem before updating the board.
 
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