Restoring A Scott 340B Receiver

I finished up the last of the actual electronic work/mods I wanted to do on the 340B prior to aligning the receiver, a final cleaning and bringing it up for a longer-term listening test.

I removed the common 3.3 ohm cathode resistors from the 7591A output tubes and installed matched 10 ohm 1/4-watt carbon film resistors on each cathode. This gives the ability to use the DC balance and bias controls now to tune each tube for proper cathode current. Unfortunately, it also reduces the utility of the nice, above-chassis test points for setting cathode current. You have to separate the cathodes for each pair in order to install individual resistors, so you lose the ability to use the test points to set the bias a pair at a time. I left them hooked up for a "quick check" feature, but they only are connected to V5 (L channel) and V105 (R channel) now.

When I put the new resistors in, I was still getting >10 ohms from each cathode to chassis, which was puzzling. Recall above that I said the original 3.3 ohm resistors had gone up in value--not so! The problem turned out to be a bad riveted chassis ground point. I traced the ground for the cathode resistors all around the chassis and finally found the chassis ground point off the common can of the C401-C404 multi. Everything was good up to the point where the can was grounded to a tie-point riveted connector, with the connection showing several ohms of resistance to the chassis! I drilled the rivet out and put a screw, nut and star washer on it and now the ground was good and all the resistors read 10 ohms to ground now. Before correcting, I would have thought I had 10 ohms for setting bias, but would have had 12-15 ohms and the bias setting would have been artificially low (under-biased) because of the higher (and changing?) resistance.

I could have just terminated the cathode resistors to a point nearer the output tubes, but I figured the Scott engineers had done it that way for a reason and I didn't want to introduce any weird ground loops. I wanted to straighten out the ground problem as well. I had read that sometimes these riveted chassis points can go bad, but this was the first I have found myself.

Then, I installed 100 ohm, 1/2-watt screen protection resistors on each output tube off a 14 gauge wire buss (picture). I put a tie strip on each end of the buss--one end off a screw holding the 8K 20W resistor to the chassis and the other fastened to a strip attached under one of the PT nuts. Took great care to make sure this buss didn't touch anything else, as it carries 430VDC, and left the insulation on the wire except at the solder points. I replaced the 330 ohm 2W carbon comp R404 with a 3 watt Vishay/Dale 500V metal film unit to bridge to the buss.

While I was in, I replaced the chassis isolation "death cap" with an XY rated unit and a new 820K resistor. Kind of tucked in back there, but got it in OK.

Powered up without the output tubes after triple checking everything and all was good voltage-wise. Put the tubes in and biased each to 350 mV to give 35 mA quiescent cathode current per tube, per the manual.

Sound check was good, but I can tell the tuner needs alignment. Even with the pointer aligned with the stops, stations are about 0.8 mHz higher than the pointer in frequency and I'm getting a couple of stations at more than one spot at the bottom of the dial, so alignment is next!

Pictures are : 1 and 2, cathode and screen resistors; 3 bad chassis ground point repaired; 4 new chassis safety cap/resistor.

Dave
 

Attachments

  • IMG_3040.JPG
    IMG_3040.JPG
    76.4 KB · Views: 175
  • IMG_3041.JPG
    IMG_3041.JPG
    69.5 KB · Views: 175
  • IMG_3046.JPG
    IMG_3046.JPG
    72.2 KB · Views: 173
  • IMG_3047.JPG
    IMG_3047.JPG
    55.6 KB · Views: 172
Riveted grounds can be a giant pain in the neck at times, especially when trying to figure out why things just don't work like they ought to. Finding them is often as much an accident as anything. Bump a part, it makes a horrible noise and then works again.

Cathode grounded to the power supply ground actually does make good sense, though few things are constructed that way. Not having to flow current across the chassis is good for noise control reasons. Thats one of the cool things you can do when building your own stuff, the ground scheme can be optimized for noise control vs just whats easy to mass manufacture.
 
