Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Paramount => Topic started by: rbc3 on April 21, 2015, 11:09:46 AM
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After many happy hours of working well and tube rolling, my left channel Paramount is now consistently blowing a fuse. The last time it was working, I was listening to headphones with a banana plug adapter. I was listening to AKG K1000, LCD2.2, and HE-500 cans to compare how well the Paramounts drive these different cans. My adapter is a banana to 4 pin XLR that I use to quickly switch between cans. I am pretty sure I was running 1950s Western Electric 396A D getter black plate input tubes which are my newest tube purchase but had been running well for at least 5 hours when driving speakers. I was also running KR VV300B output tubes, also ran successfully for at least 20 hours in the Paramounts driving my speakers.
I checked around the amp for any weird connections or crossed wires and then replaced the stock tubes. I also went back through the resistance check and it checks out exactly as before. I noticed an arc from the pins of the AC power socket to the fuse when switching the amp on which must be what is blowing the fuse.
I figure the most likely culprit is something went wrong with the WE output tubes since they're the newest factor in the equation. I did switch the OT from 8 ohm to 4 ohm, but that was a long time ago and I've been running my 4 ohm speakers for weeks now without issue.
Any and all help in chasing down the culprit is most appreciated!
Thank you in advance.
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The arcing you are seeing is the fuse blowing.
Remove the red wire going to IA on the C4S board, then measure the voltages on the 4-pin socket. Use the tubes provided with the amp; they are known to work.
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Before you apply power again, check the high voltage power transformer windings to see if either has shorted to ground. Put the black meter probe to the chassis safety ground lug and touch the red probe to each of the two right rear power transformer pins poking thru the power supply PC board (if you are standing at the front of the inverted chassis). That is the primary. The high voltage secondary is connected to the two pins forward of the primary pins on the front side of the power transformer. A near zero reading on any of those relative to chassis ground indicates that something has shorted to ground. It's not very likely but good practice to check this first.
To further Josh's suggestion, look to see if an LED has blown up on the SR/C4S board. That could indicate that the Zener string has been blown. Other potential issues are blown rectifiers, blown filter caps. Any of these could be caused by shorted tubes.
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This very well might be a completely dumb question, but I'm scratching my head trying to figure out how to determine if an LED is blown if I can't turn on the amp... Sure if it wasn't mounted to the PCB, I could test it, but on the board???
Off to probe my amp.
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it would be blowed up, in pieces.
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All four LEDs on the C4S board look fine. The four pins (two right rear and two right front) do not create a closed loop with the chassis ground lug nut. I tested with my digital voltmeter that has a closed loop beep function.
After I disconnect the power on the C4S board and check voltages on the 4 pins... just to be double sure, you want me to plug in the original input and output tubes, power on, and check the voltages across the ground lug not and each of the 4 300b pins?
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Take voltage measurements with known good tubes. We know that the ones we sent you work, because they worked for an extended period of time here. Your "upgrade" tubes may or may not be the issue, so don't use them.
If the fuse blows at power on, that is a useful data point. If not, you will need to take voltage readings. To do this, you will need the black probe on ground (use a clip lead) and put the red probe on each pin. Write down the voltage you read at each pin and post it here.
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I de-soldered the red wire from IA on the C4S board, put in a new fuse (tested before placement to be sure it's OK), turned the power on... and nothing. Pulled the fuse out and sure enough it's blown.
What should I try next?
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Now try the same test with no 300B in the socket.
-PB
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Removed the 300B. Left the stock 5670 in the socket. Red lead is still disconnected from IA on C4S board. Replaced the fuse (checked to be sure it works before). Turned it on. Nothing... fuse blown again.
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Well, this narrows things down to the power supply, though it can be tough to say with certainty where the problem might be. Can you post a couple of photos? We might be able to see where the problem could be.
-PB
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Whenever I attach a photo to a post it doesn't show up. Is there some permission or restriction that I have since I'm new to the forums?
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I see mention about the size limit of an attachment but can't find any details on what that size limit is...
Most forum applications resize images during upload to enforce size limits. I guess that's asking too much.
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The image tags in the editor aren't working either. Hopefully these links work. UGH!
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What DC resistance do you get between L and N on the IEC power entry module with the switch on? (Measure with the power cord unplugged of course)
-PB
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You don't actually upload photos here, you link them. So the image needs to be hosted online somewhere. One thing that has me puzzled in your images is that it looks like the transformer pins that are poking through the solder pads for the 6.3V heater winding aren't soldered, or at least not very well. It actually kind of looks like they have been desoldered. If they have not been altered it may just be the camera angle, as the amp ran great here for many years.
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12 ohms resistance between the two terminals of the IEC socket. Power switch is on. Amp is unplugged. IA on C4S board is disconnected. Tested and working fuse is installed in the IEC socket. No tubes are in the amp.
