Another 8080db blown - Short @ Speaker

[MitchellT]

Hi all,

I had been reading around and trying to diagnose from older/ previous posted threads but I was recommended to post my own just to cut down on confusion.

Here's the quick and dirty though.. A few years back I had the 8080db running at a house party and someone bumped one of the speakers.. this knocked the banana plugs on one of the speakers out, and I am assuming that this shorted the outputs. Whatever happened though, put the unit into protection mode and sound stopped right away (both channels even though only R channel was knocked loose).

I didn't have a ton of time to dig into the issue until now bc of moving into a new house, getting married, trying to survive, etc.. but now all that is done and I am looking to dive into this fix.

I have read that if I remove the driver board, and the unit comes out of protection, that the driver board is the issue. I plan to do that tonight. I also already printed off a Service Manual (thanks work). After the reading I have done, I assume that I'll end up having to replace some transistors, and possibly fuse resistors as well, but I just wanted some feedback and tips on what would be my best method to diagnose the issue and work backwards. Then furthermore, once I do properly diagnose this (fingers crossed), some help on suggested replacement parts.

I am fairly handy with a multimeter but a little added detail for measurements couldn't hurt LOL

FWIW, this is a late model 8080db (gold chassis, good dolby board), and it was working great prior to this issue, save for a power meter that needed (still needs) to be re-calibrated.

Thanks in advance for any help.

 

[kaplang]

Have you inspected the fuses?

If it were me and removing the driver board takes it out of protection mode I’d test the output transistors. Working from the channel that you think may have shorted first.

If it’s gets anymore coplicated then that, other more experienced troubleshooter will hopefully chime in to help you.

 

[MitchellT]

Ya, so I have inspected the fuses. Currently the F01 and F02 (7A, 125V) on the Power Supply Board are blown.

Funny enough, I went home early today bc the wife was sick.. I hooked her up with soup/ Gatorade then popped the top on the sui.

Turned it on because I am impatient, but without the fuses in place, and driver board still plugged in, no protection circuit? . I am pretty sure I had replaced the fuses a few years back when this happened and they popped again but now my memory is fuzzy :|.

Should I buy some replacement fuses and pop them in, or is there a better way to troubleshoot that? Just for good measure, I turned off the unit, let it discharge then unplugged the driver board and measured the TR11 - 14 and they seem fine? All measuring in the 0.615 - 0.630V range for all 4 of them using my multi between the BE, BC for NPN (TR11/12) and EB, CB for PNP (TR 13/14).

 

[kaplang]

Do you have a dim bulb tester?

 

[MitchellT]

Do you have a dim bulb tester?

Looks like I'll need to make one... I did already see the post somewhere else on here that shows how to make one. I think I have a majority of the parts already.

Oh and F04 is blown too... I just saw that as well.

 

[kaplang]

Looks like I'll need to make one... I did already see the post somewhere else on here that shows how to make one. I think I have a majority of the parts already.

Oh and F04 is blown too... I just saw that as well.

You should use a dim bulb tester if you plan on replacing the fuses and turning it on. It will help you avoid any further damage.

 

[spark1]

Test the output transistors first.

Given the genesis of the problem, if the outputs are good, you are most likely fine to then replace the fuses and fire it up with the driver board out. If it comes out of protection, turn it off, install the driver board, and power it up again.

 

[MitchellT]

Test the output transistors first.

Given the genesis of the problem, if the outputs are good, you are most likely fine to then replace the fuses and fire it up with the driver board out. If it comes out of protection, turn it off, install the driver board, and power it up again.

Is there any way to test the output transistors without removing them from the heatsink? Or is that the only recommended way?

 

[spark1]

Ideally they should be out of circuit when testing, but if you are able to access them without removing them, simple diode test should work ok.

 

[MitchellT]

Ok.. So I just took them out and tested.. IT was actually a lot easier than I thought.. I am a little unsure about the results though. Here's how the tested Left to Right from the Heatsink...

NPN - B/E & B/C: 0.435 & 0.434
PNP - Definitely bad (OL/OL)
PNP - E/B & C/B: 0.532 / 0.490
NPN - B/E & B/C: 0.524 & 0.509

So 1 is definitely bad. Should I just replace all 4, or what is standard practice?

Seems a little weird to me that only 1 would blow but then again I am fairly new to this stuff.

 

[spark1]

NPN diode test

• positive lead to base, negative lead to emitter...should get .450-.650V (or thereabouts...may be a bit higher)
• positive lead to base, negative to collector (the case of the TO-3 transistor)...should get close to the first reading, but a bit lower
• negative lead to base, positive to emitter...should get nothing
• negative lead to base, positive to collector...should get nothing

For PNP test, reverse the leads in all scenarios.

