TASCAM 308B (POST GENTLE EXPLOSION)

[ZLYYY]

Hello : )

So I picked up this mixer from a venue that had it laying in the corner being all sad for like forever...
It's a mixer that does not function anymore.
Poor guy was just being roasted by tobacco all night long, 365 days a year.

Anyways, fast forward...
It got deep cleaned and also it deserved a "SOLID" recap.

I work on / with TEAC Tascam Mixers and R2Rs daily...
But apparently mistakes can still happen...

The Phantom Power PCB had one leaked out capacitor. It was expanded enough to see a visual abnormality.
It was the 10uF 50V capacitor.

While I was at it, I thought let's just replace the one next to it. The 100uF 10V cap. This is one of the most common capacitors on TEAC / Tascam PCBs.
Replaced it immediately without any serious inspection.

Little did I know... Well they bounced in the most rare value cap I have ever encountered on TEAC / Tascam circuits.
It was not 100uF 10V, it was 10uF 100V...

Turned it on, smile on my face, enjoying how everything beautifully lit up and them bam...
After about a minute, just like a Glock 45 going off and I could see the cap lid shooting across the room...
Smoke, smell, etc...

Obviously devastated once figuring out what capacitor should have been there, but it is what it is now.

I have put the correct cap back, replaced the 1 and 2 Fuses but it's just a super loud buzz from this mixer and nothing else.
Fuses 1 and 2 burns out in less than a second once they get replaced.

How much damage did this mixer have received in Your opinion?
Will a new Power Transformer fix this or it most likely dead all the way though?

Thank You

 

[sweetbeats]

It’s really helpful, when asking for this kind of technical assistance, you be as detailed as possible. I know the M-300 series consoles really well, so I can reverse engineer your post to what components you’re actually talking about, but using the actual part designators is SOP.

Just to confirm, you replaced C054 and C055 on the phantom supply PCB?

100uF/10V…I think that’s pretty rare in and of itself…I see 100uF/16V and 25V more commonly.

Anyway, these things happen even when you triple check.

Unfortunately you have a not simple situation on your hands now, because F1 and F2 have nothing to do with the phantom supply.

Can you be more specific as to the hum? Do you hear it in all outputs whether or not the phantom is switched on anywhere?

What frequency is the hum?

Have you measured the DC voltage of all the regulated power rails? +15V, -15V, +6V and +48V?

Do the fuses blow even if you isolate the power supply?

Do you measure any AC volts at the power supply outputs?

These are all step 1 power supply troubleshooting measures. You have power supply problems. Don’t assume the hum is directly related to the cap blowing. Hitting an output filter cap (C055) rated for 10V with +48V shouldn’t take out the +/-15V audio power rails powered by F1 & F2, so maybe there was something on its last legs. Nor should it affect the main transformer…an overcurrent condition will damage the transformer, and generally it has to be sustained for some time depending on the current, but that filter cap isn’t even in series, so it blowing should, according to the circuit, just have some minor effect on the high frequency noise floor of the phantom rail. It’s not the main filter…nearly all the AC ripple post bridge rectifier is mitigated by the time the power is filtered at the primary filter cap (C052) and the transistor network that provides regulation. So…what you did shouldn’t cause what you are experiencing. So you don’t know what’s going on. So you have to start at square 1, isolate the supplies and start checking…first for correct DC voltage and next for any AC ripple…it should be 0VAC in all cases…a couple-three mV is okay, but anything more and it’s time to get out the scope so you can start actually seeing what’s going on.

 

[ZLYYY]

Thanks for Your response!

Correct, C054 and C055 on the Phantom Power PCB.
C054 / 10uF 50V
C055 / 10uF 100V

The one that I have missed to pay some extra attention was C055.
I have placed a 100uF 16V instead of 10uF 100V...
There's always a first time : )

I took some measurements:

/// "Power On" Switch PCB
1 /// 0.43V
2 /// 13.6V
3 /// 12.5V
4 /// 0.43

/// Power Rail
0 /// 018.1mV
+15 /// -311.1mV
-15 /// -056.3mV
+6V /// 6.1V

///Power Supply PCB
1 /// 20.7V
2 /// 0.019V
3 /// 20.6V
4 /// 9.5V
5 /// 0.023V
6 /// 9.4V

It's more like a buzz sound. I will attach it here:


It affects all the channels and is not really responsive if You twist the knobs or move the faders.
Turning the Phantom Power on or off doesn't seem to affect the buzz much.
It is not possible to receive any sound coming in at this point.

