help with old radio????

[zapped]

I hope this topic wont be rejected. I have been tinkering with an old radio on and off for a while now. Its a Philco 38-4, It was DOA when i started working on it. All E-caps changed, most paper caps, many resistors.. It was still silent. Found bad OT. Changed bad OT. Now has a high pitched squeel, so I pulled all but recto. and output audio tubes, its quiet with slight hum. if I reinstall only the 1st audio (preamp)? tube squeel returns, volume control has no effect. Could this be phase inverter or preamp related????   Thanks for any help rendered..  Ed

[jjasilli]

1st guess:  Wrong polarity/ incorrect hookup of a replacement output transformer.

Anyway, go to:  www.geofex.com > tube amp debug page > squeal 

If that doesn't work, please re-post.

[HotBluePlates]

For everyone else like me (who knows nothing about this radio), some good background info is on this page, along with some very good service data.

I don't know much about repairing radios. You might get more precise info on the Antique Radio Forum.

One thing I have found out from the ARForum, you want to assume ALL paper caps are bad. They very likely are, or will be soon.

I'd suggest you pull all tubes except the 5Y4 (rectifier), the 6K5 (driver) and the 6F6's (push-pull output tubes). You want to solve the squealing with just these in place, before tackling the radio frequency (RF), intermediate frequency (IF) and automatic volume control (AVC) sections.

You may want to lift the end of the 0.01uF cap (part 36), and the end which connects to the 330k (part 33) and 51k (part 31) resistors. This will disconnect the audio section from the rest of the radio. There is a 0.05uF cap (part 50) which runs from one 6F6 plate to a 1M resistor (part 49) and a 0.03uF cap (part 47) which may be mounted on the tone control switch itself. They form a feedback loop-based tone circuit. Killing the loop stops the tone circuit from working, but also may help diagnose your feedback problem.

The push-pull output doesn't have a true "phase splitter" but rather uses a driver tube (6K5) and a self-split scheme to create the push-pull output. Pay particular attention to resistors 62 (25 ohm), 63 (152 ohm) and cap 64 (10uF). They form a "back-bias" circuit for the output tubes, and you want to be certain they're reading roughly correct. Be sure you connect the PT center-tap back to the top of the 10uF cap and NOT to ground anywhere; this too is an element of the back-bias arrangement, and not what you typically find in a guitar amp.

If you can follow this, and be certain everything in the audio output stage is functioning, opening the tone circuit feedback loop may kill your oscillation (or not). Let's start with verifying this section.

Oh yeah... Caps 58 and 59 (both 0.003uF) from the 6F6 plates to ground may be tone-shaping, or may be a form of phase correction of the OT (I don't know enough to know which is the case; PRR can probably tell us). Either way, I'd probably make sure both those caps were replaced with brand new stuff. A leaky or otherwise malfunctioning cap in these spots will either do bad things to the power supply or possibly contribute to a squeal problem.

[jjasilli]

Good stuff.  Can't remember the repair handbook just now, but note: for our purposes, the "radio" can be considered to have 2 parts:  the radio receiver, then an audio amp.  The radio receiver picks up the air signal, and translates it into audio signal.  This is then fed into an audio amp, which we're all familiar with.  So you should be able to temporally disconnect the radio signal feed; then run outside signal into the audio amp and get good output.  If not, continue to fix the audio amp.  Then address the radio portion if necessary.

This comports with the usual plan to work backwards from the speaker.  

EDIT:  found it -- "Servicing Radio and Television Equipment with a Vacuum Tube Voltmeter" Sylvania Electric Products, Inc.  © 1951 http://www.cfp-radio.com/documentations/Sylvania-VTVM.pdf

[HotBluePlates]

One issue might be the high frequency limit of most modern DMMs.

