Tascam 388: Identical Hiss on All 8 Channels (No Tape Needed)

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pgj

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Tascam 388 with identical hiss across all 8 channels (no tape required!)​


I posted this on the main 388 fb group, but I'm not sure how much these two groups overlap, so I'm posting it here as well.
At the time this video was recorded, I had 3 noise issues:
  • channel 1 has a loud hum
  • channel 8 has a tinny hiss above and beyond the next noise issue (below)
  • all 8 channels share an identical hiss

The channel 1 problem has since been resolved. It was a loose wire connecting the rec/play head to the harness pcb.

The channel 8 problem has been somewhat isolated to its corresponding rec/play pcb, though i'm not sure which component it is on the board. (if anyone has a replacement board, i'd be interested)

Because the video was recorded with all 8 channels engaged and set to unity, and because channel 1 at that time still had the added hum, the hum is in the mix of what you'll here in the video, but the rest of the hiss is all still present. If i disengage 1 and 8 and perform the same test on only 2-7, it's nearly the same thing, just with out the added hum of channel 1 and tinnier hiss of channel 8. Still waaaaay too loud. Worth noting that after fixing channel 1 hum, it now sounds identical to 2 thru 7. Same exact hiss. It must be coming from some board that everything routes thru. My guess is the monitor pcb, but please feel free to steer me in another direction if I'm wrong.

Note that in the video there is no tape in the deck, so this is not tape hiss.

The heads are just back from JMA. I had the rec/play head relapped. 0 hours on the heads since getting them back.

Monitor, Buss A, Buss B, and PSU boards have all been recapped.


The setup for the test is:
  • all 8 on rmx
  • all 8 engaged (L/R switch)
  • all faders at unity
  • main monitors on

To avoid adding motor noise I just move the lifter up and down.

UPDATE: In the original post I said that every output, **including access send and tape out**, which are both per-channel outputs, rape The common/shared hiss, but that was incorrect. After rechecking all outputs, it looks like the 2 channel specific ones, access send and tape out, do***not*** have the common/shared hiss.

 
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Check the output of the audio power supply rails. Check DC and AC volts. Always start by checking the power supply.
 
sweetbeats said:
Check the output of the audio power supply rails. Check DC and AC volts. Always start by checking the power supply.
Click to expand...

Due to a previous power-circuit-megashock from a mesa boogie head, I'm now extra careful about dealing with power circuits. My (new) habit is to power down, unplug, drain all caps, THEN tinker, but this test will require that power is applied. So please discuss which test points in particular and how to do this safely.

Can I test a little ways down stream -- away from the power circuit -- to avoid the danger altogether?

Does the manual cover this test procedure?
 
Ah, hang on...

As it happens, my 388 has the bottom plate removed and is tipped up where I can get to other areas I've been looking at recently.

The Mother PCB, into which the Power Supply PCB is inserted, has very informative markings on the bottom...

(UPDATE: added closeup of the section in question)
 

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The 5V line is a steady 4.96 VDC.
The +15V line is a steady +14.74VDC.
The -15V line is a steady -14.74VDC.

(measured at the points shown in the closeup in the previous post)
 
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One clarification: I went back and re-checked all the exits. There are actually 2 which do not have the common/shared hiss: access send and tape out -- both are per-channel. All other outputs have the issue, even the per-channel pgm out.
 

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Hang on. I've been ignoring dbx altogether since purchasing this thing as I never intended to use it. But just for kicks I enabled it and the problem disappeared. I'm not sure how to interpret that though. This is after all the exact kind of noise that dbx is designed to cancel out. This particular problem would be right up its alley i would think. So I don't know whether to interpret this as dbx doing its job... or... that the problem is actually on the dbx boards. Hard to believe that though because it's two boards. To me it seems like it is more than likely going to be a single point of failure somewhere along the path that all eight channels travel. Is it possible that both dbx boards have the exact same problem? In my mind it's more likely that it's just doing its job and that the single point of failure is yet to be found. Anyone have any insights into this, as it relates to this all-channels-hissing problem?
 
Can someone run the same test? Because of the dbx channel 8 override on the back, let's keep it simple and just do channels 1 thru 4. Remove tape, turn monitors on, all 11 faders on unity, all channels on RMX, only channels 1-4 engaged for playback ("L/R" button), disable dbx 1-4. Start recording video here. Move the lifter up and down slowly a few times. Then enable dbx 1-4 and move the lifter up and down slowly a few times again. Very interested to hear what everyone else is experiencing in these scenarios.
 
The noise reduction only affects the signals recorded to and played back from tape. It should have no effect on signals from inputs at any outputs. Is this a new problem or did you get the machine this way?
 
I bought the machine in this condition.

It was only minimally tested by the seller, and priced accordingly.

I understand that DBX encodes data onto the tape and decodes it during playback, but what about my situation where I am in playback/remix mode and the lifter is in the playback position, though no tape is present? It's not in mic or line mode on any channel. Remix/playback across the board.
 
Hey there pgj,

Would you mind being specific about what connection was causing the hum on channel 1 in your case? I've been having a frustrating issue with hum on almost all of the channels on my 388 and from your description it seems very similar to the issue you were having. Sonically it sounds almost like a ground hum but more muted?

I *think* I've tracked the issue to the rec/play cards and have tried reseating them/reflowing the solder on the connectors on the cards themselves to no avail (I've had a bunch of problems with cold/broken solder joints on this machine).

In addition, on my machine it's also inconsistent which channels have hum on the main stereo outs vs. the tape outs. TIA!
 
