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A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26502&th=4518#msg_26502
Jonomega32018-01-27T13:20:12-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26503&th=4518#msg_26503
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.]]>chicagobill2018-01-27T17:26:06-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26505&th=4518#msg_26505
I haven't read the voltages yet, I would feel comfortable reading the voltages while the amp is on I know precautions to take so not to get hurt but I would need instruction is all.
I believe my amp to be one of the very early ones, it has the metal front with two fuses on the front one on either side of the on off switch. Guess only way to know for certain is to check pot date codes? or is there other indicators I should look for.]]>Jonomega32018-01-28T05:28:21-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26506&th=4518#msg_26506
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.]]>chicagobill2018-01-28T19:13:12-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26508&th=4518#msg_26508
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
]]>Jonomega32018-01-28T20:44:46-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26509&th=4518#msg_26509
I would guess that those resistors are in the biasing network of one of the output transistors which is making for the issue of low wattage.
Does that board still have 4 rectangular resistors on it that are about 1 inch long and 3/8s or so in diameter?
If the values for resistors R717 , 718,725 and 726 are not dead on 1 ohm and if the values of R 716,715,724 and 723 are not dead on 100 ohms then the output stage goes south wattage wise real fast!]]>stevem2018-01-29T13:18:10-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26512&th=4518#msg_26512
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!]]>Jonomega32018-01-30T02:16:22-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26513&th=4518#msg_26513
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.]]>chicagobill2018-01-30T04:48:13-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26514&th=4518#msg_26514
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!]]>stevem2018-01-30T17:07:27-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26515&th=4518#msg_26515
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 ]]>Jonomega32018-01-30T18:24:39-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26516&th=4518#msg_26516
Go over the schematic carefully and take out what should not be there as a start.]]>stevem2018-01-30T20:24:14-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26517&th=4518#msg_26517
Also if you look at pic 5 of the pics I sent there is a large ceramic .02 cap there that I cant seem to locate on the schematic, its actually hooked to ground and according to the schematics there is no .02 caps anywhere, remove that you think?]]>Jonomega32018-01-31T17:47:47-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26519&th=4518#msg_26519
This other .02 uf ciramic cap on the rear of the board that you say has one end going to ground , where does the other end land at?
I do not have a board of my own out to look at the trace side so you need to tell me where that what looks like a added cap goes to schematic wise!]]>stevem2018-02-01T13:23:00-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26520&th=4518#msg_26520
]]>Jonomega32018-02-01T16:13:11-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26534&th=4518#msg_26534
Jonomega32018-02-08T20:12:07-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26535&th=4518#msg_26535
stevem2018-02-09T13:32:58-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26536&th=4518#msg_26536
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?]]>Jonomega32018-02-09T17:54:43-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26539&th=4518#msg_26539
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.]]>chicagobill2018-02-09T20:39:55-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26541&th=4518#msg_26541
Thank you chicagobill for the tutorial on reading the voltages I will get right on that and I will be very careful!
]]>Jonomega32018-02-09T22:07:39-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26543&th=4518#msg_26543
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]]>Jonomega32018-02-09T23:30:16-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26544&th=4518#msg_26544
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?]]>chicagobill2018-02-10T04:46:30-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26546&th=4518#msg_26546
Here are the reading I have, Guess I should replace all 3 then?
Q701 B 223mv
E .824v
C -34v
Q702 B 255mv
E .82v
C -34v
Q703 B -35v
E -3.60v
C -34v
]]>Jonomega32018-02-10T17:00:39-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26547&th=4518#msg_26547
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.]]>chicagobill2018-02-11T04:54:24-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26548&th=4518#msg_26548
Yes, I wrote it down in my notes right but copied it to the post wrong, lol.
"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"
Ok, that does make sense
"Is the current problem now a fuzzy distortion that is constantly there in the background and not just a distortion on peak signals?"
That is correct
"And for now I would remove the cap and resistor from the output jack."
Yes, I did, and no change]]>Jonomega32018-02-11T05:27:49-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26549&th=4518#msg_26549
I would just replace those 3 transistors since it's a pain to get at them and hopefully be done with it!
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.]]>stevem2018-02-11T11:24:19-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26550&th=4518#msg_26550
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!
