HK 730 New Bias Trimmer Unresponsive

[Zeus12]

Greetings fellow audiophiles,

Continuing with my partially restored HK 730 I decided to change the VR401 Variable Resistor (trimmer) since it was giving me issues and couldn´t get it past 13-18 mV.

I used a Bochen 3296 W102 13 trimpot as a replacement.
https://www.audiophonics.fr/en/trim...rn-trimming-potentiometer-1k-ohm-p-13012.html

I know it´s not the most suitable orientation but for that channel, I can still reach the turning screw through the 2 transformers. Now, for VR02 will be more difficult to access, so I´ll leave the original trimmer in there until Mouser gets some new stock. According to @jheu02 this is what I need:
https://www.mouser.es/ProductDetail/Bourns/3362W-1-102LF?qs=cdlJwygV6pmEKtalJFMnhQ==

I used the approach of @Skywatcher from here:

Look close at the trimmers and board and you'll find the 3rd hole is already there along with the trace to solder to. A 3 lead trimmer is a drop-in replacement. Easy as pie.

https://audiokarma.org/forums/index...man-kardon-730-question.924705/#post-14015790

Underside:


There is another method, of "el Maestro" @dlucy here:
https://audiokarma.org/forums/index...arman-kardon-330c.781237/page-6#post-11943536

So basically this is just a simple "resistor" and needs only two legs. Does it matter which one I solder the middle leg (2)? Also is the orientation of the trimmer important? If I´m not mistaken, resistors are unaffected by orientation so it shouldn´t matter.


Unfortunately, the way it is now, is even worse than before. It is TOTALLY unresponsive and the Bias on the Right channel doesn´t go above 10 mV. I keep turning that little tiny screw and NOTHING moves on the DMM. VR02 works perfect and I can get it to the original spec of 35 mV +-.

Any ideas on what to do?

Cheers!!

 

[Zeus12]

Greetings again,

In the meantime, I decided to try and see the Ohms for the 2 trimmers. Does this mean that the old trimmer is definitely bad? It is way over spec.


Would this value for the new trimmer be OK and on the safe "low" side, around 700Ohms? From what I understand, you DON´T want it all the way down (0 mV for Bias) and a little is OK. So the higher the resistance, the lower the Bias will be? Got leg 1 and 2 joined.


Cheers!!!

 

[Skywatcher]

Just solder the 3 leads to the traces as they are. You'll see 2 of the leads are on the same trace. Always have the wiper (center lead) to one of the outer leads on this model. The only thing that will happen if the trimmers are flipped, is the rotation direction as CW or CCW. Always start with low resistance whichever direction achieves it
As you turn the trimmer, the resistance will change with one lead on the center, and one on either end. Multiturn, especially 25 turn (like you have) will need to be turned several times to make very small changes.

 

[Zeus12]

Hi @Skywatcher

Big thanks for your help!

I have soldered all three leads. The orientation that I used is as follows:

Legs 1 and 3 to the outer leads of the board and leg 2 to the middle one--which was hidden under solder.

I posted a pic of the underside in my previous post and yes, I can see that the middle lead is on the same trace as the other lead.
Shouldn´t that be the same as "shorting" the middle leg? How come NOTHING moves when I turn the screw CW or CCW?

By the way, I see that the resistor right above is also on the same trace. Is that for protection? What would happen if there was NO TRIMMER in there, for example? Would it blow up the output transformers?

Perhaps I didn´t solder it properly? Maybe one of the legs didn´t make good contact?

You also say that, "Always have the wiper (center lead) to one of the outer leads on this model." So could that be the problem? I should put the middle leg (2) of the new trimpot on one of the outer legs and NOT in the 3rd hole?


Cheers!!!

 

[mbz]

2 of the trimmer pins are shorted (blue), one of these must be the middle/wiper pin. The variable resistance is provided between the red and blue pads. With amp powered off/unplugged measure the resistance between the red and blue tabs then adjust the trimmer and remeasure, the resistance will change. Reset the trimmer to near zero (0-50?) ohms.

Have you changed any transistors in that (left?) side of the power amp. These may have slightly different properties (Vbe)
to the originals. Can you still adjust to 13-18mV before it max's out?

 

[Zeus12]

Hi @mbz

Thanks for your help from ALL THE WAY DOWN UNDER!!

Yes, those 2 blue marks correspond to leg 3 and 2 (middle) of the trimpot. Don´t understand why it´s not working for me.

I haven´t changed anything else and now I can´t adjust the BIAS AT ALL. Before, with the old trimmer, I could set it to about 13-18 mV maximum and then it would drop to around 5 mV by turning it just a bit more. So I thought that it must me the trimpot. I tested the old one out of the circuit and it would measure OK but very unstable. Here´s the beginning of my odyssey:
https://audiokarma.org/forums/index.php?threads/harman-kardon-730-left-channel-bias.994791/

Will check the resistance between the blue and red marks and post back. Could it be that it´s just a matter of soldering? What if I just clip the middle leg (2) and solder it to leg 1 or 3? That way I don´t have to disassemble the output transistors again.

Cheers!!!

 

[Zeus12]

Greetings again audiophiles!!

The Bias is now responsive and the new trimpot works like a charm!!! The funny thing is that I didn´t do anything.
Maybe it just needed to "sit" and "rest" for a few days? So yes, I can confirm that it works by using all 3 legs on the board. I set both channels at around 30 mV.

