Page 1 of 2 12 LastLast
Results 1 to 35 of 53

Thread: Marshall TSL:100 bias drift problem

  1. #1
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8

    Marshall TSL:100 bias drift problem

    Good Morning all

    Ive had a TSL100 for a service, Blown pair of Tubes, Replaced and re biased all seems ok, Then i noticed some bias drift.
    Decided to to the mod where you cut around pin 5 and wire resistors in directly to the tube base etc
    See below

    The Marshall TSL122 TSL100 thermal bias drift repair page did all this ,as ive done below with working results
    TSL100___TSL122_Valve_PCB__60_.pdf

    After about 20 mins again started to drift, stuck two DMM on and watched to see what is drifting. ( without tubes )
    V5-6 drifting V7-8 are stable
    So i changed all components in the bias circuit trim pots and all resistors and caps
    Still the same. Used the heat gun and freezer spray found by spraying around the PI tube area voltage would drop
    Changed the 82k 100k and 2 x 680K both 22nf, Removed the PI tube voltage stable, Swapped the tube still drifting

    Is there any other known issues with the board conductivity

    any help would gratfull, I know about the replacement board, but at this present time i need to repair if possible

    BBB

    0 Not allowed! Not allowed!

  2. #2
    Senior Member
    Join Date
    Dec 2007
    Location
    Poland
    Posts
    1,642
    Thumbs Up/Down
    Received: 51/0
    Given: 14/0
    Rep Power
    14
    I think that if you search in google for "marshall tsl 100 bias drift" phrase, you will get a lot of results. It seems to me that the topic was mentined here lately.

    Mark

    0 Not allowed! Not allowed!

  3. #3
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,642
    Thumbs Up/Down
    Received: 1,617/21
    Given: 1,347/0
    Rep Power
    14
    Quote Originally Posted by MarkusBass View Post
    I think that if you search in google for "marshall tsl 100 bias drift" phrase, you will get a lot of results. It seems to me that the topic was mentined here lately.

    Mark
    Indeed it was, here's your link bbb:

    The Marshall TSL122 TSL100 thermal bias drift repair page

    I've done this at least half a dozen times. The neat but time consuming way - remove output tube sockets, drill away board at pin 5, then replace sockets.

    Or if you have the patience & customer has the dosh, you can replace the whole board.

    0 Not allowed! Not allowed!

  4. #4
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    Thanks But ive done that mod that is stated in the link you posted. I have not removed the tube bases and cut the board away and point to point wired them

    BBB

    0 Not allowed! Not allowed!

  5. #5
    Senior Member
    Join Date
    Jun 2006
    Location
    Norristown state hospital
    Posts
    2,375
    Thumbs Up/Down
    Received: 200/4
    Given: 1/0
    Rep Power
    16
    I've repaired many of these in the past by isolating the socket pins but recently had one that was breaking down around the phase inverter and all the way back to V2!

    I imagine older units will have the board fail in many places.

    The only way to fix that last one was to replace the board.

    0 Not allowed! Not allowed!

  6. #6
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    Im the same Drewl. done loads of them with good results, But this one is i think is a new board. Its a local guitar shops customer and Money is an issue
    BBB

    0 Not allowed! Not allowed!

  7. #7
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,745
    Thumbs Up/Down
    Received: 436/0
    Given: 81/0
    Rep Power
    14
    Quote Originally Posted by blindboybenton View Post
    Removed the PI tube voltage stable....
    Am I reading this correctly - removing the PI 'fixes' the problem?

    0 Not allowed! Not allowed!

  8. #8
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    yes by removing the pi tube the bias is stable. Changed 82k and 100k anode and 2 x 680k + 22nf decoups

    0 Not allowed! Not allowed!

  9. #9
    "Thermionic Apocalypse" -JT nickb's Avatar
    Join Date
    Dec 2009
    Location
    Devon, UK
    Posts
    3,686
    Thumbs Up/Down
    Received: 851/1
    Given: 652/1
    Rep Power
    14
    The failure process is a consequence of conductive anodic filaments, I believe. These are copper filaments that grow primarily inside the board from one a positive plated through hole to another less positive one. Factors affecting the time to failure are the FR4 material, spacing, voltage and the temperatures involved in processing on the board with the higher temperature of lead-free soldering being a particular issue.

