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 Post subject: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Tue 06, 2018 4:12 pm 
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When I began to take the chassis out of the cabinet, I should have figured it was a bad omen when the band switch knob didn't have a regular set screw, but a phillips screw extending too far out and the knob was superglued on. Had to hacksaw the knob in half to get it off, a 1st.

Chassis out, inspection revealed the band switch cover was missing, wafers missing (no, not by me), and someone had completely bypassed the switch. Hopeful, I went ahead and replaced all bypass caps, filter caps and OOT resistors. All tubes checked good, except the 6G5 eye tube was darker than midnight. Checked the eye tube socket between pin 2 & 4 and resistor registered almost 4 megs… Quick check of the schematic (http://www.nostalgiaair.org/pagesbymode ... 012874.pdf) to see if it ought to be 1 meg, but NO, Packard Bell didn't list the eye tube in their schematic (nor on the other 46 model variations), so replaced it with the typical 1 meg resistor. Placed all the tubes back in their socket, with a good 6E5 in place of the dead 6G5 and powered the radio back up.

Whomever modified/removed the band switch had changed it to receive only on AM broadcast. I was receiving relatively well, but the audio was lower than I would have expected AND though the 6E5 was nice and bright green, it remained "open" while tuning stations in and out, not deflecting. So I'm trying to trouble shoot the problem, which is an AVC issue (?) and I suspect was caused by whomever removed the band switch and re-wired it for AM only. I was trying to read through other ARF similar problems last night until I dozed off. Not having shortwave is livable, but I'd like to at least have the eye tube deflect as it should.

Any help would be appreciated!

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Tue 06, 2018 4:49 pm 
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Dnitram wrote:
When I began to take the chassis out of the cabinet, I should have figured it was a bad omen when the band switch knob didn't have a regular set screw, but a phillips screw extending too far out and the knob was superglued on. Had to hacksaw the knob in half to get it off, a 1st.

Chassis out, inspection revealed the band switch cover was missing, wafers missing (no, not by me), and someone had completely bypassed the switch. Hopeful, I went ahead and replaced all bypass caps, filter caps and OOT resistors. All tubes checked good, except the 6G5 eye tube was darker than midnight. Checked the eye tube socket between pin 2 & 4 and resistor registered almost 4 megs… Quick check of the schematic (http://www.nostalgiaair.org/pagesbymode ... 012874.pdf) to see if it ought to be 1 meg, but NO, Packard Bell didn't list the eye tube in their schematic (nor on the other 46 model variations), so replaced it with the typical 1 meg resistor. Placed all the tubes back in their socket, with a good 6E5 in place of the dead 6G5 and powered the radio back up.

Whomever modified/removed the band switch had changed it to receive only on AM broadcast. I was receiving relatively well, but the audio was lower than I would have expected AND though the 6E5 was nice and bright green, it remained "open" while tuning stations in and out, not deflecting. So I'm trying to trouble shoot the problem, which is an AVC issue (?) and I suspect was caused by whomever removed the band switch and re-wired it for AM only. I was trying to read through other ARF similar problems last night until I dozed off. Not having shortwave is livable, but I'd like to at least have the eye tube deflect as it should.

Any help would be appreciated!

You should be able to trace the grid wire feeding the eye tube back to the AVC source node at the
IF carrier stripper filter hanging off the 2nd IF transformer. There is probably a resistor divider pair between the eye tube grid wire and the AVC bus; if so, check these resistors for correct value.
Can check AVC voltage operation with a very high-impedance measurement device like an oscilloscope or VTVM. No AVC voltage=no eye tube movement and audio distortion when tuning in strong local stations.


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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Tue 06, 2018 6:11 pm 
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"Whomever modified/removed the band switch had changed it to receive only on AM broadcast."
At the start of WW2, "aliens" were required to take their SW capable receivers into a shop and have the SW bands disabled. I've seen a couple modded that way. For me they weren't worth the effort to fix the SW bands. Good luck sorting out yours. Maybe you can get AM Broadcast going.