Big thanks again to all the gurus on here!!!

 

[Champco]

Greetings again audiophiles!!

The Bias is now responsive and the new trimpot works like a charm!!! The funny thing is that I didn´t do anything.
Maybe it just needed to "sit" and "rest" for a few days? So yes, I can confirm that it works by using all 3 legs on the board. I set both channels at around 30 mV.

Big thanks again to all the gurus on here!!!

It's called F M .... fn magic. Part of the cult

 

[Zeus12]

Hi @Champco

Yes, learning something new everyday!!! Thank God I didn´t remove the power amp board along with the output transistors again and decided to give it another try before. Was banging my head trying to figure out what I did wrong.

Would love to hear that HK 11 & 12 combo you got. Simplicity to the max!!!

Cheers!!!

 

[Champco]

Hi @Champco

Yes, learning something new everyday!!! Thank God I didn´t remove the power amp board along with the output transistors again and decided to give it another try before. Was banging my head trying to figure out what I did wrong.

Would love to hear that HK 11 & 12 combo you got. Simplicity to the max!!!

Cheers!!!

The12's were designed a lot like tube with Transistors in place of tubes with of course other necessities. The push pull design with separate power channels is good engineering so working on them is easier then later designs. It's all analogue from the turntable to the speaker, provides true fidelity if the reproduction by the speakers is able to respond to the input. The 11 is a workhorse Preamp. I have it working Two turntables, tuner, Cd, TV, and Desktop PC.
One thing I need to mention concerning replacing components on a board it is better to do a couple or one at a time and test the results. I have put in transistors wrong and just blew them and a cap or resistor. Easy to fix. But I knew what to look for. I would need to hear some of these expensive systems to determine if they perform better then the humble HK Citation.

 

[Zeus12]

@Champco Thanks for the nice info! I´m soooooo in love with the HK 730 that I can´t wait to move up the ladder.

What do you think of this combo? What about the price? HK 11 Pre and 16 Amp.
https://es.wallapop.com/item/conjunto-hifi-vintage-harman-kardon-citation-589553421

Cheers!!!

 

[Champco]

@Champco Thanks for the nice info! I´m soooooo in love with the HK 730 that I can´t wait to move up the ladder.

What do you think of this combo? What about the price? HK 11 Pre and 16 Amp.
https://es.wallapop.com/item/conjunto-hifi-vintage-harman-kardon-citation-589553421

Cheers!!!

Value is of course up to the buyer. Converting Euro to Dollars, I think it's a little on the high side, but the market there, it may be normal. I would jump on the 16 and of course as cheap as I can, but I would need some reassurance's it is not a molested piece of work. Can you get a look inside? See if it looks factory. As for the 11 it if you look as Ebay recent sold for value and again, it needs to have not been molested. The cover comes off very easily and the architecture from factory is pretty easy to spot.. Taking off the bottom is also something I would want to do. Testing output voltages on the 16 would be wise. Also IMO it will appreciate over time.
There are variations of both the 11 and the 16. The 16 has a 16A as I can remember. It is significant. You can find discussion on it here on AK. There are some here more familiar then I on the 16.

 

[Zeus12]

@Champco

Big thanks for sharing all this wonderful info buddy!!! Really appreciate your time!

I´m still fairly new to this wonderful hobby. Shame I didn´t find out about it before. Yes, the prices here are going through the roof!! That´s what I thought about the HK 16. Seems a little pricy and looks like it has had some work done on it before. I´ll keep on the lookout for another one and also the 12--which is the one I really want to try.

Do you like Sansuis? Oh, what a lovely sound these have. Right now I´m in the process of recapping my second Sansui AU-5500. This little gem has something special as well. Along with the HK 730, I am NEVER planning on parting with these two. Love them both!

As you can see, I´m still a newbie at this, but I want to learn more and more about this fascinating world. What do you think?
https://audiokarma.org/forums/index.php?threads/sansui-au-5500-recap-and-restore.1001730/

Cheers!!!

 

[Champco]

Right now I´m in the process of recapping my second Sansui AU-5500. This little gem has something special as well. Along with the HK 730, I am NEVER planning on parting with these two. Love them both!

I think that's cool. After listening to the two a report on your experience is why we are here. I was always a Pioneer guy. That being said my first listen to a 9090 had me drooling for years. It's a power thing. When I was in need of a preamp i stumbled on someone who had a Citation 11 and it seemed to fit the bill. Since then I have amassed 4 12's and two 11's. The 12's I bought just to have a pair but after going thru one and, the 11 has two outputs for amps I decided on a second one.
As I kept watching more came into my grasp and I snagged them. Mostly needing repair but I have that solved. They all work but I have learned there are some things that just need replacing. Like the trim pots and large Caps on the 12's.
There is another beast of a project in the differential transistors in each channel.
HK built their stuff to specs they engineered and had components made for a specific use. The differential transistors have 6 legs under one case. They are two separate transistors made this way so they will track each other thermally.
There may be others made this way but the challenge of replacement.
When I was told to do it the way it needs to be, I thought it was a joke. There are photo's somewhere on AK

 

[Zeus12]

@Champco

I know you´re the man to talk to for the HK 12!! I always have my eyes wide open for the "Citation" line.

All the best buddy!!!

Cheers!!