AU-9900 Function Switch
- Thread starter LBPete
- Start date
Replacing the reed relay seems to have fixed it. The relay didn't arrive until late yesterday so in the mean time I checked the diodes and the little filter cap. They checked OK but I replaced them rather than re-install the old ones. That didn't change anything. Still had about the same AC ripple. It's only half wave rectification so you can't expect a smooth line. The relay coils aren't that fussy.
With the new relay installed it comes out of protection normally and plays through both channels with no distortion.
- Pete
With the new relay installed it comes out of protection normally and plays through both channels with no distortion.
- Pete
It's a NOS surplus relay. It is almost but not quite a drop in replacement. The new relay is just a little bit longer. I had to drill a couple new holes in the circuit board and extend the solder pads. It was a straight forward mod. Here are a couple of pictures:
I had to buy a bunch of these relays to get one. If you want to try one, PM me with your address and I'll send you one.
- Pete
I had to buy a bunch of these relays to get one. If you want to try one, PM me with your address and I'll send you one.
- Pete
After playing it and turning it on and off for several days with no issues, I put the whole thing back together today. As soon as I cranked it up, it started stuttering in and out of protection. The offset and bias correct and stable so something is going on with that protection circuit. I need to dig into it more.
- Pete
- Pete
What a pain!!
Just wanted to pick up on an earlier comment a couple of posts up - the rectification employed in the protection power supply is full wave (two diodes for a single rail power supply). If you are seeing half wave rectification then there may be something wrong with one of the rectifier diodes, or associated soldering or trace(s). The ripple is higher than normally expected for a power supply rail due to the low value of filter cap - the power supply was meant to drop almost immediately on mains switch off in order to disconnect the speakers at power off. You could try a 47uf filter cap in place of the standard 33uf to see if this resolves the flaky operation.
Cheers
John
Just wanted to pick up on an earlier comment a couple of posts up - the rectification employed in the protection power supply is full wave (two diodes for a single rail power supply). If you are seeing half wave rectification then there may be something wrong with one of the rectifier diodes, or associated soldering or trace(s). The ripple is higher than normally expected for a power supply rail due to the low value of filter cap - the power supply was meant to drop almost immediately on mains switch off in order to disconnect the speakers at power off. You could try a 47uf filter cap in place of the standard 33uf to see if this resolves the flaky operation.
Cheers
John
It's better that it happened while I still have it. It shouldn't need to have a bigger filter cap. There are thousands of these units working well with the original. I'm going to check the thermistors. It also has a bunch of 2sc1313 transistors on the F-2568 board that could be troublesome.
- Pete
- Pete
It's taken another interesting turn. I checked the thermistors/positors. These devices are temp sensors that mount on an output transistor for each channel and are used by the protection circuit to detect thermal runaway. When cold they measure very high resistance, basically open circuit. I assume when they get hot the resistance drops and brings that leg of the circuit down to ground level. They seemed to check OK or at least they both checked the same. As a side note, it won't come out of protection if the thermistors are disconnected.
Then I replaced the 2SC1313 transistors in the protection circuit. They checked OK and didn't show any leakage but popped in some KSA1845s and put the board back in. It came out of protection and I played it for a couple of hours at uncomfortable high levels. It worked consistently through that whole period. I periodically switched it off and back on, and it came out of protection each time. It seems to be working.
I'm not sure exactly what fixed it. Aside from changing out the transistors, I did have to unsolder and resolder three wires from the F-2568 board to make it easier to change out the transistors and I unplugged and reconnected the thermistors. And now it seems to be working. I'll put a dummy load on it tomorrow and see what happens.
- Pete
Then I replaced the 2SC1313 transistors in the protection circuit. They checked OK and didn't show any leakage but popped in some KSA1845s and put the board back in. It came out of protection and I played it for a couple of hours at uncomfortable high levels. It worked consistently through that whole period. I periodically switched it off and back on, and it came out of protection each time. It seems to be working.
I'm not sure exactly what fixed it. Aside from changing out the transistors, I did have to unsolder and resolder three wires from the F-2568 board to make it easier to change out the transistors and I unplugged and reconnected the thermistors. And now it seems to be working. I'll put a dummy load on it tomorrow and see what happens.
- Pete
It played flawlessly all week until this morning. Flipped it on, turned it up and it immediately started stuttering in and out of protection. I installed Dr*audio's ground strap and it didn't change anything. Looked back at the power supply. When the relays are on, there is an 18.8 volt drop across the relay coil. That still seems low to me. I'm still seeing a measurable ripple in the power supply. While I had the 'scope probe in my hand, I looked at the patter. n on both sides of the rectifier diodes. The A/C sine wave coming into the rectifier diodes is clipped. That seems odd. Anyone see that before?
- Pete
- Pete
That is a bit odd, very often this can be traced to failed rectifier diodes as suggested by skippy124 in post #26. Often Sansui used more than one set of rectifiers per AC winding to derive multiple supplies, so the fault may not be with the supply you are actually measuring, but another on the shared winding.
Of course today it's working normally. No stuttering. The ripple and AC clipping bother me. The power supply for the protection circuit and pre amp have dedicated taps on the same winding. Here's that part of the schematic.
With the pre amp power supply windings disconnected to isolate the protection power supply, it still shows the same ripple in the protection power supply.
Disconnected the transformer from all the boards, all of the AC taps show some degree of clipping. Could this be an indication of a bad transformer?
- Pete
With the pre amp power supply windings disconnected to isolate the protection power supply, it still shows the same ripple in the protection power supply.
