Help finding fault with old bass amp that blows fuses

Discussion in 'Troubleshooting and Repair' started by permanentrecordstudios, Jan 12, 2018.

  1. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    Hi folks!

    I'm trying to find fault in an old Yamaha bass amp from the very early 80s, but I'm struggling a bit because of the fact that the fault is intermittent, sometimes instantly blowing the two 4 amp fuses (F2 & F3) on the main circuit board for the amp (secondary power side), sometimes working fine for extended periods.

    I've disconnected everything I can, tested as much as I can with a meter.

    I think I could probably find the fault quite easily if I could just power the board and trace it (I have the schematic), but obviously I can't do that because the fuses instantly blow when they do blow.....otherwise all is fine.

    There is no short (no continuity) between B+ and B- anywhere on the board.

    Does anyone have some good advice on how best to proceed, without unsoldering every single component and testing it?

    P.S. This amp has worked fine for 20+ years. This first started recently when the terminal on the speaker frame broke, and the speaker wires shorted, blowing the fuses for the first time.
     
    permanentrecordstudios, Jan 12, 2018
    #1
    1. Advertisements

  2. permanentrecordstudios

    duke37

    Joined:
    Jan 9, 2011
    Messages:
    5,020
    Likes Received:
    669
    Location:
    Derbyshire. UK
    Use a lamp limiter, this is just a filament bulb (40W?) in series with the mains. This enables a voltage to be applied but the current is limited, saving your fuses. Then measure the voltages in the amp.

    Since the problem is intermittent and may be voltage dependent, you might have difficulty in finding the fault.

    I found the schematic on page 13 of the manual but I could not read it. Can you post a better version?
    What do you mean by secondary power side? There are two supplies, possibly one for the main output power and one for the pre-amp.
     
    duke37, Jan 12, 2018
    #2
    1. Advertisements

  3. permanentrecordstudios

    73's de Edd

    Joined:
    Aug 21, 2015
    Messages:
    2,349
    Likes Received:
    775
    Location:
    Texas
    Sir permanentrecordstudios . . . . .

    Submitted herewith . . . is a better all encompassing referencing of the most concerned aspects of your unit .

    We can see that . . . . THE . . . . real power sucking portion of your unit are the AF output transistors in the form of that complementary / symmetry clusters of Tr123+Tr124 and Tr120 and Tr121 transistors.

    AND they DO get their own POSITIVE and NEGATIVE voltage sources from the same section of the power supply that relates to your blown fuses. . . .F2 and F3.

    My VERY highest suspicions are either a transistors time / temperature related or flow soldering development of a ring solder joint at the 12 connections of those transistors to their mini PCB connectors.
    Check out all of those solder connections with magnification assistance and a VERY bright lighting. (Example given on referencing . . . . .in case you have never seen such )

    REFLOW . . . all of the mini PCB to power transistors solder connections with fresh 60/40 solder being added.

    Time evaluate the unit . . . . for a shakedown.

    IF that doesn't solve the problem , the next suspicion would be an avalanching transistor.
    To confirm that , notice that two transistor collectors go to a + voltage source at the mentioned power supply section and the other two go to a - voltage source.
    You will have to be the eyes for confirmation now, but are there not short wire lengths that are interconnecting the mini PCBs of the power transistors, to the main chassis PCB . . . .see the shaded inset of the PCB snippet at the bottom.
    My next suggested procedure would be to insert a 1A fast blowing fuse between each of the 4 points shown as RED X's.

    Upon a run time evaluation, a specific fuse would then blow instead of the pairs that were blowing.

    Standing by for any ? 's or comments . . . . .

    LE GRAN REFERENCE . . . . .

    [​IMG]

    73's de Edd
     
    Last edited: Jan 12, 2018
    73's de Edd, Jan 12, 2018
    #3
  4. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    Thanks for the reply. The only other schematic I could find is here http://schems.com/bmampscom/yamaha/Yamaha_G100-III_sm.pdf
    I never had the original schematic, and I think these are just scans.

    So by secondary power, I mean post transformer side. There is a mains fuse which protects from the 120v main power before the transformer. There are 4 more fuses, on the board, post transformer, 2 of which protect the low-voltage circuit (15v), and 2 of which protect the circuit I'm having problems with (B+B-, like 30v last time I checked).

