| A-4 Repair [message #26502] |
Sat, 27 January 2018 08:20  |
Jonomega3
Messages: 81
Registered: November 2012
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Thank you stevem, really appreciate the help! Owned several guitar amps over the years and this A-4 has been and always will be my desert island amp!:)As electronic test equipment I have a multi function voltmeter that has capacitance test ability, Anatek blue ESR capacitor tester. Not the highest quality test equipment but hopefully they will be helpful in this project.
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| Re: A-4 Repair [message #26503 is a reply to message #26502] |
Sat, 27 January 2018 12:26  |
chicagobill
Messages: 2010
Registered: April 2003
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I went back and read the original thread that got you to this point. So to restate the distortion description is that it is on the attack transient and does not follow the note decay, correct?
Have you gone through the amp and tested the dc voltages in the power amp and compared them to the schematic? Do you feel safe in reading voltages while the amp is plugged in and turned on?
Depending upon the exact model that you have, the overload protection circuit may be the cause of your problem. Very early A's did not have this circuit, so you will need to check exactly which model power amp you have.
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| Re: A-4 Repair [message #26506 is a reply to message #26502] |
Sun, 28 January 2018 14:13 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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By fuses, do you mean white button circuit breakers?
As far as I know, there is very little documentation regarding the circuit breaker heads or the K200A series amps. I recently serviced a few circuit breaker heads and that's when I noticed that there was a version of the power amp that did not have the current limiter circuit included.
At the time that these heads were being designed, I imagine that there were a lot of production changes being made. I'm guessing that's also the reason that the circuit breaker heads didn't last that long and were soon replaced by the plexi front A-series heads with internal fusing.
The good news is that the basic power amp design is not much different from the later series amps, so the schematics will give you a good starting point for voltage value. I will look at my CB head and see if there is a pc board number on the power amp. Take a look at your amp and see if you can see one there.
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| Re: A-4 Repair [message #26508 is a reply to message #26506] |
Sun, 28 January 2018 15:44 |
Jonomega3
Messages: 81
Registered: November 2012
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Yes,sorry I did mean circuit breakers. Looks like the on/off switch has been replaced
Pc 702 for power amp.
Im going to try and get some pics posted later, there looks to be some boogering in the power amp section that might help if you see. There are 4 resistors coupled together with a 250v 4.7 electrolytic cap going to ground that doesn't look like it belongs fyi
[Updated on: Sun, 28 January 2018 16:49] Report message to a moderator |
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| Re: A-4 Repair [message #26512 is a reply to message #26509] |
Mon, 29 January 2018 21:16 |
Jonomega3
Messages: 81
Registered: November 2012
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https://drive.google.com/open?id=14Pw1SMvBWU4hT-rQzMLk3ycm_1 6aRel8
Above is a link to my google drive containing some pics of the A-4 Power amp section, I tried to get them to appear in the post but no luck, sorry but let me know if you can see them.
Yes, there are some rectangle ceramic resistors that are not original and in two sections where there is supposed to be one 1 ohm resistor they have two coupled together. 717 reads 1.3-4 ohms 718 1.3-4 ohms as well 725 and 726 both read 1 ohm.
716-105 ohms
715-107 ohms
724-124 ohms
723-which I think is where the 4 coupled resistors with the 4.7 electrolytic cap is and I couldn't get a reading from that mess. Closer look at the power section, it has had a lot done to it, ugh!
[Updated on: Mon, 29 January 2018 21:30] Report message to a moderator |
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| Re: A-4 Repair [message #26513 is a reply to message #26502] |
Mon, 29 January 2018 23:48 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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I got a 404 error when I tried to get to your link.
If there has been a lot of work done on the power amp board, then there's no way to know what has been done unless you compare it to the original schematic. It may simply be that a number of parts need to be replaced and because the driver transistors were no longer available newer style parts were used which have different casings and look wrong, but they may be perfectly fine as replacements.
Or it could be that someone thought that they could modify the circuit to make it better in some way or another.
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| Re: A-4 Repair [message #26514 is a reply to message #26502] |
Tue, 30 January 2018 12:07 |
stevem
Messages: 4778
Registered: June 2004
Location: NY
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Thanks for sending me those shots of your 702 board, and all I can say is it's a mess, but at least the good news is that your not blowing any fuses, which by the way I hope are the right amperage!?
Those 4 added resistors ( paired in series for 39.8k ohms ) with the 4.7 uf cap in the middle to ground should not be there in regards to the schematic for a 702 board, however a 703 board does have a 39k resistor with a cap going to ground.
The 702 board does however have two 3900 ohm resistors.
The first thing I would do is remove those 4 resistors and that cap and then see how the rest of the board looks in relation to the schematic.
Those .22 ohm and .82 ohm white box resistors in series are ok for now, so leave then as is until we this messed up board working better for you!
[Updated on: Tue, 30 January 2018 12:30] Report message to a moderator |
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| Re: A-4 Repair [message #26515 is a reply to message #26514] |
Tue, 30 January 2018 13:24 |
Jonomega3
Messages: 81
Registered: November 2012
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Really glad you can finally see the pics even though theyre hard on the eyes! haha. This is the model with circuit breakers no fuses. And they appear to be original with the date of 67' and also appear connected correctly.
