dc300a guru needed
Posted 09 February 2013 - 01:47 PM
Posted 11 February 2013 - 09:07 AM
Thirdly, now you can verify drivers, pre-drivers, bias, current limitter circuit, etc.
Check low resistance resistors: they're the most likely to have taken the brunt of the accident.
Make a note of the defective output transistors: If one side appears bad, it is likely that the same driver side may have crapped.
Replace components from the input to the output section, refaining from installing output devices yet.
If the driver section is operational, such as no DC at the output, and an appropriate bias voltage is detectable on the drivers, then you can install the newer output transistors.
If the isolators are of the 'mica' variety (looks metallic), replace them for either the 'plastic' or tissue (SIlpad) version. The micas, with time, leak and can generate low level hum or other electrical noises. Lastly, adjust input and output offsets.
Hope this helps!
Posted 15 February 2013 - 02:16 PM
I would say the transistor you grounded with the front panel will be dead for sure.
If you are really lucky you didnt kill the output transistors.
Dont be too hard on yourself, i have worked on hundreds of dc300a's and done a similar stunt myself, those little round heatsink can get very close to the front panel very easily! we are all human and all make mistakes.
Check the two corner transistors and the one thats almost between the two, no good me reeling off part numbers they could be any of dozens fitted over the years, check the resistors 82 and 820 resistors near lower edge of mainboard.
Obviously look for smoke damage!
Posted 22 February 2013 - 07:13 AM
Posted 22 February 2013 - 10:04 AM
Is it blowing fuses or do you have DC at the output?
If you need the service manual, let me know an address to send it to.
Posted 22 February 2013 - 02:29 PM
Posted 22 February 2013 - 03:06 PM
Posted 01 March 2013 - 07:10 AM
Posted 06 March 2013 - 01:32 AM
The 2n4929 and 2n3637 were used in later models and i would say they will be fine.
Posted 06 March 2013 - 12:06 PM
Posted 06 March 2013 - 03:07 PM
From crown parts list: D 2923-7 SERF XSISTOR, 2N3637 175V PNP
Posted 11 March 2013 - 08:44 AM
Posted 11 March 2013 - 10:05 AM
Since you have a -60V at the output it appears that the positive side is locked up and that the -60V is not being pulled up! Check for open resistors also.
Posted 15 March 2013 - 05:11 PM