Fuses blow instantly...

 

[sweetbeats]

Thanks for the reply.

/// "Power On" Switch PCB
1 /// 0.43V
2 /// 13.6V
3 /// 12.5V
4 /// 0.43

/// Power Rail
0 /// 018.1mV
+15 /// -311.1mV
-15 /// -056.3mV
+6V /// 6.1V

///Power Supply PCB
1 /// 20.7V
2 /// 0.019V
3 /// 20.6V
4 /// 9.5V
5 /// 0.023V
6 /// 9.4V

I’m not going to spend time interpolating this information. I have no idea what the bulleted numbers mean. Are those just bullets or do they related to wire terminals or…???

And are you measuring AC volts or DC volts or…?

I’m not trying to be a dick at all…it’s just I’m obviously willing to engage and give you high-level technical advice for free and I literally do not have time for a game of 20 questions…kids…busy day job…moonlight tech job paying customers waiting…I know you’re trying, but please take the time to narratively walk me through what you’re doing.

 

[ZLYYY]

Apologies : )

I have tried to be quite precise by writing down the Voltages of the wire terminals on each PCB.

The wires are numbered like that on the PCBs so there are no other terminals marked in "Numbers".
I thought it was precise enough.
I will list it again with colours : )

/// "Power On" Switch PCB
Measuring AC
Wire Terminal 1 BLACK /// 0.43V
Wire Terminal 2 BLACK /// 13.6V
Wire Terminal 3 WHITE /// 12.5V
Wire Terminal 4 GRAY /// 0.43

/// Power Rail
Measuring DC
0 /// Shows 018.1mV
+15 /// Shows -311.1mV
-15 /// Shows -056.3mV
+6V /// Shows 6.1V

///Power Supply PCB
Measuring AC
Wire Terminal 1 RED /// 20.7V
Wire Terminal 2 BLACK /// 0.019V
Wire Terminal 3 RED /// 20.6V
Wire Terminal 4 ORANGE /// 9.5V
Wire Terminal 5 WHITE /// 0.023V
Wire Terminal 6 ORANGE /// 9.4V

As You can see voltages are all of the place.
I'm just asking for a guess. Would You guess that it's the Power Transformer that is playing or the Power Supply PCB that is done?

Thank You

 

[sweetbeats]

Hi. Thanks. I don’t need the colors, but telling me what the numbers means helps, and AC or DC volts.

I don’t know what “///Power Rail” is. The other thing I can’t tell is what you’re measuring across. When we measure for voltage it’s always across two nodes or points or places right? You’ve got a positive probe and a common probe right? Your lists just detail some voltage present at one point but there’s no information what that one point is referenced to.

As per my previous post, all we care about right now is measuring for DC and AC voltage at the outputs of the regulated DC power rails; +15V, -15V, +6V and +48V. No need to measure at the power switch or inputs to the power supply assemblies, just the outputs.

So, for the main power supply PCB assembly, set your DMM for DC volts, put the common probe on terminal 20 (ground reference) and put the positive probe on:

Terminal 19 (should be at or near +15V)
Terminal 21 (should be at or near -15V)
Terminal 22 (should be at or near +6V)

Now do the exact same thing but set the meter to AC volts. The results should ideally be 0.00V in all cases. A couple-three mV is okay.

Now go to the phantom power PCB set your DMM for DC volts and put the common probe on terminal 4 (ground reference), and the positive probe on terminal 3 (should be at or near +48V). Now repeat but set the DMM to AC volts. Again the result should be 0.00V or very near that.

If you keep blowing fuses I suggest you isolate the power supplies, which means disconnecting the outputs from everything downstream. This may require desoldering wires form the terminals. If fuses still blow with the power supplies isolated let me know. Report back.

 

[ZLYYY]

Hello,
Thank You very much for Your response : )

Here are the new numbers... Looking pretty bad...

