I don't think the series resistor is the problem because you have 12+ volts on the center leg of the PS FETs.
You can't trust your meter at 20kHz on AC volts. I don't think its response is flat to 20k. The rectified DC will read significantly higher than the AC if the meter's response is down significantly at 20k.
The low rail voltage seems to be due to the vertical 6488. This transistor pulls the drive voltage down when the target rail voltage is reached. It looks like it thinks the target voltage has been reached. There's a zener diode that controls the rail voltage. Follow the circuit back from the base of the 6488 to see if it goes to a zener. If it does, check the voltage across the zener. Look it up to see what it's supposed to be.
The voltage on the A06 looks OK. It's on and passing current to the thermostat.
Is the previously missing 10 ohm resistor back in the circuit?
You can't trust your meter at 20kHz on AC volts. I don't think its response is flat to 20k. The rectified DC will read significantly higher than the AC if the meter's response is down significantly at 20k.
The low rail voltage seems to be due to the vertical 6488. This transistor pulls the drive voltage down when the target rail voltage is reached. It looks like it thinks the target voltage has been reached. There's a zener diode that controls the rail voltage. Follow the circuit back from the base of the 6488 to see if it goes to a zener. If it does, check the voltage across the zener. Look it up to see what it's supposed to be.
The voltage on the A06 looks OK. It's on and passing current to the thermostat.
Is the previously missing 10 ohm resistor back in the circuit?
the missing 10 ohm resistor is back in place.
there are 2 zeners in series from the Base of the vertical 6488
(2 x 1n4746)...they are 18 volt diodes.
the trace from the base first hits a 1k resistor which is also connected to a 100 ohm resistor that connects to the 2 series zener diodes.
measuring across both the zeners i get 18 vdc...
the A06 nearest to those zeners has an arrow etched pointing to its base....maybe the previous guy is trying to draw me a treasure map?...or ....possibly a booby trap.
at one point before the ps entered the twilight zone...i was getting 37 volts for rail voltage.
<<< this amp is taking the life out of me...
there are 2 zeners in series from the Base of the vertical 6488
(2 x 1n4746)...they are 18 volt diodes.
the trace from the base first hits a 1k resistor which is also connected to a 100 ohm resistor that connects to the 2 series zener diodes.
measuring across both the zeners i get 18 vdc...
the A06 nearest to those zeners has an arrow etched pointing to its base....maybe the previous guy is trying to draw me a treasure map?...or ....possibly a booby trap.
at one point before the ps entered the twilight zone...i was getting 37 volts for rail voltage.
<<< this amp is taking the life out of me...i measured -18vdc across both in series...black lead on one diode on the black end, red lead on the other diode at grey end.
if i measure across one diode at a time i keep getting different readings and its pretty annoying, it fluctuates between -1.0 vdc to -0.2vdc for either diode
black lead on black end of grey bodied zener diode.
if i measure across one diode at a time i keep getting different readings and its pretty annoying, it fluctuates between -1.0 vdc to -0.2vdc for either diode
black lead on black end of grey bodied zener diode.
The voltage across the two zeners in series should be ~36v.
If you measure the DC voltage across either diode, you should see ~18v.
Is one of them shorted?
You're getting negative voltage readings because you have the meter leads backwards on the Zeners. Remember that Zeners are used with reverse biasing (striped end/cathode is positive).
If you measure the DC voltage across either diode, you should see ~18v.
Is one of them shorted?
You're getting negative voltage readings because you have the meter leads backwards on the Zeners. Remember that Zeners are used with reverse biasing (striped end/cathode is positive).
if i pull one leg out and measure them i get .722 with theblack lead on the stripe end....and OL when the red lead is on the stripe end.
there is anoter regular diode that is connected to both the rectifying diodes underneath the board, it is also getting 18 volts.
so is the other regular diode connected to the secondary leg by the lone capacitor by the positive rectifier.
there is anoter regular diode that is connected to both the rectifying diodes underneath the board, it is also getting 18 volts.
so is the other regular diode connected to the secondary leg by the lone capacitor by the positive rectifier.
There are two 1n4003 diodes that connect to the secondary windings. The cathodes of the 4003s connect the cathode of the first zener. On this node, you see what is essentially the rail voltage. The anode of the first zener goes to the cathode of the second zener. The anode of the second zener goes to the 100 ohm resistor. The other end of the 100 ohm resistor goes to the base of the vertical 6488.
With both Zeners in the circuit, what's the DC voltage where they connect to each other (black lead on chassis ground)?
What's the DC voltage at the point where the cathode of the first Zener connects to the 4003s?
