So I took the amp off the heatsink. I see very small marks leading from the metal tab to the bolt hole on the two transistors that showed the goo around the bolts. I'll try to get a good photo after dinner. Looks like the
non conductive thermal compound conducted.
Is there a way to see if the transistors are OK without desoldering them and taking them from the board? I am also wondering where I might get insulators for the mt200 output devices. I am a bit wary of reusing the ones that had a problem.
Thanks for the support,
Evan
non conductive thermal compound conducted.
Is there a way to see if the transistors are OK without desoldering them and taking them from the board? I am also wondering where I might get insulators for the mt200 output devices. I am a bit wary of reusing the ones that had a problem.
Thanks for the support,
Evan
If the transistors blew they would most likely be shorted. If it's showing a shorted output I would change them all, and the drivers. If they don't measure to be shorted, you can power the amp back up with a bulb limiter and see if it operates. If it looks like it's running okay, remove the bulb and measure the voltage drop on all six emitter resistors with some bias current flowing to see if they are all operating. If there's one not operating, again I would change them all and the drivers.
I've never found pre-cut pads for MT-200 devices. You can buy sheets of thermal pad and cut your own though.
I've never found pre-cut pads for MT-200 devices. You can buy sheets of thermal pad and cut your own though.
I measure resistance in the megohm range between the pins of all the output devices. I checked the working channel and it also measures conductive in the megohm range but not the same as the broken channel. I may try it with the bulb limiter not mounted to the heatsink just for a bit to see if it runs. I will place an order for some parts I want and am including a sheet of thermal pad to do away with the mica and goop. If just the outputs and drivers need replacement that's not too bad.
Luckily I had the protection board running or I would be feeling a lot worse about this.
Luckily I had the protection board running or I would be feeling a lot worse about this.
As I said these output devices are very robust. Valery told me he blacked out the whole floor of his apartment building with one of his amps and the outputs devices survived. There's a good chance yours are still fine.
if you do need to use some thermal compound, Wakefield 120 is very good stuff and reasonably priced. I've got a few industrial electrician buddies that have been using it in 600V three phase applications for 30+ years.
The protection boards do a great job. They are a bit expensive to build, but are well worth the cost.
if you do need to use some thermal compound, Wakefield 120 is very good stuff and reasonably priced. I've got a few industrial electrician buddies that have been using it in 600V three phase applications for 30+ years.
The protection boards do a great job. They are a bit expensive to build, but are well worth the cost.
The technique I use to check the output devices in such cases - I unsolder one leg of each emitter resistor and raise it a bit. This way, you can measure the c-e resistance independently for each device.
It's also a good idea to see if the drivers are not shorted, although there is a good probability they are fine.
It's also a good idea to see if the drivers are not shorted, although there is a good probability they are fine.
This is one of the early design boards with the parallel SMT emitter resistors. Here's one of the reasons we thought it was a good idea to use through hole instead.
Thimios, Thanks for the lead on the insulators.
Valery Jeff is correct...surface mount emitter resistors. My plan is to clean the boards up, reassemble them and hope for the best. I will definitely check for continuity between the output rails and heatsinks.
If the output transistors and drivers need replacement that is not so bad.
Valery Jeff is correct...surface mount emitter resistors. My plan is to clean the boards up, reassemble them and hope for the best. I will definitely check for continuity between the output rails and heatsinks.
If the output transistors and drivers need replacement that is not so bad.
I'm trying to decide between two different thermal pads. K52-1 or K52-2. What do you guys think. The K51-1 thinner pad that transfers the heat better or the K52-2 thicker and safer.
http://www.mouser.com/ds/2/223/THR-DS-Tgard-K52_032515-1042861.pdf
http://www.mouser.com/ds/2/223/THR-DS-Tgard-K52_032515-1042861.pdf
Yes - I would also go for the thicker ones. I'm using not exactly the same, but similar principle slightly "elastic" material, not requiring the paste as it fills all the possible surface roughness by itself. Sold as big sheets here (200x150mm) - you can cut the pads of the required size and shape then.