Nice restoration. I have a 340B that I also meticulously disassembled and cleaned. Mine was in good working condition with no cosmetic issues other than dust. I gradually did much of the electrical work you are doing. I did mine for precautionary measures. These are nice looking and nice sounding receivers, the FM section is pretty good too.
 
I too was bitten by the rivet corrosion on my 340B. I had an intermittent buzz which I was finally able to find by flexing the chassis and pushing down with a heavy wooden stick. The rivet was tight but there was enough corrosion that the resistance had gone up. Drilled out the rivet and replaced it with two external star washers, screw and nut. Problem gone.

I am glad that you have brought this up because it can often cause one a lot of headache (even more than too much booze).

Nice job by the way, thanks for taking the time to share. I am also glad that your numbers did not come off of the back of the dial glass. I have had them literally disappear with one wipe! That was on my HH Scott LT-112B-1 kit that I built. Poof--- Gone!
 
Thanks for your detailed writeup Dave451. I for one would be interested to follow your alignment procedure, which is still a bit of a mystery to me.
 
Thanks, guys. Drew, yep the resistor is a little dark. I couldn't figure if that was the natural color or not, but I'll check it out whenI have the bottom off in next couple days. I moved the receiver up to the listening room after doing some alignment. Dandy, I'll describe the 'hybrid procedure' I used in next post and offer it for comments and suggestions. I will say, though, that it did improve the operation of the tuner (sensitivity and dial accuracy, in particular). Getting good tuning behavior (single peak in the middle of the station, good symmetrical drop off on both sides of the signal, good capture even of weaker stations next to stronger ones, etc), no apparent distortion on strong stations, stereo indicator operating well. I really want to hear how the amplifier sounds on my 'big speakers' as well.

I did a power check and got exactly 30WRMS (the spec level) per channel at clipping with the Tung Sol re-issues, 1kHZ, one channel driven. Distortion looked quite low on the scope, maybe a little lower than the Fishers I've been working on lately. More to come!....

Dave
 
Nice thread, thanks Dave for detailed posts.

Working on my 340B with amplification issue. The volume doesn’t change by the volume pot and no channel balance. Checked both pots there are no issues. When I turn the maximum volume on I get a little lowering the overall loudness.

I suppose that issue can be with grounding of volume/balance. I found out that there is no connection between the volume pot ground wire and chassis at all.
Please advise.
 
Vince, both those pots (two-ganged volume control, one pot for left channel, one for right channel) must be grounded on the end opposite the input signal for volume control and balance to function. Correct that situation first and see what happens.
 
Vince, both those pots (two-ganged volume control, one pot for left channel, one for right channel) must be grounded on the end opposite the input signal for volume control and balance to function. Correct that situation first and see what happens.
Thanks you Dave, finally, found broken corroded wire. 340B sings again!
 
Vince, glad your intuition about the ground was correct and that your 340B is working properly again. Hopefully the cause of the corrosion was localized to this particular problem. Good luck!
 
I've gotten so fed up with these riveted ground points and all of the problems that they cause that I just take care of them as a matter of course.Especially in tuner work,these bad grounds can really waste a lot of your time.

For steel chassis,I prep the area (usually with a Dremel tool) then solder them.For aluminum I either drill out the rivet and bolt them, or just daisy chain them all together with wire.
 
Art, sounds like this is a chronic problem. I'm going to be a lot more sensitive to checking the chassis grounds in future work. As I said, this is the first one I've encountered that (as far as I know) was causing a problem. Do you avoid random ground loops by daisy-chaining them?
 
I haven't had a great lot of trouble with riveted grounds, but it really depends on the condition of the chassis and whatever platings and materials may have been in use. I had issues with my 30s Philco that developed rust around the rivets and lost connection. I fixed all of those with a rather more archaic method, I simply re-set them with a hammer, a punch, and something heavy on the back side to work as a bucking bar. Others I've drilled and replaced with screws, or cleaned and soldered, depending on what seemed to make the most sense.

On the rare occasions I build stuff, it gets buss grounds. The only thing that the rivets or screws do is hold parts in place.
 
