Masterpiece VI

[Robert Feenstra]

The bass control on my MPVI is totally non-functional.  I can't even get resistance readings with contact directly to the resistance elements.  I can't determine if the 2 ganged controls are linear or audio taper.  Does anyone know?  The parts list does not say, nor does the schematic.  Mark Oppat will make a new control but needs to know the taper.

Thanks in advance for any expert knowledge!

Rob

[David C. Poland]

You might try a search on this site for McMurdo Silver. May be helpful.

There have been some MS threads.

[Kent King]

I suspect (but don't know for sure) that you'd want an audio (log) taper for the bass control. A linear control is going to change very rapidly - the "useable" control range will be very limited. On the schematic for the MP VI, it is P3a/b (dual pot, as you noted). The values according to the parts list I have are a 100K and 1M ohm. So I'd see if Mark can make you a 100K/1M audio taper setup for this. The wiper connected to the LC circuit off the volume control tap is the 100K pot. 

[Robert Feenstra]

Thanks Kent.  I have a control on order.  I was playing with it using a 300K audio for the 1meg.  Most of the bass control happened in the last 1/4 of the travel.  Not sure what a 1 meg will do.

[David C. Poland]

I also asked a friend (Jon) who is a very experienced high end radio restorer and has restored McMurdo Silver models, including recently an MP6, to see what he can add. Meanwhile I think Kent's suggestion is on point.

[Robert Feenstra]

I have emailed Jon but have not received a response as yet.  I also have a MP V that I purchased from him and have met him.

I've been playing/experimenting with the bass control system.  The 100K control is, as Kent said, connected to the volume control.  itis the typical bass compensation circuit using a tap on the volume control.  The thing I don't like about it is that when the bass control in in the max position, the compensation network is tied to ground through a 5K resistor.  THis cuts the treble dramatically.  I suppose it makes the bass appear to be bigger but it it is not a good sound.  i've fixed the compensation at a particular level by putting a resistor in series to ground.  I may actually leave it this way as the bass control (with the improper control) now boosts the bass without changing the treble.  I'll see what a new control brings!

[David C. Poland]

Robert - interesting resolution on the bass control. Kudos, especially that you like the result.

*

In a related vain, E H Scott seemed to shy away from bass emphasis, as reflected in the mid 1930's Scott radios in my collection. I find I have to listen carefully for the low register sounds - they are clear and they are not at all muddy, but I wish there was a bit more bass available.  With my 5 knob AW-23 of 1935 (Laureate Grande), I prefer a portable disk player with the mega bass feature into the phono input for music. My 7 knob AW-23 (Roslyn) seems a bit better in this regard. In contrast, my 14 tube Scott Masterpiece (1940/ Scott Mayfair credenza) sounds great with a normal CD player to phono input.

[Jon Ketron]

Hi Rob,

I received your voice message, apologies for not returning your call sooner.  Dave had also contacted me mentioning your question here on the forum.  I still have the customers Masterpiece VI here at the house.  Let me retrieve the tuner chassis and I can submit values here on your topic, as well as pictures of the current set up.

Will be back soon,

Jon

[Jon Ketron]

Rob,

After plotting values on a chart, my findings indicate the pot is indeed linear.  Original control within the Masterpiece VI I am servicing appears to utilize two separate pots sandwiched together. 

Front section 500K ohm

Back section 100K ohm

Pictures below of the original set up:

[Robert Feenstra]

Thank you Jon.  Your pics are helpful as they confirm a couple of components that are there but don't appear on the schematic.  The schematic and parts list calls the one a 1 meg pot not a 500k.  But, my testing would say that aq 500K would work better.

[Jon Ketron]

Glad to help Rob.  Good luck on your rebuild!

Jon