Harman Kardon HK430 refurbishment - DC offset problems

[NickArchdale]

Hi there, I hope someone can help please.

I'm restoring an HK430 twin PSU receiver and appear to be falling at the last hurdle. There was crackling and switch/pot noise so after some consultation here, I have replaced all the angular 2SC458, 2SC1344 & 2SC1345 transistors with new KSC1845 (from Farnell), replaced the 1S2076 diodes with new 1N4148 and recapped the equaliser, tone/pre-amp and main amp PCBAs - I have not touched the tuner and have no intention of doing so. Deoxit fixed all the controls.

Using a signal generator (1KHz sine on Aux) and a 'scope I examined the speaker outputs (no load) and all appeared well, no crackling and amplifying the sine wave nice & cleanly with no observable DC present, although it did "swing" up/down a bit when using the volume control but always settled which I guessed was ok (I have not worked on these before). So the final tweak was to set the bias per the manual but I could not get it anywhere near to 25mV so figured that, because I hadn't matched the KSC1845 gains, this might be the problem, especially the differential pairs Q401/403 & Q402/404.

So today I used my Peak Atlas DCA55 to select some new matched pairs with hfe=360 and replaced Q401-404. Now, with nothing else changed, I get massive (full rail) DC swings on power up with the left channel leading down then up then settling at 0V, the right channel following and heading to, and presumably staying at, the negative rail - I axed the power before finding out. I have now lost faith and need some guidance before I do some damage!

Thanks,

Nick (a newbie at this era but keen to learn)

 

[nicholas2381]

Can you adjust the bias at all? How far?

Double and triple check that the transistors you replaced are in the correct orientation. You’d be surprised how often I find I put one in backwards. If you find any that were in backwards, pull them and replace them rather than using the same ones.

 

[jhjove]

Using a signal generator (1KHz sine on Aux) and a 'scope I examined the speaker outputs (no load) and all appeared well, no crackling and amplifying the sine wave nice & cleanly with no observable DC present, although it did "swing" up/down a bit when using the volume control but always settled which I guessed was ok .....

I'm a bit confused.
So the 1st problem with the receiver was the switch and pot noise, which you repaired by cleaning / lubing with deoxit.

The 2nd is DC offset; did you measure this before swapping out all the transistors and diodes in the pre-driver stage?

And I guess 3rd, how are you measuring the DC offset; with a scope (I think difficult) or DMM (easy and accurate)?
I ask this because of your description of driving a 1khz signal, monitoring the amplitude changes via the volume control and measuring DC offset too?

I do my DC offset measurements+adjustments with the amplifier warmed-up, no load, all controls normal or flat with no input signal and my DMMs connected to the speaker terminals.

I do my amplifier signal testing with a dummy load connected to the speaker terminals along with my scope terminals.
The input signal can be sine, square or sawtooth so I can see how well the amp responds to these signal shapes.

 

[NickArchdale]

Thanks chaps, sorry if I'm not being clear. The original fault upon triage was faint crackling (all the time, regardless of volume/source etc) on one side (I forget which now, used sacrificial speakers) and the usual switch/pot noise & blown bulbs. I did look for DC on the output first and I remember checking the biases which were close enough (25mV across those resistors).

AK advised the immediate replacement of those transistors so I did the power board, then tone board then finally phono board and replaced the electrolytics on each while I was there, tested with signal generator and 'scope, no load, after each one. I also changed those diodes on the main board on AK advice.

The power rails are all good and the main filter caps are spot on (I have the Peak cap tester) - indeed all the caps I tested looked pretty good so I could probably have left them. I have not touched the power transistors nor double-diodes on the heatsink as they appear to be working and there was no sign of trouble or trauma anywhere.

Having done all that the receiver appeared to work as expected with all spurious noises banished so I thought I'd set the biases before finishing up and connecting speakers I care about. Using a multimeter set to mV with no signal (nor load) and I couldn't get either close to 25mV across the resistors so I guessed that my differential pairs might not be matched (all KSC1845 from same reel though) so I swapped them for ones closely matched using the Peak, vhe=360.

