putting new trim pots in a amp

[morbo!]

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]

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!]

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]

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]

Agreed! DC bias pots are not for the novice tinkerer to play with!

[morbo!]

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]

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/Pro...&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!]

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]

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]

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/Pro...&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!]

maybe ceramic

[Ian Mackenzie]

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!]

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]

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]

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