Tascam 38 R+P Card Potentiometer Problems

[Ruebinb]

So I'm getting around to restoring a Tascam 38 I bought a couple years ago. For reference I would say im entry level when it comes to tape machine restoration. I've fixed a couple 4trk cassette machines. Mostly basic transport control problems. This is my first reel to reel.

To catch you up, all the transport controls work fine on this machine and I've done a thorough cleaning and demagnetization on the tape path.

The problems I'm seeing are on the circuit side of things.

1) Vu meters are not reading on certain channels.
2) After recording sync plays back faint on certain channels.
3) After recording repro was playing faint to inaudible on all channels.

I started with the reseating/deoxit the R+P boards since it was the least invasive job. This is where I discovered the source of the problem. Channel 2 VU meter wasn't working so I sprayed deoxit in the adjustment pots (R141) and started to fiddle with it. It would jump whenever pressure was applied to the pot with my thumb. I decided to try the same thing on a channel card that had bad sync & repro (R122 & R120) playback. It worked! the sound would cut in and out when pressure was applied. If I put enough pressure the channel would stay on.

So this is where I'm at now. I think either the pot or the connections have gone bad. I reflowed every circuit joint that had that style of pot on it. I figured it probably happening to all of them. It partially fixed the problem. It's easier to find the spot where they don't cut the audio signal when applying pressure. I can set them there but it seems like a temporary fix.

Has anyone else had this problem? What should I do next?

 

[sweetbeats]

The motherboard of the 30 series is known to have mechanical failures of the solder joints, and I understand it can be beneficial to reflow those solder joints.

 

[Ruebinb]

Yea that was my next step and testing the relays on the channel boards. I guess my question was more towards if I need to replace the adjustment pots?

I think the joint themselves on the board are bad and I'm worried that if I keep messing with them it's gonna eventually break the circuit. I desoldered one of them just to take a look. The solder joint ring and some of the circuit freyed.

Im not too acquainted with this symptom. Either the flux eroded the joint, bad oxidation, the old solder tore up joint, or I did. Attached a pic of the bad pot connection. Channel 8 bottom 4th from the left.

 

[sweetbeats]

Those trimmers will out-last any other potentiometer on that machine. They are not the problem.

I recommend you find some scrap PCBs to use for practice with your soldering.

What type of iron do you have? What type of solder are you using, composition and diameter wire? What are the specs on your solder tip, type and size?

You’re applying too much solder, too much heat, and I suspect your tip is too big. You have to be careful with phenolic resin PCBs. You have to be careful with everything, but particularly those types of boards. Too much heat or for too long and the trace delaminates from the board and can become fragile and break off. You fried it. When you reflow solder joints you maybe add some solder, and if so just a little bit. You’re adding 4x or more solder than the factory joint. Who knows…maybe somebody shoved on the trimmers too much and cracked a solder joint or trace, but you have to assess that and then pick the right fix. If it’s a broken trace you have to remove a little bit of mask from the trace on both sides of the crack and then bridge the crack with jumper wire and solder it in. But I would not suspect bad trimmer resistors themselves. Cold solder joints, maybe, bad relays probably…but let’s figure out your equipment and supplies first and you should do some practicing to minimize the amount of time heat is applied.

 

[Ruebinb]

I'm using a Weller 40w 120v station for an iron with an st3 tip. The solder I'm using is Maiyum sn63 Pb37 Flux 1.8 0.8mm 50g rosin core solder.

Soldering is definitely not my fortay. Mostly PCB proto board for Arduino projects. First time reflowing on an old board. That trim pot I took out and I definitely overheated when desoldering. Ill be doing some practice runs before I tackle the motherboard.

I got them all working besides the fried channel 8 trimmer. When I say them I mean the three trim pots that were causing issues on each board. VU, sync, and repro gain adjustments. They're just delicate. If they get bumped they can cut out.

I agree that it's not the pots themselves but most likely the joint and trace connections. Thanks for the input, I'm kinda in the grey when it comes to points of failure. I'll take a deeper look for making any more modifications.

 

[sweetbeats]

Thanks. Not trying to be a downer.

