MAC-1700 Distortion with Tuner Input Selection from Differential Pair

[jd3rdcoast]

Hi All;

MAC-1700 S/N 37J4, No Output Monitor board.

I am just about finished with this MAC-1700 but have an issue I would like some input from the experts.
I had another thread regarding initial issues that were cleared up with replacement of some burnt resistors, Recap of P/S, PreAmp & Driver boards. Here is Link to my other post:MAC 1700 w/a power Supply issues.

My current and hopefully final issue is in Tuner mode, I am getting very large distortion from the output of both sets of differential pairs Q20-Q24 to Q25/Q26 bases. Also the outputs heat up quite hot which I believe is attributed to the distortion or excessive signal level when Tuner is used. I have scoped the Tuner input to the Driver board and it is clean although very robust levels with mild volume.
Distortion shows up on both channels equally. Of note: The Tuner signal is exponentially higher than the other inputs (AUX/Phono). The distortion sets in at about 18 W of output and Vol pot is only at about the 9:00 position. I have tested the Amp with 1Khz sine wave via AUX input and it is good up to about 55 Watts output before clipping w/Vol at about 75%.

Question:
#1. Is the high Tuner signal level over driving the Differential Pair compared to other inputs or are they just no longer up to the task? I have KSA992 sets I can match up here but wanted to see if anyone had another take on this. Edit: I just re-read Hints & Kinks thread stating all units should have Diff Pair changed out. I noticed mine has a mix of 2N4249 and 740's factory installed on both sides. I'll match 2 sets of KSA992's and get them installed.

#2. Is the Tuner signal normally so much higher signal since coming from a Tube based circuit. Or is it in need of professional alignment?
Here is pic of Diff Pair output from Tuner. Scope at 2v/div.

Power Output Schematic - Left Side:

 

[jd3rdcoast]

Well I guess that would have been too easy. New matched sets of KSA992 Differential pairs did not change anything. I do notice a buzzing from the Transformer that coincides with the interference seen on Tuner trace above. It gets louder as I increase the Volume level and distortion gets worse. I'll go back and look at the power signals from the rectifiers to see if I see anything amiss. Any suggestions would be appreciated. This is the first really old McIntosh I have attempted to repair and this one is in pretty bad shape. I also to mention there was a bad ground connection on Phono-1 Right RCA Jack to the chassis. I temporarily fixed with some solder to get make the connection more reliable. It was about 28K ohms to chassis and dropped it to about 3 ohms. So maybe there are other ground Issues I am not aware of.

 

[jd3rdcoast]

Still no love here. New rectifier diodes (UF4007) for both D19-D22 and D24-D27 with not change. FWIW when scoping the +/- 37V feed to the main power caps, as soon as I advance the Tuner volume the AC waveform goes ballistic with artifact which also coincides with the slight buzzing heard from the Transformer. Below is a Pic of rectifier outputs D24-D27 feeding main Filter Caps: zero volume vs. increased volume (3:00 position).
To recap:

1. Distortion on FM audio signal from amplifier as volume reaches 3:00 position.
2. Equal on both L & R channels only with tuner input.
3. Tuner signal out of FM/MPX is clean. Same out of PreAmp to Driver.
4. No issue with other inputs with lower input signals until clipping around 55 W and
5. I can hear Transformer buzz in sync with Tuner distortion peaks or when clipping at full power.

Anyone want to throw a dart? I feel like I'm deep in a rabbit hole here...
Thanks in advance.

 

[clinic-audio]

Houston , we have a problem

 

[jd3rdcoast]

Houston , we have a problem

Perhaps a clue?
R across the Transformer windings at D24-D27 that feed Main Filter caps rectifier is 0.2 ohms.
R across D19-D22 to C129 is 36 ohms
R across D20-D21 feeds C128 is 18 ohms

I'm not sure if the 0.2 ohms reading is accurate because it is a full rectifier or if something is amiss. I may need to lift the diodes and check the resistance again. The voltage output of the rectifier is a +/- 38.3V. When the Tuner Vol is raised to distortion level of the waveform the voltage bounces around 35.5-37.8 VDC and 0.5 - 1.2 VAC.
I am hoping the Transformer is not toast, but because the other inputs work fine has got me scratching my head.
Can I lower the output of the Tuner section?

Edit: I am going to check the Output Transistors and Emitter Resistors closely to see if a problem exists there. I can see on emitter resistor is newer but have not really evaluated closely because with an AUX input sine wave into 8ohm dummy loads it checks out to above 40 watts before clipping. Perhaps with the higher Tuner input level it's over-driving something there. But since both sides exhibit same behavior I am still leaning towards transformer.

