Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: henry909 on August 13, 2012, 01:45:57 pm

Title: Need help identifying an old CTS pot with no value markings
Post by: henry909 on August 13, 2012, 01:45:57 pm

I have an old RMI 140  amp with a broken pot (the shaft pulls out of the body).  I am trying to find a replacement pot, but the pot does not have typical marks such as  "A100K".  I tested the pot with a multimeter (dial set to  200k ohms) and got a reading of 101.1.  The only markings on the pot are  "CTS", "USA",  "204-1040",  and "1376927".  Can anyone help identify the value of this pot?  Thanks in advance for any help with this problem.

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: cbass on August 13, 2012, 02:01:05 pm

 I belive tthe 204 means its a 200k pot

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: sluckey on August 13, 2012, 03:06:03 pm

Quote

I tested the pot with a multimeter (dial set to  200k ohms) and got a reading of 101.1.

That meter reading suggests a 100K pot. What is the function of the pot? What type circuit?

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: henry909 on August 13, 2012, 04:56:27 pm

Thanks for the replies.

Re: The function of the pot:   It is a volume pot on one of the channels of the amp (a two channel amp).  Are volume pots on guitar amplifiers typically audio taper or linear taper or can they be either one? 

New tests show a 103.2 b/w  the outside terminals of the pot; and 68.4   and  35.9  between the wiper and each of the outside terminals (with the knob on the mid way point of its travel as best that midway point can be found).

I was told by someone else that "204"means 200K but the readings contradict this. Unless the fact that the shaft is damaged affects the meter reading. Can the damaged shaft affect the meter reading? 

Re: The rarity of the amp. Yes very rare. I could find absolutely nothing online about the amp or the company (except that the company, Rocky Mount Instruments, from Rocky Mount, NC) existed between 1966 and 1982.  The amp I think was high end because it still sounds very good and it came with a matching cab that has a JBL D-130F  fifteen inch speaker.

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: cbass on August 13, 2012, 07:29:00 pm

Almost every amp  I can think of has audio taper pots for the volume.

I have some old CTS pots that are labled like. 204=200k   203=20k    201=2k  104=100k  103=10k etc.

So thats why I thought the 204 might mean 200k.

sluckey's a lot smarter than me so I would listen to him.

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: sluckey on August 13, 2012, 09:25:12 pm

You can buy a complete working RMI 140  right now on Craig's List in Austin for $40. One just sold on ebay recently. Just replace it with a 100K audio taper. If you don't like it, you're only out $2.

http://austin.craigslist.org/msg/3196383888.html (http://austin.craigslist.org/msg/3196383888.html)

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: henry909 on August 14, 2012, 12:55:21 am

Thanks for the replies everybody.

Sluckey:  Thanks, I guess I'll go with A100K  but  I was looking at the pots on the chassis of a Fender Frontman 212R amp and found that the two volume pots on that amp are B100K and B50K. So I don't know what to think. Except that volume pots on amps can be any type of taper.   The problem is not the $2.00 to buy the pot but the soldering work to install it. Six wires, two on each terminal, small space to work in. I would like to get it right the first time.

BTW:  I connected the pot to the meter and turned the knob. I had 103k with the knob at "0" and about 60k with the knob at the midway point.  Does this mean anything?

Also thanks for the link. I took a look at the ad. He somehow got some old pic of the amp and cabs.

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: stratele52 on August 14, 2012, 03:40:42 am

Yes go for 100 k audio pot , A not B .Shaft damage don't change your meter reading if you test from each side lug , not the center one.

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: henry909 on August 14, 2012, 05:44:31 pm

drgonzonm:  Thanks for the reply. I am going to go with an A100K pot but finding one with the right dimensions is proving to be a chore in itself. I looked on Allen Organ. Couldn't find anything on the RMI 140.   Where on the site did you find it?

Title: Re: Need help identifying an old CTS pot with no value markings
Post by: henry909 on September 01, 2012, 12:43:09 pm

drgonzonm:

Thanks for the link. That is the amp I have. In the synthmuseum article Tom Emerich states "In my opinion, the tonal quality and reliability of the RMI product line far surpassed any other product line in its day. The list of professional customers speaks for itself." I think he is right (I wonder who the professional customers were). I was so surprised at how good this thing sounds. Only paid $35.00 for it with a JBL cab with a D130F fifteen inch speaker. When I removed the chassis to replace the pot you could tell how well built the amp is. I finally found a replacement A100K pot from Bourns which is almost identical to the original, at least dimensionally. The amp works well now.