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Old 26th February 2011, 10:47 PM   #21
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I repaired one of these many years ago. It kept blowing output transistors. With a current limited power supply I was eventually able to trace it to an open bias trim pot which had a crack in the carbon. So I recommend that as part of the upgrade that you should replace the bias trim pots

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Old 27th February 2011, 06:32 AM   #22
Mooly is offline Mooly  United Kingdom
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The BD139/140 are marginal with regard to VCE rating where the phenomena of secondary breakdown comes into play.

In practice the MJE devices will be fine, particularly given the hfe of the more modern 2N3773

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I repaired one of these many years ago. It kept blowing output transistors. With a current limited power supply I was eventually able to trace it to an open bias trim pot which had a crack in the carbon. So I recommend that as part of the upgrade that you should replace the bias trim pots

regards
Trevor
Good advice... ancient pots can be troublesome.
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Old 27th February 2011, 02:23 PM   #23
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Can't agree with you there is simply not enough current available to handle a short or even low impedance loads with the .5A devices. The BD's are off by less than 10% should be fine given typical margin for newer devices if the OP is concerned he could use 2SD669/2SB649 which would be an excellent choice or pretest/select the BD for breakdown voltage.
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Old 27th February 2011, 02:43 PM   #24
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What about MJE243/253
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Old 27th February 2011, 05:56 PM   #25
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Quote:
Originally Posted by PB2 View Post
Can't agree with you there is simply not enough current available to handle a short or even low impedance loads with the .5A devices. The BD's are off by less than 10% should be fine given typical margin for newer devices if the OP is concerned he could use 2SD669/2SB649 which would be an excellent choice or pretest/select the BD for breakdown voltage.
Have to agree to disagree on that one You can't assume a device will have better specs than the data sheet says.

Without any VI limiting and SOA protection, failure with a short seems inevitable, and the scenario that caused this very problem.

Just out of interest I looked up Doug Selfs "load invariant" design and that too used MJE340/350's despite driving parallel output pairs. I'd be quite happy with the MJE's
The 2SD/2SB's you mention are a good choice too.

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What about MJE243/253
Not familiar with those but would almost certainly be OK...
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Old 27th February 2011, 06:51 PM   #26
PB2 is offline PB2  United States
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Quote:
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What about MJE243/253
I have suggested and used these in the past but on closer inspection they have worse SOA than the BDs even with their 4A max spec, they would probably work fine and do at least meet the voltage spec.
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Old 27th February 2011, 06:55 PM   #27
PB2 is offline PB2  United States
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Quote:
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Have to agree to disagree on that one You can't assume a device will have better specs than the data sheet says.
Right say for a Military or new design, but statistically for this particular spec it should be fine.

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Without any VI limiting and SOA protection, failure with a short seems inevitable, and the scenario that caused this very problem.

The problem was that it had the wrong fuses in it, currents will soar with a short and the fuse can take about 1 second to blow so you need good current margin and particularly SOA in the drivers.

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Just out of interest I looked up Doug Selfs "load invariant" design and that too used MJE340/350's despite driving parallel output pairs. I'd be quite happy with the MJE's
The 2SD/2SB's you mention are a good choice too.
They are not a good part for this application and drivers in general.
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Last edited by PB2; 27th February 2011 at 07:03 PM.
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Old 28th February 2011, 02:16 PM   #28
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At the minute I am almost settled on STMicroelectronics 2N3773's, Central 2N5322 and 2N5320's from Mouser, and STMicroelectronics BF259's and the resistors I need from Farnell.

I'm waiting on the arrival of a new soldering iron so I can test the rest of the components surrounding before I place the orders (postage is a bitch from both companies!).

What would be the reason behind replacing with different drivers? Surely if I can find like components to the originals that would be the best bet?

Scream now if you don't like what you see, I'm still learning.

Last edited by thefamilybassman; 28th February 2011 at 02:18 PM.
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Old 28th February 2011, 02:27 PM   #29
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Perfect choice.
Reason behind me suggesting different drivers was to enable you to order all from one company, and tbh I didn't scour different companies to see if the originals were available

The design of the amp is a classic "text book" circuit and substituting parts would present no problem.

Make sure the mica washers on the outputs are OK. After all these years they may fall to pieces.

When you have replaced the parts remember you MUST use a bulb tester as that will save any major failures should anything still be faulty.
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Old 28th February 2011, 02:29 PM   #30
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Another thing that has made me think...

I have read somewhere that when it comes to the output caps you can basically put as much capacitance on as you like (within reason). The original caps in the amp were 4000uf and I bought 4700uf's as replacements. I have also read that caps can be wired in parallel to add together their capacitance, and as the caps I bought came in a pack of 4 I opted to wire a pair in on each channel, bringing my total up to 9400uf on each channel.

This wont be putting added strain on the amplifier ... will it? (a niggling doubt in my head is saying "think of the amp having to drive the caps as well...")
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