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 Post subject: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 12:04 am 
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Despite the fact that there have been some signs of life in the old SB-401 I am working on, it just didn't seem right yet (at least poor audio) so I checked all the resistors. A solid 40+% are out of spec (+/- 10% or worse). This was not a big surprise based on other posts and as others have said many are WAY out of spec. My question is, is it a better strategy to identify the bad ones and replace them or just replace them all. I wouldn't replace still functional bleeder resistors. Many are in circuit boards (very old style) so more difficult to replace without doing damage. Any ideas on the best strategy? There are probably 100 resistors in there.

Thanks for any thoughts.


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 12:43 am 
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I have only once done complete replacement of resistors and that was in only in a single section of a radio, the front end of a Pierson KP-81 receiver, which is a major pain to access and repair. In this case I replaced ALL of the passive components in that section in hopes that I will never have to go inside that portion again because it takes a few hours to either remove or install the front end module in the receiver and then several more hours with a long curved tip soldering iron to remove enough leads to separate the two case halves of this module. Nothing in any of the Heathkit SB series gear requires this sort of work, thank goodness!

My usual strategy with resistors is to do a quick in-circuit measurement of the value and go after those that measure far out of tolerance. There will be some cases where a resistor is shunted by other components making in circuit measurement problematic but these are in the minority and I will catch those during initial testing/alignment when I find a circuit or stage not functioning as it should. Resistors very rarely drop in value so when you measure in circuit resistance that is low out of spec it is usually because of other circuit components creating a parallel path.

I would go through and measure and then replace those that you find are out of tolerance. I am a little surprised that many are bad in your SB-401 but it may have had a lot of hours on it. I haven't replaced that many resistors in Heathkit SB series tube type rigs but I have found a few. When my SB-102 was still pretty new it had a couple of way out of spec plate load resistors in the audio section that were causing abnormally high distortion. I recall speaking with someone at a hamfest who found many out of spec resistors in a HW-101 but he thought that was because the original owner had built his own power supply and the B+ was far higher than Heathkit specs which created a number of problems.

Rodger WQ9E


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 12:46 am 
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Did you unsolder one end each resistor when taking the measurement? In circuit resistance tests will not be accurate due to the influence of connected components.

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 4:34 pm 
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Thanks for the information. Most resistors were tested in circuit with stable readings. There remained a few that I could not get a good reading and for those I disconnected one end. I was also surprised at the number that were greater than 10% off. I don't know enough history of the radio to guess why so many. I'll take your advice and replace only the out of spec ones. I have read advice to replace all carbon resistors but that's a lot of replacements.

With so many bad in the circuit boards this rig is probably not worth the effort in any economic sense but I shall press on. I got the radio along with a SB-303 for free so the price was right. The SB-303 works fine as is.

BTW are metal film resistors OK for replacements?


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 4:39 pm 
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Eddy wrote:
Did you unsolder one end each resistor when taking the measurement? In circuit resistance tests will not be accurate due to the influence of connected components.


In circuit is fine, provided you are comfortable reading and interpreting schematics. You have to avoid parallel paths: leaky bypass caps, electrolytic capacitors, parallel resistances, coils, etc. I usually find 90 percent of the resistors in a circuit can be checked in place, at least for vacuum tube equipment. One thing to be aware of, if a resistor reads high, it is not due to stray paths... it has aged and needs to be changed. If a resistor reads low, than I'd look to see if there is a parallel path that I missed.

Pete

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 4:40 pm 
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caseydogmusic wrote:

BTW are metal film resistors OK for replacements?


Metal oxide and metal films are fine for almost applications in vintage radios.

Pete

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Sun 03, 2017 4:51 pm 
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I have had a recent experience in resistor failure in a '29 receiver. Although the resistors passed the VOM check and the check by heating one end of the resistor via its soldered joint the radio is now experiencing fading. The degradation of the resistor is not confined to just a shift in resistance but a shift when current passes and/or when heated. Testing for those parameters is just is not practical for the cost of the time. For me it was a painstaking process to re-install the chassis into the cabinet, accurate dial alignment to prevent rubbing on the bezel.

Time is catching up to carbon resistors.

Chas


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Mon 04, 2017 12:30 am 
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In 1969 I bought a Healthkit HW-101. I put it together and it worked, more or less, but the output power/ plate current was always low and the vox would not work. It was a really popular rig at the time. I talked to a lot of HW101 owners on the air and it seemed everyone had some sort of problem, some minor, some major. But it seemed no two people had the same set of problems.

Years later I sold the HW101 to a young new ham. A couple weeks after than I got a call from his dad--- who happened to be my boss's, boss's, boss. The rig was not working could i come have a look at it. I had just got a used Fluke digital meter. I took it along. A number of critical voltages were badly wrong. I turned off the rig and started measuring resistors. Over half were well out of tolerance.

