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38-10 Low volume on stations - Printable Version

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38-10 Low volume on stations - Nick3092 - 05-18-2014

I've had a 38-10 on and off my workbench quite a bit over the past 6 weeks. I think I stumped most of the guys over on ARF with this one too. Hoping maybe someone who is over here might have some ideas.

I went through it, did a full re-cap and replaced all resistors. Upon firing it up, I find I can pick up stations either with a 10' antenna, or touching the 6A8 grid. Stations even as far away as Chicago (I'm just west of Milwaukee). But the volume on them is very low with the radio volume maxed out.

I've gone through the alignment process multiple times. I'm pretty confident the IF's are set correctly. But something weird is going on in the front end. I had a heck of a time with aligning the front. No matter how many times I went back and forth between the 580 and 1500 adjustment, I couldn't get the dial to track. After some luck, I have the dial tracking for the most part (+/- 5kHz depending on the location). Here is the weird part. Nether adjustment 7 or 9 is peaked though. If I go and peak either of them, my dial tracking goes way off again, and I wind up bouncing back and forth because every time I peak one end, it completely screws up the other (and the dial tracking).

I cleared the audio section by injecting an audio tone at the top of the volume wiper. Depending on the volume of the gen and the radio, it got quite loud. So I know the radio is capable of putting out a much louder sound than it is.

The AVC seems to be working normally. So I think the radio is receiving the signal. but something is messing with it in circuit. For example, i was able to get these AVC readings:

Normal AVC, no station tuned = -2.53vdc

620: A very strong station in Milwaukee
10' Antenna connected normally = -4.55vdc
Touching the 6A8 grid wire = -6.1vdc
Antenna connected normally, plus touching grid = -6.25vdc
10' Antenna through .001 cap to 6A8 grid = -6.5vdc

1080: My SSTRAN in the room above my workbench
10' Antenna connected normally = -5.05vdc
Touching the 6A8 grid wire = -5.4vdc
Antenna connected normally, plus touching grid = -5.76vdc
10' Antenna through .001 cap to 6A8 grid = -5.2vdc

I ran through all the voltages on the tubes. Everything is within the expected range that is noted on the schematic, although some of these voltages are not listed (radio was powered off a variac @ 116vac):

6A8 - plate = 163.1, screen = 93.8, osc grid = -15.45, osc screen 137.7, grid cap = -2.64
6K7 - plate = 162.9, screen 93.9, grid cap = -2.36
6Q7 - plate 162.9, pin 4 diode = -2.54, pin 5 diode = -1.40, grid cap = -1.9
6F6 - plate - 233.5, screen 249.2

The sets original 6K7 caused the screen voltage to drop the longer the radio was on. After a couple minutes, it would cut out all audio. So I think it was gassy. I replaced it with a NOS 6K7 and it seems to have corrected the issue. I've run all tubes through my 533A TT and they all check out.

I'm not really sure what else to do at this point. At my disposal I have a VTVM, and Eico 324 sig gen, and a B&K 1470 scope. (although, I'm not good with scopes, so I'm not entirely sure how to set them up, or interpret the results). I'm open to suggestions.

Thanks!

RE: 38-10 Low volume on stations - gregb - 05-18-2014

Sounds like a radio that I just had, yes I know you say the IF is OK but is it really. What did you use to align it, did you verify the injected IF with a counter? If the IF is off frequency you will have the exact problem you are describing.

Gregb

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

I always verify my sig gen with a counter. During one of my tests, I also fed 470 (which is the IF for this radio) into the antenna jack at full strength. The tone was pretty constant outside of I heard a fluctuation at 940 and around 1410 in the tone. So I believe that means the IF's are aligned. Not sure how else to prove that.

Also, if the IF's were off, I don't think I'd get such strong AVC on the local station. -6 seems to be a pretty strong signal from what I've seen for OTA signals. And I'm pretty sure the dial wouldn't track, and stations would be "wide" on the dial (meaning they come in over a large span of stations on the dial).

RE: 38-10 Low volume on stations - morzh - 05-18-2014

Nick

A simple-stupid way to start is change tubes one by one.
The "tubes cheked out" is not a guarantee they actually work.

Also have you cheked all RF transformers' continuites?

RE: 38-10 Low volume on stations - murf - 05-18-2014

Feel your pain.
Have a 40-180 that I can't get to play either.
Would be nice to have the tech knowlege that greg has.
For most of us it is just a hobby. It is still fun trying to get them playing again.
Greg has helped me out in the past. He knows his stuff for sure.
Good luck with your Philco.
murf

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

morzh,

I have also tried another NOS 6A8 and NOS 6Q7 in the chassis (and I already replaced the 6K7). No change in behavior. I did check continuity on the coils way back a few weeks ago when I first started this one. Everything was as expected. These coils have very low resistance on them. Most (if not all) are all "less than .1ohms".

RE: 38-10 Low volume on stations - morzh - 05-18-2014

How about the range (band) switch - have you cleaned it?

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

Bandswitch has been cleaned with a bit of deoxit. It works as it should.

RE: 38-10 Low volume on stations - gregb - 05-18-2014

Have you checked the mica caps? I have run into a few bad ones lately and they cause strange problems like what you describe.

Gregb

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

All caps were replaced, including micas. They were Micamold. Don't trust them.

RE: 38-10 Low volume on stations - morzh - 05-18-2014

Your 6K7 plate is dead wrong. I should be full rect voltage, about 260V. You have it at 160V.

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

Typo on my part, sorry about that. I accidentally typed the 6Q7 plate voltage twice. You are right, I have it written down as 248.5v for the 6K7. Which is about what I saw at the 8uF cap (248.1v) and on the 12uF cap to CT it's 325.6v.

RE: 38-10 Low volume on stations - morzh - 05-18-2014

Yes it should be same a screen grid on 6F6.

Have you looked at the volume pot and circuit? To anticipate you answer about injecting audio, unless you do know what level to inject, loud sound does not guarantee the correct curcuit perfrmance.


Oh...,and this is happening in both bands or just one?
How is your antenna connected (what terminal)?

RE: 38-10 Low volume on stations - Nick3092 - 05-18-2014

The pot is working. From wiper to ground it varies between 310 ohms at low and 350k ohms at high. As for injecting the audio, the AF output was set to less than 10% on the dial on the gen. It was quite loud when the volume on the radio was all the way up. But if you have a better way to test it, I'll do it.

RE: 38-10 Low volume on stations - morzh - 05-18-2014

350k at hi to GND, if your pot is 500k as it supposed to be (possibly higher) means you never reach full volume.

Now, if you inject mod IF signal, is the volume still loud? (grod of 6k7)?