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Need Help With a Scott 208!

Discussion in 'Tube Audio' started by ScottFan355, Dec 3, 2017.

  1. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    IMG_0118.JPG Hello all! Let me say thanks to anyone who responds in advance!

    So let me start by saying that I know squat about electronics though I have started doing some reading. My only hands-on electronic experience is putting together a kit or two from radio shack when I was a kid.

    Now that we've got that out of the way I acquired a Scott 208 a while back and after using it a few times a tube started to redplate. Of course I immediately unplugged the amp and it has sat in my closet ever since.

    From the reading I've done it seems the likely culprits are a bad tube, the voltage going to the socket being off in some way, or perhaps a problem with the socket itself. This amp has been partially recapped and was checked out by a tech prior to my purchase and was proclaimed 'in good operating condition'.

    I've been planning on taking it to a tech but having been a member here for a little while and respecting the DIY spirit that's here in abundance I've thought maybe I could give it a try on my own. My biggest priorities are not damaging the amp, my speakers (perhaps I should get some cheapies at goodwill to hook up to it until I feel certain the problem has been addressed), or myself! The only equipment I own is a cheap multimeter (which I don't really know how to use aside from adjusting bias on the st-70 I used to own). I definitely will get a soldering iron (cheap suggestions?).

    I would love to gain the confidence to go through the whole amp and replace any remaing caps (after practicing my soldering skills on something else) and make sure that it is in safe and up-to-spec operating condition. But frankly I'm totally intimidated by the idea of trying IMG_0121.JPG IMG_0112.JPG IMG_0131.JPG to learn on this rare and beautiful amp. I'm including some pics of the 208 and my 355 as well so you all can see what beauties they are.

    Any help is greatly appreciated, I love this beautiful amp!
     
    Last edited: Dec 3, 2017

     

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  2. trainbuftony

    trainbuftony Electron Herder Subscriber

    Start by never putting another JJ 7591 in it ever again. Out of 8 I had 3 fail within a year. I have had great luck with the tung sol reissue tubes. I've bought 16 of them and they all still work and don't drift. The grid resistors should be checked and replaced if they are above 220 ohms. And the 4 output stage coupling caps should be replaced along with the electrolytics. I think the main issue is the jj tubes. They tend to short grid to cathode iirc, and have proven unreliable in my fleet of scotts. Can you shoot some photos underneath the amp so we can see what has been done to it? I own one and it was pretty simple to bring it back from the dead, you should have it singing in no time. Get a hold of Jim Mcshane and see if he can get you a good set of 7591s. His website is old school, but his service is top notch and the product he sells is tested more throughly than just about anyone else.

    For soldering I would start with a 25 watt Weller (or simmilar) pencil style iron, some paste flux, and some rosin core 60/40 solder. A solder sucker of some type is nice too. You should be able to put a simple soldering kit together for under $50 which will get the job done, then keep an eye out for a used Weller soldering gun. The gun will come in handy for bigger jobs like desoldering chassis grounds on big can style electrolytic caps. This Scott amp is a great place to learn and practice your soldering. There is plenty of space under the chassis and the layout is pretty simple. Anything which you screw up can be fixed. Aren't tube amps great?
     
    Last edited: Dec 3, 2017
    Ballylongford likes this.
  3. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    I worked on lots of Scotts and Tony has great advice for you. My only input would be to get a good iron like Weller or other name brand. Stay away from the pencil tip units as they are for working on circuit boards. My everyday iron is a Weller W60 with an 1/8" 800 deg. tip. When soldering point to point circuitry like a Scott, the solder lugs can soak up a lot of heat. Excessive heat exposure is bad for components and causes metal fatigue as well. So you want to be using heat for less than 10 seconds if you are dealing with resistors and capacitors. For carbon comp resistors you should use a heat sink on the leads to prevent heat damage which can change the value of the resistor. Scott uses a lot of CCR.
    I think it would be ok if you work on the 208 since that is a simpler unit than the receiver but it would be better to practice on something a little easier like a single ended amp which would have less than half the components of the PP amp. good luck
     
    trainbuftony likes this.
  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

    Messages:
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    Location:
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    Scott -- That's a beautiful late version 208 you have! I have an early version 208 which had a number of design issues I had to deal with that I detailed in a thread of my own on this model. But the effort was really worth it, as it is now such a superb performer that I really enjoy.