What model Weller station? I generally like using something that runs at about 700 degrees. It needs to be hot enough to melt the solder. The wattage is what sets how quickly the iron can keep up with heat discharge. So a higher wattage iron will be able to heat soak the work more quickly…can recover more quickly…has more oomph at the iron’s operating temp, okay? I use a couple different little Antex pencil irons, a 12W or a 15W. 40W is a LOT of iron for what you are working on. The high wattage means you have to be very accurate with your application of the iron because the heat is going to very quickly soak into the PCB. I can’t imagine working on boards like that with that much wattage. So I think that’s part of what’s working against you. And then the other rule of thumb is you generally want your iron tip size to be about 1/3 the size of the pads you are working on. The ST3 is 1/8”; too big. So you’ve got a lot of wattage, and a large tip with a large contact area and an iron that can recover and provide a lot of heat energy continuously. And I don’t even know what temp your iron runs at. So it’s kind of an impossible situation. Your solder sounds good. You just need a smaller tip, and if your temp is adjustable I’d set it around 700 degrees. The only thing you can do about the wattage is get a different iron. But by using a smaller tip and ensuring proper temp, and some practice, you might be in better shape. You just have to work quickly. By the way you don’t need flux for reflowing, or really for any resoldering if your solder is rosin core.

If you still have signal cutting in and out if you bump the trimmers, there may be damaged traces.

 

[Ruebinb]

The weller station is a wlc 100. I'll grab a smaller tip.

I'll do some research on trace repairs. What kind of wire do you use for a jumper? Also I assume Ill have to clean off the solder to repair.

 

[sweetbeats]

The weller station is a wlc 100. I'll grab a smaller tip.

I'll do some research on trace repairs. What kind of wire do you use for a jumper? Also I assume Ill have to clean off the solder to repair.

I read the manual. So the tip temp is fixed at about 900 degrees. That’s too hot IMO. So, again, if you’re going to continue using that iron, you’ll have to work quickly. Where do you set the power knob? That controls the wattage. If it was me I’d make sure I had a right-sized tip (like maybe something around 1/16”?), and I’d set the power control to maybe 2 to 2.5.

For bridging broken traces I like to use trimmings from electronic components. Like when I recap something, for instance, and trim the tails of the new cap after soldering it in, I like to save some of those trimmings for repairs…basically it’s just small gauge tinned solid core wire. For longer jumpers, like for really compromised traces or missing pads, I have a bunch of wire harnessing from dead devices…24AWG tinned stranded hookup wire. So any small gauge wire if you need to install a point-to-point jumper for something that’s really messed up. I hope that’s helpful in some way.

 

[Ruebinb]

Yea thanks for the info. I'm gonna put a pause on working on the unit. I'm up in Wyoming visiting my parents. That's where I've had the Tascam stashed the last couple of years. Not the best place to dissect the machine.

I got a circuit wiz buddy back down in New Orleans with all the right equipment. Will sit down and geek on the service manual before I do any more prodding and prying. I want to tackle calibration myself too so there's no rush in getting things fixed here.

Also, looking a replacement parts it seems like it would be best if I had a spare to pull parts from. There's a couple 38s for sale on my route back down south.

 

[sweetbeats]

That all makes sense FWIW.

And yes…I think it’s very beneficial to have a spare machine. I even called it one of the most underrated “accessories” to a tape machine at one point. I have gobs of spares for my Ampex machine…a complete spare transport and electronics for my 3M halftrack…a, for all intents and purposes, complete spare machine for my Tascam 58…same for the 122 cassette deck and 244…and same for the Audio Technica AT-RMX64. Gobs of spares for my Studer console…I’ve salvaged as many compatible spare parts as I could for my one-of-a-kind Tascam prototype console from period consoles I’ve parted out…pots…switches…meters…etc. And complete backup unit and spare parts donor for my Yamaha AW4416…and then there’s all the electronic components…resistors, caps, transistors, ICs, etc. Through-hole parts are already becoming harder to find.

I’m sure I could find people that would say “yeah that makes sense…” I’m sure I could find a lot more that would say it’s excessive. But it’s really helpful to be able to pull spare assemblies and also to have a donor machine to explore how it’s put together or to test things out without cutting on the primary machine.

Go get your spare 38.

 

[Ruebinb]

For sure. VHS camcorders were my first dive into old electronics. I started collecting a variety of camcorders at first. Then I narrowed it down to the Panasonic AG-456. I had four of those units. Kinda bet on the wrong horse because the leaky caps in those cameras were sooo bad. Became a lost cause very quickly.

Now that I'm down the rabbit hole of tape machines it makes me wonder what horse to bet on? There's some specifics in what functions I want in a tape machine, but now I look more towards accessibility to parts and ease of maintenance. It's usually after I buy something that I find that out though. I think I'm gonna stick with the 38. I have a Marantz PMD 430 for field recordings, a PMD 740 for 4trk cassette, and now the tascam 38 with a ramsa mixer for 8trk tracking.

I was looking into the Tascam TSR-8 or the fostex r8. Mainly because of the expense of 1/2 vs 1/4 tape. I know I'm gonna have to eventually fork over some cash for a test calibration tape. Not a big enough reason for me to switch camps though.