Fixing an Aune T1 (amp/dac) TUBE DAC Board

[cowboybob]

... While I think the idea of using a valve in something like this is particularly silly (and just a con for the gullible) ...

Sadly, my conclusion as well, Nigel.
I was just dancing all around that sentiment (in deference to the OP) even though the apparent circuitry simply did not add up to a working dual triode. Either as a stereo amp (why would headphones even need that level of amplification?) or simple follower; which would make more sense if what was intended was to introduce a "tube" influence into the audio stream (which is real, but I'm not sure that everyone can discern or quantify the difference from straight digital).

... I'm even more bemused that they use an RF twin-triode rather than an audio one. ...

Exactly... A 12AX7 (with pin 9 grounded and 4 & 5 each fed 6VDC) was specifically designed for audio and pin compatible with the 6922. Cheaper also, but not as sexy, I guess.

 

[bittgata]

And since your tests indicated what I consider an appropriate Vin for the KIS-3R33S IC, this is the basis for my conclusion that the KIS-3R33S is defective, given the Vout value you noted on the failed DAC, as compared with those values noted for the working DAC.

I never tested the working DAC or mentioned any Vin value from it, just values for the defective unit with and without the tube , this is the first test I'm about to do since you told me to "perform all the exact same tests on the working DAC board":
Since I was going to test the working DAC board as well, I decided to do so at desk where I have the working Aune T1 is located and also its power supply and I've notice the Vin on the defective dac board with the tube the Vin is just 14.9VDC, 1V less than yesterday so I checked the other psu and gave me a 15.7VDC Vin which is still 0.1-.02V less than yesterday but maybe it's the cheap multimeter. After that I checked both PSUs and the PSU from the working dac is more like 16.5-01-16.5AC and the one from the defective dac is 17.5-0-17.5AC. Wondering if this might be, over time, part of the original cause of the problem.
Anyway I've tested the working board with both PSUs and it runs just fine with both but there is a difference in the Vin of 0.9VDC (21.2V vs 22.1V) but I'll try to limit the some of the info to just the 16.5-01-16.5AC PSU, unless I mention it's with both.

With the tube connected the Vin on the working board is 21.2VDC vs 14.9VDC on the defective board. The Vout on the working board is 6.2VDC(with both PSUs) whilst the Vout on the defective board is 1.58VDC(with both PSUs).
Without the tube the Vin on the working board is 22V whilst on the defective board it stars above 16.8VDC and drops to 15.7VDC when the light turns on. The Vout on the working board is 6.26VDC whilst on the defective board it stars at 6.23VDC then drops to 6.07VDC.
Judging by this I'd say the KIS-3R33S on the defective board is not getting enough power but I don't really know so please do tell me if this makes sense or not.
Defective board without the tube, negative on ground positive on:
Pin1:-0.43V at start up to -0.47V when the light turns on
Pin2:stable at around -16.85V
Pin3:-22.3V stable
Pin4:0V stable
Pin5:6.23V drops to 6.07V when the light turns on
Pin6:-0.43V at start up to -0.47V when the light turns on
Pin7:stable at around -16.85V
Pin8:-22.3V stable
Pin9:0V stable

Functional board without the tube, negative on ground positive on:
Pin1:22.1V stable
Pin2:-16.5V stable
Pin3:-21.9V stable
Pin4:0V stable
Pin5:6.26V stable
Pin6:22.1 stable
Pin7:-16.5V stable
Pin8:-21.9V stable
Pin9:0V stable

While I think the idea of using a valve in something like this is particularly silly (and just a con for the gullible), I'm even more bemused that they use an RF twin-triode rather than an audio one.

Even if the tube is only there as gimmick I've not seem anyone to say it sounds bad at the price point. Though I really have no idea what the tube does in the circuit I can tell you that it does do something that changes the sound as I've tried tubes with which it sounds horrible and even with the good ones there are discernable differences which I think was the original point of the whole device.

I was just dancing all around that sentiment (in deference to the OP) even though the apparent circuitry simply did not add up to a working dual triode. Either as a stereo amp (why would headphones even need that level of amplification?) or simple follower; which would make more sense if what was intended was to introduce a "tube" influence into the audio stream (which is real, but I'm not sure that everyone can discern or quantify the difference from straight digital).

The AMP board is separate and it's solid state only, it's still quite powerful but it's nothing special.

