Main Amp Board Adjustment Help CA-1000

F

fastfiat81

New Member
My first post! :banana:

I have been reading for the past month on recapping, etc for a CA-1000 amp I picked up. This is an awesome forum! I have replaced all caps, 2sc458s, relay, class A switches in addition to the pots on the main boards. Deoxited all the switches.

Powered it up this morning and let it warm up for 10-15 min before taking measurements. I have the service manual and am following the steps to adjust it.

First off the DC offset from the speaker ports:

Class A R= -28.8mV L= -4.0mV
Class A/B R= -17.7mV L= +9.0mV

Should I ignore the negatives (-) ??

The main amp boards are where I am having trouble (I think). The pot for the main stage difference are responsive. The others not so much. Seems I can turn and turn them with little or no change in the voltage.

Left board per SM:

a. +15.02 V main stage
b. -.004 V mid point
c. -.026 V class b idle
d. -.029 V class a idle

Right board:

a. +15.04 V main stage
b. -.32 V mid point
c. -.033 V class b idle
d. -.028 V class a idle

Suggestions? Thanks!
 
fastfiat81 said:
My first post! :banana:

I have been reading for the past month on recapping, etc for a CA-1000 amp I picked up. This is an awesome forum! I have replaced all caps, 2sc458s, relay, class A switches in addition to the pots on the main boards. Deoxited all the switches.

Powered it up this morning and let it warm up for 10-15 min before taking measurements. I have the service manual and am following the steps to adjust it.

First off the DC offset from the speaker ports:

Class A R= -28.8mV L= -4.0mV
Class A/B R= -17.7mV L= +9.0mV

Should I ignore the negatives (-) ??

The main amp boards are where I am having trouble (I think). The pot for the main stage difference are responsive. The others not so much. Seems I can turn and turn them with little or no change in the voltage.

Left board per SM:

a. +15.02 V main stage
b. -.004 V mid point
c. -.026 V class b idle
d. -.029 V class a idle

Right board:

a. +15.04 V main stage
b. -.32 V mid point
c. -.033 V class b idle
d. -.028 V class a idle

Suggestions? Thanks!
Click to expand...

Yes. Ignore the the minuses. Just concerned with absolute values.

First off the DC offset from the speaker ports:

Class A R= -28.8mV L= -4.0mV
Class A/B R= -17.7mV L= +9.0mV
Click to expand...

DC offset adjustment is the mid point adjustment and need only be done with the mode switch in normal (class AB) per the service manual.


I have replaced all caps, 2sc458s, relay, class A switches in addition to the pots on the main boards.
Click to expand...
Did you replace the 2SC458's on the amplifier assemblies ? If so, take a pix of the little transistor thats held against the heatsink with the metal spring clip.(2SC458). I need to see if its installed correctly. This transistor plays a major part in the bias circuit.
 
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Once we verify that the thermal tracking 2SC458's replacements are installed correctly. Adjust VR603 and 604 on both amp assy. full CCW. Then attach your multimeter probes to TP3 and TP4. of one of the amp assy's.

Set the volume control fully CCW. Disconnect any source and speaker load.
Place the MODE switch to the normal position. Now try to adjust VR603 for a value of 47mv. Try this on both amp assy's. If successful. Power off the CA1000 and switch the MODE switch to class A and reapply power to the CA1000 and try to adjust VR604 to 940mv. If VR603 doesn't adjust don't even bother with VR604.
 
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One other thing. Check for broken wires off of the MODE switch circuit board as well as the 4 wires on each of the amplifier assy's. marked A,B,C & S.
 
Thanks Avionic-

I did replace the 2sc458’s on the amplifier assemblies with KSC1815YTACT-ND. My old 458’s were the outhouse type and I thought I installed the new ones the correct way around. Maybe I didn’t…. The sc1815’s rounded part is up against the heat sink. See photos.

I have checked all wires on the mode board and assemblies. None were broken.

I have ~52 V on the collectors of all 4 output transistors with the ground lead hooked to the chassis.
 

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fastfiat81 said:
Thanks Avionic-

I did replace the 2sc458’s on the amplifier assemblies with KSC1815YTACT-ND. My old 458’s were the outhouse type and I thought I installed the new ones the correct way around. Maybe I didn’t…. The sc1815’s rounded part is up against the heat sink. See photos.

I have checked all wires on the mode board and assemblies. None were broken.

I have ~52 V on the collectors of all 4 output transistors with the ground lead hooked to the chassis.
Click to expand...

OK.. They look right. Continue on with directions in post #3..
 
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Is that red wire,connected to A , with the electrical tape wrap a good electrical connection ? I'd a used some heatshrink tubing there.Tape looks rather tacky IMHO.
 
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Yes I had touched the wire and melted the insulation a bit. I replaced it with some heatshrink ha.

