Tascam 58-OB Story...

[analogkid]

Somewhere in the pile of stuff I have to read over is a tech book on power supply repair specifically... geez I haven't touched any of my decks in TWO WEEKS! It was a damned miracle I got 15 minutes to play my electric guitar the other night....... time to take a LONG vacation with my Otari, one mic and my acoustic guitar........


AK

 

[sweetbeats]

Tried soldering in R5 again tonight (the resistor that got fried on the Mother PCB)...

Check out this thread I started over in the DIY forum...trying to get advice on what to do. The foil has lifted and broken, the solder is not adhering properly to the foil anyway...It's a mess.

I did get U2 (the dead +15V regulator) pulled tho'...it was no problem.

I also pulled a couple caps off of the PS PCB to check and see if they have drifted outside of spec. They all physically look really nice. I'm assuming that in the parts lists, if there isn't a tolerance noted for a cap then it is 20%. Is that correct? If so, I pulled a 220µF cap that measured 251µF, and also a 3300µF cap that measured 3690µF

What do you think?

Go ahead and recap it? Do it just in case since the power supply took some kind of hit maybe with me improperly installing amp card #8??

 

[evm1024]

good work

OK so far sounds like things are going well. The 7815 is standard part that you can find at RS in their regulator grab bags. They are commody parts do dont worry about who makes.

78 says positive reg and 15 says voltage. 79 series is a negative reg. So 7815 is a positive 15 volt regulator and 7805 is a positive 5 volt reg (as you noticed). They go up to 24 volts as I recall (7824) and can source up 1.5 amp or so.

Anyway look in your local RS for an LM7815 or some such. Being an IC regulator doinng a continnuity test would not really show if it works. Just replace it.

Those caps do a little filtering and bypassing. Values are not critical. You could put them back or change them out.

R5....I've lifted traces myself at times. Better iron helps a lot. What you cn do is put a new resistor in through the holes and using the leads of the resistor form a new trace. You just need to scrape off the green "solder mask" that covers the copper of the good portion of the trace and cut and solder the lead to take the place of the lifted trace.

Sounds like you are moving forward and startng at the basics (do I have all voltages fromt he PS)

Your heads. I had thought about that...but I think they should be fine. That is the last inthe chain of what you are doing.

--Ethan

 

[sweetbeats]

You could put them back or change them out

So in your opinion there's no sense (i.e. no reason) in recapping at this point? I realize there are no hard-and-fast rules about when to do such a thing, but my understanding is that 25 years old is kind of a typical tipping point with electrolytic caps, and that power supplies are typically where they start to go first...

 

[evm1024]

Lots of factors in cap life. PS caps are in the brute force category in my mind. Mostly when they have failed on me it is with a bang if the PS has enough raw power behind it or oozing type.

On one hand you have the PS out and it is an "easy" job. on the other is well enough alone. There have been bad batches of caps made. Work for 6 months and then high failure rates.

Should you replace the electrolytics then I would suggest that you use 105 degree caps rather than the more common 85, and that you select the next higher voltage rating cap if it will fit. Values are the same otherwise.

--Ethan

 

[sweetbeats]

Lots of factors in cap life. PS caps are in the brute force category in my mind. Mostly when they have failed on me it is with a bang if the PS has enough raw power behind it or oozing type.

On one hand you have the PS out and it is an "easy" job. on the other is well enough alone. There have been bad batches of caps made. Work for 6 months and then high failure rates.

Should you replace the electrolytics then I would suggest that you use 105 degree caps rather than the more common 85, and that you select the next higher voltage rating cap if it will fit. Values are the same otherwise.

Hmm...I'll have to think about that...I have not left well-enough alone several times already and look at the spot I'm in now, and at the same time it is a relatively "easy" procedure at the moment with the PCB hanging outside the deck...It took me very little time to remove the couple I did last night, and I guess that's what got me thinking about just doing it...Thanks for the food for thought.

