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HH IC100 overheating BD711 transistor...help !

Started by bobster, September 26, 2018, 11:46:06 AM

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hi guys , new-ish member here with a recently purchased HH IC100 amp circa 74 -75.
Amp has been working fine for long periods and is out of the shell just now for a clean and visual inspection.

The power amp board is AO3103/7 variant with two [driver i think?] heatsinked transistors numbered BD711 [ uppermost] and BD712.
Earlier on i tried a treble booster through the amp at low volume and saw smoke from the power board and amp stopped working.

Fuses all ok but noticed R26 [ 10 k ] and R10 [ 100r ] looking burnt so replaced them and amp is now working again.

However , the transistor near these resistors numbered BD711 is getting really hot even after just a short time of playing.........it's lower twin BD712 is very cool in comparison.

Can anyone with experience of these amps advise what the problem may be ?
many thanks !



Hi Enzo , thanks for coming in
i don't have the schematic for this amp but there is another similar amp made by the same company as follows http://oi197.photobucket.com/albums/aa211/bajaman002/HHVSMusicianamplifier1of2.jpg       

i think there will be differences between both but perhaps similar topology which may give some clues?

I could go ahead and replace the BD711 i suppose but from what i read online there could be other things at play causing these problems elsewhere in the circuit.
im usually a vintage valve amp non master volume guy so the SS stuff is a bit daunting for me.

fwiw the amp all works and gives very healthy and clear output....


I'm attaching that schematic as the pdf version is easier to enlarge.
The topology may be similar, but your component designators are meaningless without the correct schematic.
So tell us which resistors on this schematic you think would correspond to the ones that burnt.
Agree with you that the BD711 & BD712 should be similar in working temperature.

You might want to consider buying a schematic from MAJ:  http://www.majelectronic.co.uk/


Hi there , thanks for this.
The resistors i 'think' may apply on the VS schematic you attached are r16 r17 and r24.....
[obviously the topology is not the same as the VS schem attached has a different power board variant than my older IC100 model.]

for further info - on the IC100 I tested all the 741 opamps and get 15v and tested the main filter caps / 2n3440 / 2n5415 / BD711 / BD712 [ on power amp board ] and get 47.2v dc.

A point to make is that i was testing the chassis through an 8 ohm cab and when i played hard without a booster another resistor smoked [ 33 ohm ] .  [ r29 on my IC100 board and possibly R24 on the VS musician board ]

The actual combo cab appears to be factory wired for 16 ohms so i used that to test again once i replaced the 33 ohm and it seemed to behave ok with no more smoke possibly due to less power / current being drawn?    Bd711 still heating though
   lovely amp , amazing vibrato and a nice contrast to my VS musician head,


Going by the symptoms and using the HH IC100 schematic that *Vintagekiki* kindly posted,  :dbtu:

If R10 has burnt out then something maybe wrong around Q2. Burnt resistors means too much current is flowing.
Q2 is the VAS (Voltage Amp stage) which drives the outputs. bias is set by R15,R35 and D3.
R15 is *AOT* (which means Adjust on test) bias may have drifted.
D3 might be mounted on the Heatsink.

Before you go much further a couple of points,
With no speaker connected check the DC voltage at the speaker terminals, it should be less that 500mV,, closer to Zero is better.

As this Amp is likely a head cab then be aware that the speaker negitive is NOT Ground. If the speaker neg is accidentally grounded back to circuit ground it will overheat.

These circuits have 2 feedbacks, voltage and current FB.
R32 forms part of the Current FB.
Check that the speaker output sockets on the chassis are not connecting to chassis.


Hi Phil and Kiki , many thanks for your input and especially for the schemos which i will try and study.
[ i'm mainly a vintage valve amp guy so the SS stuff is still a bit of a 'dark art' lol ]

I tested the speaker outs and got a reading of zero so hopefully good there....

