SCOTT SLR-M questions

[n9ktw]

Hey guys:

I am working on a partial restoration of a SCOTT SLR-M. I was modified by a previous owner and has a transformer power supply. The filament string is 6 Volt PARALLEL (all the tubes are of course 6 Volt). The 25L6's are now 6V6's. I have the radio working..... AM BCB, and shortwave on all bands..... So here is my question, I know that if the #47 pilot lamp goes, I lose teh audio amp bias and therefore the audio. Does anyone have any suggestions to prevent voltage surges from taking it out? I have noticed that with new power supply caps I have to use a variac to keep things from going too high.

Any thoughts?? Maybe a ZENER across the lamp?


de Howard, N9KTW

[AJ1G]

Boy do I know about that screwy light bulb issue.  I got my SLRM (still have it) when I was 12 from my grandfather in 1964.  It was originally purchased by his bachelor brother in the fall of 1945 directly from Scott's Manhattan outlet for $250.  It was my first real short wave receiver. I previously only had ones that I JSed out of AA5 AM sets that I strung out the wire of the loop antennas on to change the resonant frequency of the LO in the pentagrid converter front end stage.

The No. 47 pilot lamp had a very short life in that set.  

For those not familiar with the set, it was a 12 tube AD DC design so it could be used on merchant ships as a super low (LO) radiation entertainment receiver, hence the SLR in the name, on either 110V AC or DC receptacle power.  It had two series filament strings of mainly 12V tubes (12A8 rf amp, 12SA7 mixer, 12J5 local oscillator, 2 12SK7s (IF), 12SN7 BFO and 1st audio, 12H6 detector and AVC rectifier, 12SN7 2nd audio and phase splitter, 1629 magic eye tuning indicator, a pair of 25L6s in PP audio output, and a 25Z5 rectifier.  The strings also had a couple of very large  glass ceramic power ballast resistors, one of which was failed open when I first got the set, and was my first radio troubleshooting and repair experience.

The No. 47 dial lamp was part of a bias circuit and as I recall, was shunted across a small electrolytic cap.  The bulb would have a big surge in brightness every time you turned the set on, and eventually every five or six power ups, would blow.  I can't recall why, but when that bulb blew, the set went dead except for the tube filaments.
I used to use a higher voltage rated bulb maybe a No. 51 that did not seem to affect the set operation except for having a much dimmer dial, in return for less frequent blown bulbs.  

Somewhere along the line I converted the filament string that contained the 25L6s to a 6 volt parallel circuit using an external transformer, and like you, used 6V6s instead.  I replaced the pilot bulb in the bias circuit with a fairly  large  power resistor pf the same approximate resistance of the 47 bulb, and powered the dial lamp off the 6V filament circuit.

I have a few other hints and kinks for you from my experience that I will send later, have to QRT for the moment.

[Opcom]

The power supply arrangement is a bit odd and contributed to lamp issues. I'd sure like to know those hints and kinks too.

For clarity I highlighted some power paths and also added voltages as measured in my set, on a separate diagram. My Scott SLR-M page is here: http://www.bunkerofdoom.com/lit/Scott_SLR-M/index.html

My readings don't seem to comply well with the voltage chart in the manual so don't take them too seriously. It says 3.5V grid to cathode on the output tubes but I get 15V. I might have the wrong lamp.

I love this because the manuals says "I101 Lamp, 6-8V 0.15 Amp. ... Contractor's drawing and part number 8225". So is this a 47 or 1847 or something else?

When I did an alignment, a band went suddenly dead. I found one of the under-chassis trimmers shorted when i turned it because the pressed-on shaft retaining 'nut' was cracked and it moved. It can be repaired and there is an article for that in case you find this eventually.

I really enjoy my SLR-M. It is my 'nightstand radio' and pulls in many mysterious stations and odd sounds at night. Its audio quality is also extremely nice. Perhaps it could be called the AC-DC version of the SX-28.

[n9ktw]

WEll here is the latest:

Got everything reassembled, and the radio is playing nicely!...... Cant hear WWV, but I have heard SW stations on all the bands!..... Going to leave it alone as far as alignment, seeing as AM BCB is about right. The dial light "issue" seems to have abated AFTER the "new" filter caps were reformed. I am still considering a zener across the dial light as a "surge supressor"..... any thoughts on that ??

Chris: which rectifier tube did you use? The one in the set I have has no makings anymore.

Patrick: I have been at your site, great site! I did download the schematic with the voltage diagram, it was a very great help.

Anyone else with any ideas, I am still fine tuning this radio.......


de Howard, N9KTW

[AJ1G]

Its been a long time since I have been inside the Scott, but I believe I used a 6X5 for the rectifier in lieu of the 25Z5. 

The BFO injection is very weak, when I first looked into why , the schematic showed no  direct coupling between the BFO stage and the IF or the detector stages.  There was just stray coupling.  This may have been by design to keep the radiated RF levels from the set low. I made a little gimmick capacitor with some twisted wires, and inserted the leads into the socket for the 12SN7 that serves as the BFO at that tube section's cathode pin (the BFO plate is grounded for RF by a big bypass cap) and at the grid pin of the second IF stage 12SK7 socket.  This made for a big improvement in the ability to copy strong CW and SSB signals without having to back the RF gain way back down.

I later built up a a product detector on a small chassis that I hung off of the big square shield can in the center of the set, the output of which I fed into the phono input.

The RF gain control would pull the received frequency a bit as the gain was varied.  I eliminated this by taking the mixer stage off of the gain control circuit,  and let it run at fixed gain, with adjustable gain applied only to the the RF and IF stages.