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Response to Doc... The only soldering I've done is to switch the OT to 4 ohm (came tapped as 8 ohm). Replaced RCA and speaker binding posts (desoldered leads from old and resoldered to new, not too difficult). And now desoldered the red lead from IA on the C4S board. I haven't touched the power board at all. I did notice that the pins on the power board don't have much solder on them. I could go through them with a touch more solder to ensure a good connection. I suppose a solder connection could have become cracked during shipment or something. <shrug>
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You can replace IA for the rest of the tests, and leave the 5670 plugged in.
The next test is to pop out the red wire that goes to the PC-2 from the 5-lug strip by the 9 pin socket. I would pop the wire out at the terminal strip. Next, turn the amp on and see if the fuse holds (with 5670 in place).
-PB
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I resoldered the red lead from terminal 16 back to IA on the C4S board. Double checked the good fuse (still good). Desoldered the read lead from terminal 16 that goes to PC-2. Plugged in the 5670 tube (300B tube still removed). Plugged the amp in. Switch on power. Nothing... blown fuse.
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There's a red/black twisted pair of wires leaving the IEC power entry module and going towards the power supply board. If there's room, side one of those wires out of the PC board, then see if the fuse pops when you throw the power switch.
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Just to be sure, I should desolder either the red or black lead going from the IEC power socket to the power board?
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I don't have easy access to the end of the wires that are soldered to the power board because the filament choke is in the way. So I desoldered the black lead from the N terminal on the IEC power entry module. I suppose that means I can't be sure if there's a short in the wiring between the power entry module and the power board though.
Installed a new fuse that I checked to confirm it was good. The red lead from PC-2 is still disconnected from terminal 16. And the 5670 is still plugged in to the amp. Turned power on and it didn't power up (expected since I disconnected the black lead from IEC power entry module). But at least the fuse didn't blow.
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I hope the silence means you guys are busy and not something worse.
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Yeah, that doesn't exactly test what I was looking for. I thought that perhaps there might be a short in the wiring going from the IEC inlet to the power transformer.
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This amp ran without issue for 7 years and also ran even after the base was knocked in shipping. So I would SWAG that the problem stems from some change that occurred after that. I would suggest checking the rectifiers on the power supply board. It's a bit of a stretch but one could have been blown when a new tube was introduced, if the tube shorted.
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OK I'll reconnect the black lead to the N terminal on the IEC power entry module and try to disconnect either red or black lead on the power supply board to check for shorts between the IEC power entry module and the power board. I'm not sure how successful I'll be in reconnecting later with such a tight space.
I'll also check all six Schottky rectifiers. Do I need to desolder them from the power board to test or is it OK to test while mounted on the PCB?
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I think you should be able to test them in place, with the tubes removed.
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Red lead coming from PC-2 is desoldered from terminal 16. The black lead to the IEC power entry module is disconnected, fuse and tubes are not installed. Here's what I got for the Schotky rectifiers in circuit.
Left Amp (looking with front of amp with badge facing me) - the one blowing the fuse
1N5820 3A Schottky rectifier black lead on silver stripe (rear outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (rear outer) 205mV
1N5818 1A Schottky rectifier black lead on silver stripe (rear inner) 161mV
1N5818 1A Schottky rectifier red lead on silver stripe (rear inner) 196mV
1N5818 1A Schottky rectifier black lead on silver stripe (front inner) 161mV
1N5818 1A Schottky rectifier red lead on silver stripe (front inner) 195mV
1N5820 3A Schottky rectifier black lead on silver stripe (front outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (front outer) 206mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (left) 737mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (left) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (right) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (right) 734mV
Right Amp (looking with front of amp with badge facing me)
1N5820 3A Schottky rectifier black lead on silver stripe (rear outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (rear outer) 196mV
1N5818 1A Schottky rectifier black lead on silver stripe (rear inner) 151mV
1N5818 1A Schottky rectifier red lead on silver stripe (rear inner) 195mV
1N5818 1A Schottky rectifier black lead on silver stripe (front inner) 152mV
1N5818 1A Schottky rectifier red lead on silver stripe (front inner) 195mV
1N5820 3A Schottky rectifier black lead on silver stripe (front outer) 151mV
1N5820 3A Schottky rectifier red lead on silver stripe (front outer) 196mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (left) 737mV
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (left) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on rear facing pin (right) OL
Cree CSD 01060 600V 1A silicon carbide Schottky rectifer black lead on front facing pin (right) 731mV
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A diode test usually reads in ohms, not mV. Were you using the diode test setting on your meter? If that is the case I would say that the consistency from amp to amp is a sign that the rectifiers are OK.
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I'm not sure why the digital display of my multimeter switches from the ohm symbol to an mV symbol when I switch it to diode test mode. The diode test setting on the dial is in the ohm section of the settings...
I was concerned that the four round rectifiers weren't working because they didn't read OT in one direction which is why I tested the right amp too.