I searched online and noticed that the BD711 transistor that is heating has an equivalent BD911 which seems to be a higher spec in terms of wattage and voltage capacity.      I may buy a BD911 and BD912 and replace that pair for insurance but that may of course leave an untraced fault elsewhere....so will check around Q2 also as you say

For now while i try to make sense of the circuit i will run the amp with a fan on back to cool and stick to 16 ohm cabinets as that may draw less power / current and maybe keep the amp in a safer zone.

Something that may be implicated is that the main filter caps are 4700uf / 50v and are original. Their vent holes are letting a little bit of dried up powdery stuff through so they look to be heading towards expiry.
I wonder if replacing them with new ones at 63v will make a difference to the fault?
It certainly won't do any harm so i will order a pair asap...
thanks again guys
rob          [ p.s. don't know why the past couple of paragraphs have lines through them ! ]


The design is for TIP29C and TIP30C drivers so the BD parts are overkill for either the 711 or 911 series. 
Use whatever is most available and cheap for your location.  TIP series are usually very cheap and common, BD types may be more common in Europe.
R26 is the base to emitter resistor for the upper driver, so likely the bad part that is overheating (BD711) got damaged along with R26.
Suggest you replace upper and lower driver, then do DC voltage checks with no load connected.
Post your DC voltages for E,B,& C of both drivers after replacement.  Also for output transistors and mV DC across each of R34 and R28 (0R33 6W).


thanks G1 ,  will be a few days till i get around to it [ kids waiting on parts etc ] but will get back with some results...

can i take it that when you advise replacing the two driver transistors you mean the Tip29 and 30 [ bd711    / 712 ] ?   or are you referring to the other two transistors elsewhere on the power board i.e. 2N5415 and 2N3440 ?


HH amplifiers series 100 as driver transistors use TIP29/30C.
Overheating the transistor driver can occurs if the replaced driver transistor has a larger hfe than the original driver transistor.

Each replacement driver or output transistor implies the setting of a quiet current (output) transistor.
Adjusting the quiet current is done with a fine change of the resistor R15 (AOT) which is connected in parallel with the resistor R14 (100 Ohm)
The large quiet current of the output transistors makes a great quiet current of the TIP29/30C transistor, which can be one of the reasons for heating TIP29/30C.
Driver transistor TIP29/30C and output transistor 2N3773  should be mutually matched within 10%.

TIP29/30C Datasheet

TIP29/30C Replacement


Quote from: bobster on October 06, 2018, 02:53:22 PM
thanks G1 ,  will be a few days till i get around to it [ kids waiting on parts etc ] but will get back with some results...

can i take it that when you advise replacing the two driver transistors you mean the Tip29 and 30 [ bd711    / 712 ] ?   or are you referring to the other two transistors elsewhere on the power board i.e. 2N5415 and 2N3440 ?

Hi Rob, yes it gets confusing.  :loco
I think you will find that *g1* means the PRE driver devices TIP29 & TIP30.
(I'm using the IC100 schem part names as I/We don't have your PCB in front of us)
I agree with g1 yes if those parts have smoked then likely damage to that and other transistors, I'd just replace them. ;)

TIP29/30 are the PRE drive and your large Power transistors are on a heat sink.
(is that Correct?)
Q2 is voltage gain (VAS) and then those pre and power transistors deliver the Current needed to drive the speaker.
Those last 4 transistors do the same job as a transformer in a valve amp, converting a voltage into a current.

Just like a Valve amp the output devices need to be biased.
which is what *vintagekiki* is talking about.

