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HH IC100 overheating BD711 transistor...help !

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

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thanks Kiki , i'll look into that.
The Aot resistor you mention sounds to me a bit like the resistor in valve amps e.g. Marshall that leads to the bias pot and can be changed to give more or less neg volts at pin 5 of power tubes . All new info to me and interesting stuff..
cheers for now guys


Hey Bob, Any chance you could post a few pictures of the circuit board so we can get a better idea of how this thing is wired up? That would speed up the process. :dbtu:


Hi , some phone pics here hopefully they will upload ok.....


I hope you saw my last post about how the output transistors and the wires to them must be insulated from the heatsink/chassis.


Yes G1 i did ,  i made sure the little mica spacers are in correct position next to chassis
cheers Rob


P.S. i have just noticed on my pics of the power amp circuit board that R15 [ the AOT one Kiki referred to ] is missing....it looks as if there has never been any resistor in these slots as there is no evidence of previous solder-

it is also missing on the board of my HH VS musician amp which works without any issues....

As far as i can make out it is [ or would be if present ] in parallel with a 100ohm resistor  'R35'.....

Maybe it was not required in these two amps and the R15 slots are there as an option for amps that are harder to bias?

It seems odd that it is missing though and i wonder if putting a resistor in that slot would help cool things down ?


Quote from: bobster on October 08, 2018, 05:33:29 PM
Maybe it was not required in these two amps and the R15 slots are there as an option for amps that are harder to bias?

It seems odd that it is missing though and i wonder if putting a resistor in that slot would help cool things down ?

Makes no sense to try that as the amp worked fine for many years. 8|


From First post;
Quote from: bobster on September 26, 2018, 11:46:06 AM
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.

OK so Amp worked fine.

Then you say;
Quote from: bobster on September 26, 2018, 11:46:06 AM
Earlier on i tried a treble booster through the amp at low volume and saw smoke from the power board and amp stopped working.

Was this before or after you cleaned it?

My point being if the amp worked fine and then you pulled the lid to clean it you may have unknowingly made a mistake which caused the problem.

It only takes a hair thin strand of loose wire that gets dislodged in a clean to cause problems.

Can you take an internal pic of the two speaker output sockets?
They MUST be Insulated from Chassis otherwise it will kill the amp.


- When replacing the output transistors, it is  necessary to clean the heatsink and insulating washer from the old thermally conductive paste with a some solvent on a nitro base.
- In order to better cool the transistors, apply to the insulating washer fresh thermally conductive paste on both sides.

Mounting a transistor to heatsink

How to mount/attach transistor to heatsink


hi Phil , i had cleaned the amp just by spraying contact cleaner to the pots and switches, isoprop alcohol to jack contacts and a general de-greasing of dirt on cosmetic surfaces / facia....

When i said it was working ok for long periods , this was after i had cleaned it and that referred to when i had it idling at low volume in the house into a 16 ohm cab but i wasn't playing much or loudly....

It was a couple of days later when i tried the treble booster [ into an 8 ohm cab at that time ] and that is when i noticed the smoke coming from the resistors [ replaced as discussed ] on the power board and the heatsink of the BD711 being hot.....

i will post some pics of the speaker jacks for review...thanks Rob


Hi gents
finally got a chance to do some DCV readings with no load.......some of them seem weird but hopefully they will make sense to those of you with the know how....

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    = 5.1mv
DC mv across  R34    = 4.9mv

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



Quote from: bobster on October 09, 2018, 03:50:45 PM
DC mv across  R28    = 5.1mv
DC mv across  R34    = 4.9mv

In your case if R28 = R34 = 0.33 Ω idle current through 2N3773 is
DC mv across  R28    = 5.1mv = 0.0155 mA
DC mv across  R34    = 4.9mv = 0.0148 mA
2N3773 are at beginning opening
For optimal bias idle current it should be increased to 25-40mA


hi Kiki , thanks for reply
it's easily possible i have read them wrong somehow as that seems very low doesn't it..

it's late here and will need to get kids to bed but will double check the reading tomorrow.
The amp sounds clear and loud fwiw lol


hi , tried the amp again today through 16 ohm. got 15 minutes then r29 [ 33 ohm ] started burning . According to schematic this is associated with the emitter of TIP29c [ BD711 ]  that is overheating. This TIP29c emitter also looks like it's tied to the base and emitter [ emitter going through R29 of 33ohm first ] of the 2N3773 ..

I don't know enough about these amps to tell what is causing excess current draw ? i'm wondering if either the TIP29c is faulty , or is one of the main output 2N3773 transistors out of commission and leaving the other one overworked?

I can't make sense of the voltage readings above that i took yesterday so if anyone can decipher anything from them i'd be glad to hear ..
thanks rob


Hi Bob,
OK so it's slowly overheating while running with what sounds like a good clean strong signal.
I would be more concerned about Q2 as I think you mentioned R10 was replaced as it was burnt.
If Q2 has an issue then of course the next transistor is going to fail/ overheat

You might have HF runaway if R10 burnt then C5 would have copped a flogging and might be dead.
That 150pF cap is for hiFreq compensation (IIRC)
if it's now open circuit then the amp might be driving some RF that only bats can hear at 100 plus watts, and would certainly overheat the output.


Yea thanks for the photos of output sockets,, they look fine. :tu: