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Leak TL/12 Plus / Point One Plus

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

  1. snowman_al

    snowman_al pfm Member

    I didn't point it out earlier on the spread sheet (as you are not replacing it it does not matter), but C7 is 0.0002uF (200pf) as in the 'Service Manual' you linked to, in the 2nd diagram 0.00002uF is a typo. Again the value of C15 is vague, but as you are keeping the originals it will not matter. You will see better when you remove R5. Likely 15pF as on the service manual diagram. The 200pF (0.0002uF) value is associated with the addition of R23 which yours do not have. Could be when the change to the 8778 output transformer was introduced. Yours have the early 3921 transformers.
     
    Tony L likes this.
  2. Tony L

    Tony L Administrator

    They are the earliest transformers, though I’ll be building with the revised resistor and cap spec as Leak changed it due to a high failure rate (the first values ran the EL84s far too hard). As I understand it Leak used the later resistor values when servicing any returned amp regardless which output transformer the amp was fitted with. The big difference is the 1m to 470k change of R16 & 17 and the 0.02uF cap to 0.1uF. Given a good set of Mullard EL84s are best part of £200 and a GZ34 £150 (I have very nice sets of both) I’m going for a somewhat conservative build here!
     
  3. Tony L

    Tony L Administrator

    Interesting. I wonder if Leak fitted that on any failures/future services? Mine are interesting as despite some signs of failure and work done (a cap in one, resistors in the other) neither seems to have been changed to the later 470k design, so I guess never made it back to Leak for service, or if they did it was before that revision. It is hard to get good unambiguous documentation for the TL12 plus, the Stereo 20 seems better covered.

    PS I’ve updated my documentation. If this all works out without fire and it seems a stable sensible build I’ll stick a link on the thread to it as it should make life rather easier for anyone coming later and wanting a simple rebuild process.
     
  4. Tony L

    Tony L Administrator

    [​IMG]

    My very simple ‘dim-bulb tester’ now part built. It was the most basic design I could think up that didn’t require any woodwork or anything. I’ll just stand it on something to keep the bulb off the carpet. I’m just waiting on a mains socket and blanking-plate landing and its done. I’m obviously teaching most people reading this to suck eggs, but basically the idea is the mains live wire goes in and out of the light bulb so if there is a dead short or similar serious issue on the amp you are working on the bulb lights up rather than anything exploding! If all is ok the amp will work and the bulb will stay very dim. There are some aspects of bulb wattage I don’t fully understand regarding how many Amps they’ll pass to the socket, I have a choice of 40W and 60W bulbs to hand. The bulb needs to be an incandescent filament type, you can not do this with LEDs, fluorescent or anything!
     
  5. snowman_al

    snowman_al pfm Member

    Ohms law, so if your mains is 240 volts, a 240 Watt bulb would pass 1 Amp. (Volts X Current = Watts). So a 60 Watt bulb will pass a 1/4 Amp (240V X 0.25A = 60Watts) and a 40 Watt bulb will pass 1/6 Amps. (240v X 0.167 = 40 Watts).

    The TL12+ will draw about 50 Watts if all is well. (HT volts 320 X 80mA = 25.6 Watts and the heaters add another 23ish Watts). Your 60 Watt bulb will glow quite a bit, even if all is well. A 100 watt bulb would be better. (Or add another lamp socket wired in parallel and have the 40 and 60 Watt bulbs in at the same time.)
    Hope that helps?
     
    Tony L likes this.
  6. Tony L

    Tony L Administrator

    Thanks. I don’t have any 100W bulbs, 60 is the most powerful (I don’t think you can buy them anymore). I do have a power meter plug, kind of similar but not the same as this one (Amazon). I assume if I were to plug it into the ‘series’ socket with the amp on top I’d have some idea what was occurring? Also I assume if the bulb glows more dull than a 60W would usually then the amp is not in a short condition?

    The main goal here is really trying to establish a) the mains transformers are ok, and b) my work is fine. My original TL12 Plus is a totally unknown quantity, I bought it at the local auction and have never turned it on. Amp #2 should be ok as HiFi Hanger claimed the pre-power combo worked but sounded distorted (to be expected as the caps will be well shot, I’d not have turned it on myself!). There is no evidence of heat damage on either amp so I’m obviously hoping all is ok with all the transformers, but I want to power up in a safe scenario to start with.
     
  7. snowman_al

    snowman_al pfm Member

    You can still buy incandescent 100 Watt bulbs, you have to specify 'Rough Service' 100W bayonet cap (B22) bulb or similar.

    Testing the mains transformer: with no valves in there should be very little current drawn, so the limiter should just flash and then go very dim. If there is a short internally the bulb will continue to glow. You can measure the AC voltages from the transformer with your test meter now.
    Testing your work: put the GZ34 in on its own first. Again the lamp will come on and all being well will dim down but not go off completely. You can measure the HT voltage on the 100Ω resistor now, it will be 360 or more volts if all is well.
    Put the rest of the valves in, make sure you connect a speaker or 10Ω resistor to the speaker terminals.
    Now when you switch on the bulb will glow brightly at first, and if you use the 60W bulb, stay at least half brightness. Voltages in the amp will not be correct as the bulb is taking a fair share of the mains voltage. If all is well for a few minutes, then full mains...
     