]]>Jonomega32018-02-11T15:58:27-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26553&th=4518#msg_26553
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.]]>chicagobill2018-02-11T20:41:53-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26555&th=4518#msg_26555
Also great tip with the heat shrink tubing on the leads, thatll make this a lot easier. I will do the voltage test on the bias diodes tomorrow and get back with the results, thanks greatly for all the excellent info too
]]>Jonomega32018-02-12T02:05:03-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26557&th=4518#msg_26557
stevem2018-02-12T12:29:04-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26558&th=4518#msg_26558
Cant seem to find these components you mentioned on my PC 702 board as well as Q705 and Q709 but they are showing up on the schematics. ]]>Jonomega32018-02-12T14:04:49-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26559&th=4518#msg_26559
Jonomega32018-02-12T15:10:53-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26560&th=4518#msg_26560
stevem2018-02-12T15:18:35-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26561&th=4518#msg_26561
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 ]]>Jonomega32018-02-12T15:41:23-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26562&th=4518#msg_26562
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.]]>chicagobill2018-02-12T17:47:45-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26563&th=4518#msg_26563
Jonomega32018-02-12T18:25:04-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26564&th=4518#msg_26564
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?]]>chicagobill2018-02-12T19:25:46-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26565&th=4518#msg_26565
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.]]>Jonomega32018-02-12T21:07:48-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26567&th=4518#msg_26567
chicagobill2018-02-12T23:08:18-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26568&th=4518#msg_26568
stevem2018-02-13T12:39:43-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26569&th=4518#msg_26569
Jonomega32018-02-13T16:04:03-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26570&th=4518#msg_26570
Jonomega3- Well, I hoped you'd found it. Keep us informed of your progress.]]>chicagobill2018-02-13T21:40:08-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26833&th=4518#msg_26833
Thanks! ]]>Jonomega32018-07-26T16:43:49-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26834&th=4518#msg_26834
Which of the 6 transistors are bad, the RCA's, the replacements? There are 4 for the power amp and 2 for the low voltage power supply. If the 4 RCA's are good, use them in the power amp and replace the power supply transistors with new ones. If some of the RCA's are bad, replace the 4 power amp outputs with new ones and use any others in the power supplies. The power supplies do not need matching transistors, while it is best to use the same type transistors in the power amp section.]]>chicagobill2018-07-27T04:02:33-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26835&th=4518#msg_26835
Two that are bad are RCA and I suspect one of the no label transistor is bad as well.
They were cheap enough so I ordered 6 today]]>Jonomega32018-07-28T18:31:00-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26836&th=4518#msg_26836
stevem2018-07-29T10:06:44-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26838&th=4518#msg_26838
Jonomega32018-07-29T18:10:26-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26877&th=4518#msg_26877
Jonomega32018-08-16T15:45:19-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26878&th=4518#msg_26878
If you are seing 34 volts positive on the speaker jack then you have one or more output transistors or driver transistors bad in the positive side of the output stage.
The normal D.C. Voltage seen on the speaker jack 2 seconds after turn on should be less then .045 volts D.C. .
Also if you are seing that level of D.C. Voltage on the speaker jack do not plug in a speaker as 34 volts is = to over 140 watts and you will likely blow the stock speaker (s)!]]>stevem2018-08-16T16:40:12-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26879&th=4518#msg_26879
Jonomega32018-08-16T17:14:36-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26880&th=4518#msg_26880
Jonomega32018-08-16T22:51:15-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26882&th=4518#msg_26882
Before we get into anything else here it's what you have posted true, that you have a plus 34 volts on the speaker output jack ?]]>stevem2018-08-17T10:51:31-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26883&th=4518#msg_26883
Jonomega32018-08-17T10:58:32-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26894&th=4518#msg_26894
stevem2018-08-19T10:15:30-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26898&th=4518#msg_26898
Jonomega32018-08-20T16:18:50-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26899&th=4518#msg_26899
]]>stevem2018-08-20T16:27:04-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26916&th=4518#msg_26916
Jonomega32018-08-24T22:22:57-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26918&th=4518#msg_26918
The usual voltage would be about -0.5 volts.
Does the distortion occur with both preamp channels?]]>chicagobill2018-08-25T04:15:51-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26922&th=4518#msg_26922
so 13 volts should at 40410 base, not 15v? I'm not getting any voltage there. I'm getting a little more than 1 volt at the input of the 1st diode]]>Jonomega32018-08-25T11:58:21-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26923&th=4518#msg_26923
Jonomega32018-08-25T15:43:04-00:00Re: A-4 Repair
http://www.vintagekustom.com/FUDforum/index.phpindex.php?t=rview&goto=26924&th=4518#msg_26924
My point was that the 15 volts is a typo. In fact one schematic that I have looked at lists .15 volts there. Maybe the decimal got wiped out at some point along the line.]]>chicagobill2018-08-25T16:41:18-00:00