MR78 mystery
 

At least it's a mystery to me. Maybe somebody can help.

I just got an MR78 (can't believe how good FM can sound). It is in fantastic condition, but the lettering above the knobs doesn't light up. I can see a dim glow in the lettering when I look at it from above, but from straight on I can't even tell there is any lighting at all. The dial and meter lights are fine, although dimmer than my newer gear.

I thought it was the bulbs, but when I opened it up, all the panel lights were good and lit up when turned on. I do see a clear plastic panel with holes for the panel lights to fit into, attached to the inside of the faceplate. I assume this is some sort of light guide to light up the lettering, but I don't think it is working. However, I do not see anything obviously wrong with it.

Any ideas? Or, is it normal for the lighting to be so dim? I will try to post pictures, but this is a busy weekend for me; my son is getting married.

Thanks,
Mike

Congrad's on your sons marriage. :thumbsup:

Not sure about the MR78

I had the same issue with my MR78. The dial lighting was fine but the lighting across the lower portion of the panel was extremely dim. I puzzled over it for some time. Then, once I acquired a service manual I figured out the reason. In series with the panel filiment circuit is a 2 ohm, 5 watt resistor that prevents the filiment circuit (which is comprised of type 1866 bayonet style bulbs) from receiving the full 6.3 volts from the secondary winding of the power transformer that powers the circuit. You need to remove this resistor, as I did, and replace it with a jumper wire to complete the circuit and optimize the brightness. The resistor is designated as R508, and is located in the underside of the tuner chassis near the rear of the unit. As I recall it is soldered onto a stand-up, hard-wired terminal strip.

I'm not sure what the reasoning was here, with having this resistor in the filiment circuit in the first place-all it does is give you a very dim bank of lights. Perhaps it was intended to extend lamp life-but I removed (bypassed) this resistor over 8 years ago, and have not expreienced any issues with a shortened lamp life. You may also want to check to be sure that the correct lamps are installed in the sockets across the lower panel-they should be type 1866 bayonet lamps.

I seem to remember taking some photos of the area of the tuner where the resistor is located. I'm not finding them right off the but but I'll keep looking.


I hope this helps-enjoy your MR78. It'san awesome and beautiful tuner.


Pete

Thanks Ivan and Pete. It's quiet here at the moment as the younger men are sleeping off the bachelor's party.

Pete, your answer makes sense. I did put new 1866 bulbs in since I had it open. Your pictures would be nice, but I think I will be able to find the resistor if you do not have them readily available.

I have attached a picture of the panel lights. It seems the camera is more sensitive than my eye, because when the room is light enough for me not to stumble over things, I cannot see much glow from the lettering. It also seems dimmer over by the volume knob on the right.

Pete.......When I read Mike's original post, my first thought was you would know how to fix that. Way to go. :thumbsup:

Dan....thanks for the vote of confidence!

Mike....the attached photo shows the general area. You''l see two white jumper wires near the rear of the unit, shown kind of under one of the meter lamps I'm holding in my hand. I'm not sure which is the one that used to employ the resistor; I'm thinking it may be where the white wire "closer" to you is in the photo. I'm not positive, it's been a while...but this is the area. It is a 5 watt power resistor, and so it is not a color-banded resistor. It will be similar to the white power resistor you see to the left of the power transistor on the small circuit board. Remember it's a 2 ohm resistor.

Good luck!!

Pete

Pete.......Most likely the 5 watt 2 ohm resistor is on a stand up terminal adapter because R508 is not shown on the Power Supply circuit board.

Here is the MR78 manual. R508 is shown in the power supply schematic, but is not shown as being part of the printed circuit board for the power supply.

http://www.tubebooks.org/file_downlo.../MR78_late.pdf

Dan....I didn't say the resistor was on the circuit board, I said the resistor is similar to the one on the board. In my original post I said the resistor was located on a stand-up terminal strip.

At any rate, I decided to have a look...I was correct about it being the white wire "closer" to us in the photo. It is actually soldered between two stand-up terminal strips....the original photo is a bit ambiguous. Here is a new photo, I opened up the tuner for this photo, you can clearly see the white wire jumper that replaced the resistor, between the two terminal strips.

Thanks for posting the manual, Mike and others may want to print it out...I paid I think $15 to McIntosh for mine.


Pete

Pete.......I was agreeing with you, not challenging your comments. :)

Thanks for going the extra mile and opening up your MR78 this morning. Not many folks would get that excited to offer that level of help. Your effort is commendable, and posting the new photo clears all confusion. Thank you. Great information.

Photo by Vintage Pete
http://audioaficionado.org/attachmen...y-dscf4896.jpg

Dan....no problem, it's all good!! :yes:

I didn't mind at all opening up the tuner....I'm glad to help any fellow AA'er any way I can. Lord knows they've been good to me!


An interesting side-note: As I look in the interior photo in the manual you posted, we really don't see a resistor here (or at least not of the 5-watt variety). I see that the file name is "MR78_late"...perhaps the resistor was omitted in later units-if so, ir was a wise decision. However, it does appear in the schematic. My manual mirrors the one you posted. Go figure... :dunno:

Pete