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Messages - smackoj

Hi friends; It's been a very long time away. A lot of things got put on hold including amp repairs. I have been working on a Peavey Ecoustic 112 amp lately. Everything is running smooth and quiet except the reverb which is important for the new owner. I have reverb when I turn the pot up but I also get like a 60hz hum. I tried re-soldering, cleaning the pot, switched out the tank with 2 diff ones but nothing quiets it down? Does anyone have a suggestion? Looking through the Peavey Forum I found evidence that Peavey amps are known for noisy reverbs. Someone on there suggested using an isolation transformer which I have not tried.
Yes I would like to know more about proper grounding techniques. I have problems with stomp boxes because of poor grounding. Ugh
I am not sure about what changed with the grounding but it is playing nice and the reverb sounds real nice too.
Thanks for all the help amigos.

Jack D
Hi Phatt; the wiring is correct but I found out where the faceplate meets the aluminum reverb pan is the noise problem. I put some non-conductive strips between the 2 pieces and it is all quiet.

thanks a million,

Jack D
I have not been testing with the aluminum plate and reverb tank sitting in posistion. I put it together outside the box just like it would be inside. Lots of hum. I disconnected the reverb leads from the preamp board. Still humming. I moved the alum. plate so it was not touching the power amp chassis. still humming. I removed the plate/reverb pan completely so not touching the preamp faceplate/board or the power amp .... dead quiet. The last piece to be moved from touching the reverb plate was the preamp faceplate. That is when it stopped humming
Yes the amp is working, however, with it sitting on the bench outside of the enclosure it sounds perfect. Very quiet outside the wooden and tolex covering. But when I put it in the case it starts humming badly. It is not one uni-piece chassis. there are 3 pieces; the power chassis (both PT and OT, along with the ss rectifier and the power amp board) that mounts into the back of the cabinet. 2nd is a flat piece of aluminum that the reverb tank is mounted on that slides in from the front and sits on the bottom of the cabinet. And 3rd the faceplate - preamp conrols that slides into the front of the cabinet. There is a 5 wire harness that connects from the power amp board to the preamp board and a 3 wire harness from the reverb pan to the preamp board. All 3 pieces touch metal to metal.

I thought maybe there was insufficient ground between the power amp and the preamp so I ran a ground wire between the 2 which did not cure the loud hum.
Yes, pin 3 of the Current Lim. pot has -8.85 vdc
Thanks g1, I will check Q26 again now that I have the pot replaced and know it is functioning properly. And I am seeing on the shematic that zener D3 limits voltage to 12v? Also, I need to try and be sure I am actually testing Q26 because the PCB does not supply any parts numbers.

Later addition to this post: Q26 has -9.0 on the E, -22.0 on the C and -9.5 on the B

muchas gracias,   Jack D
Having such a big hunk-o-iron OT was my first suspicion but thanks to mi amigos aqui (my friends here) I started checking more carefully and thoroughly the Current Limiter pot and, yes, the voltages on Q26. Note; I wasn't sure about having a load on the OT so in early tests I had it connected to load. Now however I have been working on the amp with the OT disconnected. I realize that could make changes to the voltages throughout although I think I have this amp stable again. The Current Limit pot went crazy when I gave it a small turn under power. It went from -0.5 vdc to -30 vdc with about a quarter inch turn on the pot and it immediately released a foul odor. I killed the power (I had it on the light bulb limiter) and replaced the pot and my first tests seem to indicate that may have been the only problem? I am pooped from this amp today so I will start again tomorrow with putting the OT back in and see if this old girl wants to dance or play bingo?

I will report back asap with hopefully some good news!  Muchas Gracias Amigos
The electrolytic caps can be in several diff types of packages so that might be one issue with finding a lot with the same Uf values and the same voltage ratings? The 3 most often seen are the Radial, Axial and Can Capacitor. Generally you don't see Can capacitors in SS amps but were quite common in the vintage tube amps. The Radial caps with the two long wire leads coming from one end would be most common to use in more modern amps with smaller printed circuit boards (pcb). The Axial type with one wire coming out of each end are found more often in amps with older and bigger pcbs where there is more room on the board for the caps to lay down. I am a fan of Nichicon, Spraque, Mallory, Panasonic.....just about any electrolytic caps made in the US or Japan.