Couple questions on caps:

Is the temp rating just a physical rating of what environmental temp or self-temp the component is designed to handle, and the higher the temp means a more robust component?
Voltage rating...again, is this a capacity consideration? I mean,will a cap with a higher voltage rating be more robust and offer better performance over the long-haul, and is it a general rule-of-thumb to oversize caps when replacing where space allows?

 

[evm1024]

Hmm...I'll have to think about that...I have not left well-enough alone several times already and look at the spot I'm in now, and at the same time it is a relatively "easy" procedure at the moment with the PCB hanging outside the deck...It took me very little time to remove the couple I did last night, and I guess that's what got me thinking about just doing it...Thanks for the food for thought.

Couple questions on caps:

Is the temp rating just a physical rating of what environmental temp or self-temp the component is designed to handle, and the higher the temp means a more robust component?
Voltage rating...again, is this a capacity consideration? I mean,will a cap with a higher voltage rating be more robust and offer better performance over the long-haul, and is it a general rule-of-thumb to oversize caps when replacing where space allows?

Cap life is directly linked to internal (core) temps. Higher heat rating gives longer life. Also, working voltage as a percent of rated voltage also tells us how hard the cap has to work (or how stressed) and thus how hot it gets. Less stress = longer life. But let's not over do it.

It would be safe to replace the caps given the work you have into getting the PS out. But wait till you get that new iron and get a solder sucker rather than wicking them

--Ethan

 

[sweetbeats]

Sorry, when I used the term "wick" I was refering to the solder leeching into the joint when applying the solder. I do have a spring-loaded solder sucker...had it for 20 years or so and it is one of my favorite toolbox items.

But let's not over do it.

Is there a point of diminishing or even declining return then?

 

[evm1024]

More than two parameters in cap specs. Straying too far from what tascam designed without looking at the other specs and we don't know where we will end up. Size is the obvious thing here, but there are other specs. One up in temp and voltage will not change things too much. I would select temp upgrade over voltage upgrade.

 

[sweetbeats]

R5 is populated

I, thanks to Ethan's advice and the excellent advice given to me by several members over in this thread, the resistor in position R5 on the Mother PCB is back in.

The joint with the modified trace is a little funky because, obviously, the trace is not really big enough (at least with my experience) to get a really nice looking joint, but after I cleaned things up and got the newly exposed section of trace tinned, I think the bent tail is adhered underneath...it feels solid and I tested resistance at several points on the PCB and it is good.

Thanks again everybody. Its great how one can learn so much from such a small event.

Here is a pic of the joint-side of the completed reinstall...tell me what you think:

Here is what the site looked like after I cleaned it up but pre install of the 27Ω resistor:

 

[evm1024]

Not too bad...

It will work. With a better iron you should be able to get a little less solder but we are not grading you we are taking score. One point to you.

--Ethan

 

[sweetbeats]

Progress

One point to me...

Serves that Mother PCB right...its about 2 up on me anyway...

Well, I've pretty much decided that my old soldering iron is completely worthless. I got my Antex M2 iron today and tried it out tonight. So much smaller and easier to handle than my old one and yet the tip puts out more heat that is easier to control. That, coupled with the advice I've received during the past week on this thread as well as over on the DIY forum (keep the tip clean and tinned) made such a difference tonight. My work was cleaner and faster with much less impact on the PCB. I also found that clipping the tails after sucking the solderoff the joint made it a quicker more gentle job. I got all the caps removed (C20, the 6800uF cap was a bit of a bear...). I checked them all after getting them out. Most are pretty good but there are a couple that are really pushing the 20% mark...I also noticed that one of the joints for R15 looked weird...sort of greenish, so I measured it and it came out less than half of the specified value...R16 too was quite a bit out of spec. Looking at the schematic that makes sense...those are both in the 24V rail. Not sure what may have happened to the transistor that is just upstream of them...?

Anyway, I pulled them too and tested the rest of the resistors and they all seem fine.

Looking forward to using my new iron to put the replacement components in.

 

[sweetbeats]

I don't get it...