With both Zeners in the circuit, what's the DC voltage where they connect to each other (black lead on chassis ground)?
What's the DC voltage at the point where the cathode of the first Zener connects to the 4003s?
heres a picture to check out while i decipher what you just said..lol 🙂
black lead on rca jack....
black lead on rca jack....
An externally hosted image should be here but it was not working when we last tested it.
There's a 120 ohm resistor on the yellow side of the board near the FET closest to the audio section, is it within tolerance?
See if you can decipher this...
The 18, 18, 18 transistor looks like it's defective. It's one of 4 transistors that are used for over-current protection.
The 18, 18, 18 appears to be driving the 1.12, 2.3, 3 transistor on.
The 18, 18, 18 transistor looks like it's defective. It's one of 4 transistors that are used for over-current protection.
The 18, 18, 18 appears to be driving the 1.12, 2.3, 3 transistor on.
that 18,18,18 is the same one that the previous guy drew the arrow to... i have to run out and get some more of those transistors, probably from a different shop...because ive been into this one too many times the past week....
god i hope this is the last problem....
god i hope this is the last problem....
with input voltage around 9 volts
i changed the defective a06 and now it reads
e = -3242
b = -32.43
c = -0.266
and the transistor that previously read 1.12, 2.3, 3 now reads
e =32.33
b =32.26
c =0.003
these are the measurements for those zener diodes
measuring across both
32.3
and individually
14.61
both banks of the ps fets now seem to be operating at the same temperature, ....would it be correct to assume that when the overcurrent protection kicks in, it shuts down one bank?
the vertical 6488 now seems to be at normal operating temperature, no longer gets hot.
current draw is 1.72 amps
----------------------------------------------------------
if everything is good to go, im going to go ahead and put in new outputs transistors ...a couple of mylars i scorched with my iron.....new power/ground .....and speaker wires.
i havent tested the amp with music just yet...is there anything else i should measure for safety's sake?
(crosses fingers and hopes there are no more problems)
i changed the defective a06 and now it reads
e = -3242
b = -32.43
c = -0.266
and the transistor that previously read 1.12, 2.3, 3 now reads
e =32.33
b =32.26
c =0.003
these are the measurements for those zener diodes
measuring across both
32.3
and individually
14.61
both banks of the ps fets now seem to be operating at the same temperature, ....would it be correct to assume that when the overcurrent protection kicks in, it shuts down one bank?
the vertical 6488 now seems to be at normal operating temperature, no longer gets hot.
current draw is 1.72 amps
----------------------------------------------------------
if everything is good to go, im going to go ahead and put in new outputs transistors ...a couple of mylars i scorched with my iron.....new power/ground .....and speaker wires.
i havent tested the amp with music just yet...is there anything else i should measure for safety's sake?
(crosses fingers and hopes there are no more problems)
An externally hosted image should be here but it was not working when we last tested it.
Both banks of FETs are controlled together. There's no way to shut off one bank without the other.
I'd suggest comparing the drive voltage of both banks of FETs at various input voltages. It's strange that one side would run hotter than the other when the amp had the low drive voltage.
Double-check the insulators between the transistors and the heatsink. After you mount it down and get the screws tight, I'd suggest that you make sure there is no continuity between the tabs of the transistors and the sink.
The negative TO-220 rectifiers will likely show 120 ohms to ground. but the others should be isolated. To know if there were any others that would normally have continuity to ground, you could check all of the components before you install in into the sink. Connect the black lead to the ground wire and touch each of the tabs of all of the TO-220 devices with the red lead.
I'd suggest comparing the drive voltage of both banks of FETs at various input voltages. It's strange that one side would run hotter than the other when the amp had the low drive voltage.
Double-check the insulators between the transistors and the heatsink. After you mount it down and get the screws tight, I'd suggest that you make sure there is no continuity between the tabs of the transistors and the sink.
The negative TO-220 rectifiers will likely show 120 ohms to ground. but the others should be isolated. To know if there were any others that would normally have continuity to ground, you could check all of the components before you install in into the sink. Connect the black lead to the ground wire and touch each of the tabs of all of the TO-220 devices with the red lead.
the drive voltage stays pretty much the same between 12.5 to 16 volts...around 1.98 for orange and 1.96 for yellow side , each side fluctuates about 0.02 from the voltage range i tested them.
i cant seem to shake the continuity between the transistors/fets and heatsink....
fets = 3.5m ohms
output transistors = 0.666k ohms
the rectifiers = 0.2 ohm (orange) and 6.25m ohm (yellow) middle pins for both
does that tape wear out?