The sensitivity of the set was very low on higher frequencies when I first started using it. This turned out to be due to attenuation  in the relatively high capacity series cap (I think it was a .01 uf) that the set has in series with the antenna lead in.  That high a capacity should look like a dead short for 14 MHz, perhaps it had an open internal connection, I just shorted around it so the antenna would couple directly into the RF stage's grid coil primary windings.

Somewhere along the way I developed low sensitivity on Band 3 that covers 80 and 40 meters.  This was due to an open RF stage grid coil primary, which may have had its relatively fine wire taken out by a nearby lightning strike.  I just rewound a replacement couple of turns over the original open circuited coil.

The selectivity was notably improved with a Heath Q multiplier, which was the first Heathkit I ever built.

The set had no bandspread so the tuning rate was really fast on 20 meters, a bit more tolerable on 80 and 40.  I added one of those Jackson Brothers reduction drives behind the main tuning knob which made a big improvement, but  kind of spoiled the look of the front of the set.

The original external speaker that came with the radio as sold by Scott was a Hallicrafters PM-23 sans the big h logo.  It sounded great with even with the little PP 25L6s driving it.

50 plus years down the line, I wish I hadn't made some of those hacks to a such a beautifully made radio, when I finally retire in a few years I plan on doing a  restoration on it back to its original appearance, although I will keep some of the small relatively invisible tweeks in there.

[Opcom]

I have not cared for the AVC being completely automatic except in BFO ON mode. That can be fixed pretty easily.

So, you found the pulling of the mixer's oscillator on strong signals is due to AVC?
Does this also explain the pulling of the BFO note with strong signals?

For some reason I associated this with possible B+ regulation issues due to high volume and demand from the output stage.
Have you found the B+ to be pulled down and cause frequency movement when the volume is up high?

These should all be little things that could be fixed without making a mess in the radio.

Do you have the schematic and interface hookup for your product detector? That might be worth adding if it would not mess the normal BFO usage up. Sometimes I turn the BFO ON when an AM station is weak or fluttering and it seems to help its listenability.

It seems sensitive enough. WWV on 10 and 15Mhz come in well. Radio Havana around 6Mhz is 1000 miles away and comes in well.

[n9ktw]

Apparently I have a lot of "noise" in the house. I had a heck of a time getting WWV on any of my other radios, including my other SLR-M. The best radio tonight for wwv was a Transoceanic 3000-1 on an indoor antenna!  So the SLR-M is "all good as to reception... signals on all bands!

Chris: the rectifier in my radio is a 5 volt one.... I checked the filament voltage to it.... am guessing 5Y3.... That one has a 2 A filament, not sure if the transformer could handle more. Also, the person that did the mods, swapped one of the audio tubes, and the rectifier as far as location.... bizarre to me, but hey it all works!

Has anyone thought about my idea of a zener across the dial lamp? I am thinking that MIGHT prevent the voltage surge problem which occasionally rears it head.


de Howard, N9KTW

[AJ1G]

5Y3 was a very common  rectifier in transformer powered radio receivers back in the 40s and 50s.  The power transformer they was used in your conversion likely has a separate 5 v filament winding for it in addition to the 6.3 V winding for the other tube filaments.

[Opcom]

Its on AC so a Zener diode by itself is not appropriate. The attached circuit will work to clamp AC. It might add noise when conducting during surges.

[W3RSW]

Bypass with a 0.1 uf in parallel help?

[n9ktw]

On my radio, and I believe as shown on the schematic the "infamous" dial lamp is fed with DC. So it would appear that my zener idea, with Rick's bypass cap may be the way to go.

I have found the AC POWERLINE filter caps to be leaky...... Like 50 V+ on the chassis if the ground is open. I did connect teh ground and it wasnt enough current to blow a fuse or anything, but the need to be replaced........So the question is for a line bypass cap there are X and Y types.... I forget which kind I should be using. I know I shouldnt use just a standard .01 cap.


de Howard, N9KTW

[Opcom]

My mistake, it is on DC.

AC bypass caps, so cheap no excuse not to use the highest voltage you can get. Here is a supplier who has kindly given an explanation of X and Y farther down the page.  https://www.tecategroup.com/capacitors/applications/ac-safety-capacitors.php

Here is some most basic text but PLEASE visit the link because there is much more about these, their peak voltage ratings and safety, etc. that everyone should be aware of.

X and Y Safety Capacitor Basics

There are two major types of interference suppression/AC line filter safety capacitors, type X and type Y. The function of these capacitors is to protect against surges and transients, as well as providing EMI filtering . Safety capacitors are circuit-specific, and serve to protect the circuit and the user from high voltage surges by shunting the impulse energy to ground. One common cause of such surges is lightning strikes.

X Capacitors: Also known as "across the line capacitors." Class X safety capacitors are used between the "live" wires carrying the incoming AC current. These capacitors are used in applications where failure of the capacitor will not lead to risk of electrical shock to the user. A capacitor failure in this position will usually cause a fuse or circuit breaker to open.

Y Capacitors: Also known as "line to ground capacitors" (line bypass.) Y capacitors are used in applications where failure of the capacitor could lead to the danger of electrical shock to the user, if the ground connection is lost.

X/Y Capacitors: Some safety capacitors come with a combination designation. For Example , X1/Y2. This simply means that the capacitor can be used as an X1 capacitor in an "across the line" application, or as a Y2 capacitor in the "line to ground" portion of the circuit. Tecate offers both X1/Y2 and X2/Y3 multilayer ceramic safety capacitors. [/color]