Motorboating phono input on Stromberg Carlson ASR433 - diyAudio
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Old 24th October 2009, 11:17 AM   #1
GordonW is offline GordonW  United States
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Default Motorboating phono input on Stromberg Carlson ASR433

This one is a real puzzler.

Got a SC ASR433... recapped everything, got it up and running.

The line input sounds great... but the phono input starts to motorboat, above about half-way up on the volume control (yes, it's dependent on volume control settings- both the individual channel levels and the master volume control). Also, the phono input sounds wimpy... dynamically dead. I'm also getting more hum from the phono stage, than I remember having on the last ASR433 I worked on... both channels (it's not like one channel hums more than the other).

Checked all voltages and resistor values... all within spec. Tubes in the phono stage are good... Mullard 12AX7s. Also, I have absolutely NO detectable power supply ripple at the power supply cap feeding the preamp stages... zero volts AC. Period. And, when I recapped the unit, I actually upped a lot of the power supply cap values... they're all new too. So, the power supply is definitely decoupled. I even tried tacking a few decoupling caps between hot and ground near the preamp (right before the plate resistors) and it did nothing... so, the power supply is pretty much ruled out, I think.

I'm starting to wonder it it's crosstalk, from around the source selector... the audio goes back and forth to the selector switch several times in the signal path (different "layers" of a ganged switch). I can ground the phono input RCA plugs, and it actually gets LOUDER, a bit. But, if I ground the output of either gain stage in the phono stage, I can kill the motorboating (along with, naturally, all output from the stage).

I can also drastically change the motorboating frequency (much lower frequency, but seemingly even higher amplitude), by adding additional capacitance to the .01uf (it's apparently .0033 in some ASR-433s?) at the output of the 6AV6 preamp gain section (feeding the tone controls- another potential source of crosstalk, as they're right next to the source selector as well)... which makes me think it's a positive-feedback situation somewhere.

Anyone seen anything like this? I don't remember the last ASR433 I worked on, having this kind of problems...

Regards,
Gordon.
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Old 24th October 2009, 11:19 AM   #2
GordonW is offline GordonW  United States
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BTW: To add to the intrigue... on these, the master level control (the one that controls both channels) is right below the source selector... they're almost touching. And the cables (though shielded) all run together through holes in the chassis.

This unit looks like it might have been a "Monday morning/Friday afternoon" piece... not put together as well as other SC pieces that I've seen. So, I'm just wondering if there's a wiring error in here somewhere... ground problem, etc. If there is, I haven't found it yet...

Regards,
Gordon.
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Old 24th October 2009, 11:27 AM   #3
SY is offline SY  United States
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Motorboating is 99% of the time due to interactions coupled through the power supply, as you correctly suspected. It can also be due to coupling through ground returns, which ought to be checked thoroughly, especially in an old unit.

When you re-capped, did you increase any of the values of the coupling caps? You said that you upped the power supply caps; have you increased the decoupling by using a separate resistor from the main supply to feed the decoupling caps for the phono stage? That often helps, especially when the original design has the RC decoupling sections in cascade rather than parallel.
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Old 24th October 2009, 01:00 PM   #4
GordonW is offline GordonW  United States
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That may be a good idea. Might be an opportunity to up the voltage a bit too... they're run pretty low (80v feed). Maybe if I just re-route the existing resistor from cascade to parallel... that would up the voltage somewhat (probably like up to around 120-140v, which probably would be much better for 12AX7s), too... that might help the dynamics.

BTW: I did up some coupling cap values; however, when I started having this problem, I temporarily disconnected them and tacked in the original value (new caps). No change... the only one that seems to be succeptible is the one at the output of the 6AV6 (tone control/preamp tube)...

Regards,
Gordon.
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