Hi All,
I have been frustrated by a gremlin problem with my Scott SLRM boatanchor for about a month, and no luck so far. Basically when I tune into a strong station there is a kind of clicking/rumbling--kind of like a slowly ticking geiger counter with bass tones. The 'ticks' are irregular. I've changed all the relevant electrolytic capacitors I think, but the gremlin persists. I've looked for loose wires and tried to clean switches (on the SLRM the AF Gain switch etc. has no opening to put cleaning fluid in, and the switches are covered with a kind of shellac I've seen on military radios destined for tropical/humid applications.

I was wondering if there might be a possibility that one of the tubes might be developing this noise (the rumble/ticking doesn't start until after 30 seconds after I start hearing the audio from the station, almost like something has to warm up first). All the tubes test fine, but I appreciate that 'your radio is your tube tester' and that a subtle short in one of these might be the culprit.

The tube lineup is as follows:

V101-6K7-RF Amplifier
V102-12J5GT-HF Oscillator
V103-12SA7-First Detector Mixer
V104-12SK7-First IF Amplifier
V105-12SK7-Second IF Amplifer
V106-12H6-Second Detector, AVC, NL
V107-12SN7GT-First Audio, CW Oscillator
V108-12SN7GT-Second Audio, Phase Inverter
V109-25L6GT-Output Audio Amplifier
V110-25L6GT-Output Audio Amplifier
V111-1629-Tuning indicator
V112-25Z6GT-Rectifier

Which one might be a villain (I assume that the 1629 would be innocent--it's been changed for an NOs substitute anyway). I've switched the 12SN7s around, and replaced one of the 25L6s. Could it be the Oscillator tube? I'm genuinely puzzled.

Cheers, and thanks for any advice. My Scott is otherwise a great set (excellent cosmetic condition, great sensitivity/selectivity, just this annoying rumble).
Leslie

:Hi All,
:I have been frustrated by a gremlin problem with my Scott SLRM boatanchor for about a month, and no luck so far. Basically when I tune into a strong station there is a kind of clicking/rumbling--kind of like a slowly ticking geiger counter with bass tones. The 'ticks' are irregular. I've changed all the relevant electrolytic capacitors I think, but the gremlin persists. I've looked for loose wires and tried to clean switches (on the SLRM the AF Gain switch etc. has no opening to put cleaning fluid in, and the switches are covered with a kind of shellac I've seen on military radios destined for tropical/humid applications.
:
:I was wondering if there might be a possibility that one of the tubes might be developing this noise (the rumble/ticking doesn't start until after 30 seconds after I start hearing the audio from the station, almost like something has to warm up first). All the tubes test fine, but I appreciate that 'your radio is your tube tester' and that a subtle short in one of these might be the culprit.
:
:The tube lineup is as follows:
:
:V101-6K7-RF Amplifier
:V102-12J5GT-HF Oscillator
:V103-12SA7-First Detector Mixer
:V104-12SK7-First IF Amplifier
:V105-12SK7-Second IF Amplifer
:V106-12H6-Second Detector, AVC, NL
:V107-12SN7GT-First Audio, CW Oscillator
:V108-12SN7GT-Second Audio, Phase Inverter
:V109-25L6GT-Output Audio Amplifier
:V110-25L6GT-Output Audio Amplifier
:V111-1629-Tuning indicator
:V112-25Z6GT-Rectifier
:
:Which one might be a villain (I assume that the 1629 would be innocent--it's been changed for an NOs substitute anyway). I've switched the 12SN7s around, and replaced one of the 25L6s. Could it be the Oscillator tube? I'm genuinely puzzled.
:
:Cheers, and thanks for any advice. My Scott is otherwise a great set (excellent cosmetic condition, great sensitivity/selectivity, just this annoying rumble).
:Leslie
:

Tubes are unlikely to be the cause of this low-frequency cyclical noise. Also, dirty/corroded tube contacts or a carbonized fiber socket are sometimes the cause of audio woes such as poor gain or AC hum; but not something that repeats at a sub-Hz rate. My guess is that you have a failing bypass or coupling capacitor, and that the "tick" you hear is the cap self-discharging when it reaches its breakdown voltage; at which time it starts to charge up again and the cycle repeats itself.

