Was just servicing my old Comtech 800, adjusted the bias and by mistake shorted momentarily heatsink of one channel to the chassis 🙁
After this when turning the amp on protection circuit comes on and cuts the power off to that channel; other channel works normally.
Symptoms are now, when switched on, right channel behaves normal (yellow IOC light comes on, after few sec it goes off and green ODEP lights turns and stays on).
On bad (shorted) channel the yellow IOC light comes on, after few sec the green ODEP light comes on but it immediately switches off while the yellow IOC remains on; this keeps cycling every few seconds.
I looked inside and could not detect any burned component on the output board; also there was no obvious smell/smoke that something burned when I accidentally shorted heatsink to chassis.
I would really appreciate some tips on how to troubleshoot this; there is obviously a protection circuit that turns power off that channel as soon as gets powered, but could not detect anything getting hot.
What would be the first suspected part to get damaged; I got full schematics and wanted to compare L to R channel but it is hard to do as there is no voltage to the bad chanel (it gets cut by power control protection circuit)?
After this when turning the amp on protection circuit comes on and cuts the power off to that channel; other channel works normally.
Symptoms are now, when switched on, right channel behaves normal (yellow IOC light comes on, after few sec it goes off and green ODEP lights turns and stays on).
On bad (shorted) channel the yellow IOC light comes on, after few sec the green ODEP light comes on but it immediately switches off while the yellow IOC remains on; this keeps cycling every few seconds.
I looked inside and could not detect any burned component on the output board; also there was no obvious smell/smoke that something burned when I accidentally shorted heatsink to chassis.
I would really appreciate some tips on how to troubleshoot this; there is obviously a protection circuit that turns power off that channel as soon as gets powered, but could not detect anything getting hot.
What would be the first suspected part to get damaged; I got full schematics and wanted to compare L to R channel but it is hard to do as there is no voltage to the bad chanel (it gets cut by power control protection circuit)?
You will need to check for shouted outputs, driver, pre drivers and even last voltage amp transistors(LVA) on both high and low sides .
Thanks a lot, found one bad (shorted) output on low side; removed it from the circuit and all voltages are fine now!
Now need to find a replacement, it is D 6729-4 #3 part marking; I found exact one (used but tested) on Ebay with same #3 marking.
Would it be OK to use that one and replace just this one output or is it necessary to replace all NPN outputs that are in parallel?
Now need to find a replacement, it is D 6729-4 #3 part marking; I found exact one (used but tested) on Ebay with same #3 marking.
Would it be OK to use that one and replace just this one output or is it necessary to replace all NPN outputs that are in parallel?
The Crown grading is supposed to eliminate matching issues - you should be able to parallel from the same bin. However, you have no idea how close to death the rest of the devices are after an incident, including the complements on the other side. Proper practice is to replace them ALL.
OK, thanks, makes sense! Problem is they are no longer available, so only way to find them is getting used once off ebay.
When I google parrt number that is on the transistor only ebay used once come up.
Or, is there a current part that would be suitable replacement for both, PNP and NPN?
Will also try calling Crown anb see if they still got replacements or can recomend something.
When I google parrt number that is on the transistor only ebay used once come up.
Or, is there a current part that would be suitable replacement for both, PNP and NPN?
Will also try calling Crown anb see if they still got replacements or can recomend something.
I noticed on the earlier Crown Micro - Tech amplifiers that there is a lack of DC protection in an event output transistor shorts? If there is an output device short on the low side than hopefully the fuse will blow. But what happens if their is a short on the high side?
There is nothing special about the transistors they use. They are industry standard devices. The custom part numbers are hFE and vbe groups, to simplify their manufacturing process. Hand selecting parts is no big deal to an engineer, but harder for an assembly line worker. At one time they used to be graded for voltage too, but that’s been unnecessary since Motorola introduced the MJ15024.
If you’re replacing TO-3’s use MJ21193 and 4, with devices from the same production runs. They will be close enough matches. You can always more and further down select. If you match NPN to PNP you will get less distortion (whether the difference is audible is debatable, but it will be less). Originals were usually MJ15024 in older amps like the Macrotech but their process spread is still wider, making hand matching more of a necessity. You will find outliers with single tubes of those.