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Old 2nd July 2005, 10:54 PM   #3001
VT67 is offline VT67  Belgium
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As of now only 999.

Another question: does one also need a softstart circuit when using an R-Core transformer instead of a toroid?

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Old 2nd July 2005, 11:52 PM   #3002
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Oh, and looks like I got post #3000 on this thread
Congrats, lets have a party

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Old 3rd July 2005, 02:17 AM   #3003
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Hey Stuart, I received the resistors. Those a really cool resistors. Thanks a ton!
I hope you check in pretty soon. My table is covered with stuff in hopes I can get this amp working this weekend. I hve extra parts for just about everything on the board but I just don't know what parts might be causing this. Iwas hopefup you or some of the other guys might give me a place to start looking.

Thanks, Terry
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Old 3rd July 2005, 03:53 AM   #3004
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What I do is to triple check EVERYTHING before I run anything up on the variac. I find mistakes quite easily this way and like everyone else I also make mistakes on occasion. Now that one channel is operating checking resistances against the good channel might yield you some clues. In particuluar, check for proper wiring, proper location and orientation of all transistors, diodes and electrolytic caps. Be sure that there are no minute solder bridges as this is one of the most common problems. On my first KSA-50 channel I had made the corrections to the DC balance part of the board wrong...... didn't damage anything, it just didn't allow any adjustment at all, the fix was easy.

If all is well with parts placement and wiring and you're still having problems then remove and test all semiconductors, then test the resistors in place and all other parts including both the pots.

Could you post a close up of the boards as mounted on the sink so we could see in more detail how you ahve things set up?

Thanks!

Mark
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Old 3rd July 2005, 06:29 AM   #3005
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Thanks Mark,

Here's some pics

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.


I am going to have to wait now. I was trying to set the bias on the good channel and I misread the wiki. I thought I needed to see 2vdc drop across RE1 but it was .2. I ended up smoking RE1,2 & 3 on the + bank.
I don't have any replacements for those so I will have to wait until Tues. Boy I hate learning everything the hard way.

I suppose I could just pull the boards and start looking for differences between the two. I did this before I mounted them but didn't know what was causing the differences. The voltage drop readings on the wiki were right on, on both boards, so it must be something else.

Something that would be really helpful would be if someone who has a working model could give some voltage readings as related to ground for the transistors. Hafler does this in their manuals and it is quite helpful when troubleshooting.

Blessings, Terry
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Old 3rd July 2005, 10:23 AM   #3006
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Terry,

if you had +2V across the 0.50 Ohm emitter R's it means you fried them with 4 amps going through each of them.
3 times 4 = 12 amps.

Supposing hFe of your driver is 80, current from the driver to the output was 150mA.
MJ15003/4 need about 2V on the base to open up, with 2V across the Re's the base of the outputs must have been at 4V.
Means 4/25 = 160 mA went through the emitter resistor of the driver.
Total current your driver delivered while you were biasing was 310mA.

If you measured 42vdc on the PS, voltage across the driver was 38 volts(42-4).
At 310mA current dissipation of the driver was 11.78 watts.
Your driver survives if it does not reach 150 C. With your livingroom at 25 C difference is 125 C.
the MJE's are rated at 50 watts, thermal resistance from junction to case is 2.50 C/W. (125 divided 50)
The 2.5 C/W should be listed at the MJE's datasheet for Tc.

Suppose we round the 11.78 watts off to 12.50 watts :
then total thermal resistance of the driver and heatsink should be lower than 125 / 12.5 = 10 C/W.
Insulated, TO220's have a thermal resistance from case to heatsink of more than 1 C/W.
Substracted: 10 - 2.5 - 1 = 6.5 C/W maximum for the heatsink of the driver.

The kind of heatsinks on your drivers look poorly to me, if they are folded painted steel sinks thermal resistance of those is definitely above 6.5 C/W.
Which means your driver hit the daisies too during the biasing.
hFe of your driver could have been lower than 80, which makes the presumption even more realistic.

If you measure a minus voltage on the output that confirms that the positive driver is shot, the other one may be alive.