Disconnected the transformer from all the boards, all of the AC taps show some degree of clipping. Could this be an indication of a bad transformer?
- Pete
If you have dual primaries (for voltage switching) maybe you have a single open primary? - thus only using one primary to power the unit. Or looking that way because of a damaged voltage switching plug or faulty connection in the mains wiring? - just pure guesses...
I gotta rethink this. When the protection issue isn't acting up it works fine. It sounds too good to have serious issues like a bad transformer. The intermittent nature of the problem points to a bad connection, a bad ground or a flaky transistor. I'm going to put it aside for a couple of days and hope for divine intervention.
- Pete
- Pete
Bad or failing EMI capacitor?
I had to put this down for awhile to clear my head, then life got in the way. When I turned it on last week, it's developed a slight but noticeable 60 Hz hum from both speakers. This hum was not there when I last worked on it. It's originating in the pre amp and occurs at zero volume. It does not increase with the volume control and cannot be heard with the preamp separated from the amp. It sounds like the kind of hum you get from a shielding issue. I've seen similar hums in other units when the nuts that secure the tone pots to the front rail of the chassis aren't tight enough. I've checked the retainers for all the controls on the front panel as well as the chassis screws but nothing seems to affect it. I've checked grounds for the preamp power supply. I'd like to check grounds on the various boards in the preamp but those bastards are really buried in this amp.
Since it doesn't change with the volume control, does that mean it's originated before that point in the circuit?
- Pete
Since it doesn't change with the volume control, does that mean it's originated before that point in the circuit?
- Pete
dr*audio
Fish fingers and custard!
Since it doesn't change with the volume control it must be downstream from the volume control. Could be a ground loop, or a bad ground, or power supply issue.
I'm still fighting this hum. It's a low level 60hz hum that doesn't change with the volume control. It is only heard when the preamp is connected to the amp. Separate them and no hum.
I've gone through the power supply and replace diodes, filter caps and a few transistors, no change. I've measured ground levels, comparing the main transformer ground connection to the ground traces on suspect boards and don't see any significant difference.
Looking at the schematic, the volume control is on the F-2574 board. When the tone selector in the defeat position, the signal goes directly from the F-2574 board to the connected separated switch and then out to the amp. This totally bypasses the other boards in the preamp. I've replaced the 2SC1313 transistors on the F-2574 board and all the electrolytic caps on that board are new. I even ran an auxiliary ground wire to that board with no change. I am out of ideas other than it's going to turn out to be something stupid.
- Pete
I've gone through the power supply and replace diodes, filter caps and a few transistors, no change. I've measured ground levels, comparing the main transformer ground connection to the ground traces on suspect boards and don't see any significant difference.
Looking at the schematic, the volume control is on the F-2574 board. When the tone selector in the defeat position, the signal goes directly from the F-2574 board to the connected separated switch and then out to the amp. This totally bypasses the other boards in the preamp. I've replaced the 2SC1313 transistors on the F-2574 board and all the electrolytic caps on that board are new. I even ran an auxiliary ground wire to that board with no change. I am out of ideas other than it's going to turn out to be something stupid.
- Pete
dr*audio
Fish fingers and custard!
I just went through a similar problem with my Marantz 19. Hum coming from the preamp. In the case of the 19, the problems were in the power supply. There were several issues. I realize this is a totally different animal but maybe my fixes will give you some ideas.
1. I upgraded the filter caps from 220uF and 330uF to 1000uF. Significant reduction in hum.
2. When I had replaced the multisection caps in the 19 I used individual caps and I put in jumpers to complete the ground paths that the cases of the multisection caps were used to complete. It wasn't enough. I added jumpers to connect all the places that the cans had connected. Significant reduction in hum.
3. I had to add an additional braided ground strap from the power supply board ground to the main ground at the large caps, which point connects to the chassis. After doing all 3 steps the hum was inaudible.
Another idea: you need to figure out if the hum is coming through the power supply, ground loop, too long a ground path, or inductive pick up. Put your hand near the circuit. If the hum gets worse it is probably inductive pickup. Add some shielding or simply put the case back together temporarily to see if that helps.
Did you add my ground strap mod? It couldn't hurt.
1. I upgraded the filter caps from 220uF and 330uF to 1000uF. Significant reduction in hum.
2. When I had replaced the multisection caps in the 19 I used individual caps and I put in jumpers to complete the ground paths that the cases of the multisection caps were used to complete. It wasn't enough. I added jumpers to connect all the places that the cans had connected. Significant reduction in hum.
3. I had to add an additional braided ground strap from the power supply board ground to the main ground at the large caps, which point connects to the chassis. After doing all 3 steps the hum was inaudible.
Another idea: you need to figure out if the hum is coming through the power supply, ground loop, too long a ground path, or inductive pick up. Put your hand near the circuit. If the hum gets worse it is probably inductive pickup. Add some shielding or simply put the case back together temporarily to see if that helps.
Did you add my ground strap mod? It couldn't hurt.
I'm still screwing around with this thing. The hum seems to be originating from the F-2574 board. This board has the volume control. When the tone control is set to defeat, the audio signal goes directly from this board to the pre out board F-2579 and on to the amplifier section. I'm trying to trace the audio signal back from the board output to the volume control. I can see the hum on the scope when touching the probe to the junction of R33/C07. If I move the probe to R29/C07, it kicks into protection. Clearly I'm doing something wrong. Am I miss reading the signal path?
The schematic for that board is attached.
- Pete
The schematic for that board is attached.
- Pete
