Onkyo M-506RS - Transistor wire question

D

Doug2479

New Member
Hi All,

I am new, so if this in the wrong place please help me out and tell me where or what to do.

I found an Onkyo M-506RS amp at Goodwill for $30. I ofcourse picked it up as fast as I could. I got it home and found a blown fuse, replaced the fuse and hooked it up and it sounds great... for a while.

At first it would work for a while (heat up) then goto standby mode and click back to working a few seconds later. Then it finally blows the 6 amp fuse... I'm out of those so in goes a 10 amp (for testing) I checked the idle current settings and all seemed ok. So I shut off, hooked up to speakers and cd player. turned it back on and resistor R559 got hot and smoked some. I will replace it, but it still seems ok with ohm meter. I took the covers off and found a "loop" wire going from one of the transistors to D404 (the idle current addjustable resistor) was not connected. It looks like it was "glued" on but not soldered. My question is does anyone know if it needs to be soldered or if it is only glued to the transistor ( i think its the "heat sink" leg of the transistor per Toshiba pdf file).

Welcome any help, thanks!
 
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Just to make something clear that I didn't make very clear. The wire was loose, meaning neither glued nor soldered to the leg of the transistor. I looked at the other side to compare and it was loose as well.
 
For testing you should use a dim bulb tester to current limit the amp and prevent further damage, or at least go smaller on the fuse. Going bigger is almost guaranteed to let smoke out somewhere.

Don't know that amp well enough to answer the question on the wire, though.

John
 
I admit I knew better. I did that to set the "idle" current. I have the schematics, but its so crappy you can hardly make out the numbers on it.
 
I don't think the above is an issue.

The problem is that the amp blows the 6 amp fuse. It works fine, clicks in and out of "Standby mode" while playing and then blows the fuse. Everything goes good during the initial startup. Amp sounds good when playing, no noise or anything when adjusting the output level, no hums.

What should I be looking for? The caps Look fine, no buldging or obvious damage.

Any ideas would be great.
Thanks!
 
Feel around for anything getting hot... measure the speaker terminals to see if DC voltage is present or rises as it warms up. Might also measure the power supply rails and see if they're jumping around at all before it blows the fuse.

John
 
I attached two pics that show the capacitor that was glued to the center leg of the transistor. The amp blows a fuse on the right side power supply. When I flipped it over on its back I could see the once was away from the transistor and the other side was still mostly glued to the transistor.

I don't know if that has anything to do with it. I will pickup more fuses today and check the voltage. I did check for "DC" voltage on the speaker and didn't get anything. I would guess that was due to the speaker relays not engaged. So I guess I would have to take the reading at the relay's instead. While the amp is idle? Also, I did check the idle current per the service manual and it was good and steady at 14mv after 5 minutes.

Thanks for any help.
 

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That "capacitor" should be a double diode. Schematic (not correctly listed in parts list) shows it as a vd1222 which is two diodes in series. If it were not connected or flaky, would cause that channel to blow fuse. Schematic is last page of manual listed on hifiengine.com. Normal workaround is two connect two 1n4148 in series to replace diode if needed. Diode should be mounted on transistor and are part of the bias circuit to track temperature. (Goes back to checking to see if a channel is running hot.) Good luck.
 
Thanks for the knowledge Sregor! I knew it was there for a reason and when I first looked at it, it looked like a burnt glob of solder on the leg of the transistor.

I miss spoke when I said capacitor, it was early and not enough coffee for me :) .
The schematic lists it as D404 (location) when I looked up the part # it is listed as "Silicon Epitaxial Planar Diode for Various Detector,
Modulator, Demodulator" .
I did read the board and it has the symbol for a diode and I knew that's what it called for. It just looked weird. I did feel around for heat while it was running and didn't feel anything getting hot, but I was checking the top, the pics are of the bottom.

Should I go ahead and replace the diode or should I glue it back down and test it first? Hoping your experience can save me a little time. What kind of "glue" should I use? I would think it has to be non-conductive and heat resistant?

Thanks for your help!
 
Oh, forgot one other question. Should the diode be in the sameplace? It was glued to the leg and not the body of the transistor. Both were in the same place, but the side blowing the fuse was the one that was off and the otherside just a little loose.
 