    What little I know about this, I got from this the report below.

    An interesting graph of page 34 suggests a time to failure of about 5 hours for a spacing of 1mm and 500V (btw the x axis is um not mm, I think). They state

    time to failure = k x spacing ^2/ voltage

    This implies a spacing of 10mm can fail in 500 hours but this does depend on the other factors mentioned above. If you Google CAF resistant FR4 you will see that there seem to be quite a few options available to mitigate this. I guess when the Marshall TSL was first built around 1998(?), this wasn't a very well known phenomenon. Must have been quite shock for many.
    Attached Files Attached Files

    1 Not allowed! Not allowed!
    Experience is something you get, just after you really needed it.

  10. #10
    Member mhuss's Avatar
    Join Date
    Oct 2006
    Location
    West of Philly
    Posts
    330
    Thumbs Up/Down
    Received: 33/0
    Given: 2/0
    Rep Power
    14
    I have a trio of newer DSL 100 watt heads in (joy.) "They're fine, they just keep blowing the HT fuse." The power tube ink has changed color showing clear signs of overheating. Does anyone know if Marshall ever fixed the FR4 problem in the DSL line? These are mid-2003 vintage (PCB Z03-14) and have the correct 5k6 grid stoppers on the output tube grids. Cold the resistance from pin 4 to 5 seems reasonable (220k+). I don't really want to disassemble and drill out three amps.

    Edit: Arrgh, after warm up bias voltage dropping like a stone.

    0 Not allowed! Not allowed!
    Last edited by mhuss; 11-04-2015 at 01:44 AM.

  11. #11
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,642
    Thumbs Up/Down
    Received: 1,617/21
    Given: 1,347/0
    Rep Power
    14
    Quote Originally Posted by mhuss View Post
    Does anyone know if Marshall ever fixed the FR4 problem in the DSL line?
    Presumably amps made 2006 on have improved boards where the dreaded bias drift won't/shouldn't occur. mhuss, looks like you have a very full day's work ahead. Like they say at the dentist: drill, fill and bill.

    I did the first couple of these without removing the output tube sockets. The dremel bit doesn't like to collide with the metal electrode pin on the socket. Makes for sloppy hole cut and potential busted grinding bit and/or smashed up electrode so I went for the socket removal method instead. Those who feel like it can try with the sockets in place, but for safety's sake I recommend not doing it that way.

    bbb, bizarre that removing the PI tube sets the bias straight. It's beyond me, I can't come up with an explication... just expletives. Looking forward to somebody coming up with a fix. Because inevitably one such will land on my repair bench. It's a new board you say - drat - thought Marshall had got these sorted by now.

    nickb, thanks for the link and your post. I'll have to read up on the link you gave. I thought heat on the board was a factor. If it's not heat dependent then leakage could occur anywhere, and if the new boards are failing too then all DSL/TSL owners are in deep doodoo.

    Someone made the comment a couple years back, if these amps were cars, there would have to be a manufacturer's recall to get 'em right. Looks more like a case of planned or unplanned obsolescence, a pity because the DSL/TSL amps can sound pretty dam' good.

    0 Not allowed! Not allowed!

  12. #12
    Senior Member
    Join Date
    Jun 2006
    Location
    Norristown state hospital
    Posts
    2,375
    Thumbs Up/Down
    Received: 200/4
    Given: 1/0
    Rep Power
    16
    The newer improved boards are a darker green, if it is light green it's the old bad board material.

    I had removed the filament traces electrically, ran wires, and anywhere filament traces would pass over any high voltage traces.
    It got the bias and output section stable.
    I did the same at the phase inverter and it was stable.
    By the time it was breaking down around V2 it was too cluttered to try and isolate any traces that ran over each other inside the board.
    That's when I gave up, it was a slow week at work which is why I dicked around with it to that extent!

    I imagine eventually those board will fail all over.

    0 Not allowed! Not allowed!

  13. #13
    Member
    Join Date
    Feb 2008
    Posts
    47
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    FYI: Best deal I've seen on replacement boards are here. TSL100 Main Board Hot Rox UK. The bias drift issue has been corrected. However, it has affected the tone. The distortion channel sounds better (less fizz), but the clean channel does not sound as good.