Is it starting to sound like there is a short somewhere? What happens if one of the loose pins on the cracked four pin socket were to touch the chassis plate? Could that be the problem?
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OK, your meter is simply reading mV. I think your rectifiers are OK based on your measurements. Rectifiers in a bridge circuit will always read below OL in either direction, because the meter is in effect also reading the "backwards" rectifier that is connected to the rectifier you are measuring. If they blow they can create pretty close to a dead short so you would probably see a measurement much lower on the bad rectifier than on the good ones.
Actually it has sounded like a short is the problem from the beginning, but what is shorted is still the mystery. If one of the pins of the four pin socket is touching the chassis, that's a serious shorting problem that could certainly take out a fuse. If that is something that needs to be addressed I would make it the next priority.
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Reading about diode test mode of my multimeter and it's measuring the voltage drop across the diode in diode test mode and not resistance. I can also switch the multimeter to resistance mode and measure the resistance in forward and reversed bias.
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Yup, that would be what it is doing if it reads in mV. Same thing applies as measuring resistance, it will be confused by the bridge arrangement. If a four pin socket pin is possibly contacting the chassis, that would be the best place to next focus your examination.
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OK. I wired up a new 4 pin socket. Here's the photo.
https://www.dropbox.com/s/7x4q8p7xg3xgml9/20150430_171447.jpg?dl=0
I resoldered the black lead to the N terminal on the IEC power entry module. I resoldered the red lead from PC-2 to terminal 16. The 5670 is not plugged in. I checked to confirm the fuse was good. Powered up the amp. Powered off, checked fuse... blown. :(
On the power supply board, I desoldered the red line that comes from the IEC power entry module (next to FC-1). It was a tight squeeze, but I managed to get in there with some needle nose pliers to pull off the wire. I replaced the fuse with a tested good one. Turned the power on. Turned power off, checked fuse... not blown. So there's no short in the lines going to the power supply board.
I suppose we've narrowed it down to the power supply board itself. If the Schottky rectifiers on the PS board are OK, then maybe it's something else on the board? I could also do the same for the leads coming off the PC board, which we haven't determined aren't causing a short.
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OK, now I feel I'm on to something! I resoldered the red lead going from the IEC power entry module to the power board. I desoldered the red lead from the power board that's going to FC-1. I desoldered the red lead at the power board that's going to 16U. And I desoldered the red lead from the power board that's going to the 9 pin socket B1. I rechecked the fuse to ensure it's good. Plugged in the amp. Powered it on. Powered it off, and check the fuse... it's good!
I also checked the voltage at each of those three points that I desoldered the red leads. The point on the power board at which the red lead is going to FC-1 is 6.8 volts. The point at which the red lead is going to 16U is 580 volts (seems high since the manual states 457 volts at terminal 16). The point at which the red lead is going to socket B1 is 0 volts, which seems odd.
I believe that means the power board is OK and there's a short somewhere coming off the power board. I hope I'm on to something. I think my next step is to reconnect each red lead, one at a time and then I can isolate which path the short or problem might be down.
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It's possible that the Zener string next to the nine pin socket has a blown diode or diodes. Try a diode test on each individual Zener to see if any are shorted.
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All five diodes in the Zener string show 580 mV drop in forward bias and OL in reversed bias.
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I'm astounded! I resoldered the red lead lead from FC-1 to the power board. Checked the fuse, good. Powered up. powered down. Fuse is still good! I resoldered the red lead from 16U. Powered up. Powered down. Fuse still good! I thought, well it must be a short in the heater line for the 9 pin socket. I resoldered the red lead from B1. Powered up. Powered down. Fuse still good! I plugged the original tubes in and powered up and they lit up! No blown fuse. I cant check for audio because it's too late tonight and I have kids. But tomorrow morning I might be back in business! Woohoo!
PS I swapped out the stock 4 pin socket in the right amp too so now they both have gold plated copper 4-pin bakelite sockets. I know it's way overkill but they're looking pretty good with everything but the 9-pin sockets as gold plated copper, Cardas binding posts and Vampire RCA sockets.
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Well, I wish I could say I knew what exactly happened, but I can't really find a pattern when I go back over the posts. Could have been some intermittent short, or maybe it was the socket issue and the start up inrush of the caps after being discharged for a while was just enough to pop that final fuse after you did the replacement. Anyway I am glad you got it running! Good job of staying at it, that's what separates a good tech from a mediocre one.
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Yep, they work great! I've been running the stock supplied tubes all day. Just a little bit ago, I dropped in some NOS 90's Sovtek tubes that I recently picked up. Everything is running as expected. SO HAPPY TO HAVE MY PARAMOUNTS BACK! I'm spoiled and gave up listening on my vintage solid state Yamaha M-4. It's just not the same as these Paramounts!
Thanks for all the help and putting up with my noob questions. It only cost me around $7 for blown fuses and I learned my way around the amp a lot more. If the short shows up again, I know how to proceed through the checks to try and figure it out.