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Sorta newbie 38-690 low volume - Printable Version

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RE: Sorta newbie 38-690 low volume - Hot Rod - 12-03-2017

Thanks morzh. I never claimed to be an electronics guru. I'm just a grease monkey. Everything I know about electronics is self taught (painfully I might add).

So do the tweeter pots look bad to you? I didn't disconnect them and am not sure how that would effect the measurements.

I forgot to mention the good news. All 3 speakers are absolutely pristine perfect! I was VERY happy to see that.

RE: Sorta newbie 38-690 low volume - morzh - 12-03-2017

Your 2800 ohm is ok, it is in parallel with the 500 ohm resistor so it results in 424 ohm (435 is close enough).

1.4M instead of 1M cannot be explained by connection, but then it is treble resistor, I doubt the tolerance does anything to the volume level.

Try to see 33K and 15K resistors from the anodes of the 2nd Audio tube, poke around the tube itself.
Make sure you did replace the capacitors ##82 and 136.

You're lucky - I had to re-cone both tweeters and wanted to re-cone the woofer but ended up patching it. Big gaping hole due to the broken wooden cone attached to the front panel that fell and tore the cone.

RE: Sorta newbie 38-690 low volume - morzh - 12-05-2017

Hot Rod,

Could you do me a favor and look up one thing? I am trying to fix the magnetic tuning that somehow does not work.
While investigating I noticed a GND wire that broke of some place where it was going to from the ground lug.

These are the photos. I understand your wiring could differ but maybe it is the same. I need to trace where that wire goes to.

Mike.

This is where it ends, the broken end.
   

This is where it starts (GND lug)
   

The total pic of the wire.
   

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-05-2017

Hi Mike,

I only have one wire that goes to the cable and a cap.  I'm wondering if that is a ground for the multi-lytic #110, which I would have removed.  As you can see, I'm not a re-stuffer.  But I've been called worse. Icon_lol 

BTY, you had asked if my volume goes up when I touch the antenna and YES, it most certainly does, quite a bit.  I didn't try the tube caps.  Is that the top connector on the the RF tubes?  I wasn't sure.  So does that indicate a possible open tuning coil (one of the 14 in the 690).  Please excuse if I get the terminology incorrect.  I'm still a beginner.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-05-2017

And here's the entire right (from top) side showing the #110 cap area.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-05-2017

So I found a problem, don't know if it is causing the issue of the treble causing the volume to drop, but it think it could be.  The rear #67 pot has a 0.1 cap on the center terminal, which I had replaced.  Well it measures 0.65.  It's the bottom of the two identical orange drop caps.  The top one measures 0.097.  Maybe???? Icon_biggrin Icon_biggrin Icon_biggrin

RE: Sorta newbie 38-690 low volume - morzh - 12-06-2017

0.67uF? In circuit or desoldered? In circuit it can show anything being across the resistor.


I found the photos I took before recapping, and though it doesn't show the very end, it dives under a cap, it seems it was intended to be the GND of the 5-cap lytic, and the lytic is connected, so I probably was biting off the wires and when having reconnected the lytic I used the nearest GND lug instead. Forgot to take the wire out.


Thanks for looking anyway! It confirmed what I just found.
Unfortunately this doesn't explain my problem. My magnetic tuning no workie, and I know how it works when it works from my 37-116. Pretty efficiently.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-06-2017

Glad you solved the wire mystery.  I remember that Philco has an adjustment procedure for the MT.  Mine works.  It makes the station come in worse, but at least is does something  Icon_lolno .   I already replaced the 0.7uF and two 0.1uF.

My suspicious 0.1uF cap goes from the center terminal of the treble control then directly to ground, no resistor.  I didn't disconnect it.  I know everything is affected by everything else in the circuit, but didn't think it would in this case.  I will check it disconnected before I condemn it.

I emailed Mark Oppat about a replacement treble control and a new dial but no response as yet.  I hope everything is ok. 