 

[cowboybob]

I never tested the working DAC or mentioned any Vin value from it

You are correct. My error.

In addition, please use the tube pinout chart and pin locations in post #15.

 

[Nigel Goodwin]

Even if the tube is only there as gimmick I've not seem anyone to say it sounds bad at the price point. Though I really have no idea what the tube does in the circuit I can tell you that it does do something that changes the sound as I've tried tubes with which it sounds horrible and even with the good ones there are discernable differences which I think was the original point of the whole device.

It's there to lower the quality, add distortion, add noise, and reduce the frequency response.

Anyone from back in the valve days (like me) is totally bemused by the con of modern valve equipment

 

[Les Jones]

I think the fault is the DC to DC converter (KIS-3R33S). It is drawing too much current thus causing the input voltage to drop.

Les.

 

[cowboybob]

One last easy test. Swap the tubes between the two PCBs. Does the problem follow the tubes?

If it does not, I'm now thinking that the KIS-3R33S is not the problem. I'll explain after you let us know how the tube swap worked out.

 

[bittgata]

Yes, the tube is fine(I've also done the last test with one tube), I mentioned in the beginning, before I even started the thread I have tested the PSU, tube and amp board with the good dac board and they work just fine so the problem is 99.9% on the dac board.

It's there to lower the quality, add distortion, add noise, and reduce the frequency response.

Anyone from back in the valve days (like me) is totally bemused by the con of modern valve equipment

A lot of the people that use tubes know it's distortion and they do so because they are looking for different, not better. That said this thing doesn't sound like a full valve build, the claimed SNR is stupidly high and it sounds more like a slight eq on a nice solid state dac.

 

[cowboybob]

Ok. I thought so, just didn't check back through the posts.

The KIS-3R33S appears to be supplying the filament (and probably the LED) voltage. As such, from the test voltages you posted, it is fine.

What's not good are the plate voltages, which I believe are coming from the area with the big caps.

I guess the question now is: are you up to even deeper trouble shooting and actual component replacement?

 

[Les Jones]

My guess is that the power to the board comes in via the 6 pin white connector. Can you measure the resistance between the input pin on the KIS-2R33S and each of the 6 pins on the white connector. (With the board removed from the unit.) This is to try to find out if there are any components on the board between the power input to the board and the input to the KIS-2R33S module. Another usefull test would be to measure the voltage (With respect to ground) on all 6 pins on the white connector with the board in place and powered up.

Les.

 

[bittgata]

Ok. I thought so, just didn't check back through the posts.

The KIS-3R33S appears to be supplying the filament (and probably the LED) voltage. As such, from the test voltages you posted, it is fine.

What's not good are the plate voltages, which I believe are coming from the area with the big caps.

I guess the question now is: are you up to even deeper trouble shooting and actual component replacement?

Yes, I have no problem doing that as this has been interesting we'll just have to see if I can actually do it. What did you have in mind?

I think the fault is the DC to DC converter (KIS-3R33S). It is drawing too much current thus causing the input voltage to drop.

Les.

Using my limited knowledge I have no idea if it's broken as well but I was thinking it wasn't getting enough power.

My guess is that the power to the board comes in via the 6 pin white connector. Can you measure the resistance between the input pin on the KIS-2R33S and each of the 6 pins on the white connector. (With the board removed from the unit.) This is to try to find out if there are any components on the board between the power input to the board and the input to the KIS-2R33S module. Another usefull test would be to measure the voltage (With respect to ground) on all 6 pins on the white connector with the board in place and powered up.

Les.

I can try that but just to clear a few things. By the input pin I think you mean the Vin of the KIS-3R33S however the power is not a white a connector it's a round 5 pin + a metal shroud and it's an AC 15V-0-15V 1A unit so there should be something between it and the DC Vin of the KIS-3R33S. I can measure the resistance between the pins and the pin if that would give useful info as to what is between them however would it be a good idea to put the multimiter between the AC PSU and the Gnd and what do I check for AC voltage or DC voltage?
The power connector the round black thing with the square metal shroud at the top middle of the imagine https://www.kidultdiagnostic.com/images/aune/aune-t1_06.jpg

 

[Les Jones]

I had the wrong idea about this board. I thought that this was a sub board of a larger unit such as a CD player or digital audio tape drive and the power supply part was in the main part of the unit. As the power supply is part of the board it will mean that you will have to trace out at lesat part of the schematic. As the faulty board has +22 volts on the anode pins of the valve then there must be some components betwee the raw +22 volt supply and the input of the KIS-2R33S. If I was testing it (And was not capable of tracing the schematic.) I would unsolder the input pin of the KIS-2R33S and measure how much current it was consuming both with and without the valve plugged in. I would expect it to be about 100 mA with the valve and much less without.

