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HK 430 pulsing idle bias (oscillation?) Harman Kardon 430

K

kmatch

Member
I’ve got an HK 430 on the bench that is acting strange with a pulsing idle when powered up. It came in with a bad left channel with a bad Q416, burned R424 and one output was not clamped down and may have overheated due to a missing bolt. Strange.

Replaced the bad parts and two outputs. I pre-emptively replaced Q421/Q422 (2sc458) with KSC1815 (NPN, 50V, 150mA). On the pre-amp, I replaced all six 2SC1344 with KSC1815 (and the popping noise disappeared).

Running on the 200W dim bulb with Speakers attached, the idle current oscillates several times before it settles. It takes about 20 seconds to finally settle. The idle voltage (across one of the emitter resistors) pulses three or four times every four or five seconds, with peaks at 300mV, 140mV, 50 mV, then to 20 mV (dropping to zero between peaks).

This happens on both channels.

other notes:
If I just power one channel’s B+/B- with the fuses, it still happens on the powered channel.

Doesn’t seem to pulse when speakers are disconnected.

The B4+ voltages rises to +24V at around 15 seconds.

The B+/B- supplies oscillate and droop during these pulses. I hear several waves of hum in the speakers as it pulses (modest to low volume).

Observation was the same when using a 100W dim bulb, but could see some light as it pulses. Don’t see the light with the 200W dim bulb. (I haven’t tried my 300 W dim bulb yet.)

Pulsing also happens when bringing up with a Variac + dim bulb. Idle is practically zero mV until almost reaching 110VAC, then the oscillation starts.

Question:
Is this normal for this circuit on power up? Is the dim bulb causing my problems? Did I use the wrong replacement transistor for Q421/422?

I’m really afraid to turn this one loose direct at the full line AC voltage until I understand what’s going on here.
 
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I suspect the problem you are seeing is a function of running speakers with the amp on DBT. This can throw the amp into oscillation.

Also, why are you using such high wattage bulbs? Isn't that a low-powered amp?

What did you use for output transistors?
 
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I was mainly running on 100W dim bulb and saw the issue. Out of concern that the dim bulb was causing the bias oscillations, I upped the bulb to 200W and observed the same issue.

I replaced outputs in one channel with MJ15015G. The other channel is original parts. Oscillation is seen on both channels.
 
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Minor update. I grounded the power amp inputs and the biases come up real stable without oscillation.

Perhaps these six KSC1815 I installed are not happy in the preamp?
 
Re 1815s...I think ksc1845s are the recommended option, but I can't see how the 1815s would cause problems...especially the problems you are seeing.
 
Only thing I see is the gain bandwidth of the original 2sc1344 is spec of 230 MHz while these KSC1815 have a min of 80MHz.

Perhaps BC546C are even better replacements, with gain bandwidth of 300 MHz typical.

I’ve also read that adding some base-collector capacitance may help improve stability, but haven’t tried it yet.
 
Sorry for not being clear...I was referring to the OP's use of 2SC1815s to replace the 2SC1344s.
 
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Interesting. I was recently looking for a sub for the 1344....kept finding recommendations for the 2SC1845. Wish that I had found that one instead.

I apologize for the slight thread derail, but do you also have a good sub for the 2SC1775?
 
I replaced all six tone transistors with BC548C and the same effect was observed with the idle currents going up and down, oscillating a few times until they settle. Taking about 20 seconds or so to get to a good place. (100W dim bulb and with speakers connected). note: It powers up smoothly with no speakers connected or with power amp input grounded.

So I put the factory original transistors back in and the same observation (with dim bulb). Pulled fuses to leave just the right channel operating and connected directly to the line and same idle current oscillations.

So powered up both channels direct to the AC line and same oscillation observed. Still not sure if this is a design issue or something specific with my unit. Seems like some kind of feedback due to the power supplies. It happens on both channels even though the main power amps have separate supplies.


I notice that the preamp B4+ takes 15-20 seconds to come up. It seems pretty linearly increasing to me but takes a while.

During the oscillation I see the main power amp
Supply voltages droop, suggesting to me somehow this may be feeding back to the preamp and causing the long cycle oscillations.

One other note: I hear some hum/buzz upon power up.

Anyway, it seems to be powering up even with these oscillations and it stabilizes after a few of them. Seems to be operating ok even with this.

If anyone has other ideas, I’m all ears.
 
kmatch said:
I replaced all six tone transistors with BC548C and the same effect was observed with the idle currents going up and down, oscillating a few times until they settle. Taking about 20 seconds or so to get to a good place. (100W dim bulb and with speakers connected). note: It powers up smoothly with no speakers connected or with power amp input grounded.

So I put the factory original transistors back in and the same observation (with dim bulb). Pulled fuses to leave just the right channel operating and connected directly to the line and same idle current oscillations.