    0 Not allowed! Not allowed!

  14. #14
    Supporting Member mikeydee77's Avatar
    Join Date
    Mar 2012
    Location
    Bristol, UK
    Posts
    178
    Thumbs Up/Down
    Received: 13/0
    Given: 15/0
    Rep Power
    9
    I haven't attempted the repair as I am not convinced it will be reliable. But from the ones I have seen suggest problem was centred around 2002.

    Here is picture of the good and bad board colours. Bad board top, good board bottom. Flash has got in the way but the good boards are a rich leaf green and the drifting ones a paler maybe sage green.

    http://amploft.co.uk/blog/wp-content...8-19.40.27.jpg

    0 Not allowed! Not allowed!

  15. #15
    Member mhuss's Avatar
    Join Date
    Oct 2006
    Location
    West of Philly
    Posts
    330
    Thumbs Up/Down
    Received: 33/0
    Given: 2/0
    Rep Power
    14
    Quote Originally Posted by mikeydee77 View Post
    Here is picture of the good and bad board colours. Bad board top, good board bottom. Flash has got in the way but the good boards are a rich leaf green and the drifting ones a paler maybe sage green.
    Do they have different revision/production code/... numbers? (e.g. Z03-14) Can't really tell from the pictures.

    FWIW, at least one of the failing boards looks like the darker color.

    --m

    0 Not allowed! Not allowed!
    Last edited by mhuss; 11-05-2015 at 02:09 PM.

  16. #16
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,745
    Thumbs Up/Down
    Received: 436/0
    Given: 81/0
    Rep Power
    14
    I've worked on lots of these amps and what's difficult to rationalize is that I see early revision, well-used amps that have been in regular service without any problems. Equally, I also see amps with the same board revision that have/are suffering from bias drift and other board issues. You'd think that within a production run all boards would exhibit the same characteristics.

    I guess it's like the Mini cars with CVT transmissions. They all catastrophically failed at very low mileage. Except the few identical ones that did 200,000+ miles.

    0 Not allowed! Not allowed!

  17. #17
    Supporting Member mikeydee77's Avatar
    Join Date
    Mar 2012
    Location
    Bristol, UK
    Posts
    178
    Thumbs Up/Down
    Received: 13/0
    Given: 15/0
    Rep Power
    9
    Quote Originally Posted by mhuss View Post
    Do they have different revision/production code/... numbers? (e.g. Z03-14) Can't really tell from the pictures.

    FWIW, at least one of the failing boards looks like the darker color.

    --m
    I think the revision numbers only relate to the circuit and not the parts unfortunately. Sorry about glare on the photos; it was just meant to show the colour difference.

    I have a box of old drifty boards and they are all a much paler green. So perhaps my observation has a few doubts. Usually I look for tubes with heat damaged bases and the board colour and if I am suspicious then I measure the bias and watch. Usually it starts drifting within a few minutes and I always keep an eye on them for a good while. I have seen a least one amp where the overheating got so bad that the PT was also impacted.

    When I have spoken to Marshall about this they seem well aware of the issue. I had thought it was a materials problem where the board was absorbing something from the atmosphere and then changing its insulation properties hence my belief that the workarounds might not be good long term.

    Mike

    0 Not allowed! Not allowed!

  18. #18
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    Job sorted now got a new board
    BBB

    0 Not allowed! Not allowed!

  19. #19
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,065
    Thumbs Up/Down
    Received: 606/3
    Given: 311/0
    Rep Power
    28
    How much $$ did you have to shell out?

    0 Not allowed! Not allowed!

  20. #20
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    Hi jazz bass. The job was from a guitar shop who are Marshall dealers. They brought the board. £40 + tax. £40 is about $60. Marshall UK are really good on parts and not expensive

    0 Not allowed! Not allowed!

  21. #21
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,745
    Thumbs Up/Down
    Received: 436/0
    Given: 81/0
    Rep Power
    14
    They don't deal directly with the public, though. You have to be a trade account holder and the trade prices aren't published.

    0 Not allowed! Not allowed!

  22. #22
    Old Timer
    Join Date
    Jun 2007
    Location
    Pensacola, FL
    Posts
    3,576
    Blog Entries
    1
    Thumbs Up/Down
    Received: 10/0
    Given: 5/0
    Rep Power
    16
    Hey folks.... got one in with this problem. It's a TSL 122. But as mentioned early on in this thread... the bias only drifts (high) if the phase inverter is in place. Which leads me to suspect an oscillation of sorts.

