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morbo!
10-27-2005, 03:59 PM
http://www.tnt-audio.com/clinica/bias_e.html

Has anybody here done this mod
I would like to hear your results as i have an old technics amp i would like to fixup!
And think this would be an ideal place to start
thanks
morbo!

Regis
10-27-2005, 04:25 PM
I am contemplating replacing the trim-pots on my vintage Sansui G-22000 reciever, as the old ones were very twitchy when I was setting up the DC bias and offset. Even with a precision B-K multimeter, the slightest turn would send the values rocketing from one extreme to another. Very frustrating and your post has me thinking now. To replace the junk pots with precision sealed multiturn pots would be far better than replacing them with the previous junk.

My only problem is with access. My pots are 'straight-on' adjusted through holes in the sheet metal chassis surrounding the amplifier driver boards, These would be a 90 degree change. Remember, the power is on when you're doing this and you sure don't want the screwdriver to short out anything and possibly ruin a bunch of un-obtanium vintage TO-3 cased power tranny's. Gotta do some thinking about that, but thanks, as I will look into this option.

morbo!
10-27-2005, 04:43 PM
Yeah thanks i got an old sansui down the shed packed away somewhere
It keeps tripping out when big volume is applied. z-7000 from what i could just tell from the im my dark shed
It might be the cure for this old baby too
i haven`t thought about this amp in 6 years (lol)
it was never very good anyway as far as i was concerned

morbo!

maybe in the for sale section soon

Michael Smith
10-27-2005, 04:46 PM
Be careful,some of those old Technics amps were labled "NEW CLASS A" and wouldn't pull the skin off a rice pudding".
They had some weird bias setting and when you screwed with that they went bang in a very nasty way!
Have fun
Michael

scott fitlin
10-27-2005, 05:02 PM
Agreed! DC bias pots are not for the novice tinkerer to play with!

morbo!
10-27-2005, 05:22 PM
oh i really dont want it going bang!
Maybe just new audio pots and switches if i can get them
It`s an su-7300 dont think its class a
Might have to have a good hard think about what to do with it bieng a nice old amp with a nice sound after u screw with every knob and switch on it!
wasn`t always the way i used this amp for 10 or more years and loved it

morbo!
p.s i have been looking on ebay for one here in aus thats been blown but have never seen one quite like it for sale here

rek50
10-27-2005, 06:04 PM
I have replaced the SILLY $.03 factory trim pots about a year or so ago, after reading the same "Tip". It wasn't all that bad. A solder sucker and some braid, to remove the old pots. Once removed, I measured each one and wrote it down on a sketch of the board, where each pot belonged. One board at a time, so if I got "Lost", the original was there for "Comfort". I used the Bourns 3296 Series from Digi-Key http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=118&M=3296P-1-100Regis, check out the 3296P, they should work for your "Straight On" deal. I got some of them for use in a 9090DB. I also got the other types for a couple of Eights and an Eight Deluxe. Again, it wasn't a big deal. I had never done it before (Trim pot change), but I had changed out resistors, soldered, and took measurements, before. No biggy. Work clean, (I cleaned the pot leads and board with 92% ISO before soldering). I positioned the pots in place by slightly bending the leads, to hold them in place (outboard two bent down, center bent up). Solder up one pot lead (Center), move to another pot, and on. Once you have all the center leads "Nailed" down, give the other leads the final adjust for that "Just Right" positioning. One lead, one pot, go to the next. NOTE: "Out of the Box" the pots are close to "Center". I measured them anyway, and gave them a slight adjustment, with a Crapsman Professional- 41521 (5/64 X 2-1/2), so on the initial fire-up, you don't smoke it. (receiver). You can do it......

morbo!
10-27-2005, 06:35 PM
thanks dood
I kinda thought that would be the way to do it.
Thanks for the clarification on the measuring.
I have a pretty cool multimeter i paid alot of money for when i had a pottery studio(a lurton dm-9090)for taking the temp of our kiln (incedentally where the amp was used)
Electronics has been a hoby of mine for a long time so i think i can too
thanks rek
nice clarification!

morbo!

Regis
10-28-2005, 07:26 AM
The Z-7000 while embracing eighties technology, still wasn't that bad of a unit. I restored a couple of these. Easy to open up and work on. Open it up and check the caps by the main power supply transformer. They are notorious for leaking and an easy replacement.