So, the 340B is playing right now and sounding very, very good. It is getting along quite well with my AR4x speakers--they seem to be well-matched and the extra power of the 340B helps with the fairly low-efficiency AR acoustic suspension speakers.

As I mentioned before, I completed an alignment of the tuner and put it in the listening room for some extended play and I've been very happy with what I 'm hearing so far. The tuner is working quite well after the alignment. Dandy, you asked for the procedure, so I'm going to try to pass along what I did (and what I couldn't do at this point). I'm going to attached a (quite) detailed write up of what I did and I would very much appreciate feedback from anyone brave enough to dig in and have a look. Meantime, the summary is as follows:

I just haven't been able to find much on line that gives a 'blow-by-blow' description of how do do an alignment. There are some YouTube videos that are helpful, but they tend to be for simpler radios with nothing on aligning the multiplex decoder that I can find. So, I've been in the "do and learn mode" using what resources I can find and learning by experience. Here are a couple of useful Scott references, including the Service Bulletin for the 340B (it's in the AK database):

1. “Service Bulletin for Model 340B Tuner/Amplifier,” H. H. Scott (late model version).
2. “H. H. Scott Tuners: Evaluation, Troubleshooting, Service, and Alignment,” Dave Roehr (2005-2006). I don't think he every fully completed this excellent document, but what is there is very useful.
Instruments used:
-Sencore SG-165 FM stereo multiplex generator/stereo analyzer
-Rigol 100 mHz, two channel digital oscilloscope, with distortion analysis capability (via Fast Fourier Transform)
-Hex and slot alignment tools to turn the slugs in the transformers and coils.

I had replaced six tubes overall in the unit; both of the RF/front end tubes, the limiter/detector tube, and two of the tubes in the MPX decoder and the tuner was showing signs of needing alignment.
SUMMARY PROCEDURE:

RF/Front End--aligned using RF signal modulated with 400 hZ sine wave audio from the SG-165 at 92 mHz into the antenna terminals with a relatively weak signal. Peaked the FM ANT and FM RF coils on the RF unit for max signal on the Scott meter. Then used 2. above to align the dial after making sure the dial cord was lined up properly. I used broadcast stations of known frequency at the high and low ends of the band to set proper dial frequency at the high end with the RF OSC coil, followed by adjusting the low end station by spreading the oscillator coils slightly in the 'silver box' under the RF unit iteratively until I had pretty good tuning accuracy across the band.

IF STRIP--Used low-level signal (a key to good alignment) from the SG-165 and peaked the IF transformers (top and bottom slugs) for maximum signal on the front panel meter. Worked from 'front to back' doing top slug first, then bottom: T201, T301, and then T302, then repeated. This changed the threshold for FM stereo operation, so I adjusted the "Stereo Threshold Adj" on top of the multiplex decoder unit to pick up the pilot from at least moderately strong stations to enable stereo. Anything below the threshold comes through in mono (and avoids stereo noise on weak stations).

LIMITER/RATIO Detector--This is the 6HS6 tube and T303. I used a weak signal from the SG-165 (about 4 of 10 on the Scott meter) to peak the primary (bottom slug) for maximum signal. Then, I increased the Sencore output to full deflection of the signal meter and used the Fast Fourier Transform (distortion measurement) feature of my scope to adjust the secondary of T303 (bottom slug) for minimum distortion.

MULTIPLEX DECODER--This was complicated by a frozen slug in L501 of the 38 kHz circuit, so I did the best I could working around it until I can figure out how to free up this slug without destroying it. The decoder was working well enough to start, so I didn't want to mess with it too much. I used the SG-165 with stereo-modulated MPX input into the tuner terminals with moderate signal strength and the Pilot modulation set on the 10% (max) setting. I peaked the 19 kHz pilot signal with T501 on the MPX sub-unit, but couldn't get the full adjustment and was left with about 15 V peak-to-peak on the scope, compared to the 20-40 VPTP recommended and soldiered on.