OK I'll take those resistors and 4.7 cap out, did you notice that added 22uf 25 volt cap coupled with the .47 cap that is in proximity of that mess? According to schematics its foreign as well.
Could those coupled 4 resistors and 4.7 cap be originally an 82k resistor? Im thinking If Im reading the schematics correctly it could be
[Updated on: Tue, 30 January 2018 14:32] Report message to a moderator |
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| Re: A-4 Repair [message #26520 is a reply to message #26519] |
Thu, 01 February 2018 11:13 |
Jonomega3
Messages: 81
Registered: November 2012
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Yes, the 22uf electrolytic is coupled with a small disk 1kv cap. I have a good new 10uf 16v that I can put in that spot which I believe is correct value for C702? The other end of the ground ends up at R713, 714 and 715 in that area. Im pretty certain that large .02 disk cap is not supposed to be in there, it is very sloppily affixed to the board and I went through the schematics a couple times to account for all other caps.
[Updated on: Thu, 01 February 2018 18:08] Report message to a moderator |
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| Re: A-4 Repair [message #26536 is a reply to message #26535] |
Fri, 09 February 2018 12:54 |
Jonomega3
Messages: 81
Registered: November 2012
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Not sure how to do a voltage test on transistors, any tips? Happy to replace the transistors you mentioned do you have a supply company recommendation?
I did replace the one 82k resistor on R705 which is the only resistor I replaced I then fired the amp up and the distortion is still there only the volume is a little better and the distortion is more of a fuzztone in the background now instead of a breakup.
Another strange thing is on the speaker output jack. a large 1.0 cap with a large watt 20 ohm resistor couple between the ground and tip on the jack. Would you like a pic? lol
I checked the schematics and couldn't find anything on the speaker outputs. Should I do away with that?
[Updated on: Fri, 09 February 2018 12:55] Report message to a moderator |
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| Re: A-4 Repair [message #26539 is a reply to message #26502] |
Fri, 09 February 2018 15:39 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Is the cap on the output 1uF or a 0.1uF? Typical Zobel networks will use 0.1uf and a 22 ohm resistor. Have you tried the amp with a different speaker?
To measure the voltages, set your meter to read dc voltage. The highest voltage that you will see is about 45 volts, so set your meter to the range that will cover that amount of voltage.
Set the amp up on a well lighted bench top and plug the amp into the wall socket and turn it on. If you have a light bulb limiter use it for now. Connect the black lead of your voltmeter to the metal chassis. Carefully touch the red meter lead to the different points in the circuit and see what voltages you get compared to the voltages listed on the schematic.
For example, the two large filter caps each have two screw terminal on the top of the cans. Touch the red lead to where the red wires connect to the cap and read the voltage. You should get about +45 volts dc. Next check where the green wires connect to the other filter cap, you should read -45 volts dc.
The readings that you get will not match the schematic exactly, but what you look for is something that is completely wrong. Be very careful when probing, because if you accidentally short two points together with the tip of your meter lead, you can cause problems, like shorted transistors, etc.
Watch out for the 4 metal heat sinked transistors on the power amp pc board, because the metal cases are connected to the circuit and have high voltages on them.
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| Re: A-4 Repair [message #26543 is a reply to message #26541] |
Fri, 09 February 2018 18:30 |
Jonomega3
Messages: 81
Registered: November 2012
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Because my voltmeter prongs couldn't reach the farthest prongs of the transitors I was only able to measure the collector and base of 701 and 702, both on the collector were -34. On the base of 701 216 mv and 702 was 253 mv on the base. 703 was -34 on the base and -35 on the emitter. I am thinking so far they are not that far off?
Are there special leads you can get for the multimeter to read these transistors? If so where are they available? Or would you detach the board and read them from the backside solder points? In any case, its pretty tricky I must say
[Updated on: Fri, 09 February 2018 18:46] Report message to a moderator |
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| Re: A-4 Repair [message #26544 is a reply to message #26502] |
Fri, 09 February 2018 23:46 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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Welcome to my world. Yes, there are clip adapters that can get into tight spaces like that.
Or look for what the other transistor leads are connected to and see if you can access that connection point instead. Both emitters of 701 and 702 are connected together and they connect to a 82K resistor R705. Can you reach the end of that resistor?
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| Re: A-4 Repair [message #26547 is a reply to message #26502] |
Sat, 10 February 2018 23:54 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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I think that you may have reversed the Collector and Emitter readings of Q703.
The rest of the voltages are pretty much what they should be. The 60 on the emitters is a typo, I think it should be 0.6 volts.
Is the current problem now a fuzzy distortion that is constantly there in the background and not just a distortion on peak signals?
And for now I would remove the cap and resistor from the output jack.
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| Re: A-4 Repair [message #26550 is a reply to message #26549] |
Sun, 11 February 2018 10:58 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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"I would just replace those 3 transistors since it's a pain to get at them and hopefully be done with it!"