It appears that you tested correctly. If you are confident that you did so, you should replace all four. Are the current transistors original (Sanken 2SC1116, 2SA747)?

Or, if you can locate a pair of originals (not likely), you could replace just the pair with the bad transistor. In any case, be sure to replace the mica insulators...and read up on how to apply thermal compound correctly.

Hopefully one of the Sansui masters will chime in as regards replacement transistors...but in their absence, I would say that On Semi MJ21193G (PNP) and MJ21194G (NPN) would be good (and more robust than the originals).

 

[spark1]

Why not correct the oversight, then carry on? You could have jumped in any time...but I know that your preference is critique. Glad I could give you yet another opportunity. Any complaint about the recommended replacements?

 

[kevzep]

Calm down guys, we are not saving lives here!!

Another way to tell if the output transistors may be blown, Just measure with the DMM from the chassis to the main power rails, if either the positive or negative is shorted to chassis then one or more outputs will be blown (S/C)...

 

[MitchellT]

Thanks a lot Spark. I am pretty positive that I tested correctly but for good measure I will do it again.

I had used this link originally when learning how to test a transistor and I am pretty sure its the same as you wrote above (https://vetco.net/blog/test-a-transistor-with-a-multimeter/2017-05-04-12-25-37-0700). It helped me at least, but then again I'm originally an ME trying to dive into EE so... thanks for bearing with me.

As for the replacements you mentioned, I had seen the same mentioned by LBPete in this thread (http://www.audiokarma.org/forums/index.php?threads/8080db-protection.187307/), so good call!

One more thing though... I also read about those mica insulators in that same thread mentioned above.. just like that guy though, when I took my transistors (original Sanken) off the heatsink to measure them there was no insulator.. just heatsink compound.. Was this common for original builds, or could they have been removed at some point? When I install them, do I still use the compound as well then?

Another way to tell if the output transistors may be blown, Just measure with the DMM from the chassis to the main power rails, if either the positive or negative is shorted to chassis then one or more outputs will be blown (S/C)...

@kevzep I can do that measurement right now, but not sure where to do it from, I am assuming one of the 5 - 28 pins on the PS board?

 

[spark1]

Hopefully one of the members who actually knows what he/she is talking about - as opposed to me - can address your question about the insulators. All the TO-3 transistors I've seen have them...and sometimes insulating washers for the screws, but I think the threading is insulated on a lot of them.

I guess the potential for damage without the insulator depends on whether current would flow to the die itself if the case is shorted...but again, I am only speculating while waiting for the experts to weigh in. Personally, I would use them, even if there were none there originally.

 

[kevzep]

I can do that measurement right now, but not sure where to do it from, I am assuming one of the 5 - 28 pins on the PS board?

To the big caps, this is a good place to measure the +/- rails from. One side of both caps will be to ground, the other side is what you measure.
If you detect a short circuit, you have problems.

If there are no mica insulators on the transistors, then this could be a problem, you'd be shorting the Collectors of the output transistors to ground.
They must have mica insulators.

 

[MitchellT]

To the big caps, this is a good place to measure the +/- rails from. One side of both caps will be to ground, the other side is what you measure.
If you detect a short circuit, you have problems.

Measured and confirmed that the right channel is shorted, just as i suspected from my my measurements on the T03 devices. Left Channel seems fine.

I am going to order the parts from digikey and replace all 4 of them anyways though.

If there are no mica insulators on the transistors, then this could be a problem, you'd be shorting the Collectors of the output transistors to ground.
They must have mica insulators.

A question about the mica insulators... I plan to get these from digikey as well, but since I didn't have them originally I didn't see how they are properly installed. When installing the T03 devices, do I still still use heatsink compound as well? If so, on one or both sides of the mica pad?

 

[spark1]

Yes. The insulator keeps the case from shorting to ground (that is, to the heat sink). The thermal compound has a different purpose...specifically, to aid in heat transfer from the case to the heat sink, where it can be dissipated. Be sure to purchase non-conductive compound, and apply a complete, but very thin layer to both the transistor and the side of the insulator facing the heat sink. The idea is that the compound fills any small/microscopic voids in the metal and in the insulator, making for best contact and therefore best heat transfer.

http://audiokarma.org/forums/index....rmal-compound-to-to-3-mica-insulators.716953/

 

[MitchellT]

Thanks again eh guys. I'm going to order the parts soon and then will report back on how it turns out/ if I have any other issues.

I was doing some other reading and might also end up refurbishing the entire F2624 Driver Board while I'm at it... but for now, 1 step at a time.

 

[N8Nagel]

This may be a dumb question, but does the 8080 use the "red dot" quick acting fuses on the outputs? I know they were hidden on my QRX-7001, if you didn't have a schematic you wouldn't even know they were there.