So quickly looking with the scope at the output with 1KHz on aux I'm now seeing these frightening excursions after power on first to negative then to positive then zero with the left channel moving first - I pull the plug before I let the right channel settle (or not) because I have the fear! I have the unit on a dim bulb and there's no load so I don't think that anything can get damaged but I may well be wrong!

I am going to triple check all the components that I have swapped now and report back as, yes, we've all done that before, several times sadly, I've even sort of seen myself doing it if you know what I mean, more so as I age

 

[NickArchdale]

Herewith some photos and a pdf scan of the schematic showing work in progress. All replaced components quadruple checked for orientation and correct.

What's the purpose of the 47uF caps across R417/418 on the underside of the PCBA? Should I be checking those 56R resistors R415-418?

Will now add some test points to those resistors 'cos they're dangerous to probe (hence the tape) and I'm going to pull the trim pots and check/clean - I did give them a squirt of Deoxit and wiggled them back & forth and left them at centre (no power) when I changed all the transistors and capacitors.

I do not intend to power back up until I've heard that my setup safe to do so and try set the biases.

Thanks for all your help on this.

 

[NickArchdale]

Bias trimmers pulled and checked, weird things, but reading from 300<>1400R smoothly with 750R at centre, but I don't like the wobbly right angle action.

On the subject of the transistors they all had black corrosion on their legs as though the casing had released something. Everything on the PCBAs I've worked on now clean & corrosion free.

Can't think what I've done wrong other than perhaps lose faith!

 

[nicholas2381]

Sounds like good progress. There's a chance that wasn't corrosion on the transistor legs but an insulating paint. Of course it could have been corrosion. If it was, those transistors are likely worth checking to verify that the corrosion didn't creep up into the casing and harm anything.

If the bias trimmers are reading smoothly, that's a good sign. But those open ones often wear over time. If you have equivalent sealed pots on hand, it'd probably be worth replacing them.

Regarding the bias, best to set them at the lowest setting (usually full CCW) and power everything up on a DBT. Then you can try to set the bias. If it works, turn it all the way back down before plugging directly into mains power and do the adjustment procedure again.

 

[NickArchdale]

Sounds like good progress. There's a chance that wasn't corrosion on the transistor legs but an insulating paint. Of course it could have been corrosion. If it was, those transistors are likely worth checking to verify that the corrosion didn't creep up into the casing and harm anything.

They've all been replaced and binned, folk on AK are very rude about those old trannies (ROS "replace on sight") so I followed the advice. all the replacements went through the Peak tester first to confirm that they worked.

So I should set those trims to 300-400R, power up from the DBT and try to set the bias? You sure nothing's gonna go bang!?!

Thanks for your help!

 

[nicholas2381]

Am I sure? Definitely not.

But hopefully the DBT takes the brunt of it if something goes wrong.

Ideally you want that DC bias set to 0 so you can slowly ramp it up from there. But it doesn’t sound like you eve been able to get it that low.

 

[NickArchdale]

Thanks @nicholas2381, have replaced the trimmers and checked the 56R resistors. With the trimmers set to 400R I gingerly turned it on with scope attached to see if anything has changed. Left channel swings low then high then settles at zero followed by the right hand channel that swings low and sits there. DBT remains unlit and the situation seems calm but clearly wrong, certainly on the right. Am going to pull, test and swap the diff pairs to see if the problem follows....

LATER: Q401/403 & Q402/404 pulled tested and channel swapped. LH channel now swings less widely but RH channel still sits at negative. It occurs that the hfe of all these transistors I've replaced in the 350-400 range and I think the originals may have been double that, should I be compensating somewhere? Q405 & Q406 form part of this biasing network, I guess I'll pull, test and swap them next.....

LATER STILL: Q405 & Q406 pulled, tested (hfe=360, the reel was pretty well matched) and swapped. RH still heads low. Running out of ideas but I think I'm in the correct area :-(

Any help much appreciated. Thank.