Les.

 

[Nigel Goodwin]

Just to let you know, I dug an old valve data book out at work, the common ECC81/2/3 are 300mA heaters at 6.3V, and 150mA at 12.6V. The RF version, the ECC85 is 435mA at 6.3V, with no 12.6V option. The ECC88 isn't listed in the Mazda book I found.

 

[bittgata]

If by the anode pins you mean 1 and 6 then the one with 22V on those is the good DAC.

 

[Les Jones]

I must have looked at the wrong table when I looked back at post #22. I need to look at it again as that mistake means my reasoning was wrong.

Les.

 

[cowboybob]

bittgata,

This my take on the power arrangement on the DAC PCB:

The 16 0 16 AC is fed into the AC - DC PS, which output should produce both a positive ≈22VDC and a negative ≈22VDC. (Pls note the "≈" symbol).

Those DC voltages seem to be distributed, on the good DAC, thus:
22VDC Positive:
A. the Vin pin of the KIS-2R33S buck PS, whose ≈6VDC goes to pin #5 of the 6922 (filament) and, I believe, the LED circuit and, perhaps, other components as well.
B. the plates of the 6922 and, perhaps, elsewhere as well
and
22 VDC Negative:
A. the cathode and grid circuits of the 6922 (I doubt it goes anywhere else...)

What's missing from the disabled DAC board tube pin voltages (from the test voltages you listed in Post #22) is the "appropriate" (as per the good DAC) plate voltages.

To test that theory, we'll need some very good, well lit, keenly focused closeup pictures, with very good contrast, of both sides of the PCB in this area:

Traces on the board are of significant importance, as well as any component identification you provide.

I'm suggesting is that, for starters, we disconnect the Vin pin of the KIS-2R33S from its trace and see what happens with the plate voltage.

 

[bittgata]

I'm suggesting is that, for starters, we disconnect the Vin pin of the KIS-2R33S from its trace and see what happens with the plate voltage.

Sorry, I was away for a few days.
I'll take the pictures soon, regarding the last part of your message , do you want to just me to desolder the Vin or did you have something more complicated in mind?

 

[cowboybob]

... do you want to just me to desolder the Vin ...

Yes, desolder the Vin tab from the PCB trace.

... did you have something more complicated in mind?

I'm trying to figure out why the plate voltage of 6922 on the bad DAC is whack.

My theory is that since the ≈ +22VDC appears on the good DAC at the plates and the Vin of the KIS-2R33S, but is not seen on the bad DAC, might indicate a problem with the other AC to DC PS on the opposite side of the PCB (fed by the external 16 - 0 - 16VAC transformer).

Removing the load represented by the KIS-2R33S PS from that circuitry will, hopefully, tell us something relevant.

 

[bittgata]

This is the best I can do right now. The big caps are 3300uF 25V and the small caps 330uF 35V. The KIS-3R33S is missing the top cover on both my dacs, I saw many online pictures with the cover missing as well but I attached one with the cover so you can see what part it is(they are all the same version of the dac).
After I desolder the Vin should I test anything else or just the plate voltage with no tube mounted?
https://i.imgur.com/Vjtet71.jpg
https://i.imgur.com/SlPHPub.jpg
https://i.imgur.com/NTO9spd.jpg

 

[cowboybob]

After I desolder the Vin should I test anything else or just the plate voltage with no tube mounted?

Just the plate voltage with no tube mounted, for the time being.

BTW, good pictures. Note below:

As I suspected, the KBP310 is a full wave bridge rectifier, apparently being fed by the AC voltages coming from the 6 pin connector. This lends credence to that area being the source of the ± ≈22VDC voltages. I cannot, however, ID the chip noted "???". Try googling the identifying characters and see if you can find a datasheet for it (I suspect it might be a voltage regulator, but ...).

 

[pfofit]

https://www.onsemi.com/pub/Collateral/NCP1117-D.PDF
low drop out regulator 5.0v
see page 14 for markings