Advertisement



  1. Things you need to know about the new ‘Conversations’ PM system:

    a) DO NOT REPLY TO THE NOTIFICATION EMAIL! I get them, not the intended recipient. I get a lot of them and I do not want them! It is just a notification, log into the site and reply from there.

    b) To delete old conversations use the ‘Leave conversation’ option. This is just delete by another name.
    Dismiss Notice

Leak TL/12 Plus / Point One Plus

Discussion in 'classic' started by Tony L, Dec 15, 2017.

  1. Tony L

    Tony L Administrator

    I now have an empty spreadsheet, I shall return when it is full! I’ll do every connected valve pin on both amps.
     
  2. John_73

    John_73 pfm Member

    Clutching at straws, but you didn't do the 'diode mod' on the GZ34 base did you? If so might be worth checking that too. Good luck! I'm sure it's something relatively simple.
     
  3. Tony L

    Tony L Administrator

    No, no mods at all. I wanted to get things working before altering anything. The power is definitely getting in ok.

    PS Just ready to start checking valve base voltages now, I should hopefully have them all in an hour or so.
     
  4. snowman_al

    snowman_al pfm Member

    You did swap the input plug / leads and impedance selector plug as in my post #180?
     
  5. Darmok

    Darmok "Didactic Prophylaxes"

    I was offered a faulty TL12+, low output, I declined the amp as I had other priorities at the time.

    Contacted later about the faulty amp, apparently a short on the underside of the tag board, solder can dribble through the tag holes and created a short circuit.

    IIRC, it was around the 3.3 meg resistors R8 or R14.
     
  6. Tony L

    Tony L Administrator

    [​IMG]

    Quite a lot of differences between them. I’ve no idea how significant these are (some look concerning e.g. twice or half the other), but one that really jumps out is it looks like one of the EL84s in the non-working amp (#1) has no 12-13V on pin 3, which I assume is the bias current.

    I’ve no idea where to go from here, but I suspect it means the output transformer is ok.
     
  7. John_73

    John_73 pfm Member

    Depending upon which EL34 has the 0V instead of 11/12V, I’d check the following for starters:

    R16 or 17, R19 or 20, check C11 or 12 are OK.
     
  8. Tony L

    Tony L Administrator

    I’m reading the valves in order from the EF86, so EL84 #1 is the one by the ECC81.
     
  9. Yank

    Yank Bulbous Also Tapered

    327V on the plate of the 12AT7?

    Since that's higher than the B+ for that stage, something around the grid of the EL34 must be faulted to the output stage B+. Maybe the tube socket for the output tube is arcing? Or a solder blob on one of the jumpers on the back of the board?

    Anyway, you've got one output tube that's not drawing current, and one half of the phase splitter with scary high voltage, hopefully they're symptoms of the same problem.
     
  10. Tony L

    Tony L Administrator

    My best guess is there is something wrong on the other side of the board. I can see no issue in view. Certainly no arcing or red plating that I’m aware of. It looks just like a working amp, it just doesn’t work.

    The only area I may have got a bit too hot is between the two Mills resistors, but this is amp #1, the local auction one, so it’s condition was always an unknown. It may have been dead for 50 years for all I know. The one I bought as working still works.

    Next I need to figure out how much of a PITA it is to get the board out. Annoyingly the screws for the octal bases are covered by the board, I think I can just get to the one under the input socket, I hope so as I spent so long getting that bloody resistor bent right! If need be I’ll just desolder the GZ34 base. I’ll see if I can figure it out tomorrow. Obviously a huge problem is I have no idea what it should look like, though I should be able to spot blobs, shorts, melted insulation or whatever.
     
  11. John_73

    John_73 pfm Member

    Tony, could you check the state of the soldering on the earthing tag by the original can cap, nearest to the red tag? Looks like it might have an issue, from zooming in on it... though might also just be the angle of the shot etc. Worth checking anyway.
     
  12. Tony L

    Tony L Administrator

    Do you mean the earthing eyelet bolted to the can cap mounting eyelet? It looks good to me, there are six or so black wires soldered neatly one above the other to an eyelet thing with a long vertical slot, and then that is bolted to the lug that holds the can cap in place. All looks good (it is very neatly done). The bolt is done up good and tight too. If you mean my work jumping the black lead off the can cap and to the F&T that seems fine too, certainly a solid joint.

    PS I can check continuity etc anywhere anyone suggests, just tell me exactly where and I’ll do it. All the resistors mentioned upthread seem fine, though I haven’t lifted the cap to measure that. My suspicion is something is bad above the board, though I’d far prefer it to be easier than that as getting it out looks like a PITA!
     