You likely already know that If the bias is set too hot output tubes will over dissipate and red plate.
Valves are forgiving and take a while to blow but power transistors reach a point of total destruction much faster, often instantly!! :o
Which is why you need a light bulb limiter or variac to start up a new set of transistors on a power amp.
Rail fuses are useless if something major is wrong. 8|

R34 & R28 (0R33) are a bit like the cathode resistors in a cathode biased Valve power stage. You measure the voltage drop across those and adjust the idle current of the power stage. (BTW,, 0R33 is .33 Ohms, not 33 Ohms)
Unlike Valves (which can have either fixed bias or cathode bias) transistors need that tiny resistor even though they are effectively a fixed bias.

vintagekiki uses the term "quite current" which is the same as "quiescent current". either words,, it's the idle current. :tu:

I just recently fixed a SS marshall amp that was almost identical in design with very similar issues and i just replaced the transistors and a couple of resistors,, it's faster than trying to find out what is wrong. ;)

THe reason I mentioned that speaker ground lift point is because you said this in reply 2:

"fwiw the amp all works and gives very healthy and clear output...."

So Yes,, if the speaker neg terminal is accidentally grounded in these current FB systems the power stage will run a lot hotter and IF you then load it down even more with a 4 Ohm speaker then it could do damage.
Why do I know this you ask?
Because it happened to me. :-X
the moment I removed the offending ground link the amp stabilized and worked fine.
So that is something I would be checking. 8|

Better minds here will likely know how to work out the bias voltage drop across R34 & R28 to set the correct bias.
Meantime you may want to build a simple current limiter to test it all when you get the parts.
Light bulb limiter
2nd post on this page;
Hope it helps.


gentlemen , thanks again for your informative replies , things are gradually starting to make sense to me [lol].

i'll check the speaker grounds on the jacks . I had given them all a clean but the amp was very dirty on arrival so some closer inspection is merited.....

Hopefully i'll come back with some results soon and in meantime will think about treating the old beast to new pair of driver transistors / mains caps and some attention to the biasing resistors once i do some more reading / get more confident on what i'm actually trying to achieve.

One weird thing i noticed when measuring around for DC volts was that on the output transistors 2n3773 they both had a wire coming from output board grounded to chassis on one of two nuts and bolts which secured them to chassis. ONE of these nuts read 47v but i couldn't get a reading on the other side........i followed the wire at the non-reading nut back to the circuit board and got 47v there though. [ the nut is making good contact and clean ]   
I wondered then if one output transistor is not working? But with my limited SS knowledge i would have thought that the amp would not produce any sound if one of the outputs was broken ?

For all i know if one of the outputs was goosed then 'maybe' that would cause the overheating symptoms due to other parts of the circuit trying to do all the work?        Pure speculation on my part at this stage....

cheers for now !


Danger!  It is imperative that you understand how the output devices are insulated from chassis/heatsink.  They need insulators (usually mica) between their bodies and the heatsink, as well as insulating spacers on the screw/nuts.  Make sure the casings of the output transistors do NOT measure low resistance (short) to the heatsink.

Sorry about the terminology with regard to the drivers.  There are a couple different ways of talking about them.  The way I was taught, we called the final transistors "outputs" and the ones before them "drivers" (in this case the TIP29C/30C).
Phil will have been from the alternate school of thought where "drivers" are the output devices (as they are driving the speaker).
So I was talking about the TIPs (in your case BD711/712) when I mentioned 'drivers' in a previous post.



- If you use an oscilloscope during service, be aware that the speaker is not directly connected to the ground, already is connected via the resistor R32 (0R1).

- The DC offset voltage allowed on the speaker jack is max 50-100mV

- When the power amp is defective, a DC voltage of up to 40V can occur on the speaker jack.
During repairs to protect from further damage connect speaker jack with speaker or dummy load via capacitor 2200 - 4700 uF/ 63V

- A simple current limiter to safe amp repairs can be done if instead fuse 4A in +/- 47V supply puts the resistors 100 ohm/50W

- Easy setup of the correct bias by adjusting the idle current.
a. Measurement the idle current when instead fuse 4A at point + 47V connect the DC milliampmeter (range 200mA). The idle current adjusts to value of 25-40mA by changing the resistor R15 (AOT)
b. Measurement the idle current when instead fuse 4A at point + 47V connect resistor 1R/2W and parallel with resistor connect DC millivoltmeter (range 200mV). The idle current adjusts to value of 25-40mV by changing the resistor R15 (AOT)