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7B80 : P-P AUTO Not Working Correctly
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I have yet another plugin where P-P AUTO isn't working as it should. With a six-division sine wave, I can adjust the LEVEL to the second positive division maybe to two and a half negative divisions. If I change the SLOPE to the other direction, those levels are reversed but in neither SLOPE can I cause triggering to start fully three positive divisions and three negative divisions.
The 7B80 uses a Tek Custom chip, 155-0126-00, for the "TRIGGER SOURCE SELECTOR AND P-P AUTO AMPLIFIER" and the pins for PEAK DETECTOR OUT show a - and + by those pins (14 is - and 15 is +). I presume that indicates the voltage changes expected on pin 14 is when the LEVEL control is on the "-" side (7:00 to 12:00 position) and the voltage changes on pin 15 is when the LEVEL control is on the "+" side (12:00 to 5:00). Is that correct? It at least agrees with what I'm seeing on those pins. If so, then perhaps the problem with insufficient adjustment range in P-P AUTO lies with the circuitry in/around U75 and/or U55. Is that a reasonable assumption? I took a look at the design for 155-0126-00 and it answered a couple of questions I had but I get a bit lost in that schematic pretty easily. Thanks, Barry - N4BUQ |
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Barry,
I have a 7B80 I am looking at while typing this post, if that will help you. If you have not already cleaned the switches and sockets for the active devices, do so. That may make a difference. It will not hurt. I replaced six condensers on this board; the three B+ filters with 47/25 ULD types with the leads bent to allow them to lay down, the 1mfd (C9) with a film type and the two ,27mfd (C72 and C76) with film types. The last three I listed were high in ESR and the filters I wanted better quality types. You may need to clean the post on the board. Put a pin or something outside the rotating part of the pot that will remind you where to put the pot back to its original position using one hand to hold that while turning the control back and forth a few times with your other hand right after putting the cleaner on the element. Sometimes a dirty pot is the/a problem. I will say a 1458IC is bad although doubtful. I wanted to mention it in case. Q322 and Q324 are those (dreaded) 0367 types I replaced with KSP10BU. These transistors are on the lower left half of <4>. The C to E leakage may give you problems. C214 and C215 may be giving you problems. Mine are now a film type. The two 1/2W carbon resistors that are near the top of the main board are now 2W in mine. There is a 1mfd tantalum between these two resistors that is now a film in mine. They are mounted off the board and do operate warm in operation. There are other parts I changed if you want/need to know. Mark |
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Hi Mark,
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I'm reasonably sure the rails are good and the pots are clean and LEVEL pot is relatively smooth. I'm not sure the voltages are what they're supposed to be at the top and bottom ends of R58 and R59. The schematic shows +3.89V and I'm only seeing about +3.6V causing the +3.51V at the + input of U55A to be low (around +3.3V) and I think that may be causing the input to U65 pin 20 to be too low. The voltages to U75 pin 3 and pin 5 are very close and the +15V supply to U75 is very close (+14.95V) and I'm not seeing why the outputs of those op-amps are lower then defined - especially since there should be a diode drop involved. Thanks, Barry - N4BUQ ----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@...> Barry, |
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I noticed on the TekWiki page for the 7B80 that the 7B50A uses the same custom trigger chip. I then looked at the schematic for the 7B50A and it appears the trigger board is identical to the one used in the 7B80. I'll have to test my 7B50A's P-P AUTO functionality and see if it works any better. If so, then that might give me some points of reference to help me track down the problem with mine.
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Barry" <n4buq@...> Hi Mark, |
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I tried my 7B50A this morning and it exhibits the same behavior so I'm wondering if there's not really a problem with the 7B80 and that's just how they worked. I saw a video from Zenn where he was running through the checklists for a 7B80 and I don't think this was the case with his but, on review, I'm not quite sure how that particular mode was tested. I _think_ he had a five-division sine wave and was able to trigger the entire height.
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Thinking that I may have erroneously expected a six-division signal to be fully triggerable in P-P AUTO, I tried this with varying amplitudes and the problem is there, regardless of the amplitude. It seems it's a "percentage" of + and - regardless of that amplitude which indicates (at least to me) that the P-P AUTO isn't working correctly in either my 7B50A or 7B80. Anyone else see this behavior in either of these plugins? If not, perhaps suggestions on what I might be expecting as being incorrect? Thanks, Barry - N4BUQ ----- Original Message -----
From: "Barry" <n4buq@...> I noticed on the TekWiki page for the 7B80 that the 7B50A uses the same custom |
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