thanks Kiki , that's good to know.....

Fwiw , i often put a small desk fan behind my amps at gigs , most of the places i play are low on space and the amps can end up pushed back against a wall and have little to no ventilation.

The fan is mainly for extra insurance to keep them cool as most of my amps are 40 years old plus lol

I also use lots of boost pedals etc and this can also cause amps to draw more power and put pressure on older components so by keeping them cool like this i have rarely had any failures over the years..
cheers
rob

Hi G1 , yes you were right when you thought i had read the terminals wrong in the VS.
It does in fact have the two terminals reading approx -47v

On the IC100 i worked on earlier i am getting the slight voltage ~ 0.6v difference that you state between the two terminals B and E....

thanks again for all the tips i've enjoyed getting a little bit of insight into the SS world
rob

thanks Kiki , will keep all this info in my files for the future

well guys , seem to have some progress now....

as i was suspicious of the reading from the lower 2N3773's emitter....i followed the trace on the board from it and found R33 a little 33r which corresponds to the R29 33r for the other 2N3773 which i thought had previously burned and had replaced.....

When i saw smoke previously it may actually have been this resistor [ r33 ] that was burning as it is not far under R29.
Its partner R29 was the only one that i thought was 'under pressure' at the time .
So today i popped in two new 33r 1 watt resistors to these two positions and now have a nice clear sounding amp again.

The Tip29c is not overheating any more and i am reading 4.5mv over the large 0.33 ohm power resistors which is an improvement from 0.5mv.

Hopefully this has cracked the problem..

Will take the amp to a gig soon [ with a spare ] and put it through it's paces and if it survives that we might even be good to go

i feel i have learned a bit here on SS amps and am perhaps not so intimidated by them as i used to be lol

many thanks to G1 / Phil / Kiki and everyone for the help with this it's very much appreciated

cheers - Rob

hi G1 ,
yes possibly a typo there as i measured these voltages at different times before and after changing the main filter caps.....
Also , there is no AOT resistor on the power board R15 position ,the solder holes are empty in this amp and the other two HH chassis' i have here so i'm not sure how else i can set the bias which at the moment is extremely low across the 0.33ohm resistors as in 0.5mv instead of around 11v in the working amp ..

the 2N5415 does have a heatsink as you say and shows similar voltage results to its corresponding transistor on my other HH amp [ the working VS musician head ] .

The one voltage reading that stands out as odd to me in the faulty amp is the lower 2n3773 emitter which is reading approx - 47v on the faulty amp but reads 0v on the working one.
I may have mixed up the emitter and base readings but i don't think so as the wires are colour coded and connected to the same terminals on each power board which are apparently interchangeable.

Either way , on the working VS amp i am only reading approx - 47v on ONE terminal of the 2N3773 but on the faulty amp i am reading approx - 47 v on two of the 2N3773 terminals.....

I noticed the distortion started shortly after i replaced the two main filters from a donor spare chassis which hadn't been used in awhile but the caps showed no sings of venting whereas the ones on the amp had started to let go of some dried up innards.........so it's possible these caps are skewing something...

Also i need to change out the little C5 cap again to see if it helps as the distortion appeared shortly after i changed this cap and the filters so one or both may be a factor.....

These two changes are the only ones that i've done that i can think may be clouding the issue separate to the original problem of overheating in the Tip29c which is no longer happening lol...

I'll change these parts out soon and report any changes but am still mystified as to why the 2N3773 is showing two readings of approx  - 47 v when on my other working amp its corresponding transistor is only showing one..

thanks
rob

hi G1 ,
i took these voltage readings a few days ago and have copied from another post earlier in thread...

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 ]
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.... [ it's possible my replacement C5 is faulty so will will try another one there soon ]

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 or are causing issues...

thanks rob

Hi Phil , thanks again for your help.
I never got an alert for a PM but will check the messages here...
i have also ordered two new 2N3773's and a 2N5415.....

the other amp i made the comparisons with is from the same stable and maker and it's power board is very similar in design and interchangeable as far as i am aware and also has the same two big electro caps and general topology   .
i did suspect that little 150p cap [ threw the old one out unfortunately ] and will have a look in my stash for a non ceramic replacement and try it there to see if anything gives..
i only tried the scrap amps 2N3773's as an experiment as i have to wait for parts to arrive but did think they would be likely to be dead ..
i am reading very low mv on speaker outs well within limits according to Kiki...

happy to admit i'm floundering here but am determined to get this amp working again and am not in a major hurry plus i'm learning [a little! ] along the way
thanks
rob

hi guys
just on to ask for some more advice before i attempt any more of this repair with the IC100.....

i have taken readings off the driver and power transistors in circuit in the IC100 and my working VS-Musician head with power on ..

all transistors read similar except for the lower of the 2N3773's which reads - 47.2v on its emitter while the 2N3773 in the working VS Musician amp reads 0v...

I tried another two 2N3773's from a scrap donor PA amp but got the same results with distorted output so they were both likely to be dead....

At this stage the 2N5415 is still overheating but the previously overheating tip29c is now not getting overly warm but they give very similar DCV results to the working VS amp so might still be serviceable

in your experience , does this look like i just need a new 2N3773 [ or pair of ] ?
thanks rob

ok Kiki , appreciate the safety warning...

i have a fluke meter only so hopefully it will help make sense of things when i get round to these tests soon...

cheers
rob

thanks Kiki ,
i've desoldered the power and driver transistors from a donor pa head and will test them all using the info from your links to see which ones may be feasible to use....

i'll also take readings from my VS musician head which is working fine and compare them to the corresponding parts in the IC100 i'm trying to repair and see if anything else looks weird.

I'm enjoying trying to learn here but it's certainly quite complex compared to my dinosaur valve rigs
cheers
rob

hi Kiki and Phil
tried the guitar through 'power amp in' and it remains very distorted so looks like power amp right enough.

I have replacements here for the tip29 and tip30 robbed from a dead HH pa amp and that read Hfe of 53 and 130 on my meter.

So would the plan be -
Order up a new 2N5415 , test the 2N3773's as described above and if no reading from B to E replace and see if we get a result ?

Anything els you could advise that needs done ?
Would be interested in your assessment of my updated voltage readings in post number #39 above....

Will be a few days till i get round to this as have a newborn in here plus the other monsters but will get back to all soon.
thanks again for all the help so far

cheers
rob

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

cheers rob

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



cheers
Rob 

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

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 .....

cheers!
rob