    Anyone come across this or fixed it? BBB? How did you fix those that you had this issue with?

    0 Not allowed! Not allowed!
    Last edited by lowell; 09-09-2019 at 02:05 AM.

  23. #23
    Old Timer
    Join Date
    Jun 2007
    Location
    Pensacola, FL
    Posts
    3,576
    Blog Entries
    1
    Thumbs Up/Down
    Received: 10/0
    Given: 5/0
    Rep Power
    16
    Additionally: It can't be an oscillation, because upon pulling the PI, the bias slowly goes back down, about .1ma every 3 seconds. Quite strange... I can't think of anything that could cause this. Unless the coupling caps from the PI to the power tubes are charging when the oscillation is present, and then slowly discharging when the oscillation is gone.

    There is also a noticeable hum that seems to increase over time when the PI is installed. All pots are at zero.

    0 Not allowed! Not allowed!

  24. #24
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    13,065
    Thumbs Up/Down
    Received: 606/3
    Given: 311/0
    Rep Power
    28
    It sounds like a faulty PCB.

    1 Not allowed! Not allowed!

  25. #25
    Member
    Join Date
    Jun 2006
    Posts
    89
    Thumbs Up/Down
    Received: 12/0
    Given: 0/0
    Rep Power
    14
    Hi,
    I have had C36,C37 go leaky.
    This is over and above the " bias drift " problem that has been well documented and has numerous fixes.
    John

    0 Not allowed! Not allowed!

  26. #26
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,745
    Thumbs Up/Down
    Received: 436/0
    Given: 81/0
    Rep Power
    14
    Maybe the hum is down to the increased current draw on the power tubes as the bias is shifting. Scoping the output tubes should show any oscillation. If there was any DC leakage through the coupling caps then I'd expect this to be present whether or not the PI is installed. Do your voltages look OK on the socket with the tube removed? Also, when I get unusual problems with these amps I replace the 22pf snubber cap C46. This can be the source of many mysterious problems. I use a 2Kv rated part.

    If the PCB has become conductive there may be an issue where there's a voltage leakage that in itself isn't critical, but when the tube is inserted the tube's resistance is paralleled with the board resistance to give a greater leakage. Having said that, I've worked on a lot of these amps and never encountered leakage around the PI. But this reminds me of Blackstar amps that use MOSFETS instead of a PI tube. These commonly conduct between the pins, but I've had them where I've had to chase the leakage all over the board - including between pins of the preamp tubes.

    0 Not allowed! Not allowed!

  27. #27
    Senior Member
    Join Date
    Sep 2012
    Location
    South coast UK
    Posts
    664
    Thumbs Up/Down
    Received: 27/0
    Given: 40/0
    Rep Power
    8
    easy way is to replace the board. Ive done a fair few and every time i try and repair the board it never works well
    i get the board from marshall uk.

    0 Not allowed! Not allowed!

  28. #28
    Supporting Member
    Join Date
    Dec 2009
    Location
    UK
    Posts
    3,745
    Thumbs Up/Down
    Received: 436/0
    Given: 81/0
    Rep Power
    14
    Agreed, If the board is faulty, then here in the UK it's a better cost option than spending too long on a defective one and it has a better long-term prospect. I've had good success with repairing these boards using a special router bit I made that easily isolates the power tube pins. But its only any good if the leakage is localized to the power tubes. Once you get voltage leakage elsewhere the board is pretty much finished. I would always make certain the board is the problem before replacing it and not a component failure. Even the early revision boards don't all fail because they conduct - I have a 1998 board here right now with no leakage problems - it got changed out because the owner had read about the failures and wanted it swapping.

    1 Not allowed! Not allowed!

  29. #29
    Old Timer
    Join Date
    Jun 2007
    Location
    Pensacola, FL
    Posts
    3,576
    Blog Entries
    1
    Thumbs Up/Down
    Received: 10/0
    Given: 5/0
    Rep Power
    16
    I may have found a fix for drifting bias in these amps. Tomorrow will be the true test. I found another forum where someone mentioned C46. This cap is across V8 EL34 Plate-screen. My guess is it's there to reduce ultra high frequencies. I cut one leg of the cap. Then I measured the bias for a while. For about 30 minutes it continues rising Which I admit is not ideal... It finally stopped drifting. I readjusted the bias pots after 30 mins to set both at 90mv. I left in on for another hour and It did not rise any further for the following hour.