  13. John_73

    John_73 pfm Member

    Yes it was that earthing tag with all the black wires. Zooming in it looked like there was a possible lack of solder and one of the wires might not have been making contact, but from what you’ve said it was just a trick of the light then. Bugger. That would have been too easy wouldn’t it!
     
  14. Tony L

    Tony L Administrator

    Just trying the old iPhone camera trick to see places one can’t get a head in:

    [​IMG]

    I spent quite a while looking just to get a feel what’s under there (a very substantive wiring loom!), though I can’t see any detail for fault finding. It worked a lot better for looking than trying to get a pic for here though as I couldn’t properly hold the torch, iPhone, and amp and hit the shutter!

    Has anyone pulled the board out on one of these? Am I right in thinking that if I unscrew the valve bases from the chassis, desolder everything from the two transformers, the smoothing cap and the speaker connectors I should be able to fidget everything else out in one piece, i.e. leaving the loom intact and attached to the board?
     
  15. Darmok

    Darmok "Didactic Prophylaxes"

    Indeed.

    All you have to do is de-solder ALL the tag board wires from both sides, just pull them away from the tag board ready for re-soldering.

    There are two clips each end of the tag board or two lumps of some plastic material as an alternative, melt that and the tag board will be easy to lift out upwards.

    One is concealed beneath the Green 100 ohm wire wound resistor (R22) and the other one just above, see the location at the other end of the tag board, you can see them poking through upwards.

    It appears that the tag board clips are absent imv, they are usually pressed down onto the up facing chassis studs, securing the tag board to hold it rigid preventing vibration or otherwise!

    Be careful as the solder tags are quite brittle and prone to breaking away if stressed or over heated.

    Hope this helps?
     
  16. Tony L

    Tony L Administrator

    Cool, that’s a lot more simple than what I had in mind (which was bringing everything out including the still-wired sockets). I assume it means the tag board and loom are two entirely distinct assemblies? I’ll have a go later today as I’ve a food delivery due this morning.

    PS Has anyone any ‘logic’ as to what the issue may be? If this was a computer program I’d debug it by looking at the output symptoms and from there I could hopefully narrow it down very significantly to a particular process or piece of logic, e.g. to get result X from input Y there has to be something totally screwed up somewhere around ‘here’! I’d love to know where that ‘here’ is with regards to crazy high voltage on the ECC81 and no bias to the first EL84?! Any clues would be really helpful, I’m assuming something is bridged on a power rail somewhere. Obviously it may be two totally unconnected faults, e.g. a solder blob doing for the 81 and a loose or broken wire for the 84. I’d really like to know where to run continuity checks if it isn’t visually obvious. I’ll obviously post detailed pictures as soon as I get it out.
     
  17. snowman_al

    snowman_al pfm Member

    Tony,
    Until you prove there is a problem under the board there is no point removing it. You can easily continuity check it from the top.
    Best start by testing the 'issues' you have noted.

    Have a very careful look down the valve holders before you do anything else. Look for bent or broken pins especially on pin 1 of the ECC81 and pin 3 of the EL84s. (Count the pins anti-clockwise from above.) Take some close up pictures if you can. A bad connection in the socket means the valve will not be working.

    I am trying to write up some resistance checks for you, so will get back a little later.
     
  18. Tony L

    Tony L Administrator

    Many thanks. I’ll have a really good look at the valve sockets.
     
  19. Tony L

    Tony L Administrator

    ...I really can’t see anything wrong with the valve sockets. They all have all their ‘pin grip’ things present and correct, nothing looks broken or bent. There is one individual ‘grip’ on EL84 #2 that tries to follow the valve out just a little, but it is still fine electrically, and that isn’t the EL84 with the issue anyway.

    I’m curious as to what possible scenario could put such a high voltage on pin #1 of the ECC81? I really don’t understand that. It is a really clear connection to the board (red wire). I’ve been nowhere near it!

    Here’s another perspective on the sockets:

    [​IMG]

    Higher res here.

    I can’t get the lighting right for a pic from above the amp, but the socket grips are fine, I checked them all with a loupe and a torch!

    PS The Allen Bradley 3M3s are well away from anything, they are much higher than the photo implies, certainly no bridges or anything there.
     
  20. John_73

    John_73 pfm Member

    All you can do is check the component values of likely offenders mentioned in the last couple pages with your Fluke, to make 100% sure it’s not just an incorrect resistor value that got soldered in, or a duff coupling capacitor etc. If you have an ‘expendable’ set of valves you can test with maybe try lightly tapping components with a wooden spoon or something insulated to see if anything suddenly crackles back into life.
     

Share This Page





Advertisement


  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice