80ies Poweramp repair

[nashkato]

okay , so i got this big , heavy 400W/per side convection cooled (no fans) discrete mosfet PA amp with kind of separate powersupply .
just a few parts , slow start PS with relays ,VU driver , 6-8 small signal transistor and 6 mosfets and mostly resistors on the amp board and an thermic element in the output .

symptom : one side started to crackle ,static noise like dirty high impedance circuits , no loud popping sounds that can destroy your driver .

what i´ve done :
resolder and cleaning of the whole pcb and trimpots
recap of all electrolytics , ceramics and film
swapped out most of the resistors , some are precise resistors like 825R or 4k75 , those i dont have on hand and were left in.
spray to cool down components

without ordering or even tracing down parts i´m stuck  .
even then , the symptoms may prevail cause the reason for that static noise is the PCB itself that got slightly conductive .

 

[scott2000]

okay , so i got this big , heavy 400W/per side convection cooled (no fans) discrete mosfet PA amp with kind of separate powersupply .
just a few parts , slow start PS with relays ,VU driver , 6-8 small signal transistor and 6 mosfets and mostly resistors on the amp board and an thermic element in the output .

symptom : one side started to crackle ,static noise like dirty high impedance circuits , no loud popping sounds that can destroy your driver .

what i´ve done :
resolder and cleaning of the whole pcb and trimpots
recap of all electrolytics , ceramics and film
swapped out most of the resistors , some are precise resistors like 825R or 4k75 , those i dont have on hand and were left in.
spray to cool down components

without ordering or even tracing down parts i´m stuck  .
even then , the symptoms may prevail cause the reason for that static noise is the PCB itself that got slightly conductive .

I've had bad input pots and trimmers cause these issues....

I've even had a bad power strip from the wall create some weird stuff for some reason when troubleshooting  depending on how things were plugged in.... revealing it was something else all together....

.... Does the freezing really indicate if transistors are suspect? I'm just asking....



What amp is it???

 

[nashkato]

I've had bad input pots and trimmers cause these issues....

i can exclude input pots : with the input to the amp board disconnected symptoms stay .
i suspected trimpots as well , would have to order some . however when turning the trimmers powered on the crackling stays .

I've even had a bad power strip from the wall create some weird stuff for some reason when troubleshooting  depending on how things were plugged in

doesn´t matter where i plug it in , or if i use Triac , also the symptoms would show on both channels i suppose

.... Does the freezing really indicate if transistors are suspect? I'm just asking....

that was just a guess cause symptoms most of the time showed about 10-30 seconds after it was first turned on .

What amp is it???

Solton Craaft CA800

 

[NadMk]

If you have issues on both channels, start digging around the PSU. If just one channel is affected, the problem may be just there.
Use an oscilloscope to see if the power rails are smooth.
Also, it may have an random arc somewhere.

What happens if, when the problem is present, you turn it Off and On again? Does the noise dissapear for another few seconds? wich would mean that a capacitor has to be charged to have the noise, or the noise is right there, wich would point that some heat is needed for the noise to be present.

If it has transistors screwed to chassis (or something like that) try unscrewing one by one ensureing that the "under test" one is not touching any metallic thing. Be carefull with heat..!!

 

[nashkato]

     

If you have issues on both channels, start digging around the PSU. If just one channel is affected, the problem may be just there.

first post :

symptom : one side started to crackle ...

... PA amp with kind of separate powersupply .

What happens if, when the problem is present, you turn it Off and On again? Does the noise dissapear for another few seconds?  wich would mean that a capacitor has to be charged to have the noise, or the noise is right there, wich would point that some heat is needed for the noise to be present.

No , it doesn´t . That´s why i went ahead with the freezing . i could have calculated T and at least excluded some caps
(interstingly i inserted the greek T symbol and wasn´t able to post the reply : "message body was left empty. )

If it has transistors screwed to chassis (or something like that) try unscrewing one by one ensureing that the "under test" one is not touching any metallic thing.

indeed that´s what makes working on this amp kind of PitA , TO3 housing with  one screw each  making the connection of the source (housing) to the pcb . in order to get to the solderside one needs to desolder the powermosfets

 

[analogguru]

.....
indeed that´s what makes working on this amp kind of PitA , TO3 housing with  one screw each  making the connection of the source (housing) to the pcb . in order to get to the solderside one needs to desolder the powermosfets

Yes, yes.... this is good old czechoslovakian handcraft..... tons of screws.  But if this is too complicated  for you, I am offering you EUR 50,-- for the broken one.  BTW, here you can buy a working CA800 for EUR 200,--:
https://www.willhaben.at/iad/kaufen-und-verkaufen/d/pa-endstufe-aa-craaft-ca800-257751631/

And yes, these amps are identical to the Hughes & Kettner HA880.  Solton(Craaft) was the one who let the amps be build in good old czechoslovakia (Solton sent them critical components e.g. the semiconductors) and the H&K HA880 only got a different front panel.

 

[nashkato]

solved

 

[scott2000]

Was it the flim flam valve?? ;D

 

[nashkato]

Was it the flim flam valve?? ;D

not a native speaker here , so , sorry , ´m not getting what you´re trying to say , whatever , nope , was a driver transistor .
swear it measured okay , but it didn´t , guess just needed some time away  , easy fix actually , dont know .
most annoying , besides the fact that i wasted way too many hours for THAT ! ....., after all what i´d done to that right channel , the left one sounds quite crappy in comparison  , have to rework that one , too , dough ! 

... good old czechoslovakian handcraft.

yup , speaker outputs on male XLR

 

[scott2000]

Yeah... I can't count how many times I've read that some just shotgun replace  transistors ...inputs..... if it's not too difficult to get to and source them.

It almost seems like a maintenance thing on par with replacing electro caps with some of these pieces depending on what's in them and how they've been used.........


Thanks for the update!!!