HK490i restoration going nowhere fast

[bguzz]

Hi. I’ve got an HK490i receiver that I’m finally restoring. This is my first restoration. This forum is great...I’ve learned a lot so far.

It works in general, but the right channel sounds lousy compared to the left. Not much bass and king of mid-rangy. Thus far I’ve recapped all electrolytics on the power amp board, tone board and equalizer board. I’ve also replace the bass pot and deoxit’d all the rest of the pots and switches.

I’ve done the dc offset adjustment per the manual and it was jumping all over between 0 and 75mv.

The only other odd thing is that when I power it on, I no longer hear a relay click like I used to...it would happen a few seconds after the power switch was activated.

I’m kind of stumped as to what to do next. Nothing else visually looks like it’s failed. I don’t have an oscilloscope, should I start trying to replace transistors? I hate to keep just replacing parts. I figured the caps were likely getting aged, so that wasn’t a big deal.

 

[mbz]

Most probable scenario is either failure in the protection circuit or
excessive dc voltage on the speaker line, so the amp has gone into "protect"
mode. Note, service manual from hifiengine does not show a protection relay,
possibly a late production change.

If you are comfortable working on a live amp (lethal voltages inside) then
measure the dc voltages at test point TP1 to GND/chassis. repeat for TP2.
See pic. TP1&2 or on the main board with the power transistors/large
heat sink. Appears to be a WHITE (speaker) wire coming from TP1, likewise
a RED wire from TP2. Would be good to confirm the amp has a protection
relay, it should be located at the other end of the WHITE and RED wires.

 

[xrayspex77]

I have a 495i and had a 330i and 395i, HK wasn't using relay's at that time.
If there is a Relay then someone added it.
The tiny switches are frequently the source of problems on those, especially the Tape Monitor.
It's hard to get DeOx into them. Does pushing the Tape Monitor off/on a few times make any change in the sound?

 

[bguzz]

Thanks for the great diagram!

To clarify, i cannot find any relay, so I must be mistaken. But what I have experienced is when you power on the receiver, a couple seconds go by and then I hear a click of sorts and sound starts playing...not sure if it could be something else doing that.

I did test TP1 and TP2. All was done with Aux/DAD selected, bass and treble defeated and volume all the way down.

TP1 was bouncing all over from negative 10mv to positive 50 mv. TP2 was pretty stable at about positive 95 mv. Interesting thing is when I try moving the bass pot, the TP2 would temporarily go to almost 2 volts. I'm guessing this is bad.

 

[bguzz]

I’m guessing the high voltage is problematic. Would the 4 heatsinked transistors be the first to replace to see if that’s the issue? I’m guessing it’s a transistor...though a shorted diode could be it too. Any suggestions on next steps?

Another thing I’ve noticed is that there is a constant crackle/noise in the background...it’s not loud, but it’s there.

 

[mbz]

If I understand correctly, there is no protection relay
Q405,6 and Q10,,, perform soft start?

and the "right channel sounds lousy compared to the left"

DC offset bounces between -10mV to 75mV (left)
90mV (right), +2Vdc if bass pot moved

Seems like you have multiple issues,
- the right channel sound quality is probably dirty switches
xrayspex77 post#4 who suggested pushing the tape monitor
switch a few times. Suggest trying all switches, levers, knobs

- the dc offset is lousy on both channels. Part of the problem may
be cheap trimmers eg, early NADs however I thought HK were reasonable?
Maybe replace trimmers with Bourn multi-turn types

You may need to replace the differential pair Q401,3(left) and
Q402,4(right) with a matched (hfe/Vbe) pair, this would be a pity since
the originals are nice transistors

Maybe post a pic of the dc offset trimmer, is it a cheapie or...
First step would be replace the trimmers if needed, maybe the offset
will stabilize

- You replaced the bass pot and now +2Vdc at TP2, are solder connections OK.
What did you replace it with (mouser part No?)

Have you checked the bias/idling as per the service manual?
Need other issues to be sorted first, added so you don't forget.

 

[bguzz]

The left channel is the lousy one--I mistakenly said right. I replaced the bass pot with a bourne from mouser (652-PDB182-K430K104A). The main reason was when turning the old one, it physically made a grinding noise...I tried to deoxit and didn't solve. I've actually deoxit'd all the switches and control pots and they seem to all be functioning well with no noise coming from any of them when activated. While I didn't measure the DC offset prior to the pot replacement, it sounds and does the exact same thing (woofers do a major "pulse" when adjusting the pot...i'm assuming that is the +2v spike).

I checked the bias/idle per the manual and where it should be between 0 and +-33mv, the right channel was maybe 40mv and the left was higher, like 70mv...adjusting did help a little, but only like 10mv or so.

In my measurements, should the dc be jumping around? or should it be a stable figure?

The transistors you refer to (2SC2240BL's) are obsolete now it looks like...any suggestions for replacement?

I can try replacing the trimmers, I've attached pics.

Thanks for helping BTW, I am really attached to this thing.

 

[ivandezande]

For sure replace the bias and offset trimmers.
2SC2240 > KSC1845
Just a heads up, I wouldn't use those FG caps in the amp section, it gets toasty. Use 105 rated caps, like VZ or PW.