    I will try it on in the morning and measure it again when cold. Will also play the amp as well. If it sounds good and the bias isn't so far off that there's any bad hum I'm calling it fixed.

    Will report back.

    0 Not allowed! Not allowed!

  30. #30
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,695
    Thumbs Up/Down
    Received: 1,494/1
    Given: 1,111/1
    Rep Power
    17
    FWIW: Both "the C46 problem" and the "bias drift problem" are known issues for this amp and are separate issues.

    1 Not allowed! Not allowed!
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  31. #31
    Old Timer
    Join Date
    Jun 2007
    Location
    Pensacola, FL
    Posts
    3,576
    Blog Entries
    1
    Thumbs Up/Down
    Received: 10/0
    Given: 5/0
    Rep Power
    16
    Oh I didn't know that.. what is the issue with C46 if it's not the bias drift?

    Also, do you have any tips on drift?

    0 Not allowed! Not allowed!

  32. #32
    Old Timer Leo_Gnardo's Avatar
    Join Date
    Sep 2012
    Location
    Dogpatch-on-Hudson
    Posts
    6,642
    Thumbs Up/Down
    Received: 1,617/21
    Given: 1,347/0
    Rep Power
    14
    Quote Originally Posted by lowell View Post
    Oh I didn't know that.. what is the issue with C46 if it's not the bias drift?

    Also, do you have any tips on drift?
    check post #3

    1 Not allowed! Not allowed!
    Enjoy. Every. Sandwich.

  33. #33
    Supporting Member The Dude's Avatar
    Join Date
    May 2010
    Location
    Sioux Falls, SD
    Posts
    6,695
    Thumbs Up/Down
    Received: 1,494/1
    Given: 1,111/1
    Rep Power
    17
    ^^^^ Yes, that for bias drift. The problem with C46 is that it's rated voltage is too low from the factory and often burns or shorts. I always replace it with a higher rated cap as a matter of course- even if it's not bad. Others have clipped them out and left them out without issue. It is there to prevent oscillation.

    0 Not allowed! Not allowed!
    Last edited by The Dude; 01-07-2020 at 05:04 AM.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

  34. #34
    Lifetime Member Enzo's Avatar
    Join Date
    May 2006
    Location
    Lansing, Michigan, USA
    Posts
    32,935
    Thumbs Up/Down
    Received: 2,250/7
    Given: 0/0
    Rep Power
    55
    It is generally good practice to allow and amp to warm up at LEAST 20 minutes before adjusting bias.

    0 Not allowed! Not allowed!
    Education is what you're left with after you have forgotten what you have learned.

  35. #35
    Old Timer
    Join Date
    Jun 2007
    Location
    Pensacola, FL
    Posts
    3,576
    Blog Entries
    1
    Thumbs Up/Down
    Received: 10/0
    Given: 5/0
    Rep Power
    16
    Thanks guys...

    Leo.. I should have mentioned that mod has already been done but did not fix the bias drift. It truly seems as though removing C46 fixed it. Odd.

    0 Not allowed! Not allowed!

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. JCM 2000 Marshall TSL 100 mod to DSL 100 Specs
    By Tejaus in forum Mods & Tweaks
    Replies: 10
    Last Post: 11-14-2014, 10:49 PM
  2. Marshall TSL 100...help needed!
    By acorkos in forum Maintenance, Troubleshooting & Repair
    Replies: 4
    Last Post: 04-25-2014, 09:33 PM
  3. marshall tsl bias and hum problem
    By fireman in forum Maintenance, Troubleshooting & Repair
    Replies: 18
    Last Post: 12-06-2011, 05:29 PM
  4. Marshall TSL 100 Issue
    By mikeboone in forum Maintenance, Troubleshooting & Repair
    Replies: 6
    Last Post: 05-19-2009, 02:36 AM
  5. Tsl 100 bias wacky
    By bmd in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 12-22-2007, 11:41 PM

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •