hardman Kardon..... experts

[sakis]

here is a hardman with some weird behaviour

the unit is in mint condition everything is fine all cps are replaced inside but offset doesnt want to cope in any possible way

multiturns are installed ofset can be easilly set to zero but in any temperature variation offset rides to a100 or 200 or 300 millivolt or more

hardman did some excersize on thermal junction actually bolting tranistors in some plastic to ensure full contact and thus balance of temperature between the two semis ....

though it seems that long term stability was effected since a lot of temperature rised inside the bolt resulting to thermal stress of soldering

there is a picture of the schematic and pictures of the thermal junction

Q422-426 getting quiet hot and touching the a bit is enough to alter ofset dramatically

any ideas ???

[sakis]

checked all voltages it is with in 10% according to the schematics everywhere

that is strange eee

[sakis]

temperature over transistors 426-422 is almost 65 degrees ...that is way too much i think

[spind]

Originally Posted by sakis

temperature over transistors 426-422 is almost 65 degrees ...that is way too much i think

Have you checked the value of all the fusible (safety) resistors in this amp? Maybe one (or more) of them has drifted in value.

Steve.

[sakis]

yes i did ...also i have checked all voltages according to the schematic and all seems fine ... also replaced a few resistors that looked susspicious but made no diference

[Trevor White]

Harman Kardons are prone to offset drifting because they use low over all global feedback.

You can set the offset at the correct level and then a few minutes later it may have drifted a few tens of millivolts. This is perfectly normal with this type of design. Also they are biased to run fairly warm.

regards
Trevor

[jaycee]

seems silly to me that they used 2xTO-92L devices instead of a TO-126

[sakis]

thanks for your input ...yes it could be ok with a few millivolts variations though like 300 millivolt plus or minus is too much

yet again 65 degrees over the 426-422 tranistors is also too much ... there is got to be something else wrong

as about the bias ...service manual says 30 millivolt over the emmiter resistors ...that is equal to nothing i presume ....also main heatsink stays relativelly cold only these tranistors are boilig

thanks

[sakis]

if any body cares to help or see there is a strange protection circuit that i ve never seen before ... and i thought that it might be a good idea to remove it to see if this is effecting the offset stabilty ... as you can see there is two troubles ...one is the ofset stability and the other is the overheating of the transitors

here is the protection

[jaycee]

i could see perhaps a faulty q418 or a fault in the protection circuit messing with it. might be worth pulling that temporarily?

what was the original fault?