Well, I think I will replace that 200 uF cap (C30) this weekend. I'll probably end up cutting the leads of the original, and making my own "quigs" to attach the new cap to the original leads. I will (finally!) be putting together another parts order, as well, so that I can wrap up this 700 and get ready for the next one - a 170 that is mostly working, but it looks like someone spilled some soda (or something) on it, which got into it as well and on part of the PC board that includes the amplifier and power supply.
In addition, I will also change the wiring back to original on the function switch, so that when the unit is in the FM Stereo position, it will be muted when not tuned to an FM Stereo signal.
"Tip Toe, Through the Tulips!" I usually place film caps, for anything below, 4.7uF. I especially like doing this for sound equipment, because I am a firm believer, that wine gets better with age, not electrolytic capacitors.
(This post was last modified: 04-19-2014, 03:51 PM by Omer.)
I replaced my 700-T's off-on-volume control today, with an identical dual 50K control pulled from a 175-T which has officially become a parts set. It will also be donating its AM ferrite rod antenna to my 450-T.
I sat and stared at C30 for a long while, but I'm not sure how to proceed on that one. C30 really is buried in there. I know I had mentioned using homemade "quigs" to join the leads of the old C30 to the new part. Saying that is one thing. Actually doing it is another - so much stuff in the way, so little room to get to the capacitor...
I fired up my 700-T today after replacing the volume control and rewiring the two wires I had previously moved to the proper terminal on the function switch.
Results: Yes, in the FM Stereo position, stations are muted unless they are in FM Stereo.
However, I now have pronounced distortion in the audio. I don't know if it is caused by C30 (which I still haven't replaced) or if something is up with the replacement volume control? Both are supposed to be dual 50K controls...
I would also go with the film for C776, especially since its only 0.5 uF.
Tantalums come in handy for large values and low leakage, but in general if you can physically fit the film cap you are probably better off with film.
Sound like something with the stereo muting circuit is causing the distortion. Does the voltage at the collector of Q406 drop to near zero when receiving stereo? If not it will apply a voltage to the preamp transistors acting to cut them off and inducing distortion. Don't think C30 is the problem here as it only serves to filter the applied voltage from the beacon circuit. If it leaks it would reduce the muting action, not increase it to the point where it would start muting and cause distortion.
As a side note to the discussion about transistor replacement, McIntosh is now only selling NTE substitutes for transistors.
Regarding the TR1009 power transistors in the back, what is the best thermal compound to use for these transistors, when they are returned back in to their place?
(This post was last modified: 04-23-2014, 06:29 PM by Omer.)
2 ounces is really overkill unless you plan to replace a lot of output transistors. A 1 oz. tube would probably be more than sufficient. As it is, I now have enough of the stuff to work on several (perhaps several hundred?) receivers.
Mondial, I haven't been near the bench this week. I did make some tests Sunday evening. While I failed to measure the Q406 collector voltage, I did use my signal tracer at various points along the audio chain, and found that the right channel audio drops below the level of the left channel in the control amplifier board, at the collector of Q202. Q202 is new, as are all of the transistors on the control amplifier board - all of the electrolytics were replaced, also.
So it's only been almost 10 months since my last post...
You may recall that I had traced distortion in my 700-T to the control amplifier board.
So last night, I pulled a control amplifier board from a junk 440-T. The board is very similar to the 700-T board; I felt the very slight differences would not make any difference.
The 440-T board is at the top in this photo, with the original 700-T board underneath it. I put new electrolytics into the 440-T board. Did you notice the two jumper wires in the middle of the 440-T board where the 700-T has two white capacitors? Well, I removed the jumpers from the 440-T board and put two capacitors in place of the jumpers. The capacitors are needed in the 700-T as part of the low filter switch circuit, something the 440-T does not have.
I tested the transistors in the 440-T board and all were OK, so I put them back into the board.
So I installed the 440-T board into the 700-T this morning...and then tried it out with headphones.
Verdict: Sounds good with no noticeable distortion.
Just my luck, though, in the process of installing the 440-T board into the 700-T, somehow I managed to break the Stereo Beacon incandescent bulb.
So, my dear wife agreed to pick up a couple "grain of wheat" bulbs from the Jasper Radio Shack (which is not on the list to be closed, incidentally). Later today, I will install one of them and if all is well...I'll button this set up (finally)!
Having tried the 700 out again after I replaced the Stereo Beacon bulb, and finding that all seemed well and that it still sounded OK with no distortion, I then proceeded to put it back together.
After reassembly, I tried it again with headphones.
Oh, it still works...but now when I turn it on, it makes a metallic "thump" upon power up.
And on top of that...the off-on switch has now quit working...it still clicks but is now stuck ON.
Grrrrrr....
I *might* try it with speakers sometime this weekend, but I'm inclined to set it back on the shelf and forget about it for another 10 months.
with no noticeable distortion.