Yeah thanks i got an old sansui down the shed packed away somewhere
It keeps tripping out when big volume is applied. z-7000 from what i could just tell from the im my dark shed
It might be the cure for this old baby too
i haven`t thought about this amp in 6 years (lol)
it was never very good anyway as far as i was concerned

morbo!

maybe in the for sale section soon

Regis
10-28-2005, 07:32 AM
Thanks for the tip! I appreciate it, as I was starting to think of ways to cleanly cut holes in the sheet metal to accomodate the new technology. I'm pretty good at the soldering deal (being Mil-Spec 5088 certified as an Aircraft Electrician), but the tips might help someone else as you elaborated the process quite well. Was that a plastic screwdriver you were talking about?




I have replaced the SILLY $.03 factory trim pots about a year or so ago, after reading the same "Tip". It wasn't all that bad. A solder sucker and some braid, to remove the old pots. Once removed, I measured each one and wrote it down on a sketch of the board, where each pot belonged. One board at a time, so if I got "Lost", the original was there for "Comfort". I used the Bourns 3296 Series from Digi-Key http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=118&M=3296P-1-100Regis, check out the 3296P, they should work for your "Straight On" deal. I got some of them for use in a 9090DB. I also got the other types for a couple of Eights and an Eight Deluxe. Again, it wasn't a big deal. I had never done it before (Trim pot change), but I had changed out resistors, soldered, and took measurements, before. No biggy. Work clean, (I cleaned the pot leads and board with 92% ISO before soldering). I positioned the pots in place by slightly bending the leads, to hold them in place (outboard two bent down, center bent up). Solder up one pot lead (Center), move to another pot, and on. Once you have all the center leads "Nailed" down, give the other leads the final adjust for that "Just Right" positioning. One lead, one pot, go to the next. NOTE: "Out of the Box" the pots are close to "Center". I measured them anyway, and gave them a slight adjustment, with a Crapsman Professional- 41521 (5/64 X 2-1/2), so on the initial fire-up, you don't smoke it. (receiver). You can do it......

morbo!
10-28-2005, 02:33 PM
maybe ceramic

Ian Mackenzie
10-28-2005, 02:57 PM
I have found the Spectrol brand to be good, 25 turn power rating 0.5 watts.
No doubt there are others.

Jaycar offer generous prices on OEM quantities,

In this application the trim pot, shown as the blue trimmer, is used to trim dc volt offset to under 1 millivolt in a completely dc coupled prototype active crossover.

To date it has proved very reliable.

Ian

morbo!
10-28-2005, 03:03 PM
jaycar, I love that place.Everything the mad scientist needs to disfigure himself insomeway or another under one roof way better than dickies
p.s nice looking board whats it for (a protype crossover or a preamp)that would be my 1st guesses

Ian Mackenzie
10-28-2005, 04:52 PM
jaycar, I love that place.Everything the mad scientist needs to disfigure himself insomeway or another under one roof way better than dickies
p.s nice looking board whats it for (a protype crossover or a preamp)that would be my 1st guesses

Apparently

allen mueller
10-28-2005, 07:13 PM
I want to start of saying this is my first real post. I love this site and the information is second to none when it comes to jbl. At the moment I only have 4311's and some 2404's (thanks clmrt). So here is my question. Inside my bozak which what i use for my preamp there is a trim pot on each output. Being over 20yrs old I still love the sound but this topic has me wondering if some adjustment may be needed that could lead to improvement. I measured the outputs with no source and got about 7 mV DC. So can I do better by adjusting the pots? Here is a link to the schematic for the board. If anyone would take a look follow the like I will list below and than click on the link that is 632k in size and look at the schematic for line cue monitor amp. You can see the pot indicated by the 20k variable pot. If I understand this right do I simply adjust this pot until i can get the lowest DC in my output as possible, or is there more to it. Thanks in advance to anyone who can help me apply this new found knowledge to my setup.

Link:
http://djdalelee.tripod.com/bozak.html




Allen

morbo!
10-28-2005, 11:18 PM
Im not sure about preamps. But it seems to me as long as each channel produces the exact same amount of signal you`ll be right!
Maybe a minor adjustment either way. (R4 and R24)
And if the values go crazy replace them with better ones.
I dont think its so critical to be exact (no high power componets to explode) if u mess it up.
But maybe i wouldnt do this while listening to my fav power amp!

good luck

morbo!
p.s you should really try to stick around the 7mv range as im not sure but thats lighty to be pretty close to expected output (their has to some signal out for it to work)

soundmotor
10-29-2005, 06:11 AM
http://www.tnt-audio.com/clinica/bias_e.html

Has anybody here done this mod
I would like to hear your results as i have an old technics amp i would like to fixup!
And think this would be an ideal place to start
thanks
morbo!

I haven't done this mod but I want to thank you posting the link.

