Mcintosh MA230 Bias Voltage Resistors

[Dr. Drawbar]

I've had an MA-230 for about 3 years. It recently began humming as things this old will, so I put it on the work bench and popped the bottom off of it.

First thing I noticed was three out four 15 ohm bias resistors were cracked, two of them severely so. Checked the schematic and it shows these should be 5 watt; the units installed were 1/2 watt sized, definitely factory carbon units, not wire wound.

Is this a misprint in the schematic?

Next, any pluses or minuses to using CE Distribution can capacitors in this model? They don't have exact replacements but can be arranged to be quite close in value (on the higher side) with extra WVDC than the originals. Only thing that might be a concern is the temperature ratings go as low as 55 degrees C on the candidate 60/60 mfd unit.

My background is 25 years repairing Hammond organs and Leslie speakers so I'm used to working on tube stuff and making substitutions. This is my first Macintosh repair and would like to get it right the first time.

 

[gadget73]

electrically speaking 1/2w will get the job done. 0.7v across 15 ohms is .03 watts, and realistically speaking it should drop a bit less than that much voltage since there is the winding resistance to consider. Possible they changed to or from a larger resistor at some point during production though. I'd probably put some metal film 1/4w parts in there so they have a chance of acting like a fuse in case one of the tubes shorts.

Hayseed Hamfest can do custom caps for you if you'd like to retain original values and layout. They're actually priced about the same as the CE ones most of the time.

 

[Dr. Drawbar]

electrically speaking 1/2w will get the job done. 0.7v across 15 ohms is .03 watts, and realistically speaking it should drop a bit less than that much voltage since there is the winding resistance to consider. Possible they changed to or from a larger resistor at some point during production though. I'd probably put some metal film 1/4w parts in there so they have a chance of acting like a fuse in case one of the tubes shorts.

Yeah, the math makes sense. Maybe it was just their time after 56 years. I've 1/2 watt metal film units on hand so will install those.

 

[Dr. Drawbar]

Missing (.)? It's only 4 1/2W difference...

I could agree with that, except that none of the other 1/2 watt resistors have any wattage value associated with them. The higher wattage units are called out. It's just so way off it seems like this would be a widely known error.

 

[Dr. Drawbar]

I'm uploading the schematic so you can check my eyesight. The resistors are on the cathodes of the 7591 tubes at the test point. No other components near by. Schematic calls for 5% units; I'll be installing 1% metal film units.

edit: attached schematic deleted in favor of the much easier to read copy down thread.

 

[gadget73]

its a bit clearer here

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=McIntosh-MA-230-Schematic.pdf

but it does say 5W clear as day. 5% parts too, not super tight tolerance. I'd probably go with 1% because why not.


Possibly the 5w was more about wire wound resistors than the wattage needed. CC resistors drift, wire resistors really do not. Not sure those would have been common in low wattage values back then so it may have been a 5w part was the smallest and cheapest way of getting there.

 

[Dr. Drawbar]

Thanks for the clearer schematic!

Easy to understand the perplexity... Is there even room in that spot for an old fashioned 5W tubular WW?

Here's a picture of one of them (in the middle between the tube sockets).

 

[Dr. Drawbar]

Did you notice that serial number notation? Maybe you have one made prior to the 5W parts. If so, I'd say it could be a good idea to up those to 5W because of possible high currents at turn-off if the bias voltage falls too fast compared to the B+. There could be residual uncontrolled current for a few seconds till the B+ runs down. Just a WAG.

Duh! Yeah, completely missed the serial number call out. This one is 13C66, so well before the schematic was issued.

That solves the mystery! Five watt units shall be ordered and installed.

Here is the Mouser part number: 605-RWHSE09TU015R0FS. 5 watt 1% wirewound, 194 volt rating, $1.54 each. At 12.7 mm in length they will easily fit in allowable space.

Thank you for the help!