[bhjazz]

I took in an ailing Dynahi from member iamsmrt at the last Seattle meet. His initial symptom was that one of the RCA connections had come loose, and could I solder it back together. Well, at the meet, once the lid was off, we noticed something far more serious: all six PS caps were bulging out the top, and one had quietly loosened the plastic cap...

We powered it down and started looking around. The only other major feature we could spot is that the transformer was hot hot hot. Too hot to keep your hand on. In general, there are more problems here than just the RCAs...

Now that I have taken it in, I need some help diagnosing what went wrong. It is a Trevor Networks Dynahi. The PS caps are one of the few sets of parts that appear to be non-name. The caps are 63V/4700uF, and all have failed. The other major parts on the board are (from left to right with these parts at top):
2 LM338T
BB OPA541AP
MP930
BB OPA541AP
MP930


I do have a concern about the first OPA541 in that there is a nice smear of heatsink grease below it near the pins. I can only imagine this conducting some current, a little like flux does. Not the cleanest board build I've seen. I have not removed the PS board as yet to inspect the underside.

So here are a few questions for the Dynahi builders:

1) Is the PS circuit such that these PS caps are discharged? Or will they hold their charge for days on end? If they are discharged for sure, I'll pull the board and inspect.

2) What could have caused the bulging caps? It seems like something on that board, obviously, but are there any things I can look at that might be common? Would there be something on one of the other boards I could check for as well?

3) When I desolder the caps, is the heat from the iron going to put me at risk of those things blowing chunks?

Many thanks.

 

[Pars]

Quote:

Originally Posted by bhjazz /img/forum/go_quote.gif <snip> So here are a few questions for the Dynahi builders: 1) Is the PS circuit such that these PS caps are discharged? Or will they hold their charge for days on end? If they are discharged for sure, I'll pull the board and inspect. 2) What could have caused the bulging caps? It seems like something on that board, obviously, but are there any things I can look at that might be common? Would there be something on one of the other boards I could check for as well? 3) When I desolder the caps, is the heat from the iron going to put me at risk of those things blowing chunks? Many thanks.

I've never built a Dynahi, but have built several Dynalos, etc. I would not think the grease you mentioned would be an issue. There are however some issues (documented) of Dynahi PSUs letting go, so to speak. Usually taking out one or both OPA541s, which at $20 a pop is an owwie...

If the amp has been unplugged for a day or two, the caps should be discharged. Check them with a meter. KG typically left details such as bypassing, bleeder resistors, etc. to the builder, so I would bet there are not any bleeder resistors, particularly given the rep. of the builder. As for bulging caps, defective? From a ratings perspective, they seem to be adequate. Since they are unmarked or no-name, who knows what they are, or if they are really what they are rated at? Normally bulging like that indicates overvoltage. 63V caps in that PSU should be fine IIRC. 80V would probably be better. From Dan Gardner's BOM, the Panasonic TSHA, Digikey PN P7485-ND was he recommended PSU cap, and they are really good caps.

As for removing the caps, I wouldn't think you would be at any risk if they are discharged. I hate to say this, but if you hae too many problems with the PSU, and even suspect the OPA541s as being toast, you might be better off doing an Amb sigma22 PSU instead, as it uses cheaper parts, doesn't require the massive heatsinking, and is probably just as quiet and stable. I'm not sure about the hot transformer, but that would concern me alot.

Chris

 

[Paragon]

Is it the actual metal of the caps buldging or is it the plastic? I have had one cap with a buldging top that I thought was on it's way out in my speaker amp. Ended up being just the plastic top was buldgin and the cap was fine.

 

[bhjazz]

Thank you, Pars and Paragon.

Pars: Your cap info (the panasonics) is perfect information. Certainly the step after figuring out the problem is to rebuild. The overvoltage theory works very well with the overheated transformer. You are definitely leading me in the right direction. Thanks!

Paragon: I will definitely check. As noname caps, this is possible, although the hot transformer might prove us otherwise. Still, leave no stone unturned!

 

[Roam]

Given the reputation of the builder, it would be wise to clean & inspect every single part on the board and check them against the schematic to ensure that everything is as it should be. I would not be surprised to find wrong parts, parts put in incorrectly, parts of questionable quality and other shenanigans.

 

[bhjazz]

/holds breath

I have heard that about the builder.......

Okay, then. I found Dan's info on the Dynahi here DIY RESOURCES - Kevin Gilmore DYNAMIC HEADPHONE AMPLIFIERS and POWER SUPPLIES .
Is this good information as a reference, or is there a more Gilmore-centric site? I seem to remember having a better link a year or so back. At any rate, schematics seem to be there along with board photos which should help me.

 

[n_maher]

I think Dan's site is pretty much the Gilmore library and probably your best bet.

There's also the Dynahi build thread at Headwize - HeadWize: DIY Workshop > Dynahi Amplifier Board (single channel) - Build Discussion - that might prove to be of some use.

And just a WAG as to what might be going on - given the temperature that these amps are known to run at I would be at all surprised if the cap failure is heavily related to that. If it was much more than that I would think that something pretty spectacular failure-wise would have happened by now. This amp has to be what, like 4 years old? And that also might have something to do with the trafo being upset as well. Or the trafo could be undersized, oversized, ect.

So yeah, lots for you to check out before replacing any parts (other than the caps of course). And build yourself a capacitor discharging tool, there are all sorts of instructions online.

 

[bhjazz]

Quote:

Originally Posted by Paragon /img/forum/go_quote.gif Is it the actual metal of the caps bulging or is it the plastic? I had one cap with a bulging top that I thought was on it's way out in my speaker amp. Ended up being just the plastic top .

Interesting. As it turns out, this is the case. I've popped the plasctic top off of at least one and the cap itself has no bulging metal. The other five plastic tops are super flexible, so they are likely the same.

Should I still replace the caps?

I'll be pulling the rest apart anyway, due to the hot trafo problem.




Ray Charles is rockin the house tonight! Go brother Ray!
.
.

 

[Paragon]

Glad to hear that is the situation. The caps should be fine but if you have the money.. better ones might do the amp some good.

 

[bhjazz]

I'll ping the owner with this info.

PS board, underside.

Quality is job 1™

 

[Nisbeth]

OMG, that looks like it's been soldered with a clothing iron

/U.

 

[flashbak]

OMG! Looks like the solderer never heard about cleaning ones tip! Wow, nice high quality work!

 

[Paragon]

The joints themselves actually seem to look good. Just needed to clean the flux off the board.

Not sure what is going on with the bent heatsink bolts.

 

[Fitz]

That is some nasty flux right there.

 

[tomb]

Quote:

Originally Posted by Fitz /img/forum/go_quote.gif That is some nasty flux right there.

Agreed. It actually looks like acid flux.