Ive got a weird problem with this one, which if I hunt around some more I'll probably figure out but currently has me scratching my head.
Amp came in said to power but not produce output. After fumbling a minute, I found only half the power supply fets were operating, and the other half only intermittently. This causes only half the audio circuit to see rail. I believe I narrowed the issue down to the one sides' PS fets MPSA06 drivers not (always) seeing positive rail voltage on collectors and therefore the drive signal from the TL494 is not always getting past the drivers. I say not always because I can let the amp half idle and sometimes that PS will briefly start up. When it does, the signal on the FET gates looks shaky and weak and dies almost instantly.
I removed the two sets of MPSA06/56 from the PS thats not operating correctly and I get fluctuating positive voltage on the MPSA06 NPN collector. All other pads look good including the drive signal, ground, and pulldown. To me it seems like something is dragging the positive supply voltage down after the 68Ohm 3w resistor, whereas on the good PS all NPN collectors are stable at 3.4vDC. The 10uf axial caps at these 68ohm 3w resistors are OK.
Theres a 2n6488 standing up between the PS toroids which is connected to the intermittent PS drivers, and directly connected to the MPSA06 NPN collector. If I remove the 2n6488, I get stable ~12vDC on the pad of the PS NPN collectors. Re-installed and I get anywhere from 0v to 7vDC on the NPN collector pads - fluctuating.
What could be dragging/fluctuating the drive circuit components you think? Is there a schematic or drawing for this area of these type of amps anywhere?
Amp came in said to power but not produce output. After fumbling a minute, I found only half the power supply fets were operating, and the other half only intermittently. This causes only half the audio circuit to see rail. I believe I narrowed the issue down to the one sides' PS fets MPSA06 drivers not (always) seeing positive rail voltage on collectors and therefore the drive signal from the TL494 is not always getting past the drivers. I say not always because I can let the amp half idle and sometimes that PS will briefly start up. When it does, the signal on the FET gates looks shaky and weak and dies almost instantly.
I removed the two sets of MPSA06/56 from the PS thats not operating correctly and I get fluctuating positive voltage on the MPSA06 NPN collector. All other pads look good including the drive signal, ground, and pulldown. To me it seems like something is dragging the positive supply voltage down after the 68Ohm 3w resistor, whereas on the good PS all NPN collectors are stable at 3.4vDC. The 10uf axial caps at these 68ohm 3w resistors are OK.
Theres a 2n6488 standing up between the PS toroids which is connected to the intermittent PS drivers, and directly connected to the MPSA06 NPN collector. If I remove the 2n6488, I get stable ~12vDC on the pad of the PS NPN collectors. Re-installed and I get anywhere from 0v to 7vDC on the NPN collector pads - fluctuating.
What could be dragging/fluctuating the drive circuit components you think? Is there a schematic or drawing for this area of these type of amps anywhere?
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Do a ctrl-F search on the orion page for the following:
The following diagram is for a
^^ that will bring you to a diagram that should be close.
Remember, this is a PAM circuit. The 6488 pulls the voltage down on the drivers/gates as the target rail voltage is released. Powering the amp up with the FETs in the PS circuit risks driving the rail voltage too high.
The following diagram is for a
^^ that will bring you to a diagram that should be close.
Remember, this is a PAM circuit. The 6488 pulls the voltage down on the drivers/gates as the target rail voltage is released. Powering the amp up with the FETs in the PS circuit risks driving the rail voltage too high.
That diagram just made me realize something; that this amp is running much too high rail voltage at +-51vDC were it should be +-36. Actually the installed rail caps are only rated for 50v which is wrong anyways (Someone previously tried to repair this one). Im going to install 5000uf63v caps once I figure this one through but for now I'll keep those wrong caps installed.
I believe the board has some corrosion. I'll find the disconnect. Thanks for that diagram I was not seeing the 68 ohm and 6488 at first but now I do.
I believe the board has some corrosion. I'll find the disconnect. Thanks for that diagram I was not seeing the 68 ohm and 6488 at first but now I do.
1n4751. 30v zeners. There are 4 of them; two per supply. They are connected in series like the diagram. Then there are 1n4005 from the secondary windings. All seem to be OK. I think there is something wrong down the line before the 1k resistor.
I'm not precisely sure what else would be causing +-51v rail. I changed out the rail caps and still the same rail. Also I was able to stabilize drive signal to the previously non-working PS. There was some corrosion center of the board near some 470uF caps.
I wonder is there may be something wrong with the TL494 timing. Someone previously replaced or re-soldered the cap between pins 5 & 7 with a B22k. Does that sound correct?
I wonder is there may be something wrong with the TL494 timing. Someone previously replaced or re-soldered the cap between pins 5 & 7 with a B22k. Does that sound correct?
ok back to the regulator zener diodes. On this HCCA2100 amp, the install looks like this with 2 Zeners in series 1n4751 connected to 1n4748. So thats ~52v across them.
These parts have never been serviced. Should these have been single 1n4753?
I removed 1 pair.
These parts have never been serviced. Should these have been single 1n4753?
I removed 1 pair.
It appears that that's the configuration for the regulator in the 2250 photos. You can look them up. The higher voltage doesn't seem like it would be reliable in a 2100 running lower ohm loads.
It appears that the 2100 uses 1N4753s. They're hard to read but that's what they appear to be.
It appears that the 2100 uses 1N4753s. They're hard to read but that's what they appear to be.
Thats exactly what I was thinking in looking at both the 2100 and the 2250 photos earlier this morning. Made me wonder on if this may have been a 2250 board in this 2100 chassis. If it is; is there anything else for me to 'convert' or verify that this is actually a 2100? I'm thinking not really. The toroids in this amp have green windings like the 2100 photos; which the 2250 photos do not.
I removed the original stacks and put 2 pairs of 18v Zener diodes in (Because I didn't have actual 36v units) and now the amp runs +-35.75vDC rail. Its ready to go back together and fully test now.
Thank you
I removed the original stacks and put 2 pairs of 18v Zener diodes in (Because I didn't have actual 36v units) and now the amp runs +-35.75vDC rail. Its ready to go back together and fully test now.
Thank you
I don't see any other problems but the owner (if this is a repair for someone else), may not like the reduction in power.