I picked up a 1935 Emerson tombstone model 36 last night.  SOMEBODY STOP ME BEFORE I BUY AGAIN!!! Helpsign

RE: Sorta newbie 38-690 low volume - morzh - 12-06-2017

You are talking of the 0.1uF cap that goes to that 2,800 ohm pot with 500 ohm in parallel.
This will not measure correctly at all if in-circuit. You have to measure it disconnected at least at one end. It is likely good.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-07-2017

Mike,
I have read that mica caps rarely go bad.  Do you test them?  If so, do they need to be disconnected or is there another check (open or short) to verify?  I have not checked the micas in mine.

I checked the components that you suggested and "poked around" the 2nd audio tube.  I didn't find anything.  I have a  hunch that the overall low volume output and the treble control acting as a reverse volume control are related.  Is there anything under those IF expander cans worthy of checking?  It doesn't appear to be a simple job to open them up.  I have ohmed the coils that had connections that I could get to.

I plan to spend more quality time with my 690 tomorrow night.  More suggestions are always wecome!   Icon_e_smile

RE: Sorta newbie 38-690 low volume - morzh - 12-07-2017

I never check mica caps. The overmolded type rarely goes bad. Unless a cicuit malfunctions and you specifically suspect a mica cap, they don't need to be checked.

Going back to the volume. When you say "half as expected", how do you know what's expected?
With antenna or with the finger on the Antenna input, does a local station come loud? And, no, if the finger on it increases volume, it is good, means your signal gets through.

Here's another thought:
The amplifier in the tuner chassis has two parallel channels: Bass and Treble.
Bass is the 6K7G and then it mixes with treble in the 2nd Audio 6N7G.

Pull out the 6K7, listen to it with treble (the regulator CCW), then put the tube in, see if the bass comes in.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-07-2017

It's only about as loud as normal conversation. Definitely not the volume the 690 should be capable of. Other consoles that I own go much louder. Even the 38-8 is louder.

Reception, at least on broadcast, is pretty good. I had a spool of wire (50 ft), most of it still on the spool, for an antenna. When I picked up the spool to find a good location, the volume would increase dramatically.

There is sound from the bass. The overall balance from the woofer and tweeters seems to be about right, as far as volume from each. Just not enough. And then there's that treble control thing.

Thanks for the advice. I'll try pulling the 6K7 and poke around some more tomorrow. Good luck on your MT!

RE: Sorta newbie 38-690 low volume - morzh - 12-07-2017

I noticed on mine, the bass pot does not make a whole lot of difference.
I pulled the grid cap, the tube works. I desoldered the high path (330K resistor in parallel with capacitor), only bass was left. It is there and it is ok, and is well regulated by the bass pit, but when everything is again together, the bass doesnt change dramatically. It is so low that I guess the part that goes to the high path is actually a fully balanced signal.

RE: Sorta newbie 38-690 low volume - Hot Rod - 12-08-2017

I didn't think my bass control did anything at first. Then after more listening, I realized it worked but didn't boost much. Also, at roughly 80% volume or higher, even though it's not very loud, cranking the bass up will cause audible clipping. So I think somewhere there is low voltage and somebody is getting starved for power. Sure wish there was a voltage chart for this puppy.

What you said about the two channels made me think of something. Would it be possible that the radio is only using the treble channel to power all 3 speakers and the bass channel is inop? That would make perfect sense of my symptoms.

RE: Sorta newbie 38-690 low volume - morzh - 12-08-2017

This is what I was trying to make you check. Icon_smile When I ask to pull the 6K7 Bass amp. 

But surest way to make sure the base is working is do what I did, de-solder the high path. It is very easy, the very first photo I posted here when asking you about that wire earlier shows it, but let me edit it to circle the parts.

   

The resistor and cap whose leads twisted together is the Hi-f path; the lead that has to be desoldered comes to a nice large solder post on the left (photo does not show it, it is under a wire), also circled red. Disregard the arrow - it is from the previous edit.
When you desolder it you will only have the bass left. See if it is there and if it is enough when the antenna is connected.
Mine was not that loud but I guess this is what it should be.