I have a GK MB 105-III that decided to go silent during a practice session. The red LED comes on for about 5 seconds and then... nothing. I requested and received schematics from GK and have come to a dead end. The main problem seems to be that the +15 volts is sitting at 7.6 volts, even when I disconnect the preamp board. I took several measurements around the adjustable voltage regulators and attached the results to this post. Any help will be greatly appreciated.
Thanks,
Dick
Thanks,
Dick
Attachments
iirc the +/-15 volts is also used in the driver stage of the power amp have you isolated disconnected that also?
first thing i'd try is taking c1,c4 out of circuit and see if the voltage come back to normal in an amp of this age it's not uncommon to encounter bad capacitors.
are the regulators running hot with no load?
first thing i'd try is taking c1,c4 out of circuit and see if the voltage come back to normal in an amp of this age it's not uncommon to encounter bad capacitors.
are the regulators running hot with no load?
i just noticed the list of measured voltages on the partial schematic i'd say the regulators are bad
Thanks for the quick responses... Here is more information based on your inputs. The regulator input voltages are: -22.5 to Q12 and 22.8 to Q13. I have not disconnected the power amp or C1 or C4 (which sounds like my next steps). according to the data book for these regulators, the output voltages are correct for the voltage drops measured across R1, R2, R10, and R12. Will investigate further tomorrow.
Since this is an instrument amp this thread has been relocated to I & A per forum policy.
If Q13 is unloaded (check it) and you have such low voltages, then Q13 is bad.
It's cheaper/faster replacing it than measuring it or anything else.
Then test regulation loading outputs with 100 ohms 5W`resistors or something close to that, so you make regulators work but don't risk the preamp.
It's cheaper/faster replacing it than measuring it or anything else.
Then test regulation loading outputs with 100 ohms 5W`resistors or something close to that, so you make regulators work but don't risk the preamp.
i'm curious as to what data book is telling you these conditions are normal?
i'm also wondering why the incoming ac is low seems your approximately 8 volts low.could you please disclose your test conditions and check what the incoming ac voltage is.
i'm also wondering why the incoming ac is low seems your approximately 8 volts low.could you please disclose your test conditions and check what the incoming ac voltage is.
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I, too, am puzzled by the lower han expected input voltages, but don't think that would cause the regulators to fail. The databook I'm using is from Texas Instrument for the LM317 http://www.ti.com/lit/ds/symlink/lm317.pdf Section 9.2. (The LM337 is similar.) The formula is:
Vout = Vref ( 1 + R2/R1) +(Iadj X R2)
where: Vref = .7 (as measured across R1)
R2 = 2.43K
R1 = 243 ohms
Iadj is negligible (typically less than 50 uA).
My test condition is all components on the power amp intact, preamp disconnected.
Vout = Vref ( 1 + R2/R1) +(Iadj X R2)
where: Vref = .7 (as measured across R1)
R2 = 2.43K
R1 = 243 ohms
Iadj is negligible (typically less than 50 uA).
My test condition is all components on the power amp intact, preamp disconnected.
without any load connected you won't be meeting the mimimum current draw requirement, try putting a load resistor in to give you at least 50mA current draw (the cutoff is lower but the reg's perform better with a bit higher current draw).
The other thing, have you checked that the R1 and R2 are in fact the values they are supposed to be! 🙂
Tony.
The other thing, have you checked that the R1 and R2 are in fact the values they are supposed to be! 🙂
Tony.
Thanks Wintermute. I have not checked the absolute values of R1 and R2, but based on the measured voltages R2 is 10X R1.
it's gotta be C4 or the reg.
can you post the output schematic or confirm that these supply lines aren't feeding/supplying other parts of the circuit
can you post the output schematic or confirm that these supply lines aren't feeding/supplying other parts of the circuit
referring back to post 1 it does not come out of mute the red led is on and nothing else happens no change to green. so it's staying in protect likely because of an output stage fault.the voltage for the +/- 15 (29 v in) is coming from the center rails which are part of the output stage and are likely low due to being heavily loaded.
check that dc hasn't cook the internal speakers!
check that dc hasn't cook the internal speakers!
All of these measurements were made with the speaker disconnected. Was this a mistake? I tested the speaker with another amp and it is OK. I removed C4 and measured it to be good, too.
Thank you to all who responded! Turk 182 hit it right on the head, it was the regulator, Q13. It's all better now.
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