    This circuit is mainly to power the transistors (T120-121 [B-] &123-124 [B+]), and I think the relay (r101). I don't see it used for much else.


    Question to you about the light bulb limiter is, will that protect the fuses on this secondary circuit? I've thought about building one, but it seems they only limit/protect on the mains side (120v). Again, we're only talking 30v here, and post mains power, pre circuit power.
     
    permanentrecordstudios, Jan 12, 2018
    #4
  5. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    Excellent insight & post.

    So I already removed the transistors T120-121 & T123-123 and they have tested fine per specs (my meter has special slots for transistors). I'm getting 107hFE & 115hFE on the B688's and 132hFE & 118hFE on the D718's.

    I haven't removed T122 because I can't find values for it (marked C3298B). This is one of the few components I've found on my amp (B100-115iii which I can't find the schematic for) that differs from this schematic (which calls for a 25C1953). But testing mine without any specs seems worthless.

    A bad or loosened solder connection could have escaped my eye. Guess we'll see upon resolder & inspection. It all looked pretty solid. I appreciate the faulty solder pic/example.

    What is an "avalanching" transistor?
    Can a transistor exhibit sporadic behaviors (random surges)?
    I appreciate the idea of placing fuses before the transistors, and there is a short run of wire (I've already desoldered it). I may have to do that. But before that, any other ideas?

    My thoughts were the transistors (down to T122 now), caps, or relay (ry101). I don't see much else that uses the B power supply.
    Could a faulty cap (C181 & 182) cause an intermittent excess current draw on power up & blow these fuses?

    But it looks like T122 draws from both B+ and B-? Since both fuses are blowing, would I not be looking for a common link between B+ and B-? Can this be a clue? I can read schematics to a point I'm certainly no expert.

    What else would cause such a current draw?
     
    Last edited: Jan 12, 2018
    permanentrecordstudios, Jan 12, 2018
    #5
  6. permanentrecordstudios

    73's de Edd

    Joined:
    Aug 21, 2015
    Messages:
    2,349
    Likes Received:
    775
    Location:
    Texas
    Did you even read post 3 ????
    That was the derivation of the condensed schema in #3.
    The lamp not going to help on isolating your sporadic and particular type of situation.

    73's de Edd
     
    73's de Edd, Jan 12, 2018
    #6
    duke37 likes this.
  7. permanentrecordstudios

    kellys_eye

    Joined:
    Jun 25, 2010
    Messages:
    3,800
    Likes Received:
    1,021
    Location:
    West Coast, Scotland
    What fashion do the blown fuses take? 'Just failed', 'blown to smithereens'?

    Are they the correct type i.e. anti-surge as opposed to fast-blow?
     
    kellys_eye, Jan 12, 2018
    #7
  8. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    I did read your excellent post. Perhaps you missed my previous reply specific to your post 3 (pasted below)?

    Excellent insight & post.

    So I already removed the transistors Tr120-121 & Tr123-123 and they have tested fine per specs (my meter has special slots for transistors). I'm getting 107hFE & 115hFE on the B688's and 132hFE & 118hFE on the D718's.

    I haven't removed Tr122 because I can't find values for it (marked C3298B). This is one of the few components I've found on my amp (B100-115iii which I can't find the schematic for) that differs from this schematic (which calls for a 25C1953). But testing mine without any specs seems worthless.

    A bad or loosened solder connection could have escaped my eye. Guess we'll see upon resolder & inspection. It all looked pretty solid. I appreciate the faulty solder pic/example.

    What is an "avalanching" transistor?
    Can a transistor exhibit sporadic behaviors (random surges)?
    I appreciate the idea of placing fuses before the transistors, and there is a short run of wire (I've already desoldered it). I may have to do that. But before that, any other ideas?

    My thoughts were the transistors (down to Tr122 now), caps, or relay (ry101). I don't see much else that uses the B power supply.
    Could a faulty cap (C181 & 182) cause an intermittent excess current draw on power up & blow these fuses?

    But it looks like Tr122 draws from both B+ and B-? Since both fuses are blowing, would I not be looking for a common link between B+ and B-? Can this be a clue? I can read schematics to a point I'm certainly no expert.

    What else would cause such a current draw?
     