If working on bias circuit, definitely get a dim bulb tester (Incandescent bulb in series with the AC so if problem, it won't smoke the amp. ) On the center leg is actually a good place to sense the heat of the transistor. Non conductive yes. The original part is two diodes in one package, so be careful with subs, and if two diodes, both should be attached to the transistor lead. (Center lead is the collector, which is also the metal part of the transistor which is mounted on the heatsink (and it has the supply rail voltage on it, so be careful). Good luck.
 
Since the diode was loose and not on the transistor I decided to use a small ty-wrap and a lower value fuse to test with.
I checked the vdc on the speaker output, Fluke meter 300mv dc... basically got nothing.

Have it hooked up and listening to it now. Have to wait n c if all goes well. So far so good.

Thanks for your help!!! I knew it didn't look right when I saw it, but had no idea of how it should be.
 
Any updates??? I have similar problem with mine. Bought on eBay. Tested and had it turned up pretty high for 2 to 3 hours, then 6 amp fuse in R. channel blew. Checked transistors and 3 out of 4 were blown. Just replaced (with likely Chinese copies) and turned on for 30 seconds and blew the transistors again. I checked the balancing transistor is OK. Basically I would like to know where do I measure the BIAS adjustment, I see an adjustable resistor R541 says ID Adj, but where do I measure the voltage? I also have the schematics but not sure where to measure the 10mV to 14mV as stated in the manual. Thanks, and I hope it keeps working for you.

Also where do you wire the lightbulb in to protect the circuit? B1+ and B1- ??? Thanks and sorry for hyjacking the thread....
 
Hi Mackoscout,

Attaching the Diodes to the center leg of the transisitor fixed my problem. Yours sounds a little different than mine, my amp would blow the fuse but nothing else.

The variable resistor at locations (R541 & R441) adjust the idle voltage. They actually connect to a Diode (its a double diode and looks more like a capacitor to me any way) The Diode itself acts as a "temp" sensor per Sregor (He told me how to fix my problem, Thank you very much Sregor!). The diode is glued to the middle leg of a transistor (Q515 & Q419).

The directions I found for the idle voltage adjustment is as follows:
No load, no input, no speakers attached and output (volume) turned to 0.
Turn the unit on and allow to warm up.
Read voltage from VCT and IID.
After 3 minutes should be about 10 mV
After 5 minutes should be about 14 mV

You may want to check for any DC ofset as well.
Hope this helps.
 
Which glue did you use? I have put in new transistors on r channel again and with the double diode touching middle leg outer transistor the amp so far on 20 seconds at a time (after activating from standby) with no blowing of fuse. I don't want to push it longer for now until find good way of attaching that double diode....

Also where exactly is the point at which bias current is measured?

Thanks

Hope might be same issue for me....
 
...also I did check the DC offset before connecting speakers first time and it was pretty much zero.

I do think that we might have similar problem, as before I fired it up first time I saw that double diode and I think I "tinkered with it".

Then did the 2 to 3 hr high volume test run - both channels at 7 or 8 on the gain with some blue ray jazz True HD audio into some new klipisch speakers, prior to it blowing.

When I went through the transistor replacement I did't attach the double diode first time.

Now works, but only trying few sec at a time after standby to see if anything is getting hot....
 
Take a look at the pics in one of my posts, you can see where it should be connected to the transistor. Post some pics of how yours looks, that might help. To do the initial test I used a small ty-wrap to hold the diode to the leg of the transistor.

Since your are having the transistor blow and not the fuse, check to make sure you do not have the wire leading to the diode touching the transistor leg. The diode should "feel" the temp of the metal leg but not have metal to metal contact. its a thermal transfer, not "electric".

Start with some good pics of what you have, that will really help.
Hope this helps.
 
Here are some pics:

12%2010:13:14%20PM



Closer:

12%2010:12:13%20PM


Not well attached:

12%2010:13:02%20PM


...and the good left channel:


12%2010:12:43%20PM


There must be better way to attach than ty-wraps??
I was thinking some crazy glue???
 
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The tywrap will hold it in place and is non-conductive. I only used the tywrap to test and confirm the problem was fixed. I just happened to have some small ones handy.

Most epoxies should be non-conductive, even super glue. Just read the packaging and confirm first. Personally I felt better using a two part epoxy, you can pick it up at any Lowe's or Home Depot for a few bucks.
 
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