Scott SLRM Marine Radio Boatanchor: 'rumble'

5/28/2013 12:22:39 PMLeslie

Hi All,
I have a functioning Scott SLRM Marine radio in great shape. The only problem is that when the set is tuned to a reasonably strong MW/AM Broadcast station there is a kind of 'rumble' over the signal; it's difficult to describe...not 'scratchy' like an intermittent loose wire but kind of a bass geiger-counter type sound (continuous bumps and rumbles in a non-periodic sequence). I've looked for loose wires, cleaned the band-switch, tuner condenser etc; the tubes all test fine.

Any hints/intuitions/gut feelings on how to solve this problem would be appreciated. A duff capacitor somewhere?

Cheers, and thanks as always.
Leslie

5/28/2013 12:49:12 PMLeslie

I should also add that I cannot remove the audio 'rumble' by adjusting the Audio Gain, Quality (Tone), RF Gain or Sensitivity switches. The rumble is also not present between stations--only when a fairly strong signal is being received.

Cheers,
Leslie

5/28/2013 3:32:29 PMEdd

Sir Leslie . . . .

Thats going to typically be an answering and isolating of, if its a variation of the AVC line biasing or a power supply problem.

Initially, considering you to have its schematic, track down the juncture of connected R-121 and its adjunct time constant
/ bypassing capacitor C144B.

Ground that junction with a clip lead and you will have negated the systems AVC.

Then you test the set for those previous quirks and see if they have disappeared.

The other power supply aspect could be the power supply where you could take a pair of clip leads and sub across the
filter cap sections one at a time with in the order of a 47 ufd capacitor.

Suspect units are C156 and C156 A and B, then there is a subtly hidden C-148 20 ufd unit that is filtering the B+ supply
to the V107 1st AF amp circuitry.

Standing by . . . .from afar . . . .

73's de Edd

By the time you have the right answers, no one is asking you questions.

:I should also add that I cannot remove the audio 'rumble' by adjusting the Audio Gain, Quality (Tone), RF Gain or Sensitivity switches. The rumble is also not present between stations--only when a fairly strong signal is being received.
:
:Cheers,
:Leslie
:

5/28/2013 11:00:45 PMLeslie

Hi Edd,

Nice to hear from you again. I have the SLRM manual and schematics, and I'll look for the units you mention. I have located C156 under the chassis, and I am assuming (correctly?) that the 156-A and 156-B are in the same can. Would it be a useful idea to replace C156?

The Scott radio very helpfully has various capacitors labelled underneath the chassis, but I cannot find C144; can you tell me what section it is in? When I find that one, I guess I'll see R121 close by/attached to it.

I'll look for the C148.

I'll let you know.

Cheers and many thanks,

Leslie

5/29/2013 4:19:11 PMEdd

:Hi Edd,
:
:Nice to hear from you again. I have the SLRM manual and schematics, and I'll look for the units you mention. I have located C156 under the chassis, and I am assuming (correctly?) that the 156-A and 156-B are in the same can. Would it be a useful idea to replace C156?
:
:The Scott radio very helpfully has various capacitors labelled underneath the chassis, but I cannot find C144; can you tell me what section it is in? When I find that one, I guess I'll see R121 close by/attached to it.
:
:I'll look for the C148.
:
:I'll let you know.
:
:Cheers and many thanks,
:
:Leslie
:
:
:

Sir Leslie . . . .

Parte deux . . . .

Correct . . . . your electrolytic C156 IS a dual unit . . . . . wouldn't worry about it just yet, as I feel that if we are having a marginal
electrolytic at fault, it will be so bad that it will be readily apparent after having shunted it with a good unit in testing.

On that 'ole thang, I would expect the C141 to be a metallic "bathtub" design of capacitor with at least one other "A"section also
being housed within the same case.

bath anyone ?

And by good design practices, it should be somewhat nearby to shorten its wiring run(s) and enhance bypassing effectiveness.

Try working another route by finding the secondary winding of last IF transformer T-115 and its low side.

You can see that wiring connection is routing downward and then shares a common connecting with R-119 and R-122 as well as your
quested R-121.

Once you find that R-121 physically, all you then have to do is ground its OTHER lead that ends up connected to the wayward C144B.

On the schematic, the C148 cap is located about 2 1/2 tube envelope widths to the left of V108.

73's de Edd

Down at the point, they forgot and left the lighthouse on all day. Next day the sun wouldn't rise.

6/7/2013 11:37:39 AMLeslie

Hi Edd,
Thanks for the homework, which I've diligently undertaken in my workshop. What I will say is that this Scott set is probably the best radio I've ever dealt with in terms of build quality (like a tank) and ease of finding parts (Resistors, Capacitors, Transformers being helpfully labelled).

As to the detail of the audio 'rumble' (a mild 'motorboating' perhaps?) I've tried the following, but without success:
i) I've replaced (temporarily), C148, and C156.
ii) I've tried to clean the AF switch, but it's actually impossible to get any cleaning solution inside; the switches appear to be covered with something similar to the 'tropical use' shellac used on a number of WW2 radios I've encountered. Anyhow, it's sealed shut. When I adjust the volume, it makes no difference to the 'ticking' (just makes it louder, but does not alter the speed or sequence of the noises.
iii) I found R121 (helpfully labelled on a bakelite board along with several others), and tried grounding as suggested. The audio signal disappeared and the dial light became brighter, showing a short to ground I guess.

Further indications:
i) the ticking starts to occur after the set begins to warm up. The station audio is heard, and then the ticking begins after about 20-30 seconds.
ii) the sound is less obvious on weaker signals. The more powerful the station, the more intrusive the noise.

Head Scratching:
i) Do you think this could be caused by a 'noisy tube'; one which tests OK (they all do), but has a fault which isn't shown on my tester. As someone used to say to me, 'your radio is your tube tester'...
ii) If it could be a tube with a short, which would be the likely villain? V107 (for example)?
iii) I've turned my eyes inside out looking for any loose wires, though the 'ticking' doesn't sound like that, and no moving the set around will change the nature of the 'ripple'.
iv) would there be any sense in changing out all the electrolytics just in case? I'll reconnect C148 and C156 as they seem to fine and 'if it ain't broke, don't fix it'.

Thanks for all your help, Sir Edd, and I hope I can get to the bottom of this gremlin before the Scott is put to gather dust at the back of my workshop as one of those sets requiring an exorcism...

Cheers,

Leslie