I made a list went home and the next day ordered replacement resistors. I replaced the bad resistors and turned on the rig. It worked fine, output power and plate current was on the high side of the specification and after some adjustment the VOX worked as well too.

It seems to me Healthkit got a deal on a bunch of carbon resistors but a large fraction where bad from the get go and a lot of what was left quickly drifted out of tolerance.

So my advice to you would be to replace all the resistors. One thing to look out for is a modern 2W resistor is a lot smaller than the old standard 2W resistor. You might want to check resistor dimensions before you place the order.

You can reduce the damage potential for the PCB board by cutting the body of the resistor free from the two leads and then remove one lead at a time first using solder wick to remove the bulk of the solder.


Good luck


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Mon 04, 2017 7:34 pm 
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You mention many resistors were +/- 10% out of spec ... it may be that they were actually 10% resistors to begin with. Many were back in the day. That said, I suppose there's no real harm in replacing a 10k 10% resistor with a 10k precision resistor, particularly if you are doing a re-cal anyway :)

My 2c, +/- 10% :-D

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Tue 05, 2017 12:55 am 
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I usually don't replace resistors in more modern rigs--after 1960--until after I do the capacitors and fire the radio up. If I see an obvious burnt or discolored resistor, I will change that one. A lot of times the exact value of the resistor is not critical for the circuit to function. After all, resistors only come in certain values. Once the radio is working, and I can start to find problem areas--then I measure and change resistors that aren't to spec. Sometimes, the new value I need to make the circuit work is not the original value, but a lower or higher value to compensate for other aging parts or circuits.

Norm

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Tue 05, 2017 6:15 pm 
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To all who responded thank you. A lot of knowledgeable people here.

The story Coolbrucelong told is very interesting and its an interesting theory. Interestingly, I have a an SB-303 that I think may have been the original twin (no way to tell for sure except I got them from the same SK at the same time. Unlike the 401 it works well untouched. So I wonder if it might have something to do with the heat generated in the all tube 401 - higher voltages too.

As far as how out of spec the resistors were, most were well beyond 10% though I used 10% as a somewhat arbitrary cut off point for possible replacement.

I agree if ain't broke don't fix it but this radio has had a collection of problems that have been hard to track down. I think I finally got the carrier suppression fixed (also out of spec resistors). I get the occasional fuse blow I think from arcing inside one of the 6146s. Power is a bit low but most importantly there are a number of what I will call "audio" issues. This includes the CW note being unstable and sometimes audio seems off. So at this point rather than tracking down each symptom individually I think I am going to at least replace all out of spec resistors and possibly all of them except bleeder resistors. I hope its worth the effort. Well this is a hobby and not a job so I guess I am free to waste my own time!

Cheers.


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Tue 05, 2017 6:44 pm 
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With older transmitters and transceivers, it is recommended to re-tighten all grounds. This solves many problems that are otherwise very hard to track down. I restored a Marauder a month ago that had problems which only were resolved when a nut holding the filter in place--as well as a ground lug--was tightened.

Norm

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Wed 06, 2017 3:33 pm 
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Norm beat me too it. The hardware-provided connectivity between the PC board grounds and chassis are problematic.

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Thu 07, 2017 3:24 am 
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All good advice!
What I would do, is first look for resistors that have obvious heating or other damage.
Then I would look for those that are high, and make a note of exactly how far above 10% or 20% they are.
I found that in Gonset Communicator III's, 1/3 to 1/2 the resistors were over 10%, with a minority seriously out at 100% or more.
I would then hit the critical ones that were very far out, replacing them first, then go for cathode bias resistors, then others.
The ones closer to 10%, say to 20% aren't as critical as those values that are way out.
Also, if I see discolored plate or screen resistors, I'll look to see if there is a problem, and typically replace them with higher wattage resistors of the same value.
An example of this would be the Sonar FS-23 23 channel synthesized CB set, where they economized, and used a single 410 volt B+ for all the HV, instead of a lower voltage for the receiver tubes, and a higher one for the final.
Everything but the RF final has to have good sized plate and screen dropping resistors to reduce the very high voltage delivered to the other tubes.
Typically, they use 2 watters that have gotten very hot because they were operating very close to, or at their rated wattage and drifted up significantly in value.
These are replaced with the proper resistance value 5 watt cement resistors...
Best of Luck!

-Tom


Last edited by Tom Herman on Sep Thu 07, 2017 1:07 pm, edited 2 times in total.

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Sep Thu 07, 2017 12:40 pm 
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As Tom stated in most cases a resistor that is a little out-of-spec won't noticeably impact circuit performance in most cases. If a 10% tolerance part was used initially and 50 years later the part is 12% off it probably isn't going to matter in terms of circuit performance. The original circuit designer probably used rough design center values and didn't try to optimize the value of each passive component because he knew that wasn't critical. In older radios you may well find some resistors with no color band for tolerance and this meant the original part was a +/- 20% indicating the value was not critical.