So powered up both channels direct to the AC line and same oscillation observed. Still not sure if this is a design issue or something specific with my unit. Seems like some kind of feedback due to the power supplies. It happens on both channels even though the main power amps have separate supplies.


I notice that the preamp B4+ takes 15-20 seconds to come up. It seems pretty linearly increasing to me but takes a while.

During the oscillation I see the main power amp
Supply voltages droop, suggesting to me somehow this may be feeding back to the preamp and causing the long cycle oscillations.

One other note: I hear some hum/buzz upon power up.

Anyway, it seems to be powering up even with these oscillations and it stabilizes after a few of them. Seems to be operating ok even with this.

If anyone has other ideas, I’m all ears.
Click to expand...

Two things. One. Does the amp have a signal delay built into it? Meaning the volume will reach peak a few seconds after power up? Like it is warming up??
It could be a reason for the oscillations. Second is I chased a noise in my preoutput board after a recap and a couple transistors. It caused me to dig for a problem which showed itself using the EQ defeat button. Never was able to find the cause but. After letting the unit play for a number of hours it mystically disappeared. The switch works normally. The DBT could be the cause so eliminate it. These are oscillations you are seeing?? Maybe they will not affect the play and can be forgotten. Cant hurt to try.
 
The tone/preamp supply (B4+) has a delay built in (RC delay 4.7 kOhm * 1000 uF = 5 sec) and it takes about 15 seconds to power up to its peak voltage.


Overall these oscillations die out after three or four cycles (total of about 20 seconds) and then the idle goes to its normal happy place. It plays music just fine through these cycles, with just a small hum on one of the first cycles. After settling out the idle stays constant during operation.

Also, this oscillation only happens when speakers are attached. If I turn off the speakers, it goes to “normal” idle directly.
Also, if I ground the input to the power amp, the idle goes straight to its “normal” idle.

I’m chalking this up to some oscillations among the power supplies and the amplifier stages. As best as I can tell it seems to be a consequence of the design rather than a bad component. But if others have a different observation I’d be glad to hear.

I was watching this unit really close because I had to change the outputs on one channel. Also since it doesn’t have a protection relay I could hear the hum on the audio that made me pay more attention to what was going on. Perhaps other units and designs do the same idle current oscillations but maybe I’ve never looked close enough?
 
kmatch said:
Also since it doesn’t have a protection relay I could hear the hum on the audio that made me pay more attention to what was going on
Click to expand...

The hum has a source to find. Is there a frequency to it, is it on all inputs selected, are there other filters used or not used that change it? Same with the oscillations, they should have a frequency you can pinpoint. They may just go away in time. call it burn-in but my experience is analogue components can improve with play. Tune in a radio station and just let it play for a few days or more. Low volume is fine. I will heat mine up from time to time. Work the components with current. is this a class B amp?
 
Have you replaced the electrolytic caps in the power supplies?

How about test-replaced the 2sc1212 transistors and Zener diodes in the B4 and B3 power supplies?
 
No replacement of caps but tested caps for capacitance and ESR and look ok.

Tested B3 and B4 transistors for Base to Emitter and Collector to Base junctions and resistance Collector to Emitter and checks all ok.

Did not replace zeners.

Unit is no longer in my possession so no more experimenting to be done on this one. But definitely eager to get another one in hand to see if it behaves the same!
 
kmatch said:
No replacement of caps but tested caps for capacitance and ESR and look ok.

Tested B3 and B4 transistors for Base to Emitter and Collector to Base junctions and resistance Collector to Emitter and checks all ok.

Did not replace zeners.
Click to expand...

Apart from the dual power supplies, the hk 430 is nearly identical to the 330c. I’ve never seen this behavior in a 330c… it sounds almost like a cap or transistor failing until warm… or a soft start circuit not charging up smoothly… so, since this unit doesn’t have a soft start, I stretched a bit and thought a marginal, nearly failing power supply might behave the same and deliver spotty power when first filling up or warming up.

I hear you on unit testing the power supply components; I wonder, though, what the voltage looked like coming out of them at turn on… on the way to the pre/tone board and amp.
 
The input to the B4 supply (the rectifier) comes up immediately. The Regulated B4 voltage rose slowly taking about 15 seconds to reach 24 Volts.

It seems like this is putting some DC out of the tone board that is getting amplified in the power stage. This is confirmed the interaction of the tone board because if I ground the main amp inputs the oscillation is absent.

Also it’s strange that it only occurs when speakers are connected. Maybe putting some load on the outputs causes more droop on the main amp supplies that somehow causes the oscillation.

All I can guess is that it’s some consequence of the slow B4 rise (by design) and the droop of the main amp supplies oscillating when the tone amp powers up.
 
Is this a split rail regulated supply? If those come up unevenly you might get some funny behavior like this.

Its probably just passing signal, not really oscillating. The bias reading will climb as the output stage does work.
 
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