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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 Phil ,
the resistor R10 has been fine since replacement and only looked a little burnt so i changed it out early on in this process . The one that is burning up when i play is R29 [ 33ohms ] .
Easy job to replace the 150pf cap though so will do that....

Have now replaced main filter caps 4700uf x2 but this has not made a difference to weird voltages.

I have an old HH PA amp power circuit board which i have relieved of its driver transistors
they are 2x  'D381' and 2x 'B536' which seem to be close in spec to TIP29c and Tip30c .

On my multimeter transistor hfe test setting the D381s read 53 and 15 [15 seems low? ]
the B536's read 141 and 134.

I'm up for trying these as replacements in the IC100 but would appreciate advice as to which pair will be best hfe wise ?   I'm assuming the 53 and 141...

Also , I am not sure of my previously mentioned dcv readings on the 2N3773 output transistors and whether replacing the drivers will make no difference if there is a fault with one of the 2N3773's ?


ok something interesting is happening ......
replaced c5   with a ceramic 150pf 

Now 'Q2' [ pre driver?]  2N5415 is heating up real fast and the previously heating BD711 driver is remaining cool...

Also , the sound output is now distorted and unpleasant like crossover distortion you would hear on a valve amp , still fairly loud though.........I wonder if this is because i'm only getting 0.5mv dc over the OR33 resistors and Kiki pointed out this would be an issue.....

For comparison I checked my HH VS musician amps readings and get around 11mv on it's OR33's

took some voltages on the first two pre driver transistors on the IC100-

Q1      2N3440      C = 46.9v      B = 0.6v      E = 0.1v
Q2      2N5415      C = 1.2v        B = 46.9v    E = 47.4v

also took voltage readings on my HH Vs musician head in relation to pre drivers / drivers and power transistors and get largely similar results to what i read on the IC100 amp we are looking at...

so something appears up with the biasing maybe?

any ideas ? ;)


Where is your (-) supply rail, or are you just omitting the minus sign from your voltage readings?


Quote from: bobster on October 10, 2018, 12:02:10 PM
For comparison I checked my HH VS musician amps readings and get around 11mv on it's OR33's

11mv on it's OR33's on your HH VS musician amps is 33 mA idle current and that's OK.
When replacing the transistor, check that the CBE transistor layout is the same.

Is there and how much is DC offset on speaker output jacks?

How much is the current, and how much is the DC voltage without signal at + and - supply rail (measured at Fuse 4A)


good question G1 , i freely admit to being a little out of my depth here and was omitting the - sign from the results as you pointed out .

Kiki , there is 4mv on the speaker out jacks.....

measuring over the 4A resistors i get around 0.5mv....very low i suspect

i'm not sure how to measure current though , sorry....
i have a decent fluke meter but not sure how to do the current test

thanks again guys for your patience...

next up i'm going to check the little BC204 and BC207 transistors to see what they read...

as per schematic
Q3  BC207    C = 1v   B = 0v   E = 0v
Q4  BC204    C = 0.4v   B = 0v  E = 0v

cheers for now - rob


I just whiped up a quick sim of the power amp circuit for you.
Will give some idea of voltages around the circuit.
Don't rely on this as gospel cause the devices are perfect models and real world devices will be different but gives a fair estimate of what is happening.

Not shown here but this sim shows that R34 and 28 are sitting around 40mW with a 100mV input outputting about 2 Watts into 8 Ohm load.

Regarding the bias; Yes you maybe right,,check that R35 and D3 are in fact working as that would upset the outcome.

Q 3&4 and all associated parts are the current limiting section and only turn on when the output is shorted or reaches the set limit, otherwise they are out of circuit.
I've left them and other bits out for clarity.
Sorry that C5 was not the issue and set you on a bum steer. :-[

Keep an eye out for solder cracks, at the age cracked solder pads can be an issue.



hi Phil , thanks so much for taking the time to do that it's much appreciated .
Going to get little 'un from school and will have a look later at this and also the R35 and diode .....



