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Need some input on Dim bulb tester behavior

A looong overdue update on this:

Finally got a hold of a pair of original Driver transistors (2SB647AC and 2SD667AC) and also bought some Chinese copies to compare, they look to measure about the same, higher hfe on the chinese ones but within spec.
Installed the original Drivers and no shorts.
Next I´ve hunted a long time for the FJP1943OTU and FJP5200OTU for the blown output transistors but could finally only find the FJP1943RTU and FJP5200RTU version from a reliable source, these have lower hfe range but measured around 70hfe, took a chance, I hope it is not totally out of line?

Installed them using the old batwing transistors as a clamp as seen elsewhere.

Success!
The unit sounds really good, the Bias does take a long time to settle (like 30 min) maybe because the new transistors but can live with that.
Big Thanks to all that contributed, especially @elnaldo for hanging in there even though my complete amateurishness.
 
Hello
Thanks for detailing the the long back and forth with this and leave me something to look at for my HK 560. I don't want to hijack this thread too much so I'll attempt to be brief. I'm having the exact same behavior with my HK 560 and I have yet to figure it out.

In reading, and re reading, and reading this again I want to summarize what I understand that you did to get this working so that I can apply my leanings to the same issue.

1 - You were not able to zone in exactly on a conclusive voltage test that identified the failed component that was causing the bias to not remain stable
2 - You, after the unfortunate testing issue (don't feel bad I blew the tops off of several transistors if you look at my post:
https://audiokarma.org/forums/index.php?threads/hk-560-blown-transistor-blues.1055694/#post-17691112), replaced the following:

Q421,4222SD588A
Q423, 4242SB618A
Q415, 4162SC1472(K)
Q417,4182SD667A(C )
Q419,4202SB647A(C )


Q421,422 / Q423,424 replaced with FJP1943RTU and FJP5200RTU
Q415 / Q416 replaced with BC517-D74Z
Q417,418 / Q419,420 replaced with new old stock 2SD667A / 2SB647A


These replacements ultimately resolved the unstable bias issues.

As always thanks for the help.
 
orshad2048 said:
Hello
Thanks for detailing the the long back and forth with this and leave me something to look at for my HK 560.
Click to expand...
Hello @orshad2048, sorry for the late reply, must've missed the notifications.

It is a very fine sounding unit when working so do understand that you want to get it up and running.
The unit has moved on to a new owner since I got my hands on the biggest one in the series the HK670.

I´ve had two HK450´s and one HK560 and from my amateur view there seems to be something about this series bias design that is flaw because at least two of them have displayed problems with Bias tracking and/or blown outputs.

I gave your thread a read, and just to clarify I am not an experienced tech at all, I only solved some problems thanks to the generous Techs here at AK, so I'm afraid I might not be able to give you too as much assistance as one would want..

As you might now know, the dimming down at the start and slowly getting a bit brighter while on DBT is because of this units "Soft start" circuit to prevent turn on Thump, so nothing to worry about if everything else is working correctly.

orshad2048 said:
Q421,422 / Q423,424 replaced with FJP1943RTU and FJP5200RTU
Q415 / Q416 replaced with BC517-D74Z
Q417,418 / Q419,420 replaced with new old stock 2SD667A / 2SB647A


These replacements ultimately resolved the unstable bias issues.
Click to expand...
Correct + the bias trimpot.
In this case mainly the Bias transistor solved it, However the bias took a very long time to climb up and settle at approx 32mv (about 30min) compared to a healthy unit.
I compared the new old stock 2SD667A/2SB647A to some Chinese copies I bought and it the Chinese ones measured about the same if you want to go that route, they were very cheap.
The Main amp transistors are not 100% ideal but seemed to work fine, the OTU version is preferable.

The advice I've gotten from a retired HK tech here in Sweden a while back was to look for cold solder joints around Q407,Q409, Q411, Q413.
But it seems like you already been over those?

Another possibility could be the bias Diode D407/408 which is a MV12Y Varistor and that one has a bad reputation, if you look at my last post about my HK450 you can get some guidance on replacing and extending the leads on the new ones:

Almost finished a Receiver repair, then disaster strikes. What to do?

Just read previous posts again and realized I've might got this wrong, discrepancy in Voltage drop over resistors doesn't mean that the resistor itself is bad, just an indication of something drawing more current ahead from the resistor, correct? Maybe one of the old Q506 / Q505 2SA844´s are...
audiokarma.org

My knowledge is limited so my advice kind of ends here, fingers crossed that it might steer you on a path to get it fixed.
I really hope you get it up and running and I will follow your thread for any progress, these units are tricky but worth the effort in the end:)
 
Thanks for the read of my thread and the reply.

I don't want to hijack this thread anymore then I already have so I'll post the response with all the technical details on the tread that I have going so look there for those details.

A big thank you to you again for taking a look and responding.

I do have a question about where you located the new old stock 2SD667A/2SB647A?
 
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