Hi Tom,
After cleaning, or before for that matter, if you see any pitting at all, you need a new relay. The contact plating isn't that thick, so it doesn't take much to kill a relay. If you carefully remove the plastic cover, you will see a spring running across the top. Put one finger on the back support and pop the spring off from the front. After removing the spring, the moving contacts will come right out and allow close inspection.
Given the age of this amplifier, I would simply replace the relay. You can then play with it without any stress that it might get broken or a piece lost.
-Chris
After cleaning, or before for that matter, if you see any pitting at all, you need a new relay. The contact plating isn't that thick, so it doesn't take much to kill a relay. If you carefully remove the plastic cover, you will see a spring running across the top. Put one finger on the back support and pop the spring off from the front. After removing the spring, the moving contacts will come right out and allow close inspection.
Given the age of this amplifier, I would simply replace the relay. You can then play with it without any stress that it might get broken or a piece lost.
-Chris
OK, I'm going to go ahead and put in an order for new relay. Hey, I think I've invested $10 in parts so far, what's another $7 🙂 So it's all back together, and those Bose cubes weren't cutting it to hear this thing growl, so I've moved up to my next pair of "cheap" speakers, a pair of 10 year old Sony towers. It's really sounding nice! I like the 4 tone controls, 2 bass, 2 treble...I don't think I need an EQ in the mix with this guy.
Hi Tom,
The more accurate the speakers and system, the more you will tend to leave the controls flat. You don't always agree with the engineer who mixed the music, and that's what tone controls are for once everything is singing right.
I know you'll enjoy your system. The place to sink money now is in speakers.
-Chris
The more accurate the speakers and system, the more you will tend to leave the controls flat. You don't always agree with the engineer who mixed the music, and that's what tone controls are for once everything is singing right.
I know you'll enjoy your system. The place to sink money now is in speakers.
-Chris
Hi Chris, yeah I tend to run bass and treble a bit higher than flat. Kind of what Loudness does automatically. And the reason my EQ is V shaped.
Anyway I am sticking with all vintage gear and am quite happy with my Pioneer HPM-100's that I refinished and recapped. I'm just not ready to hook this amp up to them. That's why I started w Bose cubes as Guinea pig and have moved up to Sony towers. I would like to put Mooly's protection caps idea in place w the HPMs before connecting this amp up to them.
Anyway I am sticking with all vintage gear and am quite happy with my Pioneer HPM-100's that I refinished and recapped. I'm just not ready to hook this amp up to them. That's why I started w Bose cubes as Guinea pig and have moved up to Sony towers. I would like to put Mooly's protection caps idea in place w the HPMs before connecting this amp up to them.
Chris I think there may also be some confusion as to my EQ reference. By that I meant that I may not need to hook my SG-9500 equalizer up to this amp given its 4 tone dials...
Hi Tom,
I understood that you would probably be using the Pioneer without the EQ. My comments stand equally well with just the Pioneer or the Pioneer with the EQ. As the sound quality increases, the urge to use tone controls or EQs generally decreases. Some folks may always use them, and that's fine. Most eventually find that the system doesn't need further correction. That was more something of an observation.
-Chris
I understood that you would probably be using the Pioneer without the EQ. My comments stand equally well with just the Pioneer or the Pioneer with the EQ. As the sound quality increases, the urge to use tone controls or EQs generally decreases. Some folks may always use them, and that's fine. Most eventually find that the system doesn't need further correction. That was more something of an observation.
-Chris
Just a quick update guys. I have the new relay in hand, but have not had the time to install it. I'll let you know when I do. Also, I have the 6800 uF caps Mooly. Plan to create the speaker protection assembly this weekend so I can safely hook up my HPM-100s. Thanks again for all of your help with my SA-8500! It has been running fine so far.
Good to hear its running well. The 6800uf caps are connected either PLUS to PLUS or MINUS to MINUS (it doesn't matter which).
Another SA-8500 to Fix
Hi Mooly and Chris. Are you still subscribed/listening to this thread? So my amp is still running fine since replacing the trim pots and caps on the power amp board. Thanks again! Now I've got another SA-8500 in my hands from an acquaintance that is having a similar issue. With mine the relay clicked speakers off shortly after startup. According to him, his clicks off after about an hour and then will start working again after cooling down, but only for another hour or so. So I was suspecting same components may need replacing.
But after removing the cover I noticed that the 3.6K ohm resistors (R31 and R32) have noticeable darkening and black/burn marks on circuit boards. R29 and R30 (1.2K) next to them may also be suspect Initial DC offsets at speakers showed -70mV on left and -81mV on right channel. When I successfully dialed in 0/30mV bias and idle adjustments per the service manual, those offsets both dropped to 0. But I'm seeing .609V (left) and .613V (right) at the .5 ohm resistors. Mooly, during our troubleshooting of mine, you indicated that the voltage there should be at or near 0.
Is 0.6 V too high?
After replacing the resistor pairs and same trim pots and caps as mine, I'm still able to dial in bias and idle current, and get similar numbers on the .5 ohm resistors (.608 and .611).
