Help: How to remove Damping Factor switch on Tube Amp Harman Kardon Trend 2 (A1040)

[havols]

Hi audiokarma members
Getting back to work with my original Mono Harman Kardon trend 2
This HK has been fully recapped, and resistors out of tolerance have been replaced. Coupling capacitors were replaced too.
Well, the last job missing is to remove the damping factor switch. This switch is shorted, so I need to remove it from the circuit. My speaker in use with this HK is a pioneer cs-66G.
I ask your help to fix this. The damping factor is described in the circuit that is attached. The switch is between PIN2 12AV6 tube and 16ohm leg from output TAP, with a common (ground) connection.
Thanks for your help

 

[cademan]

How do you mean it is shorted? You can remove the switch entirely and just make connections directly at the 4Ω, 8Ω, or 16Ω taps with the speaker negative at the black wire or chassis ground.

For an example, if you want to use the 8Ω tap then you would connect your speaker to the yellow and black wires or yellow and chassis.

If you do it this way, you can remove the impedance and the damping switch altogether but you will need to leave the feedback circuit intact. Just follow the line from the 16Ω tap that goes to the cathode of the 12AT7 tube.

 

[gadget73]

That damping switch is a complicated bit of kit, but the schematic showing multiple possible resistor arrangements doesn't make this easier. Can you let us know what, if any, resistors and caps your particular amp has between the output transformer and the switch, and from the switch to the 12AV6 ?

 

[havols]

Thanks for reply Cademan
This switch was shorted, so a year ago, I unplugged it from the circuit to fix other problems that amp had (it had oscillation, which I fixed when I found a shorted mica capacitor. I replaced it with a ceramic one, and OK.
This amp being stored for 1 year. Now I would like to return it to life. I pretend use it with a mono fisher FM90X tuner that I have. I am waiting for years to do it...
I was thinking to remove the impedance switch. I don't have any 16 ohm speakers. Then, I will do it.
When you talk about the 16 ohm line to the 12at7 cathode, would it be the line going to 12av6 PIN 2? I didn't see any line from 16 ohm to 12at7.
Thanks again

How do you mean it is shorted? You can remove the switch entirely and just make connections directly at the 4Ω, 8Ω, or 16Ω taps with the speaker negative at the black wire or chassis ground.

For an example, if you want to use the 8Ω tap then you would connect your speaker to the yellow and black wires or yellow and chassis.

If you do it this way, you can remove the impedance and the damping switch altogether but you will need to leave the feedback circuit intact. Just follow the line from the 16Ω tap that goes to the cathode of the 12AT7 tube.

 

[dcgillespie]

The switch should be rather easy to remove:

1. Disconnect the end of R42 that connects to the damping switch, damping switch resistors, and Common output terminal. Now, ground the free end of R42.

2. Disconnect the Common output terminal from the damping switch and damping resistors, so that it is no longer connected to anything. Now ground the Common output terminal.

3. Disconnect the damping switch and C31 from the 16Ω transformer lead.

4. Disconnect the side of network R41/C24 that connects to the damping switch and damping switch components. Now connect the free end of this network to the 16Ω transformer lead.

Remove the damping switch and all its associated components.

And that's it.

Dave

 

[cademan]

Sorry, I couldn't see the 12AV6 in your pic and I mistakenly thought it went to that first 12AT7. This might show it a little easier.

 

[havols]

hat damping switch is a complicated bit of kit, but the schematic showing multiple possible resistor arrangements doesn't make this easier. Can you let us know what, if any, resistors and caps your particular amp has between the output transformer and the switch, and from the switch to the 12AV6 ?

Thanks for reply Gadget
The switch circuit was very confuse. All those resistors on the switch were too close. I remember when I pulled it out of the circuit, I just take off. Today, It has the C24 + R41. After that, it is plugged directly into the 16 OHM tap. But it's not correct, because I did this just to fix the oscillation problem 9that is fixed)
Sorry, but I'm not sure of anything else after the C24 + R41, except that it comes in at 16 ohm.
As I intend to use modern speakers on it, i would like use some default damping factor value
Maybe following the scheme is the only way out.
Sorry if I wasn't clear enough.
Regards

 

[havols]

The switch should be rather easy to remove:

1. Disconnect the end of R42 that connects to the damping switch, damping switch resistors, and Common output terminal. Now, ground the free end of R42.

