Zach's 700B Thread

[Zach C.]

Well, my other plans for tonight fell apart, so I got a chance to review my notes (it’s been a while) and confirm that I could still duplicate the situation.

Phase Linear 700B
SN 2100
Driver T03- Motorola SJ2741
Output T03- NEC2SD555

When I bought this thing ($10), it was acting up, eventually I traced the problem to a bad T03 socket for one of the drivers. So, I changed out all of the sockets, since they were all a little crusty. Once I did that, it came up and ran like a champ. I was getting a good 350+ watts rms, symmetrical clipping, and low dc offset. Looking good!

With that done, I went ahead and upgraded the power supply caps and bridge. Afterward, the amp tested the same.

With this done, I was ready to install one of Don Imlay’s DC protect boards to save my precious speakers. When I did that, I found that the amp would not settle and idle. I also found that if I put everything back together, it was still fine. After several exchanges with Don, I found that working from the known good state, I could simply lift the zobel, and duplicate the problem. This tells me that quite clearly, it is not Don’s board causing my troubles, since it should be stable without the zobel. I would like to further thank Don for being so generous with his time in attempting to diagnose this thing. Thanks!

So, I want to get this fixed so I can go ahead and install the WO board, and Don’s board. Can anyone make some suggestions on what to check?

Thanks!

Zach

I just couldn't wait to use that emoticon.

Conclusion:

This amp was flaky, and even though it would come up in the factory configuration, attempting to install the Wattsabundant DC protect board revealed a problem. Rewiring the AC and installing a few additional caps fixed the amp, and allowed the DC board to be installed. It seems this thread may have generated some bad press for that board, so for clarity:

THIS WAS NOT DUE TO ANY PROBLEM WITH THE DC BOARD!!! It seems to be a fine product designed and sold by a very helpful guy. Seriously, Don spent quite some time with me in e-mails and on the phone trying to get this straightened out.

 

[laatsch55]

When you say:" lift the zoebel" what are you doing? 2SD 555's as outputs, 2741's for drivers....?? Do you have a scope??

I also have to ask this question, Don's 700 and 400 boards have the zoebel integral with the board, are you talking that zoebel?/

 

[laatsch55]

After the Don board what was your bias??

 

[laatsch55]

Finding a 700B for 10.00 weould almost REQUIRE you use that emoticon!!!

 

[Zach C.]

When you say:" lift the zoebel" what are you doing? 2SD 555's as outputs, 2741's for drivers....?? Do you have a scope??

I also have to ask this question, Don's 700 and 400 boards have the zoebel integral with the board, are you talking that zoebel?/

Yes, those devices are what was in it when I got it. I take it that this is odd?

I have a 100MHz Hitachi, and a 1Mhz Audio Generator.

Yes, I mean the zobel that is originally directly across the output terminals. I could be mistaken, but I thought the zobel was out of the circuit until the relays energized on Don's board. I can look if need be. I put ring terminals on the original zobel and when in circuit, everything settles, but when out of circuit, the thing continues to draw current, and "hums"

Thanks,

Zach

 

[Zach C.]

After the Don board what was your bias??

That, I don't know. It was drawing some serious current, so I can't run it on full line current.

I should mention that there was some weirdness wrt DC voltages in the amp when Don was walking me through some tests that I quite frankly did not understand. He felt there was likely a problem with an open resistor somewhere. We were looking at the grounding situation.

Not sure what to say about this, since I wasn't exactly following his meaning.

Zach

 

[Zach C.]

Finding a 700B for 10.00 weould almost REQUIRE you use that emoticon!!!

I know, right?

I also bought a Crown D150A Series II from the same yard sale. The guy said that the day before some guy bought the rack that they had been in but didn't want the amps (Say what?) so he had gotten what he wanted for the whole business and sold me the amps cheap. $20 for both, so maybe one or the other was free!

The crown worked.

Zach

 

[Zach C.]

Yes, those devices are what was in it when I got it. I take it that this is odd?

I have a 100MHz Hitachi, and a 1Mhz Audio Generator.

Yes, I mean the zobel that is originally directly across the output terminals. I could be mistaken, but I thought the zobel was out of the circuit until the relays energized on Don's board. I can look if need be. I put ring terminals on the original zobel and when in circuit, everything settles, but when out of circuit, the thing continues to draw current, and "hums"

Thanks,

Zach

I just checked, and it looks like the zobel is out of circuit on Don's board until things settle and the relays kick in. For some reason, my amp will not settle w/o the zobel.

I have not scoped the output when I have the zobel lifted. Think that might help diagnosis?

Zach

 

[laatsch55]

Yes it might, what is your offset as you bring it up. Do you have a dim-bulbtester??
Your're not running the original zoebel AND the one on Don's board are you?? The caps can go bad on the factory zoebel, so can Don's if you get some high freq oscillations..

