McIntosh - Hints and Kinks

Discussion in 'McIntosh Audio' started by dewickt, Jul 1, 2004.

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  1. dewickt

    dewickt Will fix about Anything

    Since I get to work on McIntosh equipment most every day I think a Helpfull Hints topic can benafit all of us in the McIntosh fourum. Feel free to post your hint or kink, each one of us may have a unique solution for a problem they have run into and others may need a solution for.
  2. dewickt

    dewickt Will fix about Anything

    MC30 bias

    Here is a good starter, MC30 bias upgrade for safety and reliability.
    Replace the Bias filter capacitor with a 10 to 22 uF (I use 22uF) at 450Volt, then replace the bias diode with a good silicon one, 1N4007 works well. Now that you have a new bias supply the voltage will be to much, to get it back to original replace the 3.3K series resistor (one next to diode) with a 9.1K 1/2W resistor, measured bias is exactly what it should be with these 3 changes, and the long life of your MC30 is one step further.
    Last edited: Jul 2, 2004
  3. dewickt

    dewickt Will fix about Anything

    MC30 thru MC275 distortion

    MC30 thru MC275 all, if you have increasing distortion as the volume is increased check the second stage, V2, 12AU7 Phase Inverter, pins 1 and 6 must be at the same voltage (265V), if not replace the .22 uF capacitor on pin 7. The grid bias is derived from a 2.2 M ohm resistor, if this capacitor as any leakage it will upset the bias voltage resulting in the phase inverter outputs being unbalanced.
  4. Rich Andrews

    Rich Andrews Well-Known Member

    MC2500 hum and buzz?

    The internal cable dressing is VERY important on the MC2500. Had one with a hum/buzz in the left channel. Turns out the someone had moved the leads coming from the thermoswitch on the heatsink to the other side of the wiring harness. A distance of about 1/2 inch. That was enough to cause a hum in the left channel. Moving it so that it was on the inside of the wire bundle solved the problem.
  5. Number 9

    Number 9 Well-Known Member

    Tube testers - don't rely on them

    I learned about this not that long ago.

    I had a bit of a problem with my MR67 Telefunken 6U8 MPX tube. Had it tested out on two different tube testing machines by knowledgeable technicians, and it checked out fine, but continued to run into problems.

    I replaced the tube with a new NOS, and everything is just fine.

    By my understanding, tube testers, test the tube cold, and it may perform differently when hot.
  6. Rich Andrews

    Rich Andrews Well-Known Member

    A 6U8 in a MR67? I tried a few and they don't work as well as a 6EA8 in my MR67. I have some tech notes on a MR71 and the tech recommened a 6GH8 instead of a 6U8 or 6EA8 in the MPX circuit. I personally have not tried this as the 6EA8 works great.

    I discovered that my MR67 does not match the schematic. There is an additional pot on the chassis that adjusts the MPX light. Mine was set wrong resulting in a modulation of the lamp in direct accordance to the program material.


    I can't tell you how many times I have had to test a tube hot which required me to leave it on the tester for 5-10 minutes. The most recent was when I tested tubes a few months ago that were from my MR67. I had one tube that after about 10 minutes would go so leaky that it would peg the meter and start to show short. I had another that after about 5 minutes would go quite leaky.

    My rule when dealing with a tube tester is like this. I have seen plenty of bad tubes test good, but rarely (once?) have I seen a good tube test bad.

    What was with the good tube that tested bad? It was low on emission and I think it just needed to be used. IIRC, it was NOS that had been sitting around since 1955 or earlier.
    Last edited: Jul 3, 2004
  7. PRM

    PRM Active Member


    I had Rich do his Modification on my MR-67 and he suggested using the 6GH8 for the MPX section. I have not had any problems. I also tried a couple of different 6U8 tubes.
  8. Number 9

    Number 9 Well-Known Member

    It was a BAD tube that tested GOOD. It was the original diamond-bottom Telefunken that came with the MR67. And it is as you say, because it was being tested cold rather than hot.

    Now, I had asked about the 6GH8 at the time PRM, because I saw your comment once before. The fellow who sells me my tubes (and I trust him, he's been at it for nearly 40 years and has no alterior motives) told me that they are very unreliable. He had some of those in stock too, so no skin off his back if I had bought one. He said they were designed for TV use, and said they used to fail very frequently. Miserable in fact. He would buy them buy the case once a year because they were the most common part to fail on old RCA televisions.

