Please help in identifying this large resistor - RCA Victor Model TRR-8 MARKVIII - Page 1
Forum section: electronics / repair

author: tarrence
author_rank: Newbie
timestamp: September 06, 2020, 11:37:57 pm
content: New to the forum and to projects. I believe the schematic shows the largest resistor in the picture to be 1200 ohm, 2WDoes this appear to be the case? Thanks in advance.

author: Chappy425
author_rank: Newbie
timestamp: September 06, 2020, 11:46:38 pm
content: That is a 1200ohm resistor and it probably the one. With a piece of equipment that old you may find more help on the following forum:Antiqueradios.com

author: wraper
author_rank: Unknown
timestamp: September 06, 2020, 11:49:24 pm
content: 1.2k 10% carbon crap. These resistors tend to change resistance a lot, especially at presence of high moisture.

author: tarrence
author_rank: Newbie
timestamp: September 06, 2020, 11:50:02 pm
content: Thank you. I will certainly check out the site recommended!

author: tarrence
author_rank: Newbie
timestamp: September 06, 2020, 11:51:15 pm
content: I see. Any recommendation on replacements?

author: Nusa
author_rank: Super Contributor
timestamp: September 06, 2020, 11:54:30 pm
content: You might find this useful in interpreting resistor color bands:https://www.allaboutcircuits.com/tools/resistor-color-code-calculator/Wattage is more of a size thing...bigger resistors of the same type are generally higher wattage.Why do you want to replace it? Do you have some indication that its failed?

author: tarrence
author_rank: Newbie
timestamp: September 06, 2020, 11:55:57 pm
content: Thank you!

author: tarrence
author_rank: Newbie
timestamp: September 06, 2020, 11:57:04 pm
content: Soldered near the resistor causing bubbling on the silver band. Not checked with meter yet.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: September 07, 2020, 12:48:08 am
content: HiCertainty is a 1.2K and comparing it's size to the IO octal tube base it is about 2W. These old carbon resistors go high in value with applied voltage.George G6HIG

author: coromonadalix
author_rank: Super Contributor
timestamp: September 07, 2020, 01:26:43 am
content: yep 1.2k ohms 10% 2 watts at least 1 watt for sure

author: Gyro
author_rank: Super Contributor
timestamp: September 07, 2020, 10:29:36 am
content: I'd suggest replacing it with a 3W resistor, it will have about the same body size. Modern resistors tend to have smaller body size and run at higher surface and lead temperatures. As it seems to be wired between two caps, you don't want to conduct too much heat into their terminals. Carbon Composition had (have) a very good surge capability too.

author: tarrence
author_rank: Newbie
timestamp: September 08, 2020, 06:32:31 pm
content: Thank you for the great suggestion! Another newbie question- What is the left to right orientation of this resistor? or any resistor for that matter?

author: Connecteur
author_rank: Frequent Contributor
timestamp: September 08, 2020, 07:03:28 pm
content: I might be soft in the head, but why would you need help to measure it with an ohmmeter?

author: drussell
author_rank: Super Contributor
timestamp: September 08, 2020, 07:12:14 pm
content: Quote from: Connecteur on September 08, 2020, 07:03:28 pmI think the OP was trying to verify that it issupposedto be 1200 ohms, 2W.Measuring an old carbon comp resistor with a meter doesn't tell you what it's value issupposedto be.After many, many years of drift, old carbon comps will often measure anything from in this case, say, about 1100 ohms to about 5k. They usually don't go completely open, but will often drift (usually) upwards anywhere from mildly outside tolerance all the way to sometimes several times their original value...

author: Nusa
author_rank: Super Contributor
timestamp: September 08, 2020, 07:55:11 pm
content: That said, old resistors are far more durable than old capacitors. If you still have original capacitors in that thing, they deserve to be checked first. Especially if you're trying to make a non-working unit work. Tubes, and I presume you have at least one, tend not to be affected much by simple age, and are probably as good as they were the last time the unit was used.