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Tue 06, 2018 8:53 pm 
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WWII & disabling shortwave, yes very likely since this set came out between 1935 - 1936. AM is playing pretty well, though I'd like to see a little more volume. Haven't been in the shop today (honey-do's), but will hit it in a bit. :D

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Wed 07, 2018 3:43 am 
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So here’s what I see:

6G5 grid wire goes to a terminal strip with a .05 cap to ground, the IF lead and a 5k resistor- The other terminal has the other end of the 5k, a .05 bypass cap to ground and wired to the volume control (not center wiper).

The 75 tube has pin 5 cathode with a 80 ohm resistor to ground, a wire to the other end of the volume control and is connected to the 6D6 pin 4 grid, a .05 bypass cap to ground and a 100 ohm resistor from pin 4 to pin 5 cathode and pin 5 connects to 6A7 cathode.

Riders shows pin 4 of the 6D6 is grounded. Was this part of someone disabling the shortwave?

Right now, I have less than -.7 VDC (negative.7) on the 6G5 grid. What / where am I going to get enough negative voltage to have the eye tube to deflect normally?

Thanks

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Wed 07, 2018 4:41 pm 
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Still stumped :oops:

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Wed 07, 2018 5:32 pm 
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Pin #4 of 6D6 is grid #3 and is ok grounded. You need more negative AVC voltage.

Have you replaced the AVC cap? The one far left on the schematic. Value isn't identified but .05 mf will work. Pin #3 of the eye tube should connect here.

It takes good alignment and a strong signal to have AVC negative 10 volts or more. While monitoring AVC voltage peak up IF Transformers.

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Fri 09, 2018 5:22 am 
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Cutting to the chase...
Yes, I had replaced that cap with a .05 and pin 3 of the eye tube is connected there, but less than -1 volt (the deflection issue).
On the 6d6 pin 4 grid 3, it’s not grounded at all as shown in the schematic. It has a 100 ohm resistor to pin 5 cathode and pin 4 connects the 75 pin 5 cathode... Schematic shows 6D6 pin 4 as being grounded! (Yes 6D6 is connected to 6A7 pin 6 cathode).
The 6D6 pin 4 grid 3 doesn’t match the Riders schematic at all.

I went out on a limb, disconnect everything from the 6D6 pin 4 and tied them together as shown in the schematic and then grounded pin 4. Brought it up on a variac, watching the meters and it began playing. Checked the eye tube for deflection and IT DEFLECTS! I’m getting all sorts of static I don’t normally receive in the shop (?). Tweaked the IF a little,watching the eye tube close a bit more on a stronger station. Will do it with a signal generator tomorrow and try to see where all the hash noises are coming from.

Now here’s the strange thing. When disconnecting the cap (.05 I has put there) and disconnecting the 100 ohm resistor and wire to pin 5 of the 75, from the way they were securely crimped on the 6D6 lug pin, you’d have sworn it was done at the factory. Really odd. :? Looking forward to getting back on it tomorrow.

Thanks for the help!
David

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www.sarsradio.com
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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Fri 09, 2018 8:49 pm 
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What Norm says:
Quote:
You need more negative AVC voltage.


Perhaps you are trying to repair the symptom rather than the fault. You have often said "low audio" which leads me to think that you are not generating any AGC voltage - have you measured the AGC line voltage as you tune through a station? Does it vary? If not, you been betting on the wrong horse - as my Dad used to say!

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 Post subject: Re: Packard Bell 46 No Eye Tube Deflection
PostPosted: Mar Sun 11, 2018 12:03 am 
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Two things:

It’s amazing what a good alignment can to. First, re-wiring it per Riders was the ticket to getting correct AVC, plus the alignment was icing on the cake.

Set was working great, then volume would drop. I had checked all tubes on a calibrated TV7, but the last time I had a problem like this, it was the 6D6 tube, so I swapped it with another one... been listening to the PB for hours in my basement shop and it’s playing well, picking up plenty of stations :D AVC on a good station is over -7 volts and plenty of eye tube deflection. Still wondering why someone mis-wired it.

Got a little rattle in the speaker I haven’t figured out yet. Seems aligned well and don’t see a cracked spider, but will keep trying to run it down.

Will update when I’ve found the speaker rattle.

Thanks all.

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Southeastern Antique Radio Society SARS 1991
www.sarsradio.com
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