    Ampex (Made in France) NOS 6U8s are supposedly the best but are extremely expensive.
    Last edited: Jul 3, 2004
  9. PRM

    PRM Active Member


    The brand of 6GH8 is an Amperex (France) 6GH8 which Rich Modafferi had available. I have heard from two veterans of McIntosh that 6U8 tube was notorious for failing. I myself have not had a problem with them. I did use a 6EA8 which I did have problems with. If it works for you that is good. If I had to go back with the 6U8 I would for I have a couple on hand. I am glad it works for you. Man, I love my MR-67. Great tuner. We are lucky. Enjoy.
  10. dewickt

    dewickt Will fix about Anything

    MC2125, MC2120 and others

    This series of amplifiers have 4 lamps on the panel, 2 normal (#7382) and 2 power guard (#7373), on power up the power guard lamps blink on for about a second and then the normal lamps come on. If they never come on (all 4) and you know that they are good, check the power supply board behind the front panel. This board has 2 capacitors 2200uF at 16V (C307 and C308) in a voltage doubler to drive the lamps - these capacitors tend to short - thus no lamps. While you are at it check the 2 diodes between them, if the amp has been used for an extended time with a shorted cap the diodes will be bad, 1N4004 is a good replacement.
  11. Rich Andrews

    Rich Andrews Well-Known Member

    The 6GH8A was notorious for failing in tube TV sets. It was used as a horizontal driver and they would get soft. Now whether it was due to the type of use or just crappy lifetime I don't know.

    Speaking of MR67, does everyone know about the factory tech bulletin regarding the addition of a cap and resistor to improve separation?

    look at MR71S13.JPG in the zip file for more info.

    Yes it applies to the MR67 as well.

  12. Rich Andrews

    Rich Andrews Well-Known Member

    Don't you mean Amperex?
  13. Rich Andrews

    Rich Andrews Well-Known Member

    cleaning up RCA jacks


    I cleaned up the RCA jacks on the back of my MA5100 with Brasso and they look like fine jewelry now! Thanks!
  14. Number 9

    Number 9 Well-Known Member

    Hi Rich ... yes, I meant Amperex. BTW thanks for the MR71 mod tip. I'll git it a try when I have some time.
  15. dewickt

    dewickt Will fix about Anything


    Does your MA5100 sound dull, lack bass, behave tired - replace the 100uf (4 each) capacitors on the preamp board. If yours has white Mallory caps in it, replace them as they will fail with age.
  16. Number 9

    Number 9 Well-Known Member

    Terry ... I have an MA6200 ... would the same apply?
  17. dewickt

    dewickt Will fix about Anything

    MA6200 is a full generation newer, the MAC4100 uses the same output section, very trouble free, have not seen any repeating problems in this design.
  18. KTT

    KTT New Member

    Re: MC30 bias

    I am new to DIY. I have replaced the bad filter caps with 80+40+20+10uF/525V from AES in my MC60. Can I use one of the unused 10uF for this Bias filter cap or use both 10uF in parallel (or just get one 22uF/450V cap for this purpose). If I replace the diode with 1N4007, what series resistor is need to replace the 1.8K resistor in MC60?
  19. Rich Andrews

    Rich Andrews Well-Known Member

    Re: Re: MC30 bias

    You cannot use the unused section as the polarity is wrong. This is a negative voltage that is being generated and attempting to use the unused section would likely result in the total failure of all the sections.

    Don't cheap out- get the right parts.

  20. dewickt

    dewickt Will fix about Anything

    First - the bias filter has it's + end to ground, opposite of the main filter capacitor, you will have to use a seperate capacitor for this filter. Tie the unused section of the filter capacitor to the 80uf section, more is better at this point.
    Second - you may have to put a series resistor at the transformer lead to the bias rectifier, try without one first and measure the bias voltage at the KT88 with a high impedence meter, most DVM are OK, it should be -45 volts. If the voltage is -50 or more you will need to add the resistor to bring it back down to -45.
    I do this with no input signal, and the AC plugged into a variac to set the plate voltage on the KT88 (pin 3) to 435 volts when I measure the grid (pin 5).
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