 

[nicholas2381]

Using your DMM, what are the voltages at +/- B1 and +/- B2 on the main amp?

 

[NickArchdale]

Using your DMM, what are the voltages at +/- B1 and +/- B2 on the main amp?

Just checked those at the entry to the main amp, +/-25/26V and steady. While I was there I measured the voltage at C408/R414 (LH) at -0.07V and C407/R413 (RH) at -8.0V and heading down so something is most certainly awry in the RH bias stage.

 

[nicholas2381]

Just checked those at the entry to the main amp, +/-25/26V and steady. While I was there I measured the voltage at C408/R414 (LH) at -0.07V and C407/R413 (RH) at -8.0V and heading down so something is most certainly awry in the RH bias stage.

I wish I had enough knowledge to narrow it down for you, but at this point I'd start working back through the circuit and comparing voltages on the left and right and see where they start to differ. This will give you a sense of which component is likely at fault.

 

[NickArchdale]

I wish I had enough knowledge to narrow it down for you, but at this point I'd start working back through the circuit and comparing voltages on the left and right and see where they start to differ. This will give you a sense of which component is likely at fault.

Thanks, yes, will do that but it's something I must have disturbed when changing Q401/403 the second time. I'll be leaving this for a few days now to take some holiday so will let you know how I get on when I'm back on it. Thanks for your help, it's reassuring if nothing else

 

[nicholas2381]

I’d check closely for any microscopic solder bridges in those areas. Sometimes they’re nearly invisible (I often use continuity mode on my DMM to check) but they can cause havoc.

Enjoy your holiday!

 

[NickArchdale]

I’d check closely for any microscopic solder bridges in those areas. Sometimes they’re nearly invisible (I often use continuity mode on my DMM to check) but they can cause havoc.

Enjoy your holiday!

Thanks, took rather longer than expected to get back on to this - distracted by summer, the end of lockdown and even some jury service! Have also cleared through some other projects as I was unable to see the wood for the trees on the HK430 but back on that now.

Have fitted test points for my DMM and new multi-turn top-adjust trimmers to better set the bias safely and accurately (huge improvement), the trimmers have all 3 pins connected btw since the PCB supported that. The LH channel now sets perfectly at 25mV and the output is fine, the RH stays around 12-15mW regardless of trimmer value and the output swings fully negative, the dim bulb is glowing faintly, no speaker load. The power rails measure on DMM:

LH+ 25.68V
LH- -25.46
RH+ 26.16V (hmmm, appears to rise to this as output falls negative)
RH- -25.60V

What sort of tolerance are we expecting here? Also I've seen reports on AK about folk getting caught out by the DBT messing up their power supply regulation but don't want to try taking it out without good reason. Will do some solder joint reheating next....

 

[nicholas2381]

First thing I'd check is to make sure the trimmer for the RH is installed in the correct orientation. The glowing bulb means something is amiss. And the inability to adjust the voltage via the trimmer makes me suspect it's at fault.

 

[NickArchdale]

Hoorah, all solved!

D405 (which I had replaced along with the other diodes per AK recommendations) had died shorted, presumably killed by heat when I swapped out Q401 & Q403 to match them. Diode replaced and the right hand channel is now behaving correctly.

I came back to this after working on some other vintage gear which had helped me understand the implications of a DBT, namely that unregulated supplies struggle to get what they need to stabilise. So the initial worrying output swings and inability to set the bias that prompted this post for help to AK purely down to the DBT. Doh, valuable lesson learned by newbie! Bias set to 25mV on both channels and it sounds very clear and alive, very pleased with it.

Thanks @nicholas2381 and others for your help.

Nick

 

[nicholas2381]

Well done! Glad it's up and running. Now time to enjoy the music.

 

[NickArchdale]

Some photos of the final bias checks, before and after casing up and on soak test with Marantz CD63 & Wharfedale Glendale 3XP speakers which are sacrificial enough should something go wrong (i.e. I have spare drivers!). Steely Dan's Gaucho sounds fabulous