----------------------------
how hot is that vertical 6488 suppose to get? i was testing with the inline resistor, but i took it off to see how it would be when more current passes through it...it gets warm; almost hot,not neccessarily scorching hot, but becomes uncomfortable after 10 seconds, but bearable...
anyway, i put a little heatsink on it...i dont know if this is just the equivalent of putting a band aid on it, or if there is a problem.
the pre drivers are operating at about the same temp as that 6488 also....the drivers are chillin.
just how hot are these guys suppose to get?
----------------------------
the ps is buzzing also...its hard to centralize ,but i think its coming from the area where the transformer is,you think those sideways caps could be contributing to the noise?....i was able to find some 4700uf caps with the side legs, theyre a little longer...but if they can work i will put them in.
when there is a long bass note, the buzzing sound goes away, then comes back when the bass note is finished...like the bass is drawing power out of it...cap?
----------------------------
the good point is, it plays music beautifully, no more low volume distortion, and current draw appers to be normal.
im still kinda scared to put it in the car though...🙂
i cant seem to shake the continuity between the transistors/fets and heatsink....
fets = 3.5m ohms
output transistors = 0.666k ohms
the rectifiers = 0.2 ohm (orange) and 6.25m ohm (yellow) middle pins for both
does that tape wear out?
----------------------------
how hot is that vertical 6488 suppose to get? i was testing with the inline resistor, but i took it off to see how it would be when more current passes through it...it gets warm; almost hot,not neccessarily scorching hot, but becomes uncomfortable after 10 seconds, but bearable...
anyway, i put a little heatsink on it...i dont know if this is just the equivalent of putting a band aid on it, or if there is a problem.
the pre drivers are operating at about the same temp as that 6488 also....the drivers are chillin.
just how hot are these guys suppose to get?
----------------------------
the ps is buzzing also...its hard to centralize ,but i think its coming from the area where the transformer is,you think those sideways caps could be contributing to the noise?....i was able to find some 4700uf caps with the side legs, theyre a little longer...but if they can work i will put them in.
when there is a long bass note, the buzzing sound goes away, then comes back when the bass note is finished...like the bass is drawing power out of it...cap?
----------------------------
the good point is, it plays music beautifully, no more low volume distortion, and current draw appers to be normal.
im still kinda scared to put it in the car though...🙂
If you had your black meter lead on the chassis ground, the orange side rectifier should have read 120 ohms unless you had a head unit's RCAs connected to the amp.
I'm not sure which insulating tape you have. It's probably OK. Sometimes metal splinters from the screw holes get between the transistor and the insulator causing a short to the heatsink.
If you can touch the 6488 and the predrivers for 10 seconds, they are OK. You may want to monitor their temperature when running the amp harder to make sure they don't get significantly hotter.
The buzzing is likely from ripple from your 12v power supply.
When you reinstall it in the vehicle, you may want to use a relatively small power line fuse (40A). If something fails, it's likely to do less damage than if you use a larger fuse. If it holds up with the 40 but the fuse blows under normal operating conditions, then you could go a bit higher.
I'm not sure which insulating tape you have. It's probably OK. Sometimes metal splinters from the screw holes get between the transistor and the insulator causing a short to the heatsink.
If you can touch the 6488 and the predrivers for 10 seconds, they are OK. You may want to monitor their temperature when running the amp harder to make sure they don't get significantly hotter.
The buzzing is likely from ripple from your 12v power supply.
When you reinstall it in the vehicle, you may want to use a relatively small power line fuse (40A). If something fails, it's likely to do less damage than if you use a larger fuse. If it holds up with the 40 but the fuse blows under normal operating conditions, then you could go a bit higher.
uhhhhhhh... i accidently threw away my output transistors i was going to use....all 20 of them 
if i put the black lead on the secondary ground i get 120 ohms 🙂
i was puttng it in the hole used to secure the side panels.
the ps buzzes with a battery, i thought the same thing at first...that the battery charger was causing it.
20 transistors !!!!
if i put the black lead on the secondary ground i get 120 ohms 🙂
i was puttng it in the hole used to secure the side panels.
the ps buzzes with a battery, i thought the same thing at first...that the battery charger was causing it.
20 transistors !!!!
You're correct. 120 ohms to secondary ground and 0 ohms to chassis ground is what you should have for the negative rectifier.
Threw away your outputs?... That'll teach you that no good has ever come from cleaning your work bench. 🙂
Threw away your outputs?... That'll teach you that no good has ever come from cleaning your work bench. 🙂
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