*Before Power Supply Isolation
///Power Supply PCB
Measuring DC from across Terminal 20 (Ground) to:
Terminal 19 /// -304.5mV
Terminal 21 /// -0.54mV
Terminal 22 /// 6.1V

Measuring AC from across Terminal 20 (Ground) to:
Terminal 19 /// 0V
Terminal 21 /// 0V
Terminal 22 /// 1.8V

///Phantom Power PCB
Measuring DC from across Terminal 4 (Ground) to:
Terminal 3 /// 0.40mV {Numbers keep dropping forever}

Measuring AC from across Terminal 4 (Ground) to:
Terminal 3 /// 0V

//////////////////////////////////////////////////////////////////////////////////////////////

*After Power Supply Isolation
There are no wires going downstream.
The only wires that were connected to it while I was writing down the numbers were:
Terminal 1 to Terminal 6 (1, 2, 3, 4, 5, 6)
Terminal 10, 11, 12
Terminal 13, 14, 15
Terminal 16, 17, 18
Terminal 20

///Power Supply PCB
Measuring DC from across Terminal 20 (Ground) to:
Terminal 19 /// 0V
Terminal 21 /// 0V
Terminal 22 /// 6.1V

Measuring AC from across Terminal 20 (Ground) to:
Terminal 19 /// 0V
Terminal 21 /// 0V
Terminal 22 /// 1.8V

///Phantom Power PCB (Removed)
Measuring DC from across Ground Wire (Black) to:
Pink wire /// 0.40mV {Numbers keep dropping forever}

Measuring AC from across Ground Wire (Black) to:
Pink wire /// 1.1V

Fuses F1 and F2 blows in less than a second when it gets turned on.
I suspect that's why all these measurements are so crazy.

Thank You for taking time to read this.

 

[sweetbeats]

So if wires are disconnected from terminals 19, 21 and 22 the main power supply is isolated. And if the fuses still blow you have significant issues with that power supply. Fuses protect what’s downstream from them. If something fails downstream causing too much current to draw the fuse blows, protecting stuff from getting roached from the over current condition. Fuses have nothing to do with what’s upstream from them. They don’t protect the power transformer from an over voltage condition at the mains power source for instance, though if there is an overcurrent issue downstream and the fuse blows that can prevent damage to the transformer, but the point is the fuse is trigger by and prevents damage to primarily what’s downstream. There’s not much downstream with the supply isolated.

Pull F1 and F2. Set your DMM for AC volts and measure across terminals 1 and 3.

 

[ZLYYY]

Thank You for Your response : )

After pulling out F1 and F2 and measuring AC voltage across Terminal 1 and 3, I got a measurement of 41.8V

 

[sweetbeats]

I think your transformer is probably fine. Again, they can fail either by virtue of a significant over-voltage condition at the input, or sustained over-current condition at the secondaries.

I think your power supplies need repaired.

 

[ZLYYY]

I hope so...
It's way more challenging to find a new transformer for it because of the Phantom Power than it is to repair the Power Supply PCB : )
Will start the deep dive in couple of days, I'm not sure I have some of those Voltage Regulators at the moment...

 

[sweetbeats]

They are all super-common 78xx and 79xx parts. The power transistors that are mounted to the heat sink can easily be found too, even if not identical parts but equivalent replacements. Just pull the data sheets for the original parts and look at the specs.

I may have complete replacement assemblies here for both.

The voltage at the output of your transformer looks correct. Is your meter RMS measuring or peak-to-peak? Either way 40+VAC across the secondaries looks like correct and adequate power for +/-15V rails. Again it takes a bit more to roach a power transformer…like connecting a 110V transformer to 220V power, or a significant and prolonged over-current condition, one which, in this case, should blow the fuses long before it’s a problem for the power transformer. If it doesn’t stink and the voltage is correct at the secondaries it’s fine. As an additional check you could measure the AC voltage across terminal 1 & 2 and across terminal 2 & 3. It should be roughly the same between each and around 21-ish VAC for each.

 

[ZLYYY]

I just took the measurement across the Terminal 1 & 2 and then across Terminal 2 & 3 and the measurements were indeed the same.
20.8V on both : )

I really starting to be slighlty relieved that the transformer is not cooked. Anytime it's on, there is no bad smell from it either.
I hope it's the Power Transistors or Voltage Regulators...

p.s. I'm measuring with a RMS meter.