    One of the best things you could do before starting anything is to see if you can find a schematic for your unit. There are plenty of schematics for the 208 out there, but they are invariably for the early version units like mine. Your later version unit is identifiable by having only 2 can caps instead of three like mine does, having the four pin test point plug (which mine does not), and having both DC Bal controls and Bias Adj controls -- the latter which mine did not have, as designed.

    By getting a later version unit then, it comes with the builtin ability to adjust each output tube for optimum performance, which is not possible with the earlier versions as designed. As a relative newbie then to vacuum tube electronics, it means that any restoration work you do will be greatly simplified, because you won't have to be dealing with the design short comings of the earlier versions in the process. Your's will largely be just a remove and replace effort.

    Short of finding the correct schematic, you can likely use the power supply and power amplifier sections of the 299D and/or LK-72B schematics, which will be very close. Sometimes they will come with the appropriate bias and balancing instructions for you to use in adjusting your amp for best performance. If not, some good underside shots of the unit will help us to help you better. If you will, also please post the numbers found on the top of the power transformer for your unit as well.

    Good luck with it!

    Dave
     
    tcdriver and Dandy like this.
  5. Dandy

    Dandy Super Member

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    4,531
    Location:
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    Those two Scott's of yours look in beautiful condition. With the excellent advice above you're on your way with DIY restoration of your 208.

    Re soldering, if you can afford it, get yourself a temperature controlled solder station. Weller makes nice ones where the iron tips switch the iron on and off to maintain the correct temp. These really do make soldering a pleasure and help you to make good joints.
     
    tcdriver likes this.
  6. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    Thanks for the advice (and thanks to everyone else who replied!). So do you think there is a fair chance that the problem might lie with a faulty tube? How safe for the amp (and my speakers) would it be for me to troubleshoot this by switching the tube into a different socket to see if the red-plating stays with the socket or the tube? Underside pics coming soon.
     

     

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  7. trainbuftony

    trainbuftony Electron Herder Subscriber

    I'd say there's a high likelihood that the grid shorted to the cathode. If your meter has a diode test function hook the leads to pin 5 and 6 of the suspect tube. If you get a beep you have a short. If your meter does not have the diode test feature use the ohm meter, and make sure there isn't a short. Anything close to 0 ohms is a short. There still may be more wrong, for example tube socket pins could be cleaned and tightened. In my expexperience jj 7591s tend to short grid to cathode. If someone locally has a tube tester that's even better. Consider posting your location that way folks near you can perhaps offer their help. You aren't likely to damage anything by swapping tubes around, as long as you kill the power as soon as the red plate appears. Your speakers are safe.
     
    Dandy likes this.
  8. drew_t

    drew_t AK Subscriber Subscriber

    Messages:
    702
    "This amp has been partially recapped and was checked out by a tech prior to my purchase and was proclaimed 'in good operating condition'."
    -----------------
    This could mean anything from "all the caps it's important to change have been changed" to "one bad cap that was preventing the unit from operating was changed so the tech could get it out of his shop, leaving a bunch of other 60 year old caps that are about to fail."
     
  9. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    Like I said, pics coming soon. Haven't looked at the underside yet.
     
  10. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
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    But the tech who looked at it (according to the previous owner, who I believed) I've since been referred to by a couple different sources, including Echo Audio, one of the top audio stores in Portland OR (which is where I am). His name is Bob (have his last name and number somewhere) and has been described to me as 'the guy who can fix anything tube-related'. Though I get the impression he is more fix-it guy than restorer.
     
  11. Dandy

    Dandy Super Member

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    That tech sounds nice, but let's see pics of the underside to check if the relevant coupling caps have been replaced.
     

     

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  12. trainbuftony

    trainbuftony Electron Herder Subscriber

    You can replace every capacitor in this amp for well under $100 if you do the work yourself. There's not a whole lot of em.
     