    Last edited: Jan 12, 2018
    permanentrecordstudios, Jan 12, 2018
    #8
  9. permanentrecordstudios

    duke37

    Joined:
    Jan 9, 2011
    Messages:
    5,020
    Likes Received:
    669
    Location:
    Derbyshire. UK
    Edd Thank you for a readable schematic.

    The lamp limiter will limit the input current so will limit the secondary current. This will enable you to compare one channel with the other but may suppress the fault.

    I would look at the voltage across Tr122 and the connections in that area which set the bias. Perhaps a dicky potentiometer.
     
    duke37, Jan 12, 2018
    #9
  10. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    So you're saying the lamp limiter on the mains side may prevent these secondary fuses from blowing?
    I guess I fear damage to caps C181 &182 and or damage to the bridge rectifier going that direction (but I don't know enough for those fears to be credible).

    Let me ask a stupid question...I already know a 30v current will light a 120v60w bulb, dimly. So can I temporarily wire light bulbs in place of the fuses to absorb the excess current draws without damaging anything?

    My thoughts were the transistors (down to Tr122 now, as I've already tested the others with a trans tester), caps (b181&182), or relay (ry101). I don't see much else that uses the B power supply.

    I haven't tested Tr122 because I can't find the specs for it.

    Could a faulty cap (C181 & 182) cause an intermittent excess current draw on power up & blow these fuses?

    But it looks like Tr122 draws from both B+ and B-? Since both fuses are blowing, would I not be looking for a common link between B+ and B-? Can this be a clue? I can read schematics to a point but I'm certainly no expert.

    What else would cause such a current draw?

    I haven't measured the voltage across B+ to B- on Tr122, but mainly because I'm tired of buying fuses. But that's a good suggestion and I hadn't thought of it. I just need steady/reliable current first....
     
    Last edited by a moderator: Jan 12, 2018
    permanentrecordstudios, Jan 12, 2018
    #10
  11. permanentrecordstudios

    73's de Edd

    Joined:
    Aug 21, 2015
    Messages:
    2,349
    Likes Received:
    775
    Location:
    Texas
    Sir permanentrecordingstudios . . . . .

    So can I temporarily wire light bulbs in place of the fuses to absorb the excess current draws without damaging anything?

    Yep, you can watch the brightness increase exactly at fault time considering that you are CONTINUALLY observing them until exact fault time, then that wont tell you a damn thing about the quite expansive potentially related fault area.
    The 1 amp fuses would pinpoint the particular 1 in 4 power transistors, be it avalanching /temporary voltage breakdown or a temporary hard base biasing error coming into it . . .needed to be pinpointed next.
    You don't want to be soldering to fuse caps .

    Cheeeeep fuse prep: . . .of 1A units

    Run 2 in of insulated hook up wire ( bared and tinned for 1/8 in on the PCB connection side and 1/4 inch on the fuse cap end) across the 1 A fast blow / std fuse and stop at the end of the cap and bind tight with multiple turn sewing thread wraps, knot off.
    Run the other wire across in the opposite direction and bind..
    MADE: One temporary pig tail fuse at no cost of buying a fuse holder.


    Could a faulty cap (C181 & 182) cause an intermittent excess current draw on power up & blow these fuses?

    Possibly, by either cap shorting or also having an intermittent internal open connection which would shoot sky high ripple to one section of the power amps to instill an imbalance and potential high current draw by the imposed voltage upset.
    BANG . . .fuses blow.


     
    73's de Edd, Jan 12, 2018
    #11
  12. permanentrecordstudios

    Wayne Phillips

    Joined:
    Jan 7, 2018
    Messages:
    61
    Likes Received:
    1
    Location:
    Scotland
    Lot's of helpful pointers made here to help solve this fault.

    More pointers..has the power amp stage been desconnected from the psu to check that it's psu is showing level voltage on each rail.

    Yes use the lamp limiter as it will give some time to check the circuit.

    No load or signal until the fault is found and the good thing about transistor amplifiers is..they can be worked on with no speaker load hooked up.

    A dummy resistor load is handy when repairing amps. 4 to 10 ohms at 50w for low rated amplifiers.

    Check offset voltages..if high dc is found then check the 'long tailed' pair. Ie the input tansistors.

    Then work your way towards the vas stage then driver and output devices.

    Old amplifiers are fare striaght forwards to trace out to get working again.
     
