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1180DC Troubleshooting

Bradw764

Bradw764

New Member
Hi: I bought an 1180DC and it was damaged in shipping. I was able to repair the cosmetic damage ok. The filter board was jarred loose (which I didn't notice at first. I powered it on and it blew the main fuse. Found a shorted D427 output transistor. Ordered a couple of sets of outputs and some caps for the preamp board.
I removed the output amp completely from the unit and tested the pre-amp outs to a 1060 power amp in. Works great although I did end up soldering the filter board connections due to intermittent channel dropping out.
I hooked up the power amp again and left the output transistors out of the left channel and the right channel works fine.
I then swapped the outputs to what I thought were the left channel and disconnected right channel inputs and when I powered it on, I had left channel working but distorted. Then I lost the outputs. This time both a D427 and a B557. Also burned up a 3.3ohm resistor r786 on preamp board. Is there a good way to troubleshoot the power section? I am using a variac and current meter to watch for excessive current draw. Also the schematic I have is full of errors. Are the filter board or led peak board causing problems? Are they in the circuit when I'm using it as a preamp only? Thanks for any info. I have ordered more parts, hope to get them later this week.
 
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Working off the service manual from hfe.

R786(3.3) will burn up if there is a near short on the
output transistor emitters. Check for shorts, amp powered off
measure between red and black speaker posts.

Then check at output emitters, ie measure between J726 and chassis.
Likewise J712 and chassis.
 
mbz said:
Working off the service manual from hfe.

R786(3.3) will burn up if there is a near short on the
output transistor emitters. Check for shorts, amp powered off
measure between red and black speaker posts.

Then check at output emitters, ie measure between J726 and chassis.
Likewise J712 and chassis.
Click to expand...

Thanks for the reply!
I have already pulled and tested the outputs. Found a couple shorted. Waiting for replacements. My question is more on the block diagram and which boards are involved in the preamp out. The preamp line out works fine so does that rule out all boards except the output section and driver board?
 
Main candidate is the P700 Main amp board especially for
faults like shorted outputs and smoking resistors. There
may be issues in the power supply, not a bad idea to
check the +/-45V output. Also checking the main filter
caps/voltage is needed.
upload_2019-3-26_6-42-41.png
 
mbz said:
Main candidate is the P700 Main amp board especially for
faults like shorted outputs and smoking resistors. There
may be issues in the power supply, not a bad idea to
check the +/-45V output. Also checking the main filter
caps/voltage is needed.
View attachment 1460120
Click to expand...
Thanks for the info. I hope to have new output transistors tomorrow. Can then troubleshoot further.
 
If you don't have a dim bulb tester (DBT), I would build one before you test those outputs. If something is shorted and you aren't on a DBT, you might fry the new outputs in milliseconds.
 
I seem to have all of the Shorts and open resistors replaced. Brought it up to voltage today with the variac. Got to 120vac but the relay didn't kick in.
Drawing 450ma. I believe I found an open 2SC1913. What replacements do you recommend? Also for the 2SA913 and 2SC1953 and 2SA914?
these are Q724, Q722, Q726 and Q728 on the prints.
I also picked up another 1180DC and it arrived today. Had some usual intermittent issues that I fixed. I took the Output ass'y board and Output transistor ass'y and plugged it into the
1180DC I'm working on and everything worked fine. I at least know its the output board! Thanks again for all of your help!
 
I did find all the transistors on Ebay. Thought maybe there would be a better match though.
My outputs came from Ebay and nothing is matched! I hope this will run when I'm finished with it!
 
Replaced all outputs with Toshiba replacements. Replaced all electrolytic caps. Ran it up with variac today and was drawing 500ma. At about 110vac , R779 smoked as did R783. Also lost Q725, Q721 and
all 4 outputs shorted? Back to square one. Any thoughts on what may be going on here?
 
Not to beat a dead horse, but IMO, a variac is not the way to test a unit with new components in it. It doesn't limit current flow if there is a problem somewhere. By the time you realize you have a problem (smoking parts), the damage is done. A DBT will glow bright to indicate the problem and generally limit current flow enough to allow you to shut down before damage is done to new components.
 
Do you connect that to the incoming AC?
The variac worked great for shorts before the relay kicks in! Things smoked before the relay even kicked in. I have another 1180DC and used it’s power assy on this amp and all was good.
 
Finally got all parts replaced on the output board (and all shorts gone) and amp powers up now and both channels are working. but...
I have no peak indicator on left channel and some noticeable distortion at high volume. (Right channel is peaking ok and no distortion). Low volume sounds fine on left channel.
Will that be the peak indicator board?
 
Amp was running great for over 2 1/2 hours. Then I cranked some Rush thru it it and smoked the Left channel again. This time I lost Q713 along with the 4 outputs on the left side.
About ready to part this thing out! Any thoughts on this? Are the Hitachi Output Transistors just junk? I know they are not matched pairs but I don't know where to find better outputs.
Thanks for any info!
Brad
 
I see you sourced outputs from Ebay. Lots of "Hitachi" or "Toshiba" transistors from Ebay are fake junk.

I'd put OnSemi MJ21193/21194 in it. You have your work cut out finding replacements for C1913, C1953, A913, A914. Maybe try Little Diode?
 
Jailtime said:
I see you sourced outputs from Ebay. Lots of "Hitachi" or "Toshiba" transistors from Ebay are fake junk.

I'd put OnSemi MJ21193/21194 in it. You have your work cut out finding replacements for C1913, C1953, A913, A914. Maybe try Little Diode?


Thanks for your advice! I even ordered some Hitachi outputs from China. They held up better than the Ebay outputs did. Half of the Ebay transistors were shorted when I got them.
I will MJ's in. Thanks again! I have a bag full of parts from this amp. Driving me crazy..
Click to expand...
 
Do not trust the old parts on ebay, it is just not worth it when you can get next day shipment from Arrow.
use the MJ21193/21194 & MJE15031,32 for drivers, pre-drivers/VAS use KSA1381/KSC3503, Input stage, KSA992/KSC1845
These are what we use for new amp designs too
 
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