McIntosh MAC 1700 Bias

[dbxdx5]

Based on the McIntosh service bulletin, the bias for the 1700 should be 40mV across pins 12 and 4, at least that's for the version with the adjustable 50 ohm bias trimmer. It's always been assumed that the bias should also be 40mV for the version that subbed a 10 ohm fixed resistor for the trimmer, but I'd like to confirm. I ask because the amp circuits in the two versions are actually different in a few areas. Thanks.

 

[Stevonian]

I am bumping this thread and advancing it with an added question. I have an older 1700 like described above, w/o bias pots. Each driver card is reading 12mv at startup. I assume this is an issue but want to be sure. Then, if it is a problem, discuss ways to rectify w/o a pot to adjust.

 

[Stevonian]

Bump

 

[Stevonian]

Bump

 

[dbxdx5]

To answer your question (and mine, from the first post), you do definitely want to be measuring 40mV +/- 1 or 2mV at the bias test points when the amp is cold and within the first minute or so of powering on. If you see that, all is good, even if the bias begins drifting up after that initial few minutes, which it will.

Since this version doesn't have a trimmer, I would replace the original 10 ohm resistors at R7 and R8 with 10 ohm 2 percent (or better) tolerance 1W metal film or metal oxide resistors. The original carbon comp resistors can drift, and even an additional few ohms will put the bias higher than it should be at idle. That said, your problem is low bias. Have you checked to make sure you have the correct power supply voltages, per the schematic? I would get all of those voltages where they need to be first, making sure to check the four fuses and the fuse holders underneath, too. I've had oxidation and/or weak tension from these fuse holder clips cause issues in the amp section, since the +/- 37V supplies pass through them.

I would also do the following:

• Check the rest of the carbon comp resistors on both resistors on both driver boards and replace any that are out of spec.
• Check the bias diodes, D39 and D40.
• Replace the white axial electrolytics on both boards. These don't age well at all, and will often have low capacitance.

 

[Stevonian]

There's someone out there!!
Thanks for the reply and the ideas. I'm out of town for a couple of days but I will put your thoughts to use when I return. Thank you!

 

[Stevonian]

I'm back and checking for voltages. I don't see voltages listed coming off the transformer but in case you know them, here are mine...

Prior to the D19 to D22 rectifier, Red : 101v and Blue/yellow: 50v
Prior to D24 to D27 rectifier,Green: 24v

Checking right at the power cord show 105v (seems low as the plug I'm powering from is nicely at 120v).


My model does not have any fuses save for the main one at the rear of the unit. I swapped that with no change in any numbers.

Every other voltage heading off of these rectifiers is obviously also low.

Thoughts?

 

[James n]

Based on the McIntosh service bulletin, the bias for the 1700 should be 40mV across pins 12 and 4, at least that's for the version with the adjustable 50 ohm bias trimmer. It's always been assumed that the bias should also be 40mV for the version that subbed a 10 ohm fixed resistor for the trimmer, but I'd like to confirm. I ask because the amp circuits in the two versions are actually different in a few areas. Thanks.

Where are pins 12 and 4 ?

 

[James n]

Based on the McIntosh service bulletin, the bias for the 1700 should be 40mV across pins 12 and 4, at least that's for the version with the adjustable 50 ohm bias trimmer. It's always been assumed that the bias should also be 40mV for the version that subbed a 10 ohm fixed resistor for the trimmer, but I'd like to confirm. I ask because the amp circuits in the two versions are actually different in a few areas. Thanks.

pin 12 and 4 ? Where ?