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SPEC-2 Refurb Help Please

Others made the effort when I was floundering, so in true Karmanic fashion its paid forward.
What i've learned is just how much I DON"T know. As the mechanic doesn't need to know why a part needs replaced, understanding what contributed to its failure and why and how it was integrated into the design to begin with is the LARGE CHASM i'm looking over now. It has prompted many late night searches on "mirror circuits" "current limiters" etc. Before the internet the learning process was less helter skelter but frustrating at times for the same reason. Bless AK and it's constituency for filling in the knowledge and theory holes we DIY"ers are faced with.
 
laatsch55 said:
Bless AK and it's constituency for filling in the knowledge and theory holes we DIY"ers are faced with.
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Yeah, that's one of the things I really like about the replies from Glenn & Mark... They don't just say, "I suggest replacing the C5 10uf/35V cap with a 22uf/50V." -- Instead they take the extra time to explain why is better to do it that way, by extending the "short answer" to include something like "because it provides better bypass filtering without increasing it's inital charge time to the point of causing a 'speaker thump' at turn-on."

Kinda makes you feel like you not only learned how to do something, but that you were also exposed the reasoning for the change... (Which you might be able to build upon & apply to future projects yourself!)

 
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Before you install the board I would clean the pins on the connector with Brasso
pencil eraser works fine also
 
Surgeon, AMEN. Exactly what I was refering to.
Gonna be gone for 5 days stalking the wily Wyoming trout. Good luck on your fix.
 
donberry - My wife's old silver jewelry cleaner did the trick! Applied/wiped with a few Q-Tips... Then coated with a little DeOxit... Less than a minute & now they're shinny as NEW!

laatsch55 - Good luck with the fishies...

(FYI - To everyone following this thread's sub-plot, my father-in-law is recovering well after his heart valve repair...)

 
The only thing I recommend for cleaning pins is Brasso. Many jewelry cleaners contain sulphuric acid (or some other aggressive low-PH chemicals), which attacks the metal of the pin. These chemicals are designed for use on precious metals only, not the steel & tin pins on electronics gear.
 
Two thoughts just occurred to me and one just after Echo's post, both regarding contact cleaning...

1. If the pins on the board are this badly tarnished (and this would apply to all tarnished pins/contacts) wouldn't the mating contacts inside the molex connectors also be as badly tarnished and require cleaning?

2. I have been using MAAS Metal cleaner for the 30-year-old RCA connections on the back of my amps and receivers that are tarnished/dulled. It has been working perfectly but it never occurred to me to check the chemical composition of this cleaner for long-term residual effects that might cause problems. I guess I'll have to look now.
 
The female pins are usually OK, for some reason probably best explained by a metallurgist. The male pins are the ones that end up looking bad and getting tarnished.

MAAS is fine, but slow to remove tarnish. I've tried everything, and keep coming back to Brasso.

Tarn-X is a no-no. :no:
 

OK... My 2 SPEC-2's have been (more or less) repaired! On each amp board I replaced the 6 TO-220's with the On-Semi 32/33's, replaced the 3 trimpots. replaced the reed relay RL1, replaced the 4 electrolytic caps as recommended, and cleaned & re-seated the TO-3 output transistors. The idle current & current limiters have all been adjusted to specified values.

During the idle current adjustment on my 2nd unit I had problems getting the right-amp's idle current up (it maxed out @ 27mv). The left-amp adjusted fine with plenty of range above the 30mv specs. After checking all the resistor values between the two amps, which all matched fine, I finally discovered that the voltage accross the cap C5 (controlled by zener diode D2) was 34.5V on the good amp but was down to 32.6V on the one that wouldn't adjust high enough. I swapped the zeners between the two amps & was then able to get them both to adjust properly.

Now some finish-up questions...

If I were to replace the 34V D2 zeners on both amps with 36V versions would that be ok?

If I wanted to replace transistors Q1, does anyone have a good source for the 2SA798? I found these for $0.45 here: http://www.electronicsurplus.com/commerce/catalog/product.jsp?product_id=76173 , but I only need 4 & they have a $15.00 minimum order...

Finally, when I check the DC offset on this unit I'm getting -45mv on the left-amp & +35mv on the right-amp... Any thoughts on bringing this down?

THANKS!

 
Surgeon said:
If I were to replace the 34V D2 zeners on both amps with 36V versions would that be ok?
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No. The stock zener is a 1W 34V zener, and that's what I'd stick with. Replace the zeners with new ones (mount them above the board an 1/8" or so, so they can get good cooling), and tweak the zener voltage if you need to by changing R9, the 4.7K 2W resistor to push more current through the zener and raise the voltage a bit. A 1W zener can take up to about 29mA, which would translate to a value of R9 equal to about 2K. I would not consider such a low value, but dropping the resistor to 4.3K or 3.9K or even 3.6K is fine. But I'd also go to a 3W resistor.
If I wanted to replace transistors Q1, does anyone have a good source for the 2SA798? I found these for $0.45 here: http://www.electronicsurplus.com/commerce/catalog/product.jsp?product_id=76173 , but I only need 4 & they have a $15.00 minimum order...
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That's the only source I know of. Buy $15 dollars worth, and I'll gladly buy the remainder from you. Contact them, and ask if you can pay extra for hand-selection of only 'G' gain-code parts (the two gain groupings are 'F' and 'G'...the 'G' is more desirable). And if you want to get rid of the extras, I'd be glad to take them off your hands.
Finally, when I check the DC offset on this unit I'm getting -45mv on the left-amp & +35mv on the right-amp... Any thoughts on bringing this down?
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A 'G'-code 2SA798 ought to help. ;)
 