Pretty simple the way he breaks it down.

morbo!
10-29-2005, 06:37 AM
no probs all:D
Glad to give a little something back.
As their is so much knowledge and expertese here,
It will take me the next year and 1/2 to sort through and comprehend.
This is the 1st forum i ever joined.
But the jbl`s i scored kinda changed my listening experience and thats just the k-120`s(amazing in home audio)probally abit to revealling(done some mods will post pics when i get me hands on a camera)
Cant wait to get or plan somthing better.
But this is the place that will help me do it.
And have another dedicated convert in the process
I recomend them to everybody who asks me about speakers now!
and im known in my town as that mad guy who will fix anythig especially pc`s and speakers now
thanks alot to you all for your freely given knowledge

morbo!
p.s thinking of a bigger cab for them i found the correct size on the jbl pro site

rek50
10-29-2005, 07:39 AM
Regis, that screw-my-driver is a metal blade mounted in a plastic handle with a swivel "Knob" at the end. I just touch it to my tongue (possible discharge) before I adjust. So far so good (16 pots and counting). It tastes like a screw driver, go figure.... The Bourns pots (Blue) look very similiar to those posted by Ian.

allen mueller
10-29-2005, 08:00 AM
I had always thought those pots in the past were simply to adjust the output level. A few years back I had adjusted them using a test tone and my multi meter set to find the rms ac voltage, and adjusted them to get the max from both cards and than matched them. I remember it was a real pain to get them the same level. The slightest turn would send the values all over the place. I eventually got them matched and when I hooked things back up and listened I was happy. Knowing more now I think I will go ahead and some new trim pots and get things dialed in.



Not wanting to go off topic but, can anyone recommend some good books to learn about basic audio circuit design? I have done a lot of basic things like swap caps, or pots or find bad solder joints. Basically the easy stuff. I am at a point now where I would really like to learn more.



Allen

morbo!
10-29-2005, 08:38 PM
hi allen
I don`t want to sounds like im bieng smart.
But it sounds like a technicall course is in order
I did my preapprentiship in electronics and had a great time their and learned alot.(everything from making pc boards to transistor thiery)
Then got a job paying me good money, But only cause i could use what was called a computer back then($10,000.00aud for bleep bleep chunk cunk)
Who here remembers the turbo button :banghead:
I might go back meself 15 years later and finish what i started but i fear life has crushed my spirit and it would just get in the way of my drinking time!:D

p.s maybe a earth strap rek! unless you like the taste of stainless;)

allen mueller
10-29-2005, 10:34 PM
I agree a technical course is in order for my self. That is why I asked about if there are any books I can read. I do feel that replacing and adjusting the trim pots in my pre amp is something I am capable of as long as I am given the proper walk thru and that I have the proper tools. If it is beyond having a multi meter and replacing the pots with better ones I have no problem waiting till I know more and have the proper equipment. Like I said before I am at a point where I want learn more and go beyond simple and easy electronic modifications and repairs. Thanks again for any help.



Allen

morbo!
10-29-2005, 10:59 PM
books are good
i really dont know of any off hand
but you really need to be able to experiment with alot of diffrent gear

a couple of thoughts 2 of my electronics teachers were complety mad scientist`s
the 1st nearlly burn`t down the classroom with a function generator and a build your own transformer kit (showing us how to boil water with hf )
the second used to hookup electrolitic caps backward put them under his waist basket and sneek up on this bloke who came to class to sleep (very funny and you cant learn that outta no book!)
plus things like redrawing schematics so they actutally fit together in the real world is much easier when explained by an expert!
morbo!
p.s i realy gave me technics a good cleaning and went over it with a soldering iron and its running a little better,I need some graphite spray for the sound and balace pots now i think.
Never tryed graphite on an amp before will post back how it went on tuesday or wensday(a distance to the nerest electronics shop)

p.p.s nikola telsa if you want a good read on an early electric mad man
i think i even seen a patent for a loud speaker in one of the books about him(and many weird and wonderfull devices)

rek50
10-30-2005, 04:52 AM
An earth/ground strap is on my "List", for an up-coming computer rework session. Deoxit D5 (Caig Laboratories) is a good cleaner/quieter for pots, if you can get it over there.

tomt
10-31-2005, 10:10 AM
allen mueller


Not wanting to go off topic but, can anyone recommend some good books to learn about basic audio circuit design? I have done a lot of basic things like swap caps, or pots or find bad solder joints. Basically the easy stuff. I am at a point now where I would really like to learn more.



Allen
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robert l shrader's - - Electronic Communication is a worthwhile starter book on this topic.

early editions have tube information, later editions more about things digital.

many larger librarys have copys, also online booksellers,seen priced from $2.98 to just

under $300.00


latest edition new is $132 from mcgraw-hill