Regarding the non-polarized capacitors used for coupling gain stages together, commonly referred to as tone caps. I don't know what the orange and green ones would be without seeing them. But the different brands can be found as the predominant "tone" caps in different amp manufaturer's products. Generally speaking, the less expensive the amp, the less picky the manufacturer is about the brand/quality of the parts they use. People who build the much higher priced amps are trying for the best product possible and they buy the tone caps they believe sound best in their amplifier. I don't remember most of the better tone cap maker's names right off my head, but you can always buy a few different kinds in small amounts and see which ones sound best to you. I would say go to a website dedicated to offering the best amp materials and peruse the diff capacitors they sell. (try Antique Electronic Supply, Tube Depot and EL34 World which all carry quality parts)

Big word of caution:  Electrolytic capacitors work like batteries storing energy so the amp doesn't warble in and out when the circuit calls for more power. But depending on what type of electrolyte material used in the capacitor, all of them eventually dry out and no longer hold the energy. So, that said, don't buy "vintage" electro caps because they can be no good right out of the box.
Hello gents; I am having a little trouble checking voltages because the PCBs have no markings to designate the exact position of any of the parts on the board. However, I feel confident that I know where Q26 is and I read:  C= -30 VDC
B= -9 VDC and E= +0.7 VDC   I am not exactly sure where to test for the (H) voltage by the Cur Limit pot but all three legs of the pot read +0.65 VDC  This pot is reverse taper and I had it set at 10 which equals max current limiting. I rotated the pot to six diff settings and it seemed to stay very close to +0.7 VDC at all settings. One time when I had the probe on leg 1 of this pot I got a -8.5 volts for just a second then it went back to +0.7.  Thanks again for the knowledgeable advice.

Jack D
I will get the voltages Phil, thanks.  I agree the sound would change using a chip amp Teemuk but I was working from the idea that fixing this would be to difficult and/or expensive. However it looks like I may be able to avoid going with a chip amp.
Thanks Phil; Yes, this amp has confused me quite a lot. Here is the original problem. I would turn the amp on everything would lite up (it has 5 red leds on the face. Within a minute or less 4 of the leds would turn off, all but the one indicating power on, and no signal would go thru the amp. If I shut it off and try it on again within a few minutes, nothing, power led on but no signal moving. If I would wait a long time, say half a day, and turn on, it would light up all the leds and started to amplify. Note; once when I first had it on and I turned the Master Vol. about a quarter inch, the leds went dead and so did the amp.

Also I made an error on my original post. I was measuring the voltage on the brown wire which is the Current Limiter send from the pre to the power amp (I thought it was being used as a pre-gain and thought it was carrying the guitar signal to the amp section). I did measure that pot at the time and did not get any change in the voltage as I rotated the 10k pot.

Thanks for the expert advice,     Jack D
I will double check the breed of voltage. I thought I read DC but it's been several days so I will test it again and report back. Thx mucho
I have worked on this GK 200GT for a little too long and believe the 1976 "boat anchor" output transformer is bad. Rather than searching for a replacement, I would  like to replace the whole power amp board with a chip amp, either class AB or D. The problem I have, if it is actually a problem?, is that the preamp section which I like a lot, except for the "Current Limiter" pot, has a negative -8.7 vdc signal wire to the power amp board. Is that too much voltage and needing to be changed to a positive voltage before using a chip amp power board? I believe the preamp is working correctly but I am not sure because the schemo is hard for me to read and not sure if the signal from R85, the reverse 10k pot called "Current Limiter" is showing a negative voltage feed to the power amp? Review of the Owner's Manual shows that the Current Limiter function is to change the power output of the amp (I would just as soon remove it). Would anyone care to look at this and offer suggestions?  thanks