I really like the hk series, and have restored a lot of them, but they tend to have had to get to areas and a lot of problems. Never could solve a 570i.

 

[bguzz]

Would bourns trimmers be a good choice...i found these at mouser 652-3386F-1-104LF

These are the transistors I think too...512-KSC1845FTA.

Are the trimmers in there cheap ones? I was assuming because they don't get adjusted much, that they wouldn't go bad easy...but I am a novice at this. Just trying to learn.

 

[mbz]

Trimmers look a small step up from NAD el-cheapo's. Suggests Bourns 3296 series, the 3296Y is probably best, may need to extend
the wiper leg with a bit of offcut wire. Check the datasheet to get wanted adjuster/leg orientation, don't rely on mouser graphic.

DC offset voltage should be reaonably stable, bias will creap up after power on, it certainly shouldn't jump about.

yep, KSC1845FTA, if you need to go down this path, ideally you should try and find a closely matched pair, require basic tester $10-
from china.

 

[bguzz]

Keeping you up to date on my journey. I replaced the bias and offset trimmers with bourns sealed units.

I was able to dial in the idle adjustment for both sides to exactly 33mv. I had no problem getting the right channel dc offset to about 7mv and it was steady. No luck on the left channel...it is bouncing in wide swings between -90mv and +180mv...not steady at all. I checked it repeatedly for about 1/2 hour.

Next I’m going to check and replace if necessary Q401,3 Left and Q402,4 Right. Since I would be replacing them with the KSC1845’s, do I need to do both sides? The right channel seems to be operating fine. I would like to focus just on the left side differentials if that makes sense. Why fix what isn’t broke, unless I will cause balance issues.
.

 

[mbz]

do I need to do both sides? The right channel seems to be operating fine. I would like to focus just on the left side differentials if that makes sense.

Agree, right channel is fine, no need for replacements.

Regarding the left channel, I'm a little concerned about the DC off-set bouncing around. Certainly this can be caused by a faulty
transistor in the diff pair, however it could be caused by a faultry transistor elsewhere, might even be something like a dry
solder joint. Will take a look at the schematic to see if can narrow problem down on lefty channel. I prefer to match both hfe
and Vbe on diff-pairs.

 

[mbz]

Yeah, it's probably the differential pair on the left channel however
it would be nice to get some proof, Can you try monitoring the dc voltage
at R409 and R411, both 1.8k, refer to pic for measurement point, black MM
probe connected to chassis. SM has it at 31.1Vdc but more interested if it
bounces around.

 

[bguzz]

I measured the two points. The R409 was at 31.1v and R411 was 32.2v. Both were rock steady, no jumping around at all. I’m thinking that is good news. I will try replacing the left channel differential pair tomorrow.

 

[mbz]

I measured the two points. The R409 was at 31.1v and R411 was 32.2v. Both were rock steady, no jumping around at all. I’m thinking that is good news. I will try replacing the left channel differential pair tomorrow.

Probably a bit beyond my pay scale, hopefully someone will correct me but if it was the diff pair then I would have expected the voltages
bounce around a little. Also, I would have thought the two voltages (31.1 & 32.2V) would have been closer...

If you are up for some more sleuth work then check the voltages at Q411b, Q413b, Q417b, Q419b and if needed Q421b,Q423b.
Looking for some bounce, also since we are talking about dc offset swings between -90 to +180mV it probably wont be visible on the
mentioned transistor bases. Please don't probe the transistor legs, find a nearby resistor or whatever connected to the same node.

 

[bguzz]

Ok, replaced Q401,3 and nothing changed. BTW, the original tranistors were mounted back to back with a thermal past between them and a plastic holder keeping them together. Should I do the same with the new ones? or is that not needed? or put the old ones back since they don't seem to be the problem?

Here's my voltage measurements of the transistor Base's...notice they are in Volts, not MV and where there is a range, there was some fluctuation.

Q411B 30.1V
Q413B 34.3 - 34.4V
Q417B 31.7 - 31.8V
Q419B 34.9 - 35V
Q421B 23.1V
Q423B 23.1 - 23.2V

As a reference point, Q422B was 419MV
and Q424B was 817MV Both are in milivolts and were steady.

 

[mbz]

Q401,3 were bonded together for better thermal tracking. Hopefully you replaced
them with a hfe/Vbe matched pair. Certainly test the removed transistors, you
will probably find they have drifted apart (Vbe, hfe) so your replacements are
probably better. I wouldn't unsolder the new TR's, more heat, more damage...

You have a "serious" problem on the left channel with almost 23Vdc on the spkr
line. Do not connect speakers, headphones, instant toast... This issue was
tested in post #4, TP1 & 2 voltages.

Q413b, 419b and 423b should be negative voltages, check if you overlooked the
negative or are all voltages positive.

 

[bguzz]

Yes, I didn’t note it correctly...all three were indeed negative voltages.

 

[mbz]

Was Q417b or Q421b negative. The measurements don't look as expected
for this fault. Can you redo these measurements.

 

[bguzz]

Just checked again. Q421 is -29.2 to -23 V fluctuating and Q417 is -31.8 to -31.9V fluctuating.