Looking at the hole pattern in the board it looks to me like the collector and the emitter leads have been reversed. Check the schematic, the emitter of Q710 connects to the 4K7 resistor, does it? The collector should connect to the 10K resistor, does it? I would go back and double check all of the transistors that you have replaced and be sure that they have been installed correctly.
If you look at the all of the original round plastic transistors, there will be a flat spot along the outside edge. The lead closest to the flat spot is the emitter.
What is the new resistor that just out of frame, 10 ohms? I also notice that your R743 is 1K2, it should be a 1K8.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Good eye, Bill on spotting that 10 ohm resistor. It's actually three 10-ohm Rs in parallel to make up the 2.7-ohm R that was there in the regulator. I actually replaced both R745 and R751 this way.
Looks like I am gonna have to take a real close look at all 6 of those transistors I replaced. All the transistors in the negative rail blew (Q715, Q716, and Q717) but everything seems ok on the positive side. I'll meter test them all this morning and find out for sure.
My buddy who owns this amp is driving down tonight from VA and I'd like to send this back with him in the morning. He'd be thrilled if it works the way it should. He's the original owner ...
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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chicagobill wrote on Thu, 02 October 2014 21:30
... I also notice that your R743 is 1K2, it should be a 1K8.
Bill,
I'm not sure which schematic you're looking at, but my board is a PC-703 and on the schematic I have R743 is 10K and connects to the base of Q711. In fact, I don't see a 1K2 or a 1K8 resistor anywhere in the regulator section of my schematic.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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stevem wrote on Fri, 03 October 2014 08:35
It would seem that Bill is looking at rev 2 of that 703 board.
I believe my board is a first revision (KEI-1) and that I'm working from the right schematic.
Looks like Bill is absolutely right and that I have reversed every one of the new transistors. Looking at the flat spots on both types of NPN and the PNP, all the old transistors are oriented CBE and the new 2N4001 and 2N4003 are both oriented EBC. Apparently I misinterpreted the datasheets.
Can someone double-check that Zener one more time for correct orientation?
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Got all the 2N4001 and 2N4003 transistors installed correctly, I believe.
Now the dawgone 1N3754 (CR702) has broken the leads right at the body. I plan to replace it with a 2N3440 (TO-39 case) using the BE junction and snapping the body into the original chassis clip. Does anyone see a problem with that? Here is the datasheet.
stevem Messages: 4738 Registered: June 2004 Location: NY
Senior Member
With those factory Type 2n3638 round case transistors when you hold them leads up and with the lead gap facing away from you the configuration from right to left is EBC.
In regards to the 6.2 volt diode, it appears it's in correct .
Also the schematic you posted that you are working from does not show that new cap that you have across that diode?
If you hold one of the plastic cased TO-whatever with the legs down and the base is closest to you, then it is ebc. The legs are in a triangular pattern.
Be sure to retest all of your new transistors before reinstalling them.
Edit: I just saw your post about the compensation diode replacement idea. I guess it would work. I just use a 1N4148.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Bill said the leg closest to the flat spot is the emitter, so that drawing above depicts that. In observing that orientation, every one of mine were installed with B and E reversed.
I've wired in the 2N3440 for the 1N3754 and am about to fire up the amp on the limiter.
That cap across that diode replaced a tantalum of the same value that was there before. There are actually separate holes there for the cap and the diode.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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OK, There is now 7.4V DC at the RED wire and -7.5V DC at the green wire and that voltage is getting to the preamp. Voltages at Q1-Q4 are good. Still very low output with either channel through a known good speaker. Measured 0.00V DC at the speaker jack.
660mV DC at the anode and 15mV DC at cathode of the 2N3440 that I installed in place of the 1N3754.
Good, you got the low voltage supplies up and running. Having no voltage on the power amp output is a good thing. Now that all of the power supplies are working, you need to find out what sections are working and which ones are not.
If you plug a signal into the monitor jack on the back do you hear it through the speaker? If you take a signal out from the same monitor jack and listen to it through another amp do you hear the output from the preamp?
As a warning, if the amp still has the original two wire power cord, you need to make certain that the two amps (the test amp and the listening amp) are grounded correctly before plugging in the audio cable between the two amps. I use a meter set to read ac volts and put one lead on the ground of the first amp and then the second lead to the ground of the second amp. With both amps plugged in and turned on there should be no voltage reading on the meter. If there is, then change the polarity setting of the power switch on one of the amps to make the voltage go away.
I use a battery powered headphone amp to check the monitor output, so there are no ground problems with that.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Feeding a 1K signal into the input and using my homemade probe into a small amp I am able to get strong output from the preamp (BLUE WIRES) at the power board. Can pick up a very strong signal at the MONITOR output and can pick up strong signal from Q700-Q703, but it starts getting weaker at Q706 and Q707.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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With a 150mV 1K signal into the MONITOR jack, I get zero output from the speaker. However, with a 50mV 1K signal into the preamp input, I can get a weak signal from the speaker, but I can pick up the signal strong at the MONITOR jack.
stevem Messages: 4738 Registered: June 2004 Location: NY
Senior Member
I just did a test for you on one of my 200B heads!