    Tony L likes this.
  8. Tony L

    Tony L Administrator

    Excellent, thanks for that. I will do that.
     
  9. toprepairman

    toprepairman pfm Member

    Oh No! another dim bulb conversation, I shouldn't get involved, Lol.
    Actually they are a very good idea. I have one on a box that also contains an isolating transformer. The bulb is on the primary side so the filament is slightly biassed by the magnetising current.
    Of course Ohms law applies but you need to remember that the bulb filament has rather less resistance when it is cold.
     
    Darmok likes this.
  10. Barrymagrec

    Barrymagrec pfm Member

    A lot less - I have just measured a cold 60 watt bulb at 67 ohms. (60 watts 240 volts is 960 ohms)

    Fast start circuit for a Studer C37 take up motor.....
     
  11. Tony L

    Tony L Administrator

    Dim bulb tester is complete:

    [​IMG]

    I stuck the socket on sideways so it can easily be propped against anything handy to keep the bulb off the carpet. I’ve tested it on the TD-124 (runs a bit slow as expected) and a 303 (initial glow from the bulb on power-on, again as expected). All looks good. It should do what I need it to do.
     
  12. Barrymagrec

    Barrymagrec pfm Member

    Neat - but if you`d mounted the lampholder on the blank faceplate you wouldn`t need to prop it up.
     
  13. booja30

    booja30 pfm Member

    For anyone following along in the EU who have Schuko plugs, a modification will make the dim bulb tester safer.

    Use two bulbs in the circuit, one on L and one on N (each having half the wattage of what you'd use if only using one bulb). Schuko plugs aren't polarized (well they kind of are in France and some other places), there's a 50/50 chance the 240V is on the blue wire. If your thing you're testing has a ground fault, the bulb might not even light up but hopefully the worst that will happen is some blown fuses.

    This could also be the case in the UK if your electrician was sloppy.
     
  14. Tony L

    Tony L Administrator

    I did initially intend to do that, but figured I’d likely (in fact with 99.4% certainty) crack the faceplate as I just don’t have the right tools for that kind of drilling. It isn’t perfect, but it is safe and solid, and small too so easy to fling in a box and later forget where I’ve stored it when I’m done. If I was serious about doing this sort of thing I’d have made a really nice one in a slanting lab style bench-top box.
     
    Barrymagrec likes this.
  15. Yank

    Yank Bulbous Also Tapered

    In the US constructing a dimbulb with off-the-shelf items requires less improvisation.

    [​IMG]

    And the series socket can also double as a convenient place to insert an ammeter:

    [​IMG]

    I have a selection of 150W and 200W bulbs.
     
  16. Darmok

    Darmok "Didactic Prophylaxes"

    This used to be the fix for Garrard 401s rumble cure all.
     
  17. Tony L

    Tony L Administrator

    [​IMG]

    New resistor day! I’ve now got all the resistors and most of the caps (still waiting for the eBay stuff to land). I’ve been through and labelled everything and checked the values on the bags are correct with the meter. I’m happy all is ok so I can now start work on the first amp. I bought two lots of 3M3, Allen Bradley carbons and some no-name metal film as the Takmans aren’t available in that value. I actually bought six Allen Bradleys (I only need four) in order to select, but they are all bang in spec so I’ll use them in preference to the metal films. Maybe the scare stories are over-played here? They were actually closer to spec than the very expensive Mills!
     
    torstoi and Raoul Duke like this.
  18. Tony L

    Tony L Administrator

    [​IMG]

    Amp #1 gutted. I have severe envy of people who have a proper bench to work on, I’m so pushed for space (the whole house is full of vinyl and hi-fi) that the best I can do is a Black & Decker Workmate! All came out cleanly enough. I didn’t attempt to save vintage components so cut the shorter ones in two to avoid any strain on the tag boards when removing/levering out. The desoldering gun is of little use in removing this kind of tag-board component, but great for cleaning the holes afterwards. It saved a lot of time there. I notice Leak themselves had broken one tag, one of the big TCC caps was soldered on a little lower down. They are fragile things, I broke one on the Stereo 20, but I’ve learned some stuff since then so hopefully will do no damage here.
     
    torstoi likes this.
  19. Raoul Duke

    Raoul Duke pfm Member

    Love how easy these amps are to work on! After rebuilding 4 of these and a St20 my observation is that the earlier tag boards were more fragile and tags prone to falling off if even gently bent. Enjoy the rebuild, will be ineterested how you get on!
     
  20. Tony L

    Tony L Administrator

    [​IMG]

    Here’s as far as I’m going as I made an error ordering R18 &21. I clearly need 0.5W there, the 1W I have are way too physically big to do a neat job. I’ll have to wait until they land as I’d like to get these done first before putting the caps in as there’s just more room to work.

    PS Photo doesn’t show it, but the Mills 270R are a good cm up above the board.
     
    torstoi likes this.

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