 

[laatsch55]

If you have a variac you can track the bias on the way up and have a pretty good Idea where's ity's going before it gets bad. The 2SD555 was a hot rod and still is. A VERY GOOD output. Equal to or better than the 21196, IMHO.... the 2741's I'd trade out for some RCA-410's./

 

[laatsch55]

If you have a variac you can track the bias on the way up and have a pretty good Idea where's ity's going before it gets bad. The 2SD555 was a hot rod and still is. A VERY GOOD output. Equal to or better than the 21196, IMHO.... the 2741's I'd trade out for some RCA-410's./

Nothing wrong with the 2741's, but Joe has recounted many times the 410's have more gain that is really needed at that point.

 

[laatsch55]

Don's board doesn't kick oput till about 90-125MV dc. If you are getting that at the outputs, replace q1 thru q4. What board do you have in the amp?? The PL14B or the PL020??

 

[Zach C.]

Yes it might, what is your offset as you bring it up. Do you have a dim-bulbtester??
Your're not running the original zoebel AND the one on Don's board are you?? The caps can go bad on the factory zoebel, so can Don's if you get some high freq oscillations..

Only running one zobel at a time.

Yes, bringing it up on a DBT.

Haven't checked the caps on either.

Zach

 

[Zach C.]

If you have a variac you can track the bias on the way up and have a pretty good Idea where's ity's going before it gets bad. The 2SD555 was a hot rod and still is. A VERY GOOD output. Equal to or better than the 21196, IMHO.... the 2741's I'd trade out for some RCA-410's./

I see, and I do have a variac, so I'll get that info for you as soon as I can.

Likely Friday evening.

I'll look into the RCA-410's as drivers even if that's not the current problem.

 

[Zach C.]

Don's board doesn't kick oput till about 90-125MV dc. If you are getting that at the outputs, replace q1 thru q4. What board do you have in the amp?? The PL14B or the PL020??

It's the PL20. THe weird thing is that with everything hooked up "normally" the thing only has 13-14mV offset. I was geting some variance in DC offset depending on the grounding scheme. i.e. With the factory arrangement but without the zobel, DC is good, as well as with the zobel is good (as I just mentioned) but with the modified grounding scheme proposed for Don's board, I get strong offset. I probably should have mentioned that, and maybe that's where we should focus our attention. It's sure what Don was focused on when he tried to help me. Sorry for that.

I still don't understand why it won't come up w/o a zobel.

Thanks again,

Zach

 

[laatsch55]

Going out to dinner, be back in a bit.

 

[Zach C.]

Going out to dinner, be back in a bit.

Not sure what this means ^ ?

Well crap. I just went to take the measurements you wanted, and discovered that I have broken one of the legs on Q4 for the left channel. I'm certain this is new damage. It's fixable (is there a drop in replacement?), but I might just go ahead and get the rest of what I need to install the WO board, and go from there. Not sure yet.

Also, in looking up the RCA-410 transistors you mentioned, I found this post from D3imlay over at AK

"The only problems I've had have been using the wide band outputs, specifically the 2sd555/2sb600. Last year I got a 700II to repair. From the factory it had 2sd555's. The output R/C network was toasted in both channels. It was oscillating at a megahertz or so. I tried everything in my bag of tricks to stop it. Nothing worked. Then I put in MJ15024's. Ran like a champ.

Gibsonian: "Have you actually installed the 15024/5 in a series 1 with good results?"

There's nothing special about the drivers. They're just lower rated versions of the outputs. At the time these amps were in productions I suppose they were cheaper then the outputs and worked perfectly fine. The cost of MJ15024's is so reasonable these days it makes sense to use them as drivers which operates them well within their tollerance.

To answer your question, yes I've used the 15024's as drivers in series I amps with no problems."

Now, my zobel is OK, but it makes me wonder if mine isn't oscillating with the zobel out?

I'd really like to know what is going on with this thing rather than just plunking parts in there until it's fixed, even if I was going to replace those same parts anyway. Does that make sense?

Your thoughts, guys?

Thanks,

Zach

 

[laatsch55]

Yes it does. It certainly can oscillate without the zoebel as well as it hooked up. Q4 is an MPSA 93. Still a good number and they make em every day. Q3 could be a TIS-93(now obsolete) or a 2N5087(current).

Did you separate the hardwire ground from the RCA input ground plane from the speaker out grounds. When installing Don's board necessitates running a new ground wire from the board to the star ground point between the PS caps. Run a new ground wire from the input RCA ground plane to the the star ground point.

Do you have the problem with the Normal/DC coupling switch in either position?

 

[laatsch55]

I still don't understand why it won't come up w/o a zobel.




With a problem with the ground scheme, or another component the zoebel could be supplying the only ground path available.

 

[laatsch55]

Zach, getting the Pl020 board up and stable is a good thing before putting in the WO board. If you have chassis issues that new board could take a hit. The single sided 020 is A LOT EASIER to replace components than the two sided WO board.