HA3500 blowin' ye ol' fuse

[lowell]

hey all,
have an ha3500 here that has a dead short somewhere. I found some shorted output transistors and replaced all output xistors and drivers as well as all ballast resistors. short is still there. the 8amp fuse blows immediately when I turn this thing on. When I disconnect the connector to the output board the amp is fine so it's definitely in the output section somewhere. I don't know how to troubleshoot this since I cannot get the amp on to test anything. I have a lightbulb limiter if that would help. any suggestions as to what to try?

[52 Bill]

When I disconnect the connector to the output board the amp is fine so it's definitely in the output section somewhere.

Does this disconnect the power amp from the power supply? Is the power supply working correctly?

Take resistance readings of the power supply rails to ground at various points in the power amp and look for low readings. Double check the replaced output devices, to make sure that you haven't blown one of the new ones.

[lowell]

yes this connector connected the p supply to the output section. the power supply is ok.

[Enzo]

Make sure you installed the right transistors in the right spots. There are not that many parts on these modules. Get out your meter on diode test and check EVERY semiconductor on it. GO down the board and test each diode, do junction drops on the transistors.

ANd resistors OFTEN burn open when these blow up. Check the resistors especially in the output stages and driver stages

With the power cable unplugged, measure resistance at the module connector between each power rail and ground and bewteen the two powr rails. ANything shorted?

ANy chance there is a missing insulator behind a transistor?

Your problem is not necesssarily a short, an open copuld also do this. For example, the bias circuit serves to hold the bases of the power transistors close together electrically. If that opens, then other resistors will pull their bases towards their power rails and that will turn them all on HARD and fuses will pop.

This is EXACTLY the place for a variac and ammeter.

[lowell]

can i do some troubleshooting w/ the light bulb limiter? I will test those parts Enzo thanks!

[Enzo]

If parts are immediately burning up, you have to do something to limit current.

[Alex R]

I happen to have in my possession a new HA3500 power amp board, populated except for the transistors that bolt to the heatsink. I should say that I have this because I bought it before Enzo showed me how to fix the bias circuit on the old one . In fact I bought two by mistake and have already sold one on Ebay. They were not cheap and I would have to ask 50 quid for it, so you too should try to fix the old one first. PM me if you do want it.

[lowell]

Enzo,
With power module disconnected and heat sink removed: There is only resistance of 30k between the 2 power rails. Is that normal? There is not short from either rail to ground. Do the insulators actually serve an electrical purpose? There is one that is tearing apart and probably not insulating the back of one transistor from the heatsink. Took module out, plugged everything in w/o heatsink attached and VOILA! Replaced that bad insulator w/ some fish paper. will reassemble and test tomorrow.

[Enzo]

Most certainly the insulator serves an electrical purpose - it is an insulator. As in prevents the transistor from conducting to the metal heat sink. The bare bak of the transistor is its collector. Collector is wired direct to the power rail. Collector touches heat sink, it is grounded, so it would ground off that power rail. That will take fuses in a heartbeat.

The insulator also has to be a good thermal conductor, so don't stick fish paper in ther and leave it. Testing for a minute maybe, but I wouldn't trust it thermally. The whole point of the heatsink is to keep the parts cool. You need a good mica insulator or one of those gray silicone ones. Called Silpads. Mica needs silicone heat grease on both sides. It is an electrical insulator, not a thermal one.

I just looked at a power amp schemo close to yours. Far left, by the input to the power amp, from top rail to bottom, I see a couple 15k resistors and a few low value opnes, all in series across the rails, so yes, 30k sounds about right.

[lowell]

wow, after your previous post about the insulators I was thinking... "hmmm... wonder if that metal on the back of those output transistors is the collector.. that would be wild! cause then the + supply rail would be a dead short to ground...ha!" And now you say that it's the case. Man, that's wild that I did that... learn somethin' new every day. I see them on Mouser, but not sure which ones to get. Can you suggest where to get these silpads?

[Enzo]

I get them from my drawer.

Look at Allied page 1779, there are some, nit sure which size you need, TO247? see if 935-6527 fits.

Or better yet, try Digikey. enter SILPAD in the search. COnsider stock # BER157 or BER178

[lowell]

Enzo,
Got the silpads... BER178 from digikey was right on. Threw it in and powered up w/ the light bulb limiter - bulb was dim, reassembled the amp and voila! Thanks so much for your help!