Power Transistors in Power Amp Getting Hot !

Thread Starter

BamBam57

Joined Nov 14, 2019
20
Thank You Ylli ! Will do that in the morning ! Thanks for the comprehensive instructions, can you just clarify what IOW means and what across R47 means. I think Im getting there ! And I think everyone who shares this post will benefit from a great learning curve also ! Thanks to everyone so far who have added their twopenneth worth ! Best Regards Paul
Wl. Kt
In other words..... R47 is one of the two parallel emitter resistors. It is the voltage across that resistor.

I mentioned that option in my post.
Well it was all going very well, until for some reason while checking the mV over those resistors and the output, the amp decided to switch in the protection circuit and blew the internal fuse.! Hahaha! Will have some replacements fuses on Tuesday! Then spotted ALL those R47 resistors are ZERO Ohms 4W Cement resistors. The output voltages on each channel were showing 27mV on one and 1.9mV on the other channel. Next episode on Tuesday when I get some more fuses. I will crack this though! Haha Best Regards Paul
 
0.47 ohms is tough to read on most multimeters. It's on the order of lead resistance.
The 1.9 mV is going to be cool and the 27mV is going to be warm.
1.9 mV is probably too low.

If the bias is too low, you can get crossover distortion.
 

Ylli

Joined Nov 13, 2015
1,027
Looking at that schematic again, 'R47' is not a designator, but rather the value of the resistors. 'R47' translates to 0.47 ohms. Two 0.47 ohm resistors in parallel gives you an equivalent resistance of 0.235 ohms, and most ohmmeters do not do a good job measuring that low. So it may look like zero ohms, but the resistors are likely still OK.

You mentioned you ordered new D200 and D100 transistors? Exactly what transistors and from where did you order them? I would have expected a 'D200' to actually be a 2SD200 and a 'D100' to be a 2SD100. But these do not make sense in your posted circuit. The 2SD200 is a silicon NPN and the 2SD100 is a Germanium NPN transistor with ratings way too low to be used as an output transistor.
 

Thread Starter

BamBam57

Joined Nov 14, 2019
20
Do you mean they measure zero ohms?
At least one of them that I can see in the picture is marked 'R47' - 0.47 ohms.
Yes. They are all rated Zero ohms. The R47 is just the name. I was surprised by it also. Checked the spec sheet to verify. Best Regards Paul.
 

AlbertHall

Joined Jun 4, 2014
11,108
You mentioned you ordered new D200 and D100 transistors? Exactly what transistors and from where did you order them? I would have expected a 'D200' to actually be a 2SD200 and a 'D100' to be a 2SD100. But these do not make sense in your posted circuit. The 2SD200 is a silicon NPN and the 2SD100 is a Germanium NPN transistor with ratings way too low to be used as an output transistor.
Curiouser and curiouser.
The D200 should be a PNP (but as pointed out earlier it is drawn with collector and emitter swapped) and the D100 should be NPN.
 
I can't find a D100 or 2sd100.

The D200 should be a 2sd200 http://pdf.datasheetcatalog.com/datasheets/savantic/1267.pdf This is silicon and NPN/

2sd100 substitute https://www.nteinc.com/specs/100to199/pdf/nte102a.pdf
2sd200 substitute https://www.nteinc.com/specs/300to399/pdf/nte389.pdf

Something is WAY off. Could be house numbers.

I do have a Japanease transistor databook, I could try to look there.

What you don't want is a "crash course" in amplifier repair. When I built my version of the Leach Amp (google it), I accidently mirrored the PCB layout. It took me eons to find that mistake. Since the amp is so symetrical, I was able to use the boards by swapping the N-channel devices with the p-channel ones and making two cuts in the PCB to swap the orientation of the bias regulator (Q7 et. al.) in your amp.

I later as a side job repaired audio amplifiers professionally. I took on the shops harder repairs. One amp I repaired (300W/ch_was for a disco where 5 shops refused to repair it before me. Metal film resistors actually melted and became a puddle on the PCB. The guy really wanted it repaired no matter what the cost because the amp was borrowed from a friend or relative.

The speaker terminals were directly shorted by some clown that hooked them up. Excess strands of speaker wire everywhere. I think I also added thump suppression to the amp as well.

Powering up amps with a 100 W incandescent bulb in series is one trick. It acts as a current dependent resistor. If the amp has a short, the lamp will light and limit the current.
 
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bertus

Joined Apr 5, 2008
21,248
Hello,

Looking at the schematic the D100 and D200 seem to be darlington transistors.
They have drawn 2 arrows at each.
Is there a logo on the D100 and D200?
This might also help finding replacements.

Bertus
 

Thread Starter

BamBam57

Joined Nov 14, 2019
20
Hello,

Looking at the schematic the D100 and D200 seem to be darlington transistors.
They have drawn 2 arrows at each.
Is there a logo on the D100 and D200?
This might also help finding replacements.

Bertus
Here are the ones Ive ordered as a backup ! NPN and PNP as per the schematic ! The ones installed are marked ST D100 and ST D200 and on the covering spec sheet they are specified as D100/200 (SGS) ! This amp was designed as an audiophile amp which won awards back in 1989 ! Hope this clarifies ! I was only intending to replace these Darlington Transistors if they were the cause, but from all your knowledge so far, that may not have to be the case ! Best Regards Paul
Capture.JPG
 

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Thread Starter

BamBam57

Joined Nov 14, 2019
20
Curiouser and curiouser.
The D200 should be a PNP (but as pointed out earlier it is drawn with collector and emitter swapped) and the D100 should be NPN.
Yes I believe that is correct ! Dont know how it got drawn incorrectly though ! Checking another of their designed amps and it is drawn correctly on that other diagram ! Well spotted everyone ! I was really lucky to get hold of these diagrams in the first place from someone who use to work fo the company years ago ! Best Regards Paul
 

Ylli

Joined Nov 13, 2015
1,027
Congratulations on a good job of tracking down the actual parts. None of my searches turn up these SGS parts, but they certainly appear to be the right ones.
 