Be sure to note the (-) sign on any readings when it appears.  We are expecting it for about half the amp and will think there is a fault if you omit it where it should be noted.


Quote from: bobster on October 11, 2018, 06:12:34 AM... there is 4mv on the speaker out jacks ...

The DC offset voltage is OK (allowed max 50-100mV)

Quote from: bobster on October 11, 2018, 06:12:34 AM... measuring over the 4A resistors i get around 4.5mv ...

idle current power amp with high accuracy can be measured by measuring the idle current 2N3773 described at

a. recommended value measurement with DC milliampmeter 25-40mA
b. recommended value measurement with DC millivoltmeter 25-40mV


Hi guys , thanks again for your patience...i feel like i'm learning something here lol.

here are some revised [ amp unloaded ] readings including negative voltage results-

741 opamp on power board

pin 1  =  - 15.2v         pin 8   = 0v
pin 2  =   0v               pin 7   =  + 14.8v
pin 3  =   0v               pin 6   =  + 0.7v
pin 4   =  - 15.2v        pin 5   =   - 15.2v

TIP29c [ BD711 on my amp ]    C = 46.5v  B = 1v      E = 0.5v
TIP30c [ BD712 on my amp ]    C =    - 46.1v  B = - 0.6v   E = 0v

DC mv across  R28    = +0.6mv     [ on 200mv scale , i had previously mistaken these as ~6mv ]
DC mv across  R34    =  +0.6mv

Power Q's
Q1    2N3773      C = 46.5v    B = 0.5v     E = 0v
Q2    2N3773      C = 0v         B =  - 46.8v   E =   - 46.8v

2N3440     C = 46.9v   B = 0.6v   E = 0.1v            [ all positive voltage readings ]
2N5415     C = 1.2v     B = 46.8v     E = 47.4v      [ all pos voltage readings ]

*2N5145 is now heating up fast and the Tip29c is no longer heating up since i replaced C5 of 150pf as suggested in case of high freq osc....

R10 of 100 ohms has +47 volts at either end of it
C5  of 150pf [ replaced ]  has +47 at one end and + 1.2v at the other end which is collector of 2N5415 i think

the junction between R35 of 100 ohms and D3   has + 109mv which then after a short time drifted down to around +100mv

According to the power amp sim Phil put up this reading between R35 and D3 should be negative and around -155mv?

hope all this gives a clue as to what parts may need replaced

p.s. amplifier is still working but sound remains distorted at any volume
thanks  rob


From your readings I would suspect Q2 is faulty as there should be about 0.6V DC difference between the B and E terminals. To me it looks like internal dead short between base and emitter. I would replace Q2 anyway on that evidence.


thanks for this , solderer

Just to confirm , when you say Q2 , does that mean the 2N5415 or is it one of the power pair 2N3773?

And would still be interested to hear from Phil and the others if there is anything else amiss with these readings as i'd hate to replace this transistor or another part for it just to pop again a minute later ;)

i have added the 741 opamps readings to the post 2 or 3 above for reference also



I am talking about the lower of the two 2N3773 output transistors. If it was myself working on the amp I would first remove all the connections from this tranny and do a resistance check across the B/E terminals. If it reads zero on resistance range that is your problem (or one of them!) and the device must be replaced. Hope this helps anyway.


thanks , will look into that and deconnect  /  test them soon......family stuff getting busy .. :duh

cheers rob


Quote from: bobster on October 12, 2018, 10:06:38 AM
p.s. amplifier is still working but sound remains distorted at any volume
Turn guitar to poweramp input jack (rear amp).
In this way, you separate the preamplifier from the power amp.
The sound from the amplifier will be much smaller.

- If the sound is further distorted, fault is in the power amp.
- If the sound is clean (non-distorted) fault is in the preamplifier.