Kenwood KA-7300 low-frequency popping noise

[Ed400]

Hi there,
I am refurbishing a KA-7300 amp. I noticed a random low-frequency popping noise. It is noticeable in low volume or when not playing anything. I replaced several caps with low ESR in the power supply as it was running too hot. I also replaced the RK1 and RK2 with the original values of 560Ohm and 330Ohms as per the circuit diagram. (I used 5W resistors) After that, the Zener diodes run much lower temperature. I am considering mounting RK1 and RK2 on the heatsink (like a post I saw in this forum which was very nice)

In any case, the popping noise is still there. Has anybody out there experienced this or has any suggestions? Thanks

 

[analogwisdom]

Welcome to AK.

Firstly, you should have replaced RK1 and RK2 with the values that were originally in your unit. Kenwood's engineers changed those for a reason. There are several values throughout the KA-7300 that differ from the service manual. To my knowledge, a service bulletin or updated schematic hasn't surfaced for this amp.

As far as the popping problem, the first step should be to use the normal/separate switch to locate which section of the amp the problem is in.

 

[Ed400]

Hi Analogwisdom,

Thank you for your message.

I will look at replacing the resistors with the original values rather than what is written in the schematics. (This change didn't affect the popping noise)

I did some research work by isolating different sections (pre amp, amp and PS) and I can see the noise is coming from the power supply which affects the rest of the amp and the zero voltage of the speaker lines. I can see on the oscilloscope the random fluctuations of the voltage coming from the PS which coincides with noise coming out from the speakers. I replaced the bridge diodes and capacitors. Haven't replaced Zener diodes and small filter caps. The Kenwood engineers must have run out of 24v zener diodes and used 2 x 12V in series.

Anyhow I am sure there is one component that is causing that popping noise, I just haven't managed to identify it..

 

[analogwisdom]

Good work doing research to track it down.

Can you let me know everything you've replaced on the power supply board, and what you've replaced it with?

Also, could you provide photos if you have them? I wonder if anyone else has ever been in there because I've never seen series diodes in a stock KA-7300.

 

[Ed400]

Thank you.
I spent more time on the unit and realised that despite the PS being noisy it does not correlate with the popping noise from the speaker..
Please see the images below:
1) Original Zener diodes (2 x 12V to make 24V) strange arrangement. Image 1899.
2) Items replaced on the PS are marked with a red dot. (Dk1, Dk2, Ck5, Ck6, Ck7, Ck8 and Ck9 all the green elec capacitors are new). Image 1898.
3) Also replaced Rk1 and Rk2. The original Rk2 had a parallel configuration again which was strange. All these 3 green resistors were very hot. Image 1890
4) Image of the PS output in Channel 1 (top) and Speaker output in Channel 2 (bottom). The spikes are audible and they don't really correlate with the spikes from the PS. Image 1920.

So back to square one. I'm certain it is not the Amp stage. So focusing on the pre-amp now. Any suggestions will be welcome.

 

[analogwisdom]

Thanks for all of the great info and photos. I think you have a very early serial number KA-7300 - I've also never seen a TO-72 at Qk1. I have seen the parallel resistors mounted on the back, though. High heat is common with this unit. I'm about to offboard the dropping resistors in mine, too. It's not totally necessary, mine has been doing great since the rebuild, I just want to do it for extreme longevity.

I'm not sure what brand of capacitors you've got in there, but I see that they're 105C rated so that's good. I can't tell, have you got a bi-polar cap at Ck8? That needs to be BP. Also, Ck9 should be a low leakage cap. I'll go ahead and link everything that has been recommended to be used in a rebuild of this board. Wouldn't hurt to do it to ensure long-term reliability. I would at least definitely replace Dzk1,2; Qk1,2 (all 4 of those are rode hard and put away wet) and the two caps I just mentioned. Be careful with the pads for the Zeners, and mount them high off of the board, like my photo here:

DzK1,2: 1N5934BRLG
Dk7: UF4004
Qk1: KSC945YBU
Qk2: KSC2690AYS
Ck8: UEP1E101MPD (BI-POLAR)
Ck9: UKL1E101KPD1TA (LOW LEAK)
Relay: MY4-02 DC24 (recommended to replace relay)

I replaced a lot of the transistors in the control/pre-amp section on my amp. You probably already know, but I would replace any that have black legs or are listed on the Top Ten Worst Transistors thread. If the KA-7300 you're working on is low serial like I think it is, it might match up with the service manual more, but just make sure you double-check installed part values. I made a thread about the differences here: Kenwood KA-7300 control board differences. I'd bet those spikes you're seeing are coming from a transistor on the control board. I'd also check out the caps on those boards, some of the ones in mine had extremely high ESR.

Good luck, please keep us updated.

 

[Sgt Shultz]

Is there a modem or router in the vicinity?

 

[analogwisdom]

Is there a modem or router in the vicinity?

Can't believe I didn't think of that - or a phone. My phone makes my KA-7300 pop if it's close by and Bluetooth is on.

 

[Sgt Shultz]

Can't believe I didn't think of that - or a phone. My phone makes my KA-7300 pop if it's close by and Bluetooth is on.

It's a schveet amp, with the rack handles a piece of art.

 

[Ed400]

Hi
Thank you analogwisdom for your suggestions. Very kind of you.
All Caps have low internal impedance and 105 temp. I did use a polarised Cap on Ck8. I can probably put back the original one as it seems the performance is ok.
The relay circuit seems to be working well so I am not overly concerned about that for the moment.
Also, the Caps are not a high fashion brand but I'll deal with that later.
For the moment the popping noise persists.

While waiting for the Zeners to arrive, I decided to think of a strategy to find the source of the noise.
When I set the unit in Separate mode using the S12 it works perfectly fine (no noise). As soon as I switched back to normal the noise returned.
Then I decided to remove both Ci51 and Ci52 which isolates the entire pre-amp except the earthing...I was surprised to see the noise was still there. (see image below)
So I don't think the pre-amp circuit is causing the popping noise, it most likely comes through earthing fluctuations.

​

Thank you Sgt Shultz for your suggestion too but there is no Bluetooth or other EMF source nearby.

I keep searching.

 

[Ed400]

Hi
Thank you analogwisdom for your suggestions. Very kind of you.
All Caps have low internal impedance and 105 temp. I did use a polarised Cap on Ck8. I can probably put back the original one as it seems the performance is ok.
The relay circuit seems to be working well so I am not overly concerned about that for the moment.
Also, the Caps are not a high fashion brand but I'll deal with that later.
For the moment the popping noise persists.

While waiting for the Zeners to arrive, I decided to think of a strategy to find the source of the noise.
When I set the unit in Separate mode using the S12 it works perfectly fine (no noise). As soon as I switched back to normal the noise returned.
Then I decided to remove both Ci51 and Ci52 which isolates the entire pre-amp except the earthing...I was surprised to see the noise was still there. (see image below)
So I don't think the pre-amp circuit is causing the popping noise, it most likely comes through earthing fluctuations.

​

Thank you Sgt Shultz for your suggestion too but there is no Bluetooth or other EMF source nearby.

I keep searching.

I couldn't edit my previous post but a more accurate representation of the popping noise can be seen below. This was measured at the power supply output -24V (Pin 12)
It happens randomly. Perhaps on average once a minute. The fluctuation is strong enough to hear it on the speakers.