Announcement

Collapse
No announcement yet.

Blues Jr III Universal issue

Collapse
This is a sticky topic.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • MichaelNuzum
    replied
    I just had this exact problem on my bench last week. To add a bit of perspective, here's what happened to me:

    I had a Blues Jr come in with a blown screen resistor. Power tubes tested as one weak and one shorted. I replaced the resistor and installed new power tubes.

    I run all diagnostics and signal tracing into a dummy load. During the normal testing procedure, the amp passed all tests. I check voltages at all legs, I inject signal and trace with an o-scope to look for signal degradation or noise. I measure at output and look for max power before distortion. On every test the amp was rock solid. This was all on the bench, chassis out of cabinet, into dummy load. Very stable and consistent readings.

    I put the amp back together and fired it up and it sounded glorious. But, I did notice that smell of power tubes running a little hot (which I guess these amps are known for), so I popped the back off to take a few measurements. This was now chassis in the cabinet, back removed, running into onboard speaker. That's when the fun began.

    When I would probe the output transformer leads, I would get the left side (brown) reading around 330VDC, and the right side (blue) would flutter between 290-320, mostly hovering around 300VDC. At the same time, I was getting almost 18VAC across the speaker terminals!

    I looked around and saw the advice regarding lead dress, and keeping the output transformer leads as perpendicular as possible to the ribbon cables, adding a 100pf cap, etc. Nothing helped.

    THE SOLUTION: I found someone saying the amp can be very unstable without the foil-covered back in place. Something about the phase inverter ribbon cable needing to be located next to the plane of the back foil was necessary for the ribbon cable to be properly shielded from oscillations. (?) The suggestion was to mimic that property with a folded piece of aluminum foil, maybe 2x3 inches or so, alligator clipped to the chassis, extending up to just cover the p.i. ribbon cable. It worked!!!!

    I ran the rest of my tests and measurements, this time with the chassis in cabinet, plugged into the onboard speaker and back removed, and again the amp passed with flying colors! Both output transformer leads resting stable around 330VDC, with 0VAC at speaker. So, if you are working on one of these on the bench with the back off, it seems you need to either run it into a dummy load, or have that piece of aluminum foil in place, or the amp will go into scary instability.

    Hope that helps someone else and saves them from the rabbit hole I went down trying to figure that one out!

    Leave a comment:


  • christarak
    replied
    Originally posted by Chuck H View Post
    This is such an obscure piece of very useful information for most DIYers (including me) and even some experienced techs that I chose to "sticky" it for a time to keep it visible. Right on guys!
    You wrote this comment 2 years ago, Chuck, but I just discovered it in this Corona world gone crazy. I'm right with you, mate. Every single time I get on this site, I leave with new gem of wisdom and insight.

    Leave a comment:


  • diydidi
    replied
    I'm only seeing this thread now. There is a fix for this. I have been doing on Blues jnr's for the past two years.
    It is due to lead dress. The cream boards where the worst.
    Solder a 100p capacitor across one of the 100k PI plate load resistor. This will stabilise everything even for measuring with no change of freq response.

    Leave a comment:


  • J M Fahey
    replied
    Originally posted by lowell View Post
    Have 3 Blues Jr III's in the shop from a local church. All have that same obscure symptom. All have the same repair as well - open Rscreen. Pulling the PI cures the oscillation/measurement issue! Thanks guys! Awesome thread thanks for the input.

    JM, do you think after pulling the PI and mitigating the oscillation, that it still may kill your meter?
    No, no oscillation no problem

    Leave a comment:


  • Helmholtz
    replied
    I would like to add that needle meters may cause oscillation as well. But they are much more robust than DVMs.
    Its mainly the radiated field of the hot lead that capacitively couples to a non-inverting input which causes positive feedback.

    Also, prolonged high amplitude oscillation stresses the insulation of the OT's primary wire.
    Last edited by Helmholtz; 11-05-2018, 05:23 PM.

    Leave a comment:


  • lowell
    replied
    Have 3 Blues Jr III's in the shop from a local church. All have that same obscure symptom. All have the same repair as well - open Rscreen. Pulling the PI cures the oscillation/measurement issue! Thanks guys! Awesome thread thanks for the input.

    JM, do you think after pulling the PI and mitigating the oscillation, that it still may kill your meter?

    Leave a comment:


  • Dave H
    replied
    To measure bias current I put a shorting plug in the speaker jack and use the transformer shunt method. It doesn't oscillate when it's shorted.

    If it oscillates when measuring plate voltage try using a "stopper" resistor. Clip a 10k to 100k resistor to the end of the meter lead and probe with that (and short the speaker)

    Leave a comment:


  • J M Fahey
    replied
    Agree that directly measuring plate voltages can often lead to unwanted and dangerous oscillation.

    For a long time now I have shied away from directly measuring plate volts ... lost a couple cherished digital multimeters in that task, clearly amp oscillates and 1500V peak HF voltage smashes my meter input to death.

    Usually measure centertap which is +B, stable and well decoupled so no oscillation at all, and trust plate voltage is a few V below that.

    I really do not need to exactly know whether plate voltage is 453 or 449V, it´s close enough (and much safer) to know it´s "a few V less that centertap 460V" or whatever you have there.

    And if I ever needed to really know it, in the extreme case I suspect an open primary or something, I pull an old needle meter and go at it.

    Leave a comment:


  • Chuck H
    replied
    This is such an obscure piece of very useful information for most DIYers (including me) and even some experienced techs that I chose to "sticky" it for a time to keep it visible. Right on guys!

    Leave a comment:


  • drewl
    replied
    As much as possible and position them away from other PT leads.

    Leave a comment:


  • lowell
    replied
    Twist the OT primaries?

    Leave a comment:


  • drewl
    replied
    Yup, been there.
    If you re-do the wiring, twist the pairs real nice and push the ribbon cables back towards the chassis you won't get the HF oscillation and can measure the plate voltage/bias as normal.

    Leave a comment:


  • lowell
    replied
    Ahhh… right. Hence the disaster that breaks loose when you connect primaries in reverse when replacing an OT.

    Leave a comment:


  • Helmholtz
    replied
    Question though, since global feedback is NEGATIVE feedback, how is this being injected back into the PI and causing an issue. I'd think the issue would null itself.
    The two halves of the OT primary are 180 degrees out of phase with respect to AC ground. So one side may produce negative feedback with the meter leads and no oscillation while the other gives positive FB and oscillation.

    Leave a comment:


  • lowell
    replied
    BRILLIANT... thanks guys! I knew it was something more obscure. Perfect solution, if pulling the PI works.

    Question though, since global feedback is NEGATIVE feedback, how is this being injected back into the PI and causing an issue. I'd think the issue would null itself.

    Leave a comment:

Working...
X