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HK 730 with Odd DC Offset problem

xrayspex77

xrayspex77

Super Member
HK 730 has High DC Offset on the L Channel for the 1st 60 secs after Power On.
It seems to be partly related to the state of charge of the Main Caps.
Readings are from the Speaker Terminals.
If the Main Caps are fully discharged the sequence is
-> 4-5 Secs then 16V which drops until after 60 secs reads 2mV.
If there is some charge in the Main Caps the sequence is
-> 4-5 Secs then 6V which after 60 secs has dropped to 2mV.
If I cycle the Power after 60 secs the DC Offset peaks about 10mV and drops back to 2mV.

I have replaced the Q402,Q404, Q406 and Q408, Q412, with the Subs suggested in other threads. I have also replaced all the Electrolytics on the L Amp Board.

The R Amp Board is untouched and DC is in the acceptable range though I’ll probably recap it.

My Audio Tech knowledge is still very limited, I might just replace Q410, Q414 & Q416 but I’m wondering if there’s anything else I should check. I would guess that at least one transistor is “stuck” open but no idea how to check that.

The 4 sec delay before the 16V spike suggests the Slow Start is working but would appreciate guidance.

These threads helped me to get to this point. It does sound fine once it settles down but counting to 60 before I engage the Speaker Switch is a little too much :)


http://audiokarma.org/forums/index.php?threads/harmon-kardon-730-high-dc-offset.552435/

http://audiokarma.org/forums/index.php?threads/harman-kardon-730-issues.416189/

http://www.audiokarma.org/forums/index.php?threads/left-channel-issues-with-hk-730.372163/

EDIT

I was wrong about the state of charge of the Main Caps. The L Caps take about 24 Hours to drain (The R take about an Hour), this was measured on the +B1 Pin on the Connector. This may not have anything to do with the Issue but not sure why it takes 23 hours longer for the L Caps to discharge.
 
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Rail Voltage is 39.7. Just checked with the pins on the PS Board with the Amp Board removed. B1 is the L and I got +-41.8V checked B2 the R and got +-41.3V. Not sure if this is related but 10 mins later with it unplugged I still get 41.5V on B1, 0.8V on B2. I did replace the Main Caps 4700uf with 6200uf

I replaced Q402,Q404, Q406 and Q408, Q412 over a year ago, it's been out of rotation because I got tired of waiting 60 secs for it to be stable. I replaced the Transistors after a TT with a bad RCA Cable below all the PS Fuses and apparently took out some of the L Channel. I was getting 40V at the Speaker Terminals which would drop to around 100mV after 5 mins so replacing Q402/4/6/8/12 has helped. I think I tested the Output Transistors afterwards and was getting very close to 39.7V. Think I've just proved my lack of knowledge
 
Well I guess I'll be shotgunning the rest of the Left Amp

I do have subs for

MJE243G for the 2SD415
MJE253G for the 2SB549

From another thread sub for the 2SC853 http://audiokarma.org/forums/index.php?threads/marantz-varistors.445501/#post-5720179

As far as a replacement for the 2SC853, Digikey has some Panasonic 2SC39390SA devices left in stock that would work well (#2SC39390SACT-ND). (Digi still has these)

From a search a 1N4148 for the 1S1211

and will probably replace the Trimpots while I'm at it.

I'll order parts this week and hope that fixes it cause I know a some point I will forget to wait a minute before engaging the Speaker Switch
 
I have a 330c with high offset for about 30 seconds after startup, and then it styles back down. Did you ever solve the issue with your 730?
 
The KSC2690A/KSA1220A combo is a better substitute for the 2SD415/2SB549. You might check to see if those are available.
 
Didn't get around to ordering parts until last week so it's been sitting since May. I plugged it in and the issue seemed to have improved. Slow start was still too fast on the L Channel, it came on about 2 sec before the R but but V peak varied from less than 100mV to 2V. The R Channel has been good all along. It seemed that the problem was with the Slow Start so I replaced R406 (1M) although it tested good out of circuit, in circuit it tested at about .5M vs 1.1M for R405 on the R amp. After doing that the original problem came back, it varies but up to 30VDC not mV, to the speaker terminals which settles down to about 9mV after about 60 secs. I was still getting V on C406 after powering down which is why I thought R406 was the problem, now I think it was because it was pulling V from the Main Caps.

So I removed the Power Amp board and checked the B+/- (Pins 4/5) on the Main Rectifier PCB

R Channel from Power on climbs to 44V and starts dropping immediately after Power Off to .5V

L Channel from Power on climbs to 44V and stays there after Power Off

So what is supposed to discharge the Main Caps? There's no Resistor in the Main Rectifier circuit.

I think what's happening is that with the Main Caps fully charged Powered off, C406 in the Slow Start Circuit is pulling at least a partial charge from the Main Caps so when Powered On, the slow start is not happening because C406 is already charged and with the Main Caps already charged a large DC Spike is going to the Speaker Terminals.
 
Old tread and don't know if you solved this issue, but I had the same problem with an 730.

The right channel is discharging faster as the preamp is driven from the same power supply as the power amp.
The reason for the huge DC offset at start is probably due to the main caps for B+ and B- is deferentially charged at startup (if not fully discharged) causing imbalance between B+ and B- during the first seconds at start up.

I put bleeder resistors cross all the main caps so the discharged fully in a few minutes, that solved the problem.
 
Old thread but ... . Have you done it like this?

+B electrolytic capacitor: resistor from +B → ground

–B electrolytic capacitor: resistor from –B → ground

Can you show me how you did it in your 730 please?
 
xrayspex77 said:
I upped the main Caps to 6000uf. I used a Capacitor Discharge Calculator to try to balance how aggressive I wanted to be

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html

I went with 3.3K 2W. You want to use at least 2W to dissipate the heat
Click to expand...
Old thread but ... . Have you done it like this?

+B electrolytic capacitor: resistor from +B → ground

–B electrolytic capacitor: resistor from –B → ground

Can you show me how you did it in your 730 please?
 
I just put the Bleeders across the +- terminals of the Caps with some tubing to give them space to dissipate heat and help keep them away from wiring etc.
I'm not knowledgeable enough to know if
ikarus75 said:
+B electrolytic capacitor: resistor from +B → ground

–B electrolytic capacitor: resistor from –B → ground
Click to expand...
has any advantage

My 730 is now with a Realtive about a 5hour drive away so I can't take photos
 
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