author: drussell
author_rank: Super Contributor
timestamp: September 08, 2020, 08:19:43 pm
content: Quote from: tarrence on September 08, 2020, 06:32:31 pmPolarity doesn't matter with resistors.Quote from: Nusa on September 08, 2020, 07:55:11 pmYes, indeed and, especially in tube equipment, resistors that have drifted upwards in value, even to several times their original value, will seldom make the apparatus totally not work. It may not workwell, but it will usually at least dosomething.QuoteYes, most certainly! Old capacitors are often bad to the point of causing something to not work at all or even cause severe, permanent damage like burning out an otherwise good transformer winding or blowing up rectifiers, simply due to a shorted capacitor.TheOPshould also note that many types of capacitors are polarized, so you need to be careful with those to be sure you get them the right way around when it matters, but with resistors it never matters.QuoteIndeed. Unless something like storage in wet conditions has corroded the pins, worked its way up and destroyed the glass-metal seal around the pins, tubes don't really "age" when being stored and not in use. When theyarepowered up, you're slowly using up the cathode's ability to emit electrons and slowly wearing out the heater or filament, but when un-powered in reasonable storage conditions, they should last virtually forever.Good points for theOP. ThanksNusa!

author: tarrence
author_rank: Newbie
timestamp: September 08, 2020, 08:25:23 pm
content: Thank to you all for the helpful suggestions!

author: Gyro
author_rank: Super Contributor
timestamp: September 08, 2020, 08:46:26 pm
content: Indeed, tubes have almost indefinite shelf life. The leakage rate at the pin seals is infinitesimal, the only potential long term issues are inert gas infiltration (primarily Helium) that the getter(s) can't capture. Other leakage is controlled by the getters until they are exhausted.The comment about storage conditions is well made though. There are many excellent performance former Soviet NOS tubes available cheaply, however they may have spent several decade in very cold ex-military warehouses. Being in cold storage, these tubes sometimes suffer reduced vacuum due to very low getter activity. As the rate of reactions typically doubles for every 10'C rise in temperature, these tubes benefit from a certain amount of sunbathing (literally) before being put into service. If you're in a hurry, baking at 100-150'C for 24 hrs is an effective method of allowing the getters to restore full vacuum before the tube is powered up. This treatment is particularly important of power tubes, where voltages are high, otherwise catastrophic flashovers and cathode bombardment can result. The 'obvious' solution of using the heaters to warm up the tubes isn't a good idea, zero bias operation can cause cathode poisoning.As long as there has been no corrosion, even non-ideal storage can be corrected.

author: drussell
author_rank: Super Contributor
timestamp: September 08, 2020, 08:48:45 pm
content: Quote from: Gyro on September 07, 2020, 10:29:36 amAh, yes. Another great point for theOP. ThanksGyro!Replacing a resistor with one rated for a higher wattage is completely fine, and in cases like this is actually desirable for multiple reasons.A common misconception with beginners understanding resistor wattage ratings is that the resistor rating is what it is supposed to be able tohandleat some elevated temperature without burning up. It is not like a light-bulb wattage rating where the specification is how many watts the bulb will draw (due to its resistance) with the rated voltage applied to it.You can use a 5 watt resistor in place of a 2 watt and it's just fine, not like putting a 5 watt light bulb where a 2 watt maximum should be. Likewise, with capacitors, you can use a higher voltage rated part without issue, it is the rating for the maximum voltage it will withstand. You want the capacitance to be the same as an original, but you can go up inmaximum working voltagewithout skewing anything in the circuit.Just a couple thoughts to help a beginner...

author: tarrence
author_rank: Newbie
timestamp: September 08, 2020, 10:10:48 pm
content: Great teachings for a newbie like myself! Thank you for offering such thorough fundamentals. I feel much more comfortable with the project at hand.

author: drussell
author_rank: Super Contributor
timestamp: September 08, 2020, 10:31:31 pm
content: Quote from: tarrence on September 08, 2020, 10:10:48 pmWe all had to start at the beginning, once upon a time.If the helpful man at the repair shop across the street from my elementary school hadn't sat and spent about an hour testing the carriage motor power driver board for our Diablo 1620 daisy-wheel printer, and explaining to me (a 10 year old at the time) about resistors' wattage ratings (it was also a 2 watt carbon composition resistor, which I wouldn't be able to just get at the local Radio Shack),whatand where our local Active Electronics store was, (so my dad and I could go on the weekend to get a new 2W, which then quickly became my favorite store as a kid instead of Radio Shack, no toy stores for me!) I have no idea how long it would have taken me to learn and understand all these things!Having someone to ask questions of is incredibly useful. In this case, my computer teacher didn't know enough about the actual electronics to be of any help on that one, but said "Try the guy at F&H across the street, that's where we get our floppy drives repaired." (Although he did get me started on 6502 assembly with documentation he brought from home, just for me.Thanks, Mr. Foster!) This all would have probably been in 1986.If you have more questions about components in this unit or the repair/refurbishment, I suggest you try to keep them in this one thread so we all stay on the same thought process throughout your project.Happy learning, and good luck!!