  13. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    Will do my best to have underside pics tomorrow!
     
  14. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    IMG_0374.JPG IMG_0375.JPG IMG_0376.JPG IMG_0377.JPG IMG_0379.JPG Actually, no better time than now!
     
  15. trainbuftony

    trainbuftony Electron Herder Subscriber

    So, I see a few things which look off to me. There is a blue non polar electrolytic attached to the bias supply rectifier and the rectifier tube sicket, which is not present on any of my scott amps. I'm not sure what they've got going on there, but that little rectifier provides negative voltage to the grids of your 7591s in order to bias them. (Restrict current between the cathode and plate) I'm also not seeing the AC wiring feeding that rectifier. My scotts all have an additional can capacitor which has 4 sections and filters the bias supply. It doesn't look like yours was ever equipped that way. I'm also seeing a electrolytic cap paralleled with a resistor in the feedback loop, which I don't recall having on my unit. There are 4 white Ceracap coupling capacitors liked up in a row, those are your output stage coupling caps and a short on any one of em would cause a red plate tube. Hopefully someone chimes who is more familiar with this unit as wired. It has a lot of differences which is par for the course with HH Scott gear.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    This later version 208 is in fact very close to the the power amplifier and power supply circuits of the early version Scott 299D integrated amplifier. The bias supply operates with a single (half wave) rectifier and single filter cap, but now includes bias controls for each channel, that the previous version of the 208 did not have. The power transformer bias supply winding leads are attached to the rectifier tube socket terminals 3 and 7, using them as tie points only. Pin 7 is connected to ground, while pin 3 has a lead extending over to the selenium bias rectifier. I believe that close inspection discloses that the blue cap is in fact a polarized cap, being the single filter cap in the bias supply system.

    The small electrolytic cap in parallel with the 220K resistor networks are actually part of the input stage screen grid bleeder circuit in each channel, being connected between the screen grid and cathode elements in that section of the driver tubes.

    If the B+ voltages are as indicated on the early 299D schematic (430 vdc to the OPT), then the output tube cathode current test points should be adjusted for 0.25 vdc in each channel under quiescent conditions.

    Dave
     

     

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  17. drew_t

    drew_t AK Subscriber Subscriber

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    I would replace all those white tubular "Ceracap" capacitors, and, unless there is some indication that they have already been replaced or restuffed, the two multisection can caps.
     
  18. ScottFan355

    ScottFan355 AK Subscriber Subscriber

    Messages:
    684
    First I want to say once again, thanks for all of the replies, I really appreciate it. Some of it is gibberish to me but I'm working on that through Google. I see that one poster here recommended a pencil tip soldering iron and one person recommended against one. I was wondering if the Weller wlc100 40-watt soldering iron and station on Amazon would be adequate? My thought was to order it (or another, but funds are limited) and practice desoldering and resoldering caps on a mid-fi 80s or 90s receiver that I own. The thinking was that it's likely to be more crowded and delicate (pcbs and such) that if I can get comfortable doing the work on it the Scott should be no problem. Since it only cost ten bucks I'm not too worried about screwing it up. Does this seem like a good idea to all of you?

    And please, keep the responses coming, I'm doing my best to keep up!
     
  19. trainbuftony

    trainbuftony Electron Herder Subscriber

    I was unclear about my recommendation for a soldering iron. Pencil style should be read as the style without a pistol grip and trigger.
    I was not referring to the pointy tip which resembles a pencil tip. You want something a bit wider than that, or one with an assortment of tips. I think your plan to practice on some old stuff is a good one. Dave is much more knowledgeable about the circuit in this specific amp. It is quite a bit different than any of my hh scott products.
     
  20. drew_t

    drew_t AK Subscriber Subscriber

    Messages:
    702
    I had a Weller wlc100 for a few years; it's OK. I replaced it with a nicer Hakko station that has a digital temperature readout.

    If you're buying it online, make sure to add some solder and some desoldering braid, and maybe a flux pen to the order so that you don't end up paying multiple shipping charges.
     

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