[FINAL UPDATE] BassBase 400 Lamp Wiring Diagram Assistance

Greetings fellow H&K inquiring minders!

So, I have an old H&K BassBase 400 that I recently opened up so I can service a weird disturbance that was happening, and since I had the hood/bonnet popped, I figured I'd replace the T10 lamp that's been burnt out for about the past 20 years since I've owned the amp.

To my dismay, I learned the actual lamp wasn't in fact burnt, but the socket itself was rigged by either the previous owner or a service tech who bypassed it completely, as well as added a few sand resistors to compensate for something -- which led to the lamp never glowing while in my possession... Now, on to the question...

Does anybody have a BB400 or 600 they can pop the top and snap/post a pic of the far-left lamp and its wiring? I'd like to return this baby back to stock, and the BB600 Schaltplan pdf I have doesn't really help me... well, the fact that I've never been trained to read schematics in the first place doesn't help either -- but I know a *little* enough to be dangerous and to kind of make sense of some things.

Here's a shot of what the lamp in question looks like in my BB400:

Any assistance on this would be incredibly appreciated.

Cheers all!
~Phil T

Hi Phil. Can you give me a little more info on what we are talking about here? I know nothing about the BassBases in specific terms but, from the schematic I have found of the BB600 at least, they look like fairly standard beasts in terms of working on them.

Let's get things absolutely clear. The lamp is a stand alone thing isn't it, you are not meaning the one in the mains switch? There is so much gunk on the socket in your picture that I can't even make out that it is a lamp socket. That isn't a valve socket is it? The one to the left of the resistors looks very similar and I can see a valve on the other side of that one.

Can you post a couple of higher resolution pictures of both the lamp socket and the resistor setup from closer in? As much detail as possible would help as there don't seem to be any other pictures out there of the innards of your amp.

Nope, looks like I'm wrong in a couple of assumptions.  Is this attached picture the same as your own amp?  If it is I can see you have a line of 5 lamps in sockets along the white divider strip.  They should be pretty easy to wire up assuming there has been no damage to the other sections of the circuitry.

You say these are T10 bulbs.  That should mean they are simple auto types commonly used for a car.  Are these lamp holders a plug in and twist type fitting into the white panel they are mounted in?  If they are, is the unused one with the silicone gunk around it still serviceable?

I can only work from the BB600 service manual which should be fairly similar in this area. Circuitwise you can see the 5 lamps are fed direct from a 62V AC winding of the transformer. They are in series so we can presume that they are 12V types meaning they soak up 60V. There is then a series 10R 5W resistor acting to control their current. The resistor, R1, is mounted on the power supply PCB in between the two sets of 3 large caps. The lamps simply have a wire running along between them and to the terminals Plug1 and Plug7 on opposite ends of the power supply PCB. This may be a little different for your BB400 but you should be able to chase that through to see if it is exactly the same.

Check out where the cable to the other sockets leads to and see if it is as I have described. Also look and see if you can see the 10R 5W resistor on the power supply PCB and check if it shows any sign of damage.

And finally here is the wiring that you need to look into.  You can see that it is a straight run of 5 lamps in series.  This is the BB400 this time so compare it to your own amp to make sure we have the same model, particularly where the wiring runs mount onto the PCBs.  Let us know what you find.

bordonbert, I knew you'd chime in!
So from the images I posted: yes, the left-most socket is a simple push-in socket for the generic T10 automotive lamp. The images you posted do look slightly different from my BB400, however, they are similar enough in a way that showed me that all the lamps are wired in series, and the final lamp socket in question (the one that's drowning in silicone) only has one lead, which is wired to the sand resistors. Someone has seemed to have disrupted the lamp circuit on that last socket by adding (for some reason) those wirewound resistors.
I can now make sense of the wiring based on the image you posted, and I'm going to return the circuit to its intentional series path and I'll update you as soon as I can!
Thanks again bordonbert, you champion of champions!

That sounds good Phil. Glad a little bit of an alternative view helped. Mind you, I still wonder what that mod was supposed to achieve. It's pretty much a bit of butchery to my eyes.

Certainly, rewire to the design layout but I would make sure to check out the original power resistor which is required to set the current through the lamps before using them again. If that resistor has been damaged or modded in some way and its value is now too low you could end up going through a lot of bulbs. In the schematic for the BB600 I found it is shown as 10R/5W. That would be selected for a particular wattage of the bulbs. You would need to be sure that the bulbs you use match that requirement in terms of their power.

We can assume that the bulbs are meant to drop 60V leaving 2V across the resistor. At 10R that would mean it is passing 200mA (2V/10R = 0.2A). That puts the bulbs at 12V x 0.2A = 2.4W. That's a starting point at least as it assumes the same 62V tap from the transformer as shown in the BB600 schematic which just could be incorrect. Are the other bulbs marked in any way as to wattage so you can be sure?

Nowadays you could change the older incandescent bulbs out for LED types. This could give you more light if that were needed. It looks to me as if the idea is just to flood the interior chamber with light so it comes through the front panels sections pretty evenly. Often if the bulb is directly behind the front panel you get a light patch with a slightly darkening effect as you move away from the bulb point.

Yes, do make sure to let us know how you get on. It's great to have a little info on the rarer models in the H&K catalogue.

Yeah, I'm currently working on getting you the photos to show how much butchery is going on...

Looks like the bulbs are listed as T10 12V/5W in the BB600 Service Manual which is available online.

On a side note -- I popped in some LED replacements, and they worked just great (side for that last dead socket).

Ok, hope this makes sense...
A lead from Plug 7 on the PCB is going to the IN/START contact of Lamp Socket 1 (obviously 1 thru 4 are wired in series) so S1 > S2 > S3 > S4: then a lead from the Socket 4 OUT is going to the resistor cluster IN. Then a lead from the resistors' OUT is going to Plug 21 on the PCB. Also from the OUT of resistors is a small lead going into the nonfunctional Socket 5. The lead from Socket 5's OUT contact has been cut (which you can barely tell in the pic), which is covered in the silicone gloop.

Yep, that should be a straight up repair for you now with what you have found. I can't think why this has been done in the first place. Replacement parts would have been easy to find to fix anything broken. I can only assume that the owner didn't like the lighting effect with that fifth bulb in place so he removed the socket from the chain and replaced it with a resistor matching the bulb's resistance. But hey, that is a Chernobyl class resistor! It would have been a lot easier to just block the bulb with something opaque.

Anyway, it's late here and I'm sure you can take care of this yourself from here on in. Post a couple of pics of the finished job and I'll catch up later.

Ok, I couldn't for the life of me figure out why this mess was added in the first place -- your guess was as good as mine.

I went ahead and removed the wirewounds and simply completed the circuit, popped in the new LED, powered it up... et voilà! All LEDs lit up without issue.

Man, I was thinking this was a weird mod -- but what the hell did it do(?)!

Anyway -- thanks again for being the champion that you are bordonbert. Here's to rockin' out and keeping us young!

Cheers!
~Phil T

Yes, it is an oddity Phil. Anyway, issue sorted now. Glad it was such a simple fix. And we both learned something about the amp on the way. Keep it banging and thumping!