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

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 ?
rob

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 ?
thanks
rob

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 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
cheers
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

thanks
rob

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

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 ?

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

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

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
rob

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 !
rob

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 ?
thanks
rob

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 ! ]

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,