2. Disconnect the Common output terminal from the damping switch and damping resistors, so that it is no longer connected to anything. Now ground the Common output terminal.

3. Disconnect the damping switch and C31 from the 16Ω transformer lead.

4. Disconnect the side of network R41/C24 that connects to the damping switch and damping switch components. Now connect the free end of this network to the 16Ω transformer lead.

Remove the damping switch and all its associated components.

And that's it.

Dave

Great Dave
I will do
Now I'm late. I have to run to work
Soon i post the result here, with pictures.
Thank you very much

 

[havols]

Sorry, I couldn't see the 12AV6 in your pic and I mistakenly thought it went to that first 12AT7. This might show it a little easier.

Thanks Cademan.

 

[havols]

Tonight I will make the suggested changes.
Another question that came to me.
If want to remove the switch speaker (A, B, AB), and directly connect the 8 ohm taps to A, and 16 ohm taps to B. How can I do this.

However, the OutputTap secondary has 4 wires:
Black = ground
Black/White = 8 ohm ???
Yellow = ???
Green = 16 ohm.
Should I just connect Black/White on A and Green on B?
Yellow wire refers to what?
I appreciate your help
Thank you very much.

 

[dcgillespie]

First off, you're dealing with yet another Sam's schematic with an error: Speaker selector switch M10 is not shown in position A as indicated, but in position AB, so you might correct your schematic to reflect that.

If you want to remove the Speaker Selector switch, then you really need to remove the Speaker Impedance switch as well, because it exists because of the existence of the Speaker Selector switch. Without the Speaker Selector switch, then you don't need the Speaker Impedance switch.

If you remove both of these switches, then the transformer secondary leads are as follows:

Black = Common, which must remain grounded.

Blk/Wht = 4Ω Tap.

Yel = 8Ω Tap.

Grn = 16Ω Tap -- which must also remain connected to the NFB network of R41/C24 as explained earlier.

With all the old wiring removed from both switches, connect output terminal A to the transformer Yel lead for an 8Ω output, and the Grn transformer lead to output terminal B for a 16Ω output. However, since traditionally, the lowest output impedance tap is always located closest to the Common terminal, it might make more sense to let terminal B be the 8Ω output, and terminal A be the 16Ω output.

Dave

 

[havols]

Hi guys
First, I removed both switches (Speaker selection and impedance selectioN) as Dave mentioned in the post above. I believe in best performance without the switches.
I also readjusted the circuit without the damping factor, (following the Dave's steps)
1: I disconnected and grounded the free end of R42.
2: I grounded the Common output terminal, disconnected by Damping switch / resistors
3: I disconnected the damping switch and C31 from the 16 ohm transformer lead.
4: I disconnect the side of network R41 / C24 which connects to the damping switch and damping switch components, and connected it to the 16 ohm transformer lead (green wire)
All 4 steps have been taken, so I believe everything is in order.
That done, lets test:
After 4 seconds, an irregular buzzing / whistling / wheezing begins, increasing in intensity to the point where the speaker may be damaged. So, i turned off
Then I replaced the 12AV6 tube by a Know good one, and the problem persisted.
For testing, I disconnected feedback from the 16 ohm leg. The amp power normally, it has sound normally, with only a strong hum, like bad ground. It was power on for about 20 seconds.
SO... Looks like this Harman Kardon doesn't want to work properly with me. From the little I hear it, sounds amazing. I am really anxious to putting it to work

I apreciate your help
regards

 

[havols]

I can't increase or decrease this irregular sound as buzzing / whistling / wheezing with the volume pot. Even if I turn the volume control up or down, the buzzing / whistling / wheezing remains the same.

 

[dcgillespie]

Sounds like bad power supply caps.

Dave

 

[havols]

Thanks Dave and everyone who helped me on this forum
I review power supply caps. Replaced one of them. The buzzing / wheezing practically disappeared. I identified a problem with the HUM noise potentiometer. Maybe it's too dirty. I will take it apart and clean it.
greetings