Other measurements:
Left:
Input voltage (Point 17) to R29 (1.2K): 36.4V
Voltage on the other side of R29: 27.7V
Voltage on the other side of R31 (3.6K): 1.1V
Voltage across D3 STV-4H: 2.19
Right:
Input voltage (Point 45) to R30 (1.2K): 36.4V
Voltage on the other side of R30: 27.7V
Voltage on the other side of R32 (3.6K): 1.1V
Voltage across D4 STV-4H: 2.2V
(Mooly, you indicated earlier the voltage across the STV-4H must not be over around 2.3 to 2.4 volts. If it’s much over that then the device is faulty.). All of these numbers are very close to my amps numbers once we got it fixed.
I ran the amp yesterday afternoon for a few hours and it never clicked off.
So what might cause those resistors to overheat? Were they affected by high bias/idle current settings that may have been high for an extended period (months or years?).
Would the high bias/idle current and DC offset (-70mV on left and -81mV on right), eventually cause the amp's relay to click off after warming up?
Hi Mooly and Chris. Are you still subscribed/listening to this thread? So my amp is still running fine since replacing the trim pots and caps on the power amp board. Thanks again! Now I've got another SA-8500 in my hands from an acquaintance that is having a similar issue. With mine the relay clicked speakers off shortly after startup. According to him, his clicks off after about an hour and then will start working again after cooling down, but only for another hour or so. So I was suspecting same components may need replacing.
But after removing the cover I noticed that the 3.6K ohm resistors (R31 and R32) have noticeable darkening and black/burn marks on circuit boards. R29 and R30 (1.2K) next to them may also be suspect Initial DC offsets at speakers showed -70mV on left and -81mV on right channel. When I successfully dialed in 0/30mV bias and idle adjustments per the service manual, those offsets both dropped to 0. But I'm seeing .609V (left) and .613V (right) at the .5 ohm resistors. Mooly, during our troubleshooting of mine, you indicated that the voltage there should be at or near 0.
Is 0.6 V too high?
After replacing the resistor pairs and same trim pots and caps as mine, I'm still able to dial in bias and idle current, and get similar numbers on the .5 ohm resistors (.608 and .611).
Other measurements:
Left:
Input voltage (Point 17) to R29 (1.2K): 36.4V
Voltage on the other side of R29: 27.7V
Voltage on the other side of R31 (3.6K): 1.1V
Voltage across D3 STV-4H: 2.19
Right:
Input voltage (Point 45) to R30 (1.2K): 36.4V
Voltage on the other side of R30: 27.7V
Voltage on the other side of R32 (3.6K): 1.1V
Voltage across D4 STV-4H: 2.2V
(Mooly, you indicated earlier the voltage across the STV-4H must not be over around 2.3 to 2.4 volts. If it’s much over that then the device is faulty.). All of these numbers are very close to my amps numbers once we got it fixed.
I ran the amp yesterday afternoon for a few hours and it never clicked off.
So what might cause those resistors to overheat? Were they affected by high bias/idle current settings that may have been high for an extended period (months or years?).
Would the high bias/idle current and DC offset (-70mV on left and -81mV on right), eventually cause the amp's relay to click off after warming up?
Yep, still subscribed.
If those resistors have the correct voltages across them then the discolouration is going to be normal and it looks as though they are, plus you say it is all basically working.
Maybe this amp has a lot more hours on the clock or has been in a less ventilated area and also boards discolour with heat to. So I don't think there is a problem there.
High bias current would not affect those resistors... if you measure the voltage across each resistor then you will find it unchanging (apart from minimal variation) with changes in bias current.
The DC offset should be adjustable to zero with the offset trim preset. Measure the offset across the speaker terminals.
To trip the relay, the offset would need to be way over 70 or 80 millivolts (and those values aren't out of the way for an amp of this type), but a high bias current could trip it as it appears to be monitored and sensed. I haven't got the full manual and diagram in front of me but I can see a take off from across the 0.5 ohms which will go to the protection circuitry.
If those resistors have the correct voltages across them then the discolouration is going to be normal and it looks as though they are, plus you say it is all basically working.
Maybe this amp has a lot more hours on the clock or has been in a less ventilated area and also boards discolour with heat to. So I don't think there is a problem there.
High bias current would not affect those resistors... if you measure the voltage across each resistor then you will find it unchanging (apart from minimal variation) with changes in bias current.
The DC offset should be adjustable to zero with the offset trim preset. Measure the offset across the speaker terminals.
To trip the relay, the offset would need to be way over 70 or 80 millivolts (and those values aren't out of the way for an amp of this type), but a high bias current could trip it as it appears to be monitored and sensed. I haven't got the full manual and diagram in front of me but I can see a take off from across the 0.5 ohms which will go to the protection circuitry.
Hi Mooly. The -71mV and -81mV were at speaker terminals. Those numbers dropped to 0 when I dialed in bias/idle current. But even with 0V there I see .6V at the .5 ohm resistors.
It sounds like something is going astray with the measurement. Whatever you read at the speaker terminals should be the same as on L1 and L2 on the main amp diagram. L1/L2 also connect to the 0.5 ohms.
