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.