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 Post subject: Troubleshooting Sherwood S1000
PostPosted: Nov Fri 16, 2018 4:46 pm 
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It came to me humming and with a loud pop on warm up. I replaced all the electrolytic capacitors and replaced C30,C31 400v .1uf with 630v .1uf caps. The hum is almost gone and it no longer pops during warm up. The remaining hum may be from powering it up with the classy split in two with jumpers connecting the two halves. This is the only way to get to the circuits.
It has two remaining symptoms. It draws 800ma when the Sams says it should be 550ma and there is no output without the rumble filter (M5) switched on. I'm going to replace everything between the 12ax7(v3) and the 12au7(v4) but I'd like to learn how this area works. It looks like when (m5) is closed the signal jumps over R36,37 and C25,27 and goes through C26. Or do I have that backwards.

Thanks.





Google+ Sherwood S1000 Schematic

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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Fri 16, 2018 7:16 pm 
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If it draws too much current then you still have a problem.


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Fri 16, 2018 9:40 pm 
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C25, R37, C27, and R40 make up a high pass filter, passing mids and highs more than very lows, like 100hz. For the rumble filter to work, it would be in the position it's in now. When it's switched, you bypass the 10MOhm resistor sending a lot more signal to C26, which has more capacitance and will pass the lower frequencies.

I would be worrying about the 250ma extra current draw. The amp should be using 63.47 watts. With the over current condition, something's gonna be getting hot.


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Fri 16, 2018 10:40 pm 
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Tin Omen wrote:
C25, R37, C27, and R40 make up a high pass filter, passing mids and highs more than very lows, like 100hz. For the rumble filter to work, it would be in the position it's in now. When it's switched, you bypass the 10MOhm resistor sending a lot more signal to C26, which has more capacitance and will pass the lower frequencies.

I would be worrying about the 250ma extra current draw. The amp should be using 63.47 watts. With the over current condition, something's gonna be getting hot.


So (M5) in the schematic is drawn as if the rumble filter is switched in/on.

The current draw is a lot better then the 1.02A it was pulling before in replaced the .1uf caps on the power tubes. Those caps checked more like resisters then caps. Before the next power up I'm replacing the aforementioned parts and C13,14.

Thanks

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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Sat 17, 2018 12:51 am 
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If you continue to use that Sherwood while it draws too much current then something is going to burn up.


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Sat 17, 2018 12:51 am 
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Location: Rochester NY USA
The hum pot can short (resistor element to case) and result in high current in the output stage. One more thing to check...

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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Sat 17, 2018 2:18 am 
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Are you sure that the 6L6's do not have grid emission? That could cause the extra current draw. Is the cathode voltage at or near 30 volts as shown in the schematic?

Dave


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Sun 18, 2018 8:45 pm 
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+1 on what Johnny said and David said

Check the rest of the tubes and finish recapping.


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Tue 20, 2018 3:57 pm 
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It still pulls 750ma. The power up acts normal at first. A large spike followed by a rapid drop off to about 300ma then a very slow crawl to 750ma. Once stable the 320V reads 360V. The 30V that should be on pin (8) of the 6L6's is 22V. All voltages on the 6L6's match each other.

Work so far.

Resistance readings on pin (2,8) of 5Y3 that should be 150K is very high. At first I thought it was open. Used a different meter that has a 20meg range that showed 15meg that slowly rises out of range. Looking at the schema I don't see a path to ground that doesn't go through a cap. Not sure why that's on the chart. Pin (3,4) of the 6L6's should be 11,2.5 ohms read 97,22.5 ohms. Bad primary in the output transformer? Checked all the tubes except the ef86. Can't find settings for the ef86/z729 for 6000a or 666 testers. Although it will power up without it and still has the same draw.


Attachments:
File comment: Resistance Readings
Sherwood_S1000_TP_50.jpg
Sherwood_S1000_TP_50.jpg [ 60.15 KiB | Viewed 903 times ]

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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Tue 20, 2018 4:30 pm 
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Location: Cortez, Colorado
What's the voltage going into the 5Y3? Is it a 5Y3?
With the 6L6's cathode voltage low, the current should be lower. Pull all of the tubes and see what the current draw is.

You can try removing the 6L6s, powering it up and see what the primary current is, should be about 175ma.


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Tue 20, 2018 11:57 pm 
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Spiking currentl on power up sounds to me like charging of the power supply electrolytic capacitors, probably normal. However slowly increasing current sounds like the output tubes starting to conduct, possibly more than they should. Are they biased correctly?

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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Wed 21, 2018 1:46 pm 
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Location: Utica, NY 13502 (USA)
If one or both 6L6's begin to draw more current than they should due to gas or grid emission, then the cathode voltage would be higher, not lower as measured.

Is the electrolytic that replaced the cathode bypass cap installed with the correct polarity?

360 volts at the 5Y3 cathode seems rather high. What is your AC line voltage?

Regarding the OT primary, is your ohmmeter working correctly on it's low resistance range? If so, maybe there is also leakage to ground after power is applied to the amp causing the high current draw.

Dave


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 Post subject: Re: Troubleshooting Sherwood S1000
PostPosted: Nov Wed 21, 2018 2:58 pm 
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Tin Omen wrote:
What's the voltage going into the 5Y3? Is it a 5Y3?
With the 6L6's cathode voltage low, the current should be lower. Pull all of the tubes and see what the current draw is.

You can try removing the 6L6s, powering it up and see what the primary current is, should be about 175ma.


360v to the 5Y3GT with the 6L6GC's and 480v without. It draws 300ma without the 6L6GC's and 750ma with. The plates of the 6L6GC's read 373v,375v. Do you mean power up with out tubes and monitor current as you add tubes? If so in what order?

Dave Doughty wrote:
If one or both 6L6's begin to draw more current than they should due to gas or grid emission, then the cathode voltage would be higher, not lower as measured.

Is the electrolytic that replaced the cathode bypass cap installed with the correct polarity?

360 volts at the 5Y3 cathode seems rather high. What is your AC line voltage?

Dave


I'll check the connection of C1 section (c). I didn't install the exact values. The 40uf's became 47uf and the 50uf became 68uf. I adjust the AC as it warms up to maintain 117V.

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