Bottlehead Forum
Bottlehead Kits => Eros Phono => Topic started by: John on May 22, 2014, 06:15:37 AM
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Hi all.
I am having a voltage problem with both CS4 boards. All tube heater light. All 4 leds on both boards light. All voltages are right on the money, except for OA which measures at 103 vdc and not the 170 volts spec. Any thought on this.
Many Thanks
John
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If OA measures 103V, what does OB measure? What about Breg on the populated side? (A side)
If Breg is 100V and OA is 103V, please let us know, as that would be most likely from a miswire
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OA = 103v
OB(a side about 103v
Breg 99v
OA 103v
IA 223v
Thank you for your reply.
John
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I would strongly suspect a miswire at the 6922 socket.
-PB
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measuring about +.4 volt grid to cathode on 6922 tube both triodes.
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very carefully checked 6922 wiring and all seems good. Took some dc resistance readings on the 6922 and all seem good.
Cathode to ground with ""negative lead on cathode"" 25.5k ohms both triodes
Both grid to ground with either meter polarity 75K ohms
Both plate to ground 44K ohms.
Many thanks
John
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Hmm, could you post a photo of the C4S board on the offending side?
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Will post pictures tomorrow morning. Both sides of the preamp are getting the reading same readings.
I really appreciate your interest in solving this problem.
John
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Both sides of the preamp are getting the reading same readings.
Ah, I didn't know that, that's helpful information. Hopefully the photos can reveal what's amiss.
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Here are the C4S pictures
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Hi all.
After being out of town I'm back digging into the Eros preamp again. I have attatched a detailed schematic with voltage measurements.
Many Thanks
John
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Please do not post schematics for Bottlehead products. I will forward your measurements to PB.
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My apologies. No harm intended it won't happen again.
John
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I have been inspecting the pre amp all day. All wiring beeps out perfectly from all socket to C4S boards. However I may have run into a hint as to what is happening. Resistance measurements from T2 and T14 to ground read 85 K ohms and T4 and T12 read 75 K ohms to ground. The 10.5 K ohm resistor is installed correctly double checked. Any thought about this.
Many Thanks
John
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My apologies for this late reply - I've been out of town and out of contact for the last week. Just got home last night.
It seems likely that there are shorts between the grid and plate of the 6922 - i.e. C1 to C2 and C6 to C7 - or perhaps both stoppers (220 and 100 ohms) accidentally are going to the same pins. Inspect those pins closely looking for possible connections or bridging. If the problem does not appear there, it would be relatively easy to measure the resistance between C1 and C2, and again between C6 and C7. This will test the short-circuit theory, even if the connection is not at the tube pins. If you can post a picture or pictures of the C socket showing the wiring to each pin, we can see if anything seems amiss.
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Many Thanks Paul, No problem on the delay as you are helping me. I found no shorts between C1, C2 and C6, C7. I next removed the C4S boards and triple checked the Terminal strip wiring, and rewet all C4S solder joints. When unit was reassembled still had same problem of low OA voltage. I next removed tube A,B, and C and voltage raised to around 136 volts. I think my next step may be to remove and test transistors on C4S board although that will have to wait till Monday.
Tons of thanks for your help
Charette
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Here is a picture with C4S's boards removed. All sockets where tested for tubes A,B, and C.
Thanks Again.
Charette
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Bingo!
If the voltage at the plate terminal (2, 7) rises to 136 volts with the 6922 removed, then the current through the 47K resistor to ground is 2.88mA - which should be the value on the OTHER SIDE of the PC board. Check the R1 which should be 113 ohms on the A side, and 301 ohms on the B side. I think you have 301 on the A side, but check both. You must have built both boards the same way.
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That gets my vote for "Deerstalker" award for this week...John
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Many Thanks go out to Paul. I did reverse the 113 and 301 ohm resistors....bad me. Unit fired up and passed signal, however one last problem. Gain mismatch with -30db input @ 1100 hertz signal on both channels I get output reading of 13.66 db on the left and 20.25 db on the right. This ratio stays consistent across the full frequency spectrum of frequencies. 6 db is a great number in balanced audio as pin 2 or 3 could be dropped but I am at a loss for this problem. I have change positions of the pentodes and tried a new dual triode to no avail. All DC voltage look equal except for the usual variations in Kreg which measure around 1.7 volts. Oh boy. Any ideas.
Many thanks
You guys have been the best
Charette
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Did you also maybe swap a 47K input resistor for a 470K output resistor on one channel?
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nope 47k ohms on input 470k ohms on output.
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wow measuring 55 ohms across input RCA with 47k ohm and red wire disconnected form tag board to ground. Does nylon washers go to junction of heater noise caps?
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My nylon washers are on the audio ground and not on the heater noise caps.
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Post is unclear - do you mean the RCA jack measures 55 ohms all by itself? In that case you probably have a solder blob or wire whisker partially shorting the jack.
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John,
All 4 RCA Jacks have nylon washers that "Float" the outer conductor, signal common, from the chassis. The signal common and shield are taken to ground elsewhere.
I hope this is what your question was about.
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Hi all,
Here are the results so far. The amp has no intermittancies when I tap on the circuit boards or sockets. However the gain at 1000 hz is Left chan 42.85 db and Right chan. 49.33 db. What components set the gain for the circuit. Any suggestions greatly appreicated.
Many thank again
Charette
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A good idea would be to swap EF86's to see if it follows a tube. 6922's can also account for some imbalance, but 7dB is pretty large.
In the Eros, the 75K plate load resistors are (in my opinion), most responsible for gain. Next up would be the EQ (if you have a connection issue within the EQ, it can present as gain imbalance), and lastly the tubes themselves.
-PB