I went through the Service Bulletin procedure to adjust the 38 kHz circuit, but had trouble figuring out how to use the scope to "zero beat" the signal, so left it pretty much "as is" (see the write up) and proceeded to adjust stereo separation as well as I could without being able to touch up L501. Using alternating left and right channel 400 Hz input from the Sencore, I adjusted the "R Sep Adj" and "L Sep Adj" pots on the decoder unit for maximum separation of the channels. I think my Sencore needs better calibration on this function, so I was able to get good but not great measured separation, limited I think by the Sencore calibration (yet more work to do). I noticed this deficiency when adjusting my Fisher 400 as well.

RESULTS--so, the tuner appears to be working very well. Sensitivity is considerably improved and dial accuracy has been re-achieved (it was showing 0.8 mHz high across the band versus true frequency when I started). Tuning behavior is quite good with one peak in the middle of the signal and good, symmetrical fall-off on both sides of the signal. FM Stereo light/function working well and no evident sound distortion in stereo mode vs. mono with a full quieting signal, even on strongest stations. The awful tuning noise I heard when turning the knob initially is gone now (cured by new front-end tubes and cleaning the tuning cap, no doubt).

The tuner is just a tiny bit "hissy," even on strong stations. I expect this is from the pre-emphasis RC values being used and it is mitigated by the "Sub Channel Filter" setting. Anyone else see this or want to comment?

I hope folks will take a look at the attached write up and provide feedback, as well as any questions or comments on this summary. Thanks!
Dave
 

Attachments

  • Aligning_340B_Tuner.PDF
    767.9 KB · Views: 32
Dave, thank you very much for taking the trouble to write up your alignment procedure. As you say, it's hard to find such guidance on the web. I've never done such work, but I'm really grateful to be able to be read about it as a first step. It's good to hear that you are satisfied with the results.
 
Art, sounds like this is a chronic problem. I'm going to be a lot more sensitive to checking the chassis grounds in future work. As I said, this is the first one I've encountered that (as far as I know) was causing a problem. Do you avoid random ground loops by daisy-chaining them?

In my experience,this is a chronic problem. We're an ''all weather area'' (emphasis on frickin winter!) where I live,so humidity can be a problem.When you consider the age of most tube units,and that many of these guys have been stored in an attic or damp,unheated basements for decades before one of us rescues them,corrosion is inevitable.

When using the daisy chain method,you just need to respect the ''from highest to lowest'' convention. In most cases,if you wire from the farthest physical point through all intervening points to the actual power supply ground,you should be ok.The circulating currents in tube circuitry are generally so low as to be of no concern vis-a-vis looping as they generate no appreciable voltage differentials.

And nice write-up on the alignment,well done!

Art
 
Thanks, guys. ONE TYPO (of course) in the above summary--the second adjustment of the ratio detector transformer (T303) for minimizing distortion is the TOP slug, not the bottom. The detailed write-up is correct. Sorry about that.

Art, the Scott engineers apparently agree with you on the daisy chain of the grounds. The 340B cathode ground wire goes through the first B+ multi filter cap, then across the chassis to the second PS multi (near V1) for that string before going to the (faulty) tie point ground, thereby grounding the cathode tie point and both multis. This takes it through that whole B+ string before running to ground and it does run "highest to lowest." Something to consider for new DIY construction as well!

Dave
 
So, cleaned up the chassis and the tube shields, plugged in the tubes and am declaring this one "In Service". It's in the listening room and sounding and looking great. Awaiting knob brights, but in the meantime have the small Marshall knobs installed on both sides of the dual tone controls. Sounding great and looking pretty good, too. A bit more on knobs in next post. Pics!
 

Attachments

  • IMG_3060.JPG
    IMG_3060.JPG
    112.4 KB · Views: 153
  • IMG_3062.JPG
    IMG_3062.JPG
    116.9 KB · Views: 151
  • IMG_3063.JPG
    IMG_3063.JPG
    108.2 KB · Views: 146
  • IMG_3067.JPG
    IMG_3067.JPG
    45.6 KB · Views: 148
What a beautiful machine,well done Dave:)
When I found mine,I almost passed on it,thinking it was a solid state model.Boy am I ever glad I looked!
 
Back
Top Bottom