Look at the circuit to make sure you follow the paths. Keep the black meter lead on the negative speaker terminal for the channel you are measuring the offset for and then transfer the red lead to L1/L2 as appropriate. You should see the same voltage there.
Look at the circuit to make sure you follow the paths. Keep the black meter lead on the negative speaker terminal for the channel you are measuring the offset for and then transfer the red lead to L1/L2 as appropriate. You should see the same voltage there.
Ok. I have been measuring voltage of .5 ohm resistors on the topside at the front of the resistors, not on the other side where they connect to the output transistors.
I will take measurements again tonight.
I will take measurements again tonight.
Just a quick update as I left my measurements at home. But seeing some fluctuating voltages.
Last night I reset bias and idle current. Page 17 trying to set voltage to 0 and could not get it to stabilize at 0. Fluctuates back and forth a couple of mV. Page 18 set idle current to 30 mV and it tries to creep upward a mV every second of so then appears to correct itself and go back towards 30.
I then checked speaker DC offset and saw fluctuating voltage between 0 and 5 mV on both channels. Checked L1 and L2 and saw similar voltages.
Voltage at .5 ohms are still steady at about .6v
I then checked Vbe on Q1-10 and all were .6xx v so they look good.
Checked voltage at 17 and 45 and have steady 36.5v.
Checked voltage at 4 and 13 and have steady -44.5v.
I'm going to recheck/reflow my solder points for all the resistors, trim pots, caps I replaced. What else should I look at?
Last night I reset bias and idle current. Page 17 trying to set voltage to 0 and could not get it to stabilize at 0. Fluctuates back and forth a couple of mV. Page 18 set idle current to 30 mV and it tries to creep upward a mV every second of so then appears to correct itself and go back towards 30.
I then checked speaker DC offset and saw fluctuating voltage between 0 and 5 mV on both channels. Checked L1 and L2 and saw similar voltages.
Voltage at .5 ohms are still steady at about .6v
I then checked Vbe on Q1-10 and all were .6xx v so they look good.
Checked voltage at 17 and 45 and have steady 36.5v.
Checked voltage at 4 and 13 and have steady -44.5v.
I'm going to recheck/reflow my solder points for all the resistors, trim pots, caps I replaced. What else should I look at?
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Bias current varying... is normal because of the thermal inertia of the heatsink. The output transistor junctions heat up before everything else. The bias current will tend to see-saw around the set value for quite some time until thermal equilibrium is reached.
So that's normal.
Offset will also fluctuate for a variety of reasons... I suspect the most common being small changes in mains supply voltage, where that change is reflected in the power supply voltages and as a consequence the operating conditions change slightly. 5 millivolt change is nothing to be concerned of, particularly when you consider amplifiers of this era often had offsets of -/+100 mv. That was considered an acceptable target to aim for back in the day.
If the voltage on the 0.5 ohms is still 0.6 volt then something is amiss with the measurement me-thinks. The voltage on either end of L1/L2 has to be the same as the output terminals because its the same point. The inductor appears as a short to DC anyway and so it shouldn't matter which end of the coil you measure on.
So that's normal.
Offset will also fluctuate for a variety of reasons... I suspect the most common being small changes in mains supply voltage, where that change is reflected in the power supply voltages and as a consequence the operating conditions change slightly. 5 millivolt change is nothing to be concerned of, particularly when you consider amplifiers of this era often had offsets of -/+100 mv. That was considered an acceptable target to aim for back in the day.
If the voltage on the 0.5 ohms is still 0.6 volt then something is amiss with the measurement me-thinks. The voltage on either end of L1/L2 has to be the same as the output terminals because its the same point. The inductor appears as a short to DC anyway and so it shouldn't matter which end of the coil you measure on.
Beyond checking for dodgy caps and the usual dries there is little to check or give problems.
Going back to the mains... if you could measure it accurately then you would see it changing constantly by small amounts. If you have an electric shower or oven then go and switch that on and watch the mains voltage fall. Those changes (and they are also caused by other neighbouring properties and so on that are on the same feed constantly changing their demand) reflect back into the measured voltages within the amp.
The bulging cap may be nothing more than a warped plastic top or disc placed on top of the cap to insulate it. Checking caps to ensure they meet their specification isn't straightforward as you need to be able to measure the caps E.S.R. (equivalent series resistance) at the specified test frequencies. A rough and ready check is to ensure the ripple across them is low as measured with an oscilloscope. With the amp idling I would guess around 100mv pk/pk ripple voltage if the caps were say 4700uF
Going back to the mains... if you could measure it accurately then you would see it changing constantly by small amounts. If you have an electric shower or oven then go and switch that on and watch the mains voltage fall. Those changes (and they are also caused by other neighbouring properties and so on that are on the same feed constantly changing their demand) reflect back into the measured voltages within the amp.
The bulging cap may be nothing more than a warped plastic top or disc placed on top of the cap to insulate it. Checking caps to ensure they meet their specification isn't straightforward as you need to be able to measure the caps E.S.R. (equivalent series resistance) at the specified test frequencies. A rough and ready check is to ensure the ripple across them is low as measured with an oscilloscope. With the amp idling I would guess around 100mv pk/pk ripple voltage if the caps were say 4700uF
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