Thread Starter

BamBam57

Joined Nov 14, 2019
20
I can't find a D100 or 2sd100.

The D200 should be a 2sd200 http://pdf.datasheetcatalog.com/datasheets/savantic/1267.pdf This is silicon and NPN/

2sd100 substitute https://www.nteinc.com/specs/100to199/pdf/nte102a.pdf
2sd200 substitute https://www.nteinc.com/specs/300to399/pdf/nte389.pdf

Something is WAY off. Could be house numbers.

I do have a Japanease transistor databook, I could try to look there.

What you don't want is a "crash course" in amplifier repair. When I built my version of the Leach Amp (google it), I accidently mirrored the PCB layout. It took me eons to find that mistake. Since the amp is so symetrical, I was able to use the boards by swapping the N-channel devices with the p-channel ones and making two cuts in the PCB to swap the orientation of the bias regulator (Q7 et. al.) in your amp.

I later as a side job repaired audio amplifiers professionally. I took on the shops harder repairs. One amp I repaired (300W/ch_was for a disco where 5 shops refused to repair it before me. Metal film resistors actually melted and became a puddle on the PCB. The guy really wanted it repaired no matter what the cost because the amp was borrowed from a friend or relative.

The speaker terminals were directly shorted by some clown that hooked them up. Excess strands of speaker wire everywhere. I think I also added thump suppression to the amp as well.

Powering up amps with a 100 W incandescent bulb in series is one trick. It acts as a current dependent resistor. If the amp has a short, the lamp will light and limit the current.
I have put the D100 and D200 details above ! They were hard to find so hope maybe one day I will be able to use them, if not sooner ! haha ! Best Regards Paul
 

Thread Starter

BamBam57

Joined Nov 14, 2019
20
I don't know what the real scheme you have, but it's not drawn correctly (with a mistake). The D200 transistor is drawn in reverse!
Thankyou and noted ! You are absolutely correct the emmitter collector are around the wrong way in D200 on the diagram, but any ideas on a solution ? Thanks Best Regards Paul
 

Thread Starter

BamBam57

Joined Nov 14, 2019
20
JUST A SUMMARY UPDATE ON WHERE WE ARE AT THE MOMENT ! !

1. Have already ordered replacement D200 and D100s and 10 fuses after somehow when testing, the internal one blew !
2. Voltages at output for speakers with no load for the GOOD AMP are Left 25mV and Right 14mV ! The Poorly Amp I am trying to fix FAULTY AMP are Left 29mv and Right 1.8mV ! It is the Left channel on the faulty amp that gets hot but at only 4mV more than the Good amp would that really be any indication !
3. Those 8 x R47 Large White Cement Resistors ARE Zero Ohms !
4. The Power Transistor D200 is shown with the collector and emitter around the wrong way in Diagram ! As Pointed out by you guys !
5. Should be back on the case on Tuesday when the Fuses turn up, if anyone has any other suggestions by then Ill add them to the list !
6. This Power amp must be about as "simple as it gets" in minimalist design looking at how "few components" there are inside, but it has become a project of giving it a new life, if I cant eventually fix this I will throw the towel in, but I am at chapter 1 so far ! Amp Sounds great when working !
Best Regards Paul
0-02-0a-d70efae03a6c0dc060bc2a7234783180558bcd176aed605cc8ba36799d42d0a3_659edbda.jpg
 
Last edited:

Dodgydave

Joined Jun 22, 2012
9,917
I would say the 1.8mV transistor isn't conductive 4mV won't make much difference, which transistor is the 1.8mV Pnp or Npn , what are the voltages across the C and E terminals?

Note white resistors are 0.47 ohms each in parallel that's, 0.235 ohms total, hence you're seeing Zero ohms..
 

Ylli

Joined Nov 13, 2015
1,027
General experience:
1. Emitter resistors rarely die
2. If the output transistors pop, you have to get everything else RIGHT.
3. Low value resistors (100-250 ohms) in the output ckt ALWAYS need to be checked.
4. Bias regulator always set at minimum during unknown start-up. You can FORCE it to be zero by shorting the C-E junction of Q7
5. Initial tests are no speaker connected. Input signal connected. Variac. Scope on output. Monitor line current.
6. Bias best set with distortion analyzer. Never had the luxury.

DC out, no signal under 0.750 V. Usually mV. It's rare to find an amp with a DC adjust.

if you don't know the bias value, you need tosit at idle and no signal and watch the emitter voltage, line voltage currrent, the temperature and the voltage across the E-C of Q7. If your way off on bias, it will start heating up really fast.

I usually check for symetrical clipping and at full power.


It's POSSIBLE that with 4 emitter resistors in parallel, that there could be a bad one that cause the jump in the voltage across Re to be larger.
 
Attached is a pic of the Leach Amp version I builtin the mid 1980's. To service, the boards can be taken outside the case and extended with a harness. The front mounted PCB implements a slow turn-on with logarithmic ramping of the volume and thump suppression. The circuit could be smarter and disconnect power if it fails to power up within a certain amount of time.
There is no power switch.
 

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