    Wayne Phillips, Jan 12, 2018
    #12
  13. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1

    Yeah, lots of helpful suggestions. That's why I came here. Lot's of folks much smarter than myself.

    But the voltage has been checked on the psu with the fuses out (so, minus the power amp side), and the voltage (no load) is stable.
    Not sure what exactly you mean by "power amp stage" (are you talking just transistors?), I haven't checked the voltage with only the power stage (transistors) disconnected. The power amp and pre amp are mostly on the same board, so disconnecting just the power amp stage in it's entirety would be tough, that I can tell.

    But I have checked the individual transistors, and they all test fine (within hFE tolerences).
     
    permanentrecordstudios, Jan 12, 2018
    #13
  14. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    You keep going back to the transistors...so just a question, even though they test fine by my meter (within hFE tolerences) can they still be bad?
    I like your idea, it's just a lot of work if the transistors are good (as they test). But minus any better ideas, it could possibly save me a lot of time too.

    I've read in other posts that when both these fuses blow it's often an indicator of a problem the output amp side....which I'm guessing goes back to the transistors.
     
    permanentrecordstudios, Jan 12, 2018
    #14
  15. permanentrecordstudios

    Bluejets

    Joined:
    Oct 5, 2014
    Messages:
    2,761
    Likes Received:
    518
    I'm wondering if you read Kellys_eye #7.

    It is possible you don't have any fault , just incorrect fuses.
     
    Bluejets, Jan 13, 2018
    #15
  16. permanentrecordstudios

    Wayne Phillips

    Joined:
    Jan 7, 2018
    Messages:
    61
    Likes Received:
    1
    Location:
    Scotland
    Cold checking the power output transistors if all is good there ie base to collector point and b to emitter leg..

    A amplifier circuit has few stages input signal transistors (your amp has fet input pair) followed by vas stage then driver stage and power transistors..
    Removing the outputs is wasting lots of time here so leave them in and if need be order replacement ones.

    The incorrect fuse fitted will pop quick.

    Look for time delay fuse types.

    If the bais trim has been touch this can over set the output devices making draw to much, and the output voltages across the emitter resistors should be reading say 10mv or so.

    See how you get on..
     
    Wayne Phillips, Jan 13, 2018
    #16
  17. permanentrecordstudios

    duke37

    Joined:
    Jan 9, 2011
    Messages:
    5,020
    Likes Received:
    669
    Location:
    Derbyshire. UK
    My very limited experience has shown that if a semiconductor has failed , it has done the job properly and is dead.

    We have not yet seen the voltage across Tr122 which controls the output bias. As previously said, a bad connection here or a dodgy potentiometer can turn the output transistors hard on.
     
    duke37, Jan 13, 2018
    #17
  18. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    Thanks for the reply.
    Yeah, the fuses were the first thing I checked. I haven't touched inside his unit for over 20 yrs. The fuses that first popped were SOC ST6, 250V4A. LittleFuse recommended their 313 series (again, 250V/4A) as replacements. Both are slow blow types.
     
    permanentrecordstudios, Jan 13, 2018
    #18
  19. permanentrecordstudios

    Wayne Phillips

    Joined:
    Jan 7, 2018
    Messages:
    61
    Likes Received:
    1
    Location:
    Scotland
    Bias transistor can be mje340 npn or bd139 both can be mounted to the heatplate..

    When powering the amplifier up are you doing so with unloaded.

    Looking at the circuit dig..them transistors just in front of the bias tr are parts of the amps short circuit limiters and have these been tested..

    It is a basic amplifier and there are ways to isolated it if need be. I have had devices leak under load thus making the unit do crazy stuff.
     
    Wayne Phillips, Jan 13, 2018
    #19
  20. permanentrecordstudios

    permanentrecordstudios

    Joined:
    Jan 12, 2018
    Messages:
    26
    Likes Received:
    1
    Yeah sorry, and thanks. I didn't mean to ignore the post, I was just focusing on other things..... but a good suggestion nonetheless. Sometimes we can get so caught in the fine details that we miss the obvious. But sadly not the case here (yet).

    I replied to Kellys_eye #7. The new fuses are cross-referenced with the old. Both are time delay.



     
    permanentrecordstudios, Jan 13, 2018
    #20
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.