Of course there are cases where the value is fairly critical but normally in those cases you would see a tighter tolerance specified and in those cases if you don't understand the circuit function thoroughly then I would replace tighter tolerance parts that have gone out of spec. Another case where a slight change out of tolerance will cause issues is when the original part was running close to its rated power dissipation. Most 1/2 watt resistors in vintage receivers aren't running anywhere close to 1/2 watt dissipation but the 1/2 watt size was used because it was the commonly available smallest wattage resistor. But there are cases where these 1/2 watt units are actually run close to that rating and that is true of the often troublesome screen resistors in the RF and IF stages of the National NC-183D receiver. Once these resistors drift up slightly in value then they will go beyond their rated power capability which further heats them causing the value over time to go even higher. This "Chernobyl like" behavior continues until the resistor value drifts so high that it effectively cuts off the stage and current draw and dissipation drop to very low values. The first NC-183D I restored had most of the 47K screen resistors in the 220K-270K range and as a result the receiver gain was extremely low.

The advisement to tighten the hardware is very good and this was a common issue with all of the vintage Heathkit gear that uses circuit boards screwed down to the chassis. With these SB and HW series rigs I go around each board and slightly loosen and then tighten every mounting point which helps to clear corrosion and provide good contact again. A number of the Trio/Kenwood rigs are also well known for having this sort of issue. And in the "there is nothing new under the sun" department it turns out that the largest single cause of multiple unusual DTCs (diagnostic trouble codes) and unusual symptoms in modern vehicles is due to poor or missing ground connections. In recent years both GM and Ford have had a number of cars with intermittent electric steering system issues caused by poor or degraded grounds in this high current circuit.

Rodger WQ9E


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Feb Thu 22, 2018 6:51 pm 
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I had missed some of the replies until today, all useful advice. I replaced maybe 40 resistors in the 401 and it was far easier than I would have thought. It is relatively easy to get them off the old style PC boards without damage. Typically the resistors replaced were off 30 - 50%, often screen and plate resistors as mentioned. There were a lot of 10 ohm resistors in the cathode circuits of the finals that were ALL way high. After replacement and re-seating all ground hardware I was able to get the carrier suppression to work much better though still not perfect. I have had several successful QSOs with the radio (SSB and CW) so its working pretty good. There are still issues though and in the process of working on them the loading o ring broke (old age). Waiting on dial cord to fix that and then the work shall continue including tying to figure out why the meter never shows any grid current but does seem to work well for HV, plate, relative power, and ALC.

Thanks again for your help.


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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Feb Thu 22, 2018 7:27 pm 
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Barry H Bennett wrote:
You mention many resistors were +/- 10% out of spec ...

Being off value does not mean a resistor is "out of spec". You must consider its tolerance.

If a resistor is banded as 10%, then any value within 10% of nominal is "in spec".

One thing many folks fail to realize is that if a company made both 10% and 20% resistors,
no 20% resistor would have been within 10% of nominal when it left the factory.
All that were within 10% were banded and sold as such.

- Leigh

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Feb Fri 23, 2018 2:26 pm 
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Greetings to the Forum:

Two comments on all the above discussion. First, about carbon composition resistors. I have run into a few occasions (not many, it is true) where the resistor begins to exhibit diode-like behavior. In other words, you will see two different resistances for the same resistor if you reverse the ohm meter lead polarity. This behavior can lead to all sorts of really difficult to find and strange problems.

Secondly, the Heathkit board ground issue. This is sound advice, but it brings up a story anout a dilemma faced by our local ham club... one in which I was personally involved. The club had been given a Heathkit SSB transceiver (I do not now recall the model) which was one of the rare factory-assembled rigs. As such, all of the circuit boards were riveted in, rather than fastened down with screws and nuts as in the kit-built versions. Indeed, this is one way one can determine that the radio was factory built. Unfortunately, the grounding problem was very much an issue in this particular radio. What to do? To be sure of good grounds, one would need to drill out all the rivets, clean the chassis and circuit boards at the interface points and then re-install the boards with screws and nuts. But.... if one does this, one destroys the evidence that it was a factory built radio.

I was able to fix some of the grounding issues by bonding various ground planes on the boards together and adding some jumper wires to ground lugs where I could. However, neither I nor another local ham were able to get the thing running really right, and I don't know whatever happened to it. It does bring up an interesting question though... lower the value of the radio by removing evidence of the factory build, or lower the value of the radio because its not working right? Any appraisers out there?

Regards,

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 Post subject: Re: SB-401 Resistor Replacement Strategy
PostPosted: Feb Fri 23, 2018 3:13 pm 
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Quote:
lower the value of the radio by removing evidence of the factory build, or lower the value of the radio because its not working right? Any appraisers out there?

Or ..... RAISE the value because it is both a working unit, and updated!! As always, this is going to be in the eyes of the beholder.

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