MJE15032/3 need 1 volt to open. With the 0.40 amp bias per device you mentioned voltage on the base of the outputs should be around 2.8 volts. (for 0.50 OHM Re's)
With 1 volt added, voltage at the base of the MJE drivers should be 3.8 volts. The pre-drivers have again 1 volt more, 4.8 volts. Think i posted the voltages for the entrance earlier.
A 2SC3955 needs 1 volt on the base to open up. When you are adjusting the trimmpot once you reach 1 volt on the base of the device it remains this value if you raise bias voltage even more. Should current increase, that would raise the bias voltage then the 2sc3955 has more than 1 volts on its base, it opens and the extra current will flow in its collector.
This will decrease the current on the base side, base voltage will drop and the collector current will drop.
So if it functions correctly base-emitter voltage should remain constant at around 1 volt



Walter,
with a toroidal transformer above 300va you'll need a softstarter to prevent the fuse to blow, inrush peak of a toroid can be 10 times the nominal value.(got a inrush graph pic of a toroid somewhere).
It depends on the size of your transformer whether you need a softstarter too.
Behind the transformer are the capacitors, if you are running really big boys a softstarter will gently load them instead of giving it to them straight-on, which lengthes their lifespan.
So i'd use a softstarter either way.
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Old 3rd July 2005, 02:20 PM   #3007
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Terry,

What I would do is to remove the boards fomr the heatsink and repair them, leave them off the main sink and just run some wires to the supply and fire them up seperately one at a time off the heatsink. I use a Tektronix PS-503 +/- 20 volt variable bench supply and set the current limiting to about .25A max current so nothing can be damaged by a mistake. When I run the main DC level knob up on the supply if there is excessive current draw fomr the main board being tested then either or both the output overcurrent current LEDs will begin to light up and I can go back and double check well before smoke and flames errupt, or any damage occurs. Once I know its working correctly then I increase the current limit to maximum and let the board run for an hour or so. Then I connect the main board up and repeat the same procedure for the whole channel...beginning with the bias set at its lowest point I bring the DC back up and if all is well then slightly increase the bias and let the channel run for a while to be sure all is well. Keep in mind that with this bench supply its almost impossible to damage an incorrectly wired amp as there is no way under the sun that this supply can run it at full bias or power.... but just enough to test it.


Before you fire them up again be sure that the bias pot is set to the minimum bias point(according to the WIKI) and the DC balance pot is centered in its range.

After repairing what ever needs be and the board seems to be working bring the bias up a little bit at a tiime to about 1/3 range... or just run it up and down say 1/3 of the pots range while measuring across the drivers emitter resistors. Setting the bias to a normal class A level, or letting the bias get way too high will overheat those drivers in a short time.... the sinks are way too small, but ok for testing purposes at lower bias levels. I'd plan on running it in class AB for now till you can move the drivers and bias sense down to the main sink, or at least till you get some sizable sinks for the drivers.

BTW: The Tektronix supply can be had very inexpensively off of E-Bay and you would also need a TM-500 main frame to power it.... but all in all it would be a good investment for you to make as other plug-ins are always available inexpensively and can be purchased at your own wim to complete the main frame.... DMM's, Scopes, etc. The TM-500 series make a really nice compact test set up, are ultra high quality, and dirt cheap.

PS-503 On E-Bay


Note: I paid less than 50 bucks each for both my TM-504 main frames. You just need to keep an ele out for them

TM-504 Frame On E-Bay

Mark
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Old 3rd July 2005, 03:22 PM   #3008
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Quote:
Originally posted by jacco vermeulen
MJE15032/3 need 1 volt to open. With the 0.40 amp bias per device you mentioned voltage on the base of the outputs should be around 2.8 volts. (for 0.50 OHM Re's)

Oops, those are my numbers for 12 devices and 0.68 Re's !
If you are planning on 23 watts in class A( 1.2 amp bias) then voltage is 0.6 volts lower.
2.2v at the base of the outputs, 3.2v at the base of the driver, 4.2v at the base of the predriver.
Sorry, i am such a dork.
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Old 3rd July 2005, 03:55 PM   #3009
VT67 is offline VT67  Belgium
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Walter,
with a toroidal transformer above 300va you'll need a softstarter to prevent the fuse to blow, inrush peak of a toroid can be 10 times the nominal value.(got a inrush graph pic of a toroid somewhere).
It depends on the size of your transformer whether you need a softstarter too.
Behind the transformer are the capacitors, if you are running really big boys a softstarter will gently load them instead of giving it to them straight-on, which lengthes their lifespan.
So i'd use a softstarter either way.
Basically it comes down to, regarding what type of transformer (toroid, R-core, EI-core.....) of 300VA and up it's always better to use a soft-start circuit to make life easier on the trafo itself and the cap's behind it. Thanks jacco

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Old 3rd July 2005, 04:42 PM   #3010
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Yes, and speaking of current limiting this can easily be accomplished with a $2.17 CL-60 current limiting device. There is no real need nor gain by utilizing any of the rediclous - extravgant turn on delay circuitry being discussed here unless you're building some really huge ampifiers. Also if you don't like the CL-60 in series with the AC mains all the time it is easily shorted with a simple R/C driven SPST 10 amp relay. Keeping things simple does have many advantages.

Mark



Thermometrics CL Type Data

Mouser CL listings, 2nd Down on left

Mark
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