Sorry for all the questions and my main goal is to get the amp working properly again but Im curious to know... would the quality or transistors effect tone in the power amp circuit or in just this circuit? Ive been doing a search on transistors being sold, looking at NOS Germanium transistors available which are probably 3 times the price of the standard that I bought from radio shack several years ago. Don't mean to open a whole other can of worms but Im just thinking in terms of keeping the great tone the amp had before it went south. If it doesn't matter then great! but if it does I would be happy to go extra couple $ and get the NOS.
"For voltage test purposes I have long 3 wire extension leads that I solder in that start off in Molex connectors so I do not have to fret about them shorting out to each other."
Great idea, the set of leads Im using are pretty old and kinda bent up, I could modify them to work on these hard to reach transistors and buy a new set of leads, thanks!
[Updated on: Sun, 11 February 2018 11:01] Report message to a moderator |
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| Re: A-4 Repair [message #26553 is a reply to message #26502] |
Sun, 11 February 2018 15:41 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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First off, there are no Germanium transistors in these amps. Only the very early Lot-O-Tone heads had them and they failed so often that they were dumped for Silicon transistors. NOS transistors are great if you are working in a museum and want to keep things absolutely original. I use modern replacements that are cheaper, quieter and often higher rated that the originals. Your amp sounds like it has already been worked over a few times, so I wouldn't worry about originality.
If I fully understand what you describe as the fuzzy distortion, it sounds like a perfect description of crossover distortion in the power amp. At least that is what I would look at as a starting point.
Crossover distortion occurs when the power amp circuit is under biased and the two halves of the audio signal don't transition smoothly from positive to negative polarities. Instead of transitioning perfectly in the middle, the first half shuts off before the second half turns on, creating a flat spot between the two signal halves.
Check the voltages at each end of the string of three bias diodes. There should be somewhere between 1.5 volts to 2 volts difference between the two ends of the diode string. This is a very delicate part of the power amp circuit because of the temperature sensing diode that is thermally connected to the heat sink. Be careful if you move this diode as the leads have been known to snap off at the case and the ends can also come unsoldered from the pc board. If the diode string is broken and the amp is on or turned on, the two halves of the power amp both turn on at the same time, causing the driver and power transistors to go up with a puff of smoke.
You can add heat shrink tubing to the metal tips of your meter leads leaving only a small tip exposed to avoid the tip from shorting things as you probe. I have a set of spring loaded clip adapters that I can use when things are really tight. With these you really have to attach the lead with the circuit power off and then turn it on and get your reading.
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| Re: A-4 Repair [message #26561 is a reply to message #26560] |
Mon, 12 February 2018 10:41 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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No, no holes and no sign of any missing components on the board at all.
I think the reason for this is mine is an older model with the two breakers on the front (no fuses) and the schematics are for the later PC 702 with the fuse. I have the later A-4 too and just pulled the chassis and confirm the schematics match it identically.
I say later but according to the tranny and pot numbers its only later by a couple months
[Updated on: Mon, 12 February 2018 11:20] Report message to a moderator |
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| Re: A-4 Repair [message #26562 is a reply to message #26502] |
Mon, 12 February 2018 12:47 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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As I mentioned much earlier in this thread, early 200A's did not have the protection circuit that would use those missing diodes. So at least now we know that yours is an early 200A without it. And a couple of months could be a lifetime in terms of production changes.
Yes, the bias voltage is about 1.3 volts rounded up. I would like to see that up a little higher, just to see if that cleans up the distortion.
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| Re: A-4 Repair [message #26564 is a reply to message #26502] |
Mon, 12 February 2018 14:25 |
chicagobill
Messages: 2010
Registered: April 2003
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Senior Member |
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You will have a problem replacing the 1N3754, so check it but you will need to try and fix the bias with the other two didoes (see above warnings). If your meter has a diode test function test all of the diodes and see what voltage readings you get when you test them. The meter will read the voltage at which the diodes conduct. You can use these voltages to select the right mix of diodes to get the bias right.
I know that the board had been messed with previously, did you get it all back to normal? Have you checked the other voltages on the board? Any signs of burned resistors or wrong value components?
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| Re: A-4 Repair [message #26565 is a reply to message #26564] |
Mon, 12 February 2018 16:07 |
Jonomega3
Messages: 81
Registered: November 2012
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Member |
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"If your meter has a diode test function test all of the diodes and see what voltage readings you get when you test them. The meter will read the voltage at which the diodes conduct. You can use these voltages to select the right mix of diodes to get the bias right."
CR701 .5v,
CR702 .0000 both ways
CR703 .49v
I guess the open reading on CR702 may explain it?
"I know that the board had been messed with previously, did you get it all back to normal?"
Yes, I clean off all extra resistors and caps and replaced the four .82 ohm block resistors with proper value 1 ohm as well as the one 82k resistor as per the schematics. There no sign visually of burned resistors
"Have you checked the other voltages on the board?"
Havent measured the boards other voltages yet but will do that tonight carefully
"Any signs of burned resistors or wrong value components?"
No signs of burned resistors or leaky E caps and they test correctly with my ESR meter. I went through all the values of other components compared to schematics and all is as it should be.
[Updated on: Mon, 12 February 2018 16:15] Report message to a moderator |
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