Did you scope the outputs of the audio power supply? It’s usually best practice to, step 1, check the power supply…make sure the regulated DC outputs are at the correct voltage (which should be around +15 and -15V), but also check to make sure there are no AC components. The best way to do this is with a scope. You should see no AC in the waveforms at the output of the audio power rails. If you don’t have a scope or know how to use one you can also use a multimeter, ideally one rated for audio systems (accurate across the audio band 20Hz to 20kHz). You set it to AC volts and measure across each power rail to the power rail ground (the 0V reference), and the AC volts should ideally be 0.00…a couple-three millivolts is okay. I would start there especially with something like global hum in an audio system. You might have a failed bridge rectifier or main filter cap in the power supply, etc.
 
Hi sweetbeats,

First of all, thank you for your reply and your dedication to helping out 388 owners such as myself - advice from you on other threads here has helped me with probably a dozen other problems with my machine. You rock!

I'm currently at work, but I'll run that test when I get home. I have already recapped this power supply, and my only thought as to why it wouldn't be AC noise in the main power rails is that the channels are all pretty quiet when in MIC or LINE mode - they only make the loud hum noise in RMX. Could be that the noise is there and is being amplified by something when in RMX rather than the other two modes?

Also, on the chance it is a failed bridge rectifier in the power supply, do you have any recs on a replacement? I tried to find one when ordering parts to recap the power supply but couldn't find any in that form factor.
 
I’ll have to review what’s installed as far as replacing the bridge…

If it’s only in RMX mode and therefore sourcing the tape section, it could be one of the other power rails. You should check all of them for clean and correct DC voltage. Do you hear the hum in the TAPE OUT jacks as well?
 
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Here are the results of testing:

+15v rail: 17v DC, 36v AC
-15v rail: -17v DC, 0v AC
+5v rail: 5v DC, 10v AC

Granted I don't own a scope and this is with my Harbor Freight multimeter (college student budget!) but I think we've found our issue; I stand corrected sir. Additionally, the same hum is present in the TAPE OUT jacks.
 
Can you be more specific on from what pin to what pin of what connector on the outputs of the power supply PCB you were measuring? Also, have you visually inspected to ensure there are no bent pins on the motherboard? Also have you reflowed the solder joints of the power supply PCB edge connectors? Your meter should be *okay* for measuring power supply voltages. Can you share the make and model of the meter?
 
Measured pins 1-3 on P5 for the 15v sources, and pins 1-2 on P32 on the mother 2 pcb for the 5v sources, and also double-checked the 15v sources on pins 3-5 there as well.

Visually inspected the pins on motherboard pcb and nothing stuck out to me. Did the same for the edge connectors on the power supply pcb and didn't see any evidence of cold solder joints, but if I have some time in the next couple days I might reflow them anyways for a bit of PM.

Couldn't find the make and model of the meter but here's the link:

Maybe it is time to upgrade to something nicer...
 
So, I think it would be worthwhile to check ALL the DC rails. I know some of this is duplicative to what you’ve already done, but here’s where you measure each rail:

+5VDC (logic): measure between pins 5 & 6 of P7 on the MOTHER 2 PCB

+15VDC (audio): measure between pins 1 & 3 of P5 on the MOTHER 2 PCB

-15VDC (audio): measure between pins 1 & 2 of P5 on the MOTHER 2 PCB

+15VDC (transport servo): measure between pin 4 of P5 and pin 2 of P6 on the MOTHER 2 PCB

+24VDC (transport servo): measure between pins 2 & 3 of P7 on the MOTHER 2 PCB

+35VDC (transport servo): measure between pins 1 & 2 of P4 on the MOTHER 2 PCB

Again, measure for DC volts and AC volts of each of the above. In all cases, except for the +35VDC rail the AC volts should measure at or very close to 0.00. The +35VDC rail will have a little more AC ripple and the DC voltage measurement may be +/- a wider range from +35VDC because it is not a regulated power rail, but only rectified and filtered.

Regarding your meter: I would personally not use that meter for what you are doing. I actually would t use it for anything. No judgment. I used to have one like that. It’s not rated to be very accurate, and it’s only rated to measure AC volts from 45Hz to 450Hz, so it is very limited, and if you have any kind of oscillation happening with anything you are measuring it may throw it for a loop and give you very inaccurate results. Again, no judgment and I appreciate you sharing the detail.
 
Sweetbeats, thank you for the detailed reply. Don't have a ton of time to troubleshoot today but I'm headed to microcenter tomorrow for another project and wanted to run these multimeters by you in case one of them would fit the bill for this type of work:

Klein Tools MM325: https://www.kleintools.com/catalog/multimeters/digital-multimeter-manual-ranging-600v-0
Triplett MM520: https://www.triplett.com/products/mm520-true-rms-digital-multimeter-with-lpf-loz
Triplett MM525: https://www.triplett.com/products/m...1&_sid=1ef895c30&_ss=r&variant=39874284060838
 
None of them are designed for audio band measurement. What I mean by that is, for measuring audio signal (like test tone, and audio signal is AC, not DC), generally you look for a meter that is rated for relatively accurate measurement from 20Hz to 20kHz, which is the standard audio band frequency range. And it should also be a true RMS measuring voltmeter, because that’s the standard measurement convention in audio system signal measurement, vs. peak-to-peak. This doesn’t mean you can’t use a non-true RMS meter or a meter that is not rated for accurate measurement across the audio band, it just means you have to do some math to convert your AC voltage measurements from peak-to-peak to RMS, and also you have to pay attention to the frequency range accuracy limits of the meter and ideally measure signal within that frequency range. Out of the three you listed, the one with the best frequency range is the Triplett MM520, which is a true RMS meter rated for accurate AC measurement from 45Hz to 1kHz. So you can at least measure 1kHz test tone with it, but you wouldn’t want to use it to, for instance, calibrate a tape machine, which typically requires measurements far above 1kHz.

Hope that helps.
 
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