Motorboating phono input on Stromberg Carlson ASR433 - diyAudio
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Old 24th October 2009, 12:17 PM   #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, 12:19 PM   #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, 12:27 PM   #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, 02:00 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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Old 8th December 2014, 08:51 PM   #5
RWood is offline RWood  United States
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Gordon,

Old post, I know, but do you recall what it took to solve this?

I'm having the exact same symptoms on a Harman Kardon A500 that I have recapped, where all voltages and resistances are within spec. The line level inputs sound good.

The phono sounds fine until around 11 -12 o'clock on the Loudness control, then the motorboating starts. It is on both channels, so "power supply" sounds logical. I am not sure what I would need to do to further decouple the B+, which for the phono section (as you well know from your work with A500s) is the end of the line on the power supply, separated from the previous section by a 150K resistor and filtered with a 47uF cap.

The phono section on this one is similar to the Sams schematic (which has differences compared to the HK schematic)

Any ideas on how to fix this or troubleshoot it?

Thanks!

RWood
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Old 9th December 2014, 10:14 AM   #6
DF96 is offline DF96  England
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When recapping people often find it irresistable to 'improve' the circuit by raising some cap values. This changes LF phase shifts, gain and parasitic coupling (e.g. via supply rail decouplers). In some cases the result is better stability, but sometimes it can result in poorer stability.

Start by reviewing every cap value change. Exactly what will be the effect on LF phase shift, coupling etc. of the change? Calculate the RC time constant of every relevant pole. Look for similar time constants - the designer may have deliberately separated them.
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Old 9th December 2014, 10:39 AM   #7
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^ what DF96 says. Also modern electrolytics can have lower ESR cf older types.
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Old 9th December 2014, 02:20 PM   #8
RWood is offline RWood  United States
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Thanks for the reply. I kept all the values the same as the schematic, at least as close as modern equivalents will allow (.047 for .05, .22 for .2, etc).

There's a PEC in the cathode of the first triode, and I left that as is. I may replace that with discrete components, at least for the Mag Phono, not the Tape Head input.
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Old 9th December 2014, 02:26 PM   #9
DF96 is offline DF96  England
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Check whether a decoupling cap is missing a ground connection. Also, check for decimal place errors (by you or your cap supplier): 0.1uF and 0.01uF could have very different LF behaviour yet are easily confused.
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Old 10th December 2014, 05:41 PM   #10
RWood is offline RWood  United States
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Quote:
Originally Posted by DF96 View Post
Check whether a decoupling cap is missing a ground connection.
Getting closer - this notion led me to a discovery that might help solve the problem.

The final coupling cap, a 47uF 350v axial, had been grounded with the previous sections' can cap. This amp uses a ground buss and there is only one chassis connection, at the reservoir cap. All of the small signal tube grounds run to the buss which in turns goes back to the power supply and ultimately, at the rectifier, chassis ground.

I disconnected the phono section cap ground and put it on a gator clip to try some other locations. I was hoping that grounding the cap closer to the phono bypass caps, etc would improve things (as I learned in Merlin's book.) But this is where it gets strange.

The symptoms actually improved with that cap not grounded at all! Flying in the air. By improved, I mean it didn't eliminate the oscillation but it allowed for much more gain before the motor boating occurrs. Now I can crank it to between 1:00 and 2:00 on the Vol before onset. With the cap connected anywhere - chassis, first cap ground, phono section, the onset is much sooner.

Something is clearly going on with the grounding scheme; I'm just not quite sure what. Ideas appreciated!
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