I was gearing up to reinstall the Mother PCB, but after finding those two bad resistors on the PS board I thought it would be a good idea to check the remaining resistors on the Mother PCB.

R1 ~ R4 are spec'ed at 1.2kΩ. I measured all of them and they came out at about 770Ω. They are rated at 5% tolerance so I figured they were shot...they are directly connected to the same 24V rail that cooked R5 so I figured they were goners too. So I pulled one, which was a little complicated because I had to pull one of the RCA jack input/output assemblies as the joints for R1 ~ R4 are directly underneath 4 of the jack assemblies ...anyway I got R1 pulled and just for poops and giggles I decided to measure it again...1.22kΩ!

I reinstalled it (new iron worked great) and now it measures 770Ω again like the rest of its ilk.

I spent some time studying the Mother PCB schematic, but it is not making sense why the measured value changes when it is installed...Can anyody help me with this? I'm trying to determine if I've got more components to replace...

R1 ~ R4 feed the power-up mute circuit which includes 4 diodes (D1 ~ D4) and 4 relays (K1 ~ K4)....

 

[analogkid]

Hmmmmm. Not sure why it would measure LOWER when installed. Then again, my electronics knowledge is VERY rusty - I'm still wading through the most basic theories ATM in an effort to relearn everything lst through a lifetime of beer .


AK

 

[evm1024]

Not shot

When you measure a resistance in circuit you will ger a lower reading than the speced value of the resistor. This is because you are measuring all the ways from one lead to the other. Every path has its own resistance and they add up according to Kirchhoff's law. The short of it is that 2 1k ohm resistors in parallel measure 500 ohms. When you measure in circuit you have more than 2 resistors in parallel.

--ethan


http://en.wikipedia.org/wiki/Kirchhoff's_circuit_laws

 

[sweetbeats]

AK,

In the inimitable words of Homer Simpson, "Mmmmm...Beer."

Ethan,

Huh. So is it possible that when I measured all the resistors on the PS PCB the circuits were disrupted enough with all the caps removed that I got true readings from the resistors?

I didn't realize resistors ha to be removed from the circuit to test their values...I know you have to do that with caps, but resistors...I'll follow the link and read...

 

[analogkid]

DOH!


in circuit.... brain short circuit... need beer!









AK

 

[evm1024]

AK,

In the inimitable words of Homer Simpson, "Mmmmm...Beer."

Ethan,

Huh. So is it possible that when I measured all the resistors on the PS PCB the circuits were disrupted enough with all the caps removed that I got true readings from the resistors?

I didn't realize resistors ha to be removed from the circuit to test their values...I know you have to do that with caps, but resistors...I'll follow the link and read...

The voltage from your meter might DOA a very sensitive component but not your typical cap/resistor/diode. Your safe - but send beer anyway. You just need to remove one lead from the board to break th circuit and get a true reading.

For high value resistors your fingers, grease and dust on the resistor can effect the value. THink modding mic's.

--Ethan

 

[sweetbeats]

Your safe - but send beer anyway.

Noted...thanks.

DOA

Dead on arrival?

Is this a caution not to go poking around willy-nilly with the DMM?

For high value resistors your fingers, grease and dust on the resistor can effect the value.

I understand beer has this potential as well...some of which I think I've found on one of the spare channel cards I got for my M-520.

 

[sweetbeats]

Bit by bit...

Got the Mother PCB reinstalled this evening...no small feat considering all the connections and the tight access for routing connections...

Also pulled the secondary PS PCB "B" and pulled the caps off of it. Pulling caps is getting easy...sometimes I can get it so that when I suck the solder off the cap literally drops off the PCB...and I know now how to tell when the heat is on too long...you can hear a little crackle...I think it is the binder for the trace starting to cook. With my new iron I'm having no problem avoiding over-doing the heat.

Waiting patiently for some additional response to my latest post (#38) to this thread before ordering replacements...still trying to get a handle on prudent selections (brands, spec value upgrades, etc.)