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  #1  
Old 5th October 2019, 11:52 PM
magmouse magmouse is offline
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Default K5881 MkII channel imbalance issue

Hello everyone,

I joined the forum recently as I am the process of bringing my old WAD K5881 amp back into use after a long period sitting in a cupboard. I built the kit when they first came out, and later bought but never fitted the MkII conversion kit.

So far, I have replaced all the valves and fitted the conversion kit, all without problems. I have also planned some upgrades, drawing on various K5881 threads on the forum and the upgrade article in the HiFi World supplement.

Today, I replaced the rectifier diodes and the power supply capacitor. On turning it on, I noticed that there was a substantial channel imbalance, with the right channel significantly louder than the left.

A visual inspection showed nothing obvious, so I checked the voltages - see attachment.

Highlighted = voltages that seem significantly off the design voltage.

* These values started a few volts higher, and dropped to these values over a few seconds of measuring.

When measuring V2 pin 7 on the right channel, I found the loudness of that channel dropped, causing the stereo image to shift the centre, or even to the left. On removing the meter probe, the loudness returned almost immediately.

Over time, sometimes the effect went away, so the balance centred, but metering V2 pin 7 would again reduce the loudness of the right channel, shifting the image to the left.

Metering the same point in the LH channel circuit has no effect.

So, some questions for the assembled wisdom of the forum:

1. any idea what is causing this problem, and how to fix it?

2. How worried should I be about the off-specification voltages? And if I should be worried, what can I do about them?

Many thanks for any help you can offer.


Nick.
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File Type: jpg K5881_voltages_5_Oct_2019 copy.jpg (53.5 KB, 13 views)
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  #2  
Old 6th October 2019, 08:07 AM
snowman_al snowman_al is offline
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Default Re: K5881 MkII channel imbalance issue

Hi Nick,
First I do not have a K5881, but replacing the rectifier diodes and caps is not the problem, it would affect both sides the same. It might have caused it by disturbing something that has resulted in a short or open circuit etc.

We assume both channels were working equally before you started work on it? ''So far, I have replaced all the valves and fitted the conversion kit, all without problems.''

1 - Swap V1 and V2 from left to right sides. Does the 'loudness fault' follow the valves or stay on the same side?
2 - Make sure you have not accidentally disconnected the feedback loop components on the loud side. (The 20K resistor and 220pF cap.)

Then:
3 - Ensure V2 pins 3 and 8 are still joined. Measure R6 (136K) on both channels are similar.
4 - Check measure R9 and R10 (150K) are similar on both channels.
5 - Measure R7 (1 Meg) and check the voltage on V2 pin 2 on both channels. Make sure C4 is OK and connected.

Other will be along to help soon I'm sure. Alan
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  #3  
Old 6th October 2019, 11:11 AM
magmouse magmouse is offline
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Default Re: K5881 MkII channel imbalance issue

Hi Alan,

Thanks for your rapid response.

I should have mentioned in my previous post that I already tried swapping the valves between left and right channels, which made no difference, so it isn't a valve problem.

Like you, I'd come to the conclusion the components in the power supply weren't the issue, since they are shared between channels and measured OK.

In terms of the other checks you suggested:

2. feedback components and circuit all looks OK.

3. Yes, V2 pins 3 and 8 are still joined. R6 (136K) on both channels measure the same.

4. Yes, R9 and R10 (150K) are similar on both channels, although R9 on the L channel measures 152k, and the others 150k - how critical is this value?

5. R7 (1 Meg) measures OK. C4 looks OK on both channels. However, the voltages on V2 pin 2 are:

L = 110V

R = 120V

In summary, the anomalies are:

- off-spec voltage measurements as per the attachment to my original post

- the different voltage measurements for L and R channels of V2 pin 2 as above

- the curious behaviour that the channel volume shifts when measuring V2 pin 7; last night this only seemed to affect the R channel, but this morning effects both

- furthermore, this morning the balance is generally better, but a bit biased to the right, unless metering V2 pin 7, when it shifts strongly to the L channel.


Any further thoughts or suggestions for more testing gratefully received...
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  #4  
Old 6th October 2019, 03:16 PM
snowman_al snowman_al is offline
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Default Re: K5881 MkII channel imbalance issue

OK I'm working off page 23 of the Nov '96 supplement, hope the diagrams are the same...

The way I see it is that the 110 / 120 volts on V2 pins 2 are derived from the cathode of the top triode, V1 pin3. (The junction of the two triodes of the 'totem pole'.) There could be 10 volts difference in the two 7025 valves. You can prove that by again swapping V1s around and measuring at V1 pin 3. If the voltages swap too, it is a difference in the valves. If the voltages stay in the same place it is something to do with the circuit of V1.

Can you measure the voltages round both V1s? (Pins 1, 2, 3, 6 and 8).

Not sure why you have such a large variation on the V2 pin 6 though...?
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  #5  
Old 6th October 2019, 10:17 PM
magmouse magmouse is offline
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Default Re: K5881 MkII channel imbalance issue

Thanks for these further suggestions.

I measured the pin voltages for V1:

Pin L R
1 210 226
2 105 116
3 107 119
6 105 117
8 1 1

Swapping the V1 valves gave the following result:

Pin L R
1 217 212
2 110 107
3 112 110
6 110 108
8 1 1

Much better voltages, however the volume of the left channel dropped to almost nothing - I had to put my ear to the speaker or disconnect the right ch input to hear it.

It then occurred to me that although valves V1 and V2 are in theory the same, two were the ones that came with the MkII upgrade kit, and two were the ones I had bought to replace the originals that came with the amp. All are Sovtek 7025s, but of different vintages.

As it happens, the two newer ones were the V1s (L & R) and the old ones the V2s. I swapped V1 and V2 on each channel, and the result is the sound balance is back to normal - presumably I had got these swapped around when replacing the power supply components.

So, all is good. Except...

Voltage readings are still pretty crazy - see table (also attached in case this doesn't show up):



It seems the valves do have something to do with this, so I am going to dig out the old V2 valves, which I still have around, I think.

I also wonder if the electrolytic capacitors may be an issue? I have seen recommendations to replace these - mine are the originals from the 1990s. They were on my list to do in any case. Could they cause or exacerbate some of the voltage issues?
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  #6  
Old 7th October 2019, 07:19 AM
snowman_al snowman_al is offline
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Default Re: K5881 MkII channel imbalance issue

I'm glad it is working, but I cannot say I can make any sense of what is going on? Logically it could be a poor connection in the V1 valve holder or poor solder joint that is moved by removing / inserting the valves...

The electrolytic caps might need replacement, but the only one that could have any effect as you describe would be the V1 cathode bypass one (the one across the 2.2k ohm resistor off pin 8 - no value on my circuit.)

Maybe some one with experience of the K5881 might have an idea if the other caps generally have a good reputation or need replacement by now? Like C4, C5 and C6 etc.

Alan
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  #7  
Old 7th October 2019, 09:53 PM
magmouse magmouse is offline
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Default Re: K5881 MkII channel imbalance issue

Hi Alan,

Thanks for your further reply. I am rather baffled by it all as well. I have some work I need to do tonight, but I have found the old valves, so I will try swapping valves around to see how that changes things when I get time, hopefully tomorrow evening.

More info when I have it...
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