Most likely this failing cap is somewhere aft of the 12H6 demodulator diode tube (i.e., in the first or second audio amp stages); to include the AVC circuit which sets bias voltages on the RF front end.

You did replace the 25L6 cathode AC bypass electrolytic cap(s)- right?

If this is a set with a power tranny, start pulling the tubes one at a time, starting from the front end, and see if you can isolate the stage where the noise stops.

:If this is a set with a power tranny, start pulling the tubes one at a time, starting from the front end, and see if you can isolate the stage where the noise stops.
:
Believe it or not (it being an EH Scott product) it has no power transformer. Tubes are all series-string.

"Tubes are unlikely to be the cause of this low-frequency cyclical noise. Also, dirty/corroded tube contacts or a carbonized fiber socket are sometimes the cause of audio woes such as poor gain or AC hum; but not something that repeats at a sub-Hz rate. My guess is that you have a failing bypass or coupling capacitor, and that the "tick" you hear is the cap self-discharging when it reaches its breakdown voltage; at which time it starts to charge up again and the cycle repeats itself.
:
:Most likely this failing cap is somewhere aft of the 12H6 demodulator diode tube (i.e., in the first or second audio amp stages); to include the AVC circuit which sets bias voltages on the RF front end.
:
:You did replace the 25L6 cathode AC bypass electrolytic cap(s)- right?"

HI,
Thanks for the input, which I'll puzzle out in the workshop. With regard to your question about the 25L6 AC bypass...there appear to be two capacitors relating to V109/V110 (which are a pair of twin 25L6s). One is a .01 Mica capacitor used for "V109/V110 cathode bypass". The other is an electrolytic (I'm reading from the SLRM manual); 25MF/25Volt in a 'bathtub' casing--used for 'V109/V110 grid return bypass'. Is the latter the one you are talking about (the mica is not an electrolytic and unlikely to be defective, though I have encountered defunct micas before)? It's just that there's no mention of the exact phrase 'AC bypass'. Sorry if I'm being slow witted here.

I agree that the 'ticking' suggests a capacitor problem, and it's good to know that it's probably one of the electrolytics (as opposed to any of the other capactors). That will help to focus my search.

Cheers,
Leslie

:Thanks for the input, which I'll puzzle out in the workshop. With regard to your question about the 25L6 AC bypass...there appear to be two capacitors relating to V109/V110 (which are a pair of twin 25L6s). One is a .01 Mica capacitor used for "V109/V110 cathode bypass". The other is an electrolytic (I'm reading from the SLRM manual); 25MF/25Volt in a 'bathtub' casing--used for 'V109/V110 grid return bypass'. Is the latter the one you are talking about (the mica is not an electrolytic and unlikely to be defective, though I have encountered defunct micas before)? It's just that there's no mention of the exact phrase 'AC bypass'. Sorry if I'm being slow witted here.
:
:I agree that the 'ticking' suggests a capacitor problem, and it's good to know that it's probably one of the electrolytics (as opposed to any of the other capactors). That will help to focus my search.
:
:Cheers,
:Leslie
:

The electolytic cap to which I was referring is the one that is in parallel with the 25L6' cathode resistor. It is used to shunt AC around the cathode resistor, which is there to ensure that the output tubes' grids are biased slightly negative with respect to the their cathodes. So, the big cap provides a low-impedance path to ground for the audio signal, while allowing the DC component to do its thing with respect to self-biasing of the tube. Older designs tied the cathodes directly to ground and used voltage dividers from the center tap of the power transformer (which this unit doesn't have, being an AC-DC set) to create a negative voltage with respect to chassis ground.

Anyway, all of these electrolytics fail eventually.
If you replace it, you can at least rule it out as a source of your noise problem.