HK 450 with very low Bias, should I be concerned?

[Skywatcher]

Thank you.
Set both the BIAS and DC offset yesterday, however neither the BIAS or DC offset would settle down and become steady, constantly bouncing around up and down even after 10-20 min monitoring (between acceptable values though)..
I know from the initial test I did before I touched any of the trim pots that Both would settle down reasonably well.
Is this a sign of aging trimpots?



Appreciate all the extra info/explanation you are throwing in, really makes it a whole lot comprehensible.
Gave it a listen yesterday, the sound has become more balanced between Left and Right and clear for sure, but maybe less coherent, like mid and highs don't Jive and it´s also missing some of it´s bass & depth I experienced before, or maybe I just became accustomed to crossover distortion...
I´ll give it a couple of days to get used to.

DC Offset usually does drift around somewhat so that's nothing to worry about. The BIAS however shouldn't be bouncing around. Although a slow steady rise and fall over a few minutes is quite normal. Just blowing on transistors will change readings you get so avoid any "drafts" when doing adjustments. Like @Binkman mentioned, it is possible you didn't have quite as solid a connection as before. Leads do oxidize and sometimes enough to give fluctuating readings. I always unplug, and reinsert my leads into my meters to be sure they have a "clean" connection. When I clip onto something, I give them a bit of a "wiggle" to be sure the connection is solid, then power up the unit. If it were me, I would most definitely replace the BIAS pots at a minimum, and even the DC Offset pots as well. They are well known to go "intermittent" with age, and some outright fail. The carbon track ages and drifts higher in resistance over time. Many people don't realize it, but trim pots have a limited cycle lifespan for how many times they can be adjusted. Each "turn" forward and back becomes one cycle and many trim pots have a lifespan of somewhere around 100 cycles. And being this old, you have no clue how many cycles they have been through before you touched them. I cringe every time I read someone used DeOxit and "turned them many times" to clean them. So if you try again to read the BIAS, and are sure your leads have a solid connection, and the BIAS is unsteady and constantly fluctuating, then I would replace the trim pots for sure.

 

[Donguri]

how much? 1-2mv? as long as it hovers at spec. but keep an eye on it. I oft. see meter fluc. wiggling my test mini grabbers.

The BIAS bounces mostly between 1-2 mV (32mV-34mV) but at short times down to 31,4 and up to 34.3 mV.
The DC offset ranges -6mV to 5,6mV in a quite irregular manner not that slow, other Amps I've tried mostly settles down to a value after 10-15 min

DC Offset usually does drift around somewhat so that's nothing to worry about. The BIAS however shouldn't be bouncing around. Although a slow steady rise and fall over a few minutes is quite normal. Just blowing on transistors will change readings you get so avoid any "drafts" when doing adjustments. Like @Binkman mentioned, it is possible you didn't have quite as solid a connection as before. Leads do oxidize and sometimes enough to give fluctuating readings. I always unplug, and reinsert my leads into my meters to be sure they have a "clean" connection. When I clip onto something, I give them a bit of a "wiggle" to be sure the connection is solid, then power up the unit. If it were me, I would most definitely replace the BIAS pots at a minimum, and even the DC Offset pots as well. They are well known to go "intermittent" with age, and some outright fail. The carbon track ages and drifts higher in resistance over time. Many people don't realize it, but trim pots have a limited cycle lifespan for how many times they can be adjusted. Each "turn" forward and back becomes one cycle and many trim pots have a lifespan of somewhere around 100 cycles. And being this old, you have no clue how many cycles they have been through before you touched them. I cringe every time I read someone used DeOxit and "turned them many times" to clean them. So if you try again to read the BIAS, and are sure your leads have a solid connection, and the BIAS is unsteady and constantly fluctuating, then I would replace the trim pots for sure.

OK! I will give it another go tomorrow and follow your advices, this time I'm thinking of connecting directly to the emitters of the output resistors as instructed by the Service manual, the emitter resistors were a bit tricky to get a good grip on. My leads are new but I´ll try unplug and reinsert them.
Never tried to replace trim pots but I´ll put an order on Mouser and see if I can give it a go, thanks to the posts here I kind of know what to look for.
At the same time I think I´ll order some matched output resistors, I actually do notice some difference in sound between the repaired one and the other.

 

[Donguri]

So went ahead yesterday, seems like I got better grip around the Output transistors, managed to get a more stable BIAS measurement around spec, also put a couple of drops of Deoxit on the trim pots just in case but stayed away from Rocking them around the clock

DC offset however is moving up and down quite fast not settling down, so the trim pots gotta go I believe.
Noticed at power up a big inrush (?) of voltage on the DC offset, initially going from over around 1V for a couple seconds and dropping a couple of hundreds mV and after some time down to round Spec, is this ok for the speakers to receive that high amount?

 

[Skywatcher]

So went ahead yesterday, seems like I got better grip around the Output transistors, managed to get a more stable BIAS measurement around spec, also put a couple of drops of Deoxit on the trim pots just in case but stayed away from Rocking them around the clock

DC offset however is moving up and down quite fast not settling down, so the trim pots gotta go I believe.
Noticed at power up a big inrush (?) of voltage on the DC offset, initially going from over around 1V for a couple seconds and dropping a couple of hundreds mV and after some time down to round Spec, is this ok for the speakers to receive that high amount?

That is pretty "normal" believe it or not. There is very little current so it won't hurt the speakers. How much DC offset is it fluctuating by? If it is less than +/- 10mV it's fine and pretty normal. But if it is bouncing a lot higher, the input transistors may be beginning to go wonky. Leaky (internally) old electrolytic caps can cause fluctuations too. Especially if many are the old "Elna Gray Bombs" that are known to have issues at 40+ years old.
On Edit; The old main filter capacitors can also cause a huge DC offset until they get charged up. I would be recapping the unit if it were mine and planning on keeping it.

 

[Donguri]

That is pretty "normal" believe it or not. There is very little current so it won't hurt the speakers. How much DC offset is it fluctuating by? If it is less than +/- 10mV it's fine and pretty normal. But if it is bouncing a lot higher, the input transistors may be beginning to go wonky. Leaky (internally) old electrolytic caps can cause fluctuations too. Especially if many are the old "Elna Gray Bombs" that are known to have issues at 40+ years old.
On Edit; The old main filter capacitors can also cause a huge DC offset until they get charged up. I would be recapping the unit if it were mine and planning on keeping it.

Interesting, thank you. Well it fluctuates +/- 10mV so then I can write it off as being a problem, great!
Hah, Didn´t know that the grey Elnas went under that nickname, kind of cool, and yes there are a bunch of them in there...
I would have love a recap and keep it and wish this was a finishing post..

However, something doesn´t seem right. I´m suspecting that my attempts of setting the BIAS haven´t been correct or optimal.
Spent some listening time and firstly I noticed that it doesn´t entice me as before, like some magic, motion and bigger soundstage has disappeared, before it was a favourite of mine but now just meh, a plain experience. It should be the opposite way after changing the BIAS to spec correct?

Secondly and a bit worriesome, it started off with better balance between channels I thought but after a while I noticed that the left channel played weaker and lost some "energy" (is this possible after some time of warmup that the outputs loose energy because of overheat?) so was forced to move the balance knob to about 10:00 to gain a kind of coherant sound.
It has always been a little unbalanced but not by this much. Maybe setting it to spec revealed and opened up a can of underlying problems.
My plan is to turn back the BIAS to half and then turn off, wait for it to cool down and set the BIAS from cold as the SM states and see what happens.