The IF signal frequency is 455 kHz, and I want to say my Fluke 87 starts showing drooping readings by 200 kHz or so. What that means is that if you had an oscillator producing a constant output voltage, as you raise the frequency, the display's reading will appear to drop (from, say, 3vac to 1vac to 0.25vac) as you keep raising the frequency. In reality, the voltage level is not changing, but the meter has reduced response for one of several reasons.

So it's probably best to check d.c. voltages and take them as an indication of health. The AR forum will likely be able to provide alignment instructions, but you may need test gear you don't have available to accomplish it.

However, if things aren't very wrong, you can likely get the radio to work mis-aligned. It just might pick up weaker stations better with a proper alignment.

[zapped]

Thanks HPB for taking the time to explain what you think may be the problem. Your response is much more detailed then I have seen from ARF so far. I plan on trying your suggestions tonight. Thanks also to jjasilli for the link to cfp-radio.com I will try to download the manual..I will be changing out those old paper caps tonight.  Thanks again for your help..

[PRR]

> a high pitched squeel..., volume control has no effect.

How about tone control?

Is the lead to 6K5 top-cap tight and right? Was it shielded? Is the shield grounded?

> Wrong polarity/ incorrect hookup of a replacement output transformer.

In first two positions of Tone control there is no NFB. Also the NFB to Tone switch comes from a 6F6 plate, not from OT secondary. It would take considerable kludging to mess-up NFB polarity.

However.... excess plate-lead length on OT will radiate large output plate signal back into sensitive audio stages. New OT wiring must be at least as tight/right as before.

All wax-caps. Caps 45 58 59 C64 are very suspect. Use 0.005uFd 400V poly for 58 59. C64 may be 10-20uFd over 35 volts. Note that Negative goes to chassis.

DC voltages on all audio stages. Some tube voltages on sheet which HBP linked (thanks!). Exact voltages not critical; we looking for way-wrong voltage like zero where should be 160V. In addition, 6F6 pin K should be zero, pin G should be at negative 15V to 20V; 6K5 pin G should be -2V to -3V.

With volume down, you can trouble-shoot the audio while leaving the RF/IF tubes in places. I suspect you want these tubes working because their current is part of the bias for the 6F6 stage (back-bias in resistors 62 63).

Do NOT even try to "align". Your symptoms don't suggest any mis-alignment. Also an all-iron AM radio stays in alignment better than any home "touch-up" can achieve. If it isn't aligned, someone tinkered and broke a trimmer.

455KHz voltmeter should not be needed; we didn't have them back in the day.

[zapped]

Ok, after changing out cap 45, 39 and 3. resestor 38. All others mentioned in above post were changed prior. Powered-up squeal with tone in position 1&2 unchanged by volume control but, with tone in position 3 or 4 it's quiet and will pick up signal. However the volume is still inop. I tried spraying contact cleaner but no change.???????????

[zapped]

 I took a .o47 cap and test lead it to ground and poked around the tone sw. and found if i contact the tone sw. term. that goes to cap. #50 the squeal stops on position 1 & 2 of the tone sw. The volume pot seems to sweep smooth on the meter but only has minimal effect on the volume, it's low in all positions.  Thanks for helping   Ed

[PRR]

Your model has a switch 43 across the volume pot wiper to ground. I am guessing this is to mute while tuning. Try working the tuning knob in/out, then study the wiring at the volume control and trace to the switch. Cut that wire.

However the instability could also be a bad ground in the audio amplifier, perhaps involving the volume pot and tone switch. Check that points which should be ground (vol pot end, tone switch wiper) really do get to ground.

[zapped]

I had forgot about the tube shields. After they were reinstalled the squeal that was present in tone pos. 1 & 2 stopped but I found when fingers are close ( 3in. ) to the 1st audio or 2nd det. tube it produces a much louder feedback. Still cant figure out low and ineffective vol. pot. Reception seems good.. Thanks   Ed

[PRR]

....could also be a bad ground in the audio amplifier, perhaps involving the volume pot and tone switch. Check that points which should be ground (vol pot end, tone switch wiper) really do get to ground.