When I drive the normal channel with .152 volts @ 1K and show at the amps output into a 4 ohm load 18.95 VRMS ( clipping) the AC signal level I read with my voltmeter at the monitor jack is15.9 volts.
When I drive the monitor jack with that same .152 volt test signal I show at the output 2.62 VRMS into that 4 ohm load.
Now we know that the problem is in the power amp. Q706 and Q707 are the protection circuit transistors don't look for the signal there. Better to look at the drivers Q704, 705, 709 and 709.
If the signal is strong at the collector of Q703, then see what happens at the bases and collectors of Q704-709. Oh and nice signal tracer probe.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Yes, thanks for those numbers Steve. I'll use those for comparison once I get this power section sorted out. I'm gonna poke around and take some readings as Bill directed and will report back. I would love to get this sorted out this morning.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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The signal is VERY strong at the collector, emitter, AND base of Q703. Signal is very weak at bases and collectors for Q704, Q708 and Q705, Q709. I tested for that signal at the appropriate RED, BLUE, and YELLOW wires at Q1-Q4 connectors.
Q703 is the pre-driver, after that the the signal goes to Q705 and Q704. It can be that the signal path is broken or that the driver transistors are dead or are not turning on. Odds are against all four drivers being dead, so I'd look to see if the signal is getting to Q705.
Check R718 is it 470 ohms? Is there continuity from the resistor to both the collector of Q703 and to the base of Q705?
Next start taking voltage readings on the driver transistors. The schematic shows that Q705 should have -.7 vdc on the base, -.1 on the emitter and probably -38 volts on the collector. What voltages do you get? What are the voltages on Q704, Q708 and Q709?
I don't remember what transistors do you have in there for the drivers? Have you tested them with your meter?
For some reason the voltages on Q705 and Q709 are off and I'm trying to figure out why. Do you have the protection circuits Q706 and Q707 hooked up right now? Did you replace these two transistors?
Right now, I guess most important is the circuit around Q707. Did you test/replace diode CR704?
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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I did not replace Q706, CR703 or Q707, CR704. Just tested CR703, CR704 diodes again and they test fine (in circuit).
Pulled Q706 and Q707 and tested them just now. Q707 (2N3638) tests fine, but Q706 (2N3567) is open from B-E. Gonna swap in a 2N4401 and see what we have.
You have replaced all of the diodes CR700-CR701-CR702. CR702 is a transistor and the other two are what diodes? Try replacing the transistor with a generic 1N4148 diode.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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chicagobill wrote on Wed, 08 October 2014 17:54
You have replaced all of the diodes CR700-CR701-CR702. CR702 is a transistor and the other two are what diodes? Try replacing the transistor with a generic 1N4148 diode.
Yes, I have replaced CR700, CR701, and *CR702. The schematic calls *CR702 a diode, but what the hey. I only have 1N4448 here and will make the swap and see what happens.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Good call on CR702, Bill. Replaced the 2N3440 with a 1N4448 and she's up to full power now. So, apparently subbing in a 2N3440 (B-E junction) for the 1N3754 is not the way to go.
Great news! And I bet that the voltages on the drivers are back to normal too.
I started to experiment with mounting 1N4148 diodes into short pieces of aluminium tubing as replacements for those temperature compensating diodes. I never got around to heat testing them in circuit though. Maybe this Fall that can be one of my to dos.
Anyway, glad to hear that it's back up to volume again.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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Yeah, and thanks Bill and Steve for your help and patience. This amp likely took a lightning strike or was run on an inferior generator at some point in its life. It had been sitting for 15 years or more in my friend's basement, so who knows.
Kustom_Bart Messages: 601 Registered: October 2010 Location: Greenville, MichiGUN
Senior Member
Don, open an account at photobucket.com and you can upload pics (just one or multiple at a time)right from your computer to your photobucket account, then it will give you different options for links to use, ie hold your mouse over the pic you want to share and it will bring up a little gear looking icon, it will bring up a drop down menu and click on "Share links" then it gives you 4 different options of link types. The bottom link is the one you want for forums, when you click the link, it automatically copies it , come to your post and right click and click paste and you are done and submit your post. You will have your pic in your post. If you want multiple pics, do it multiple times.
This should be made a sticky post in a HOW TO Section of the forum.
Tone Meister Messages: 42 Registered: August 2014 Location: NC
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My buddy picked up his 200 B1 a few days ago and called me yesterday with a report. He wasn't specific about the speaker cab and speakers he's using, but said the amp sounds great with a Strat. But he went on to rave about the tone with an SG plugged in. Instant John Fogerty - CCR tones and very quiet!
Many thanks to Bill and Steve for their patience in helping me sort out the multitude of small problems I encountered in the amp. Basically, it was a complete circuit restoration and the end result exceeded expectations.