Couple switch turned to OFF on the back panel and all others set as you stated. I backed out 603 and 604 several turns and turned the amp on. After about 10 minutes the voltages slowly dropped and stabilized to around -23 mv on both boards. Turning 603 clockwise several turns didn't change the values at all.
 
fastfiat81 said:
Yes I had touched the wire and melted the insulation a bit. I replaced it with some heatshrink ha.

Couple switch turned to OFF on the back panel and all others set as you stated. I backed out 603 and 604 several turns and turned the amp on. After about 10 minutes the voltages slowly dropped and stabilized to around -23 mv on both boards. Turning 603 clockwise several turns didn't change the values at all.
Click to expand...

Those are 20 turn pots aren't they.Adjust R603 the full twenty CCW then try to adjust it CW at least 18 turns.If it still don't adjust to 47mv on either channel then something in common with both channels is fouled up.

The only components in common besides the power supply is the Mode slide switch that sits in between the two microswitches.
 
They are 25 turn pots. :D

Mode slide switch between the microswitches huh?? I missed that one. Deoxited it pretty well and ohmed the pins on the switch to make sure it was working like it should. Turned 603 and 604 all the way CCW and was able to set 603 on the right board to 47 mv. On the left board's 603 started to rise, but cut out and quickly dropped back down to a nominal voltage and would not change through the whole 25 turn range.

I changed to Class A and tried setting 604 on the right side. It rose to about 850 mv for about 30 seconds and it too cut out and dropped back down to nominal voltage. I didn't try the left yet.

My plan to deoxit the switch again and try again tomorrow unless you think there is something else going on.

Thanks!
 
Do you have this unit hooked up to a dim bulb tester ?
 
fastfiat81 said:
They are 25 turn pots. :D

Mode slide switch between the microswitches huh?? I missed that one. Deoxited it pretty well and ohmed the pins on the switch to make sure it was working like it should. Turned 603 and 604 all the way CCW and was able to set 603 on the right board to 47 mv. On the left board's 603 started to rise, but cut out and quickly dropped back down to a nominal voltage and would not change through the whole 25 turn range.

I changed to Class A and tried setting 604 on the right side. It rose to about 850 mv for about 30 seconds and it too cut out and dropped back down to nominal voltage. I didn't try the left yet.

My plan to deoxit the switch again and try again tomorrow unless you think there is something else going on.

Thanks!
Click to expand...
What does your regulated +/- 50 volts, ( violet and brown wires on the top edge of each main amp board) , look like when the bias drops out ?
 
I cleaned the switch again and reflowed the joints on the board so I think I am good there.

As far as the +/-50...... I used to have +/-52 on both amp boards when I first started this thread. Now they are 100v on the -50 and 2.8v on the +50. Looks like I burned something on the power board. Voltages from the transformer to the power board are 48v. I probably cooked TR701-704 somehow. Pull and check each diode, resisitor and fusistor along the route or?

Thanks!
 
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fastfiat81 said:
I cleaned the switch again and reflowed the joints on the board so I think I am good there.

As far as the +/-50...... I used to have +/-52 on both amp boards when I first started this thread. Now they are 100v on the -50 and 2.8v on the +50. Looks like I burned something on the power board. Voltages from the transformer to the power board are 48v. I probably cooked TR701-704 somehow. Pull and check each diode, resisitor and fusistor along the route or?

Thanks!
Click to expand...

How do you feel about rebuilding the +/- 50 regulated supply. I'm at work right now, but I believe I have a Mouser parts list at home for everything you would need.All transistors , zener diodes and resistors.
 
I rebuilt the powerboard. Hooked it up to a 60w dim bulb tester and turned it on. The bulb went dim but I still burnt the R701 and R702 fire proof resistors. F703, F704 and F705 check ok.

I pulled D701 and D702 and they check ok. I am pretty sure I got the orientation correct when changing them out per the schematic.

I will pull the rest of the diodes and transistors to make sure they are ok.

What burned the fusible resistors? Too much current, but how? More researching to see if anyone has had this problem before. :D

Thanks!
 

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Update. I checked the rest of the zener diodes and transistors. Reflowed all solder joints and replaced the two burnt fire proof resistors. Hooked up to a DBT and turned it on. The 60w bulb stays dim and the relay clicked on about 8 seconds later. The downside is that the +/- 50v outputs are still off on the power board. The two amp boards have the same readings.

-50 is 2.5v
+50 is -92v

Not sure what the voltage is supposed to be from the transformer, but the two yellow wires measure 45v.

I will pull C701-704 as well as R701-708. They are the only thing I have not checked yet. Anyone have any idea on how to proceed? :D
 
I really appreciate Avionic taking the time to help those newbies that find this hobby interesting and fun. Learning how these amps work by pouring over the schematics, electronics 101 and reading the archives has been a fun challenge. Everyone starts someplace.

inductor said:
yes, get it to someone who can repair it
Click to expand...
 
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