 

[jd3rdcoast]

I lifted the rectifier diodes for the +/- 37V feed for the output transistors. Resistance across that section of transformer secondary winding is 0.2 ohms. I suspect I am chasing a shadow here as the amp functions fine other than distortion past the Diff Pair on FM signal. But I also have no experience with faulty transformers. I assume they would not put out the correct voltage if was bad.

Pulled the outputs and they all test good with following gain from basic Atlas DCA Tester: Righ Hfe: 31/16, Left Hfe: 21/55. Not very well matched but I don't think that matters so much here.

I can tell one of the 0.47 emitter resistor has been replaced on Left output (Q36). My meter (Fluke 77) resistance is only accurate to 1/10 place. So measuring these 0.33 and 0.47 is not really accurate. But for the 0.47 I get 0.3's and 0.33 I get 0.2 after a few seconds. The replacement 0.47 is installed with markings down so cannot read the markings.

This 1700 is treating me like...

 

[KentTeffeteller]

Are the tuner tubes good? It does have a tube tuner. Tubes can have shorts or go gassy.

 

[jd3rdcoast]

Are the tuner tubes good? It does have a tube tuner. Tubes can have shorts or go gassy.

With my admittedly non existent knowledge of tube units, that is what I am leaning towards since everything else is functioning. I just don’t understand why the scoped output from the Tuner pre and post Preamp looks fine yet the signal clipping levels show up in both amp channels after the diff pair (new diff pair no change). And the +\- 37 signal gets wonky with the peaks.
Is my next step to find a tube tester or some replacement tubes to swap in? Any ones I should target based on symptoms?

 

[jd3rdcoast]

Does the 12AU7 look problematic? This is what’s installed.

 

[ramjet]

Is this tube a 12AU7 at all? The numbers I read are 5741 or 7 and 3223, neither is a 12AU7 sub (that I know of). Too hard for me to tell from just the pictures (although it does appear to be a dual triode). The green verdigris on some of the pins is troubling as well as it means it is also on the tube socket. Having said that, a visual inspection of a tube (and anything else for that matter) does not give the necessary information needed to answer whether or not that tube is part of your problem. An easy solution is to install a known good 12AU7 (once the tube socket is cleaned (and the others inspected)). As to your earlier question about "professional alignment?", with this tube being the input to the MPX decoder, I would say yes, at least for T7. I've not performed an alignment on a MAC1700 but I have done so on a MAC1500 and the tuner section does look somewhat similar.

 

[jd3rdcoast]

Is this tube a 12AU7 at all? The numbers I read are 5741 or 7 and 3223, neither is a 12AU7 sub (that I know of). Too hard for me to tell from just the pictures (although it does appear to be a dual triode). The green verdigris on some of the pins is troubling as well as it means it is also on the tube socket. Having said that, a visual inspection of a tube (and anything else for that matter) does not give the necessary information needed to answer whether or not that tube is part of your problem. An easy solution is to install a known good 12AU7 (once the tube socket is cleaned (and the others inspected)). As to your earlier question about "professional alignment?", with this tube being the input to the MPX decoder, I would say yes, at least for T7. I've not performed an alignment on a MAC1700 but I have done so on a MAC1500 and the tuner section does look somewhat similar.

Thanks Ramjet. Yes this is from the 12AU7 socket space. Unfortunately the lettering is worn off the tube and cannot read the full ID as are most of the tubes. This was the worst as it not covered. Unfortunately I do not have access to a tube tester at this time. I cleaned all the sockets and pins. This one was only one that looked dirty and oxidized. I did not see any verdigris on the other tube pins.
The unit owner has decided he will take the unit as is and not drive the tuner hard until he gets the tubes and alignment sorted out. He has a tech who can help him get new tube/s to test in it and there are a couple places here in Houston that can do the alignment. He will update me with the outcome of that so I can hopefully have closure for this thread.

As a footnote, after pulling the outputs yesterday I noted the left side thermal past was extremely dried and crumbly compared to right which had been replaced during previous service. I cleaned and reinstalled with fresh paste on both sets of outputs. I then rechecked the FM signal distortion level and found it had improved slightly. Not as prominent and coming into play with smaller peaks and at a bit higher volume . I recommended to the owner he get a set of MJ15003G outputs on order (back ordered to June) which he has the skills to replace or I'll do it for him. Overall the amp is sounding great and operating as it should other than if drive the FM above normal listening levels.