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Surgeon said:

... I finally discovered that the voltage accross the cap C5 (controlled by zener diode D2) was 34.5V on the good amp but was down to 32.6V on the one that wouldn't adjust high enough. I swapped the zeners between the two amps & was then able to get them both to adjust properly.
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The reason why you couldn't have the correct bias current is that with this lower voltage from the zener D2, the tail current in Q1 was lower and the current in Q4, Q5, Q6 and Q7 was also lower. Finally, the current in the clamping varistor D11 was lower and the clamping voltage was lower, too low to have the proper voltage between the bases of Q12 and Q13.

I wouldn't recommand to replace R9 by a lower value because it takes a large increase in the reverse current of a good zener diode to have a significant change in the zener voltage, at the cost of a large increase in the power dissipated by the zener, and a higher risk of failure. Can you imagine the consequences of a failure of this zener diode for the rest of the circuit, if the diode gets open?

I would replace R4 by a 13.9 kOhms resistor in place of the original 15 kOhms, it would be much safer.
 
My first suggestion was to simply replace the zener. I'd think that would be sufficient to bring the voltage back in line unless R9 has been previously replaced with too large of a value. But even the smallest resistor I suggested would result in a current well below the maximum that a 1W 34V zener could take. I don't like changing resistor values either, but was simply suggesting what I considered the lesser evils of the available options.
 
Surgeon, Fishing was great. a lot of Browns in the 3-4lb range, one Rainbow topped 5lb's,on a 7 wt G Loomis fly rod. Quite a time landing it.
Lee
 
laatsch55 said:
Surgeon, Fishing was great.
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Glad you had a good time Lee. We do mostly bass & crappie fishing around here... $25,000 boat + $3000 in gear for $10 worth of fish that doesn't even come pre-battered... But hey, there's BEER! :thmbsp:

EchoWars said:
A 'G'-code 2SA798 ought to help.
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Glenn, I got a reply back from electronicsurplus.com and it seems all they have are the "F" series and no "G"'s. They ALL have "A798 45F" stamped on them according to the email reply from "Nancy". Since this appears the only choice available do you think they would help with my high DC offset problem?

Also, I can't seem to source any 34V zeners anywhere. 33V & 36V are the only "single" choices (unless you can advise on a possible source?). Is it OK to use a 30V + 4.3V or even 2 X 17V in series? What about the zener's temp drift? Would it be doubled with two devices?

 
Surgeon said:
Glenn, I got a reply back from electronicsurplus.com and it seems all they have are the "F" series and no "G"'s. They ALL have "A798 45F" stamped on them according to the email reply from "Nancy". Since this appears the only choice available do you think they would help with my high DC offset problem?
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Take what you can get. They'll be fine. If you want to get rid of any extras that you had to buy to make the minimum, I'm still game.
Also, I can't seem to source any 34V zeners anywhere. 33V & 36V are the only "single" choices (unless you can advise on a possible source?). Is it OK to use a 30V + 4.3V or even 2 X 17V in series? What about the zener's temp drift? Would it be doubled with two devices?
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If you have to double-up zeners to get a voltage, you always want them to be the same voltage, or as close as possible. Thus, two 17V for 34V is cool. A 30V plus a 4.3V is not. And, since the dissipated power will be split between two devices, two 17V 500mW zeners replace one 34V 1W zener.
 
Surgeon - Did those reed relays work OK for you? I tried them in a working SPEC 2 and had problems with protection and the quality of sound. Switched back to the other replacements recommended by EW and problems immediately went away.
 
miklemke said:
Surgeon - Did those reed relays work OK for you?
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They're working absolutely perfectly in the 2 SPEC-2 amps I refurbished. I even double-checked them by "bypassing" them when I had a slight problem adjusting bias on one chanel (which was a weak zener diode problem). Are you sure you didn't have a poor connection somewhere?
 
I double checked the connections, but maybe I switched them around. Pins 1 and 2 are the coil and 3 and 4 are the contacts, with pins 1 and 2 closest to RL2 on the board, correct?
 
miklemke said:
Pins 1 and 2 are the coil and 3 and 4 are the contacts, with pins 1 and 2 closest to RL2 on the board, correct?
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NO! Pins 1 & 2 are indeed the coil, but the coil is oriented AWAY from RL2, as in this image:


011_Reed_Relay_Repaired.jpg



Since you had it installed "reversed" the effect was to bypass the unit's "protection" system with a 2K-ohm resistor (the relay's coil), which would also screw with the amp's bias current yeilding bad sound reproduction.

Just install the new relay so the text printed on it is oriented the same as the Pioneer text on top of RL2...

 
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Thanks,

That was my problem. I put the relays in backwards, duh!

You're right, they do look right at home.
 
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