TAMA Techstar TS305 - Sound Cutting Out

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TAMA Techstar TS305 - Sound Cutting Out

Postby Bobert_74 » Thu Jun 14, 2018 8:50 pm

Hi everyone! New to the group here and quite a novice when it comes to analog hardware.

This is NOT about a key synth, but about a drum machine that TAMA made in the 80's similar to the Simmons. It's called the Techstar and had different models with various numbers of pads/triggers. This type of unit is unique in that each pad's trigger bank is an analog synthesizer and has it's own IC card slot per bank.

Anywho, I came here because I have a unit I'm having trouble with. Got this unit used many years ago and it worked fine. Then it went into storage for a few months and moved across 4 states. While in storage, the brain component apparently fell to the floor from about 4 - 5 feet (~12 to 15 meters).

When I powered it back up, it would play great for a little while and then the sound would cut out after about 10 minutes. Not for just one bank, but all of them. The power light is still on, but no sound. I turn the unit off, come back the next day and it does the same thing.

When it happens, it sounds like a very short puppy whine before the sound goes totally out. The sound doesn't have to be triggered, it just makes the sound on its own when it cuts out.

I have done some basic troubleshooting - mainly looking for loose connections/missing components, etc. All OK. I also gave it a thorough cleaning with contact cleaner to no avail.

My engineering background tells me this might be a capacitor issue or similar component that builds up and releases, but I have only basic electrical engineering troubleshooting skills.

Has anyone run into a similar issue on this unit or an analog key synth? These units are rare *and awesome* and I'd hate to brick it. This seems like an *easy* enough issue to fix, I just need some help digging a little deeper.

Here's some pics. Thanks in advance!!

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Re: TAMA Techstar TS305 - Sound Cutting Out

Postby Rasputin » Thu Jun 14, 2018 11:56 pm

Yeah, definitely don't brick it. I think you're right that it will be an easy fix (famous last words).

First thing I would do is disconnect the drum modules and just leave the PSU and mix module powered. Then connect something to the Line In and monitor the outputs. If it still does the "puppy whine" then move to a rigorous investigation of the PSU.

The PSU connector going from the PSU to the mix module should output +6V, +15V, -15V, and also have two ground connections. Monitor this voltage for a while (mean time to failure is 10 minutes?) and make sure it's rock solid. Now monitor the same from the mix module connector side.

I'll think about it more when I have some more time.

EDIT: The headphones versus the mixout/XLR are relatively independent circuits so it will tell us a lot if they both fail with only input from the Line In or if they're both dead.

IC403 (NJM45580) and the +15V and -15V DC rails are the most critical components if neither the Line Inputs or drum modules appear to sound on both the mix outs and the headphone outs.

In other words, if none of the outputs pass any signal regardless of Line Inputs or drum modules being used then IC403 or the voltages feeding the op amps are the first place to look.

IC403 is a dual op amp, so that one chip handles both the left and right channel at the same time. Seeing as the signal comes and goes instead of being permanently out, permanently distorted, or only one channel being lost, it's highly unlikely that IC403 is the point of failure.

As such, I would say that either +15V or -15V is dropping out. You should see a 815 and a 915 regulator on the PSU. When the unit fails, test for DC voltage on pins 1 and 3 on the five pin header located on the PSU (probably silkscreen printed with "CN54").
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