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Philco 620B alignment woes - Printable Version

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Philco 620B alignment woes - wonderwrench - 06-19-2008

I was given a model 620B cathedral as a gift. I was told the set had a recap recently and was in good working order. I had a look and found all paper and electrolytic caps had been replaced. I powered it up and it does function though very poorly. (using a long wire ant BTW) I'm no old radio pro but to me it looks like who ever did the recap was a newbie or a hack. At this point I checked all tubes using a HICKOK 533 and found all tubes check good.

By looking at the trimmers they had been played with recently, not a good sign if you ask me considering the poor performance. So an alignment check was in order. I found the dial scale set screw loose so I need to calibrate the dial scale to the tuning cap before proceeding with the alignment but can find no info on how to go about doing so. I downloaded the Rider's manual but find no info on how to go about this.

Any help appreciated.

- wonderwrench - 06-20-2008

no bites yet. bump

- BDM - 06-20-2008

Bill, just align the lighted pointer, with the first line of the bottom edge of the scale. Another words, the first line after the last freq indicator dot for lack of a better word. Here's another thing I've learned. My 650X wasn't aligning up originally as I liked. I "tweaked" the dial scale just a hair, and the alignment went very smooth. I've had to do that on several sets in the past. A little fudging can go a long way Icon_wink Anyhow, that's where I would start.

- wonderwrench - 06-20-2008

BDM Wrote:Bill, just align the lighted pointer, with the first line of the bottom edge of the scale. Another words, the first line after the last freq indicator dot for lack of a better word. Here's another thing I've learned. My 650X wasn't aligning up originally as I liked. I "tweaked" the dial scale just a hair, and the alignment went very smooth. I've had to do that on several sets in the past. A little fudging can go a long way Icon_wink Anyhow, that's where I would start.
Does the bottom edge? = low end of the scale? That is what I was guessing but I wanted to be sure. Thanks

- BDM - 06-20-2008

Yes, here is a pic. Align it where I'm indicating the thick black line.
[Image: http://antiqueradios.com/gallery/d/78128-1/620+dial.JPG]

- wonderwrench - 06-20-2008

Got it! thanks much.

- wonderwrench - 06-21-2008

More alignment woes. As I suspected who ever worked on this radio had no clue. The alignment was so far off it was a nightmare to perform. Once completed the radio performs much better, not great but good. I did uncover two problems.

First the RF trimmers act strange. It is not possible to peak any one of the three trimmers during the alignment. From tight as I loosen up the trimmer I get a rise at about 1 half turn but never a peak. I ended up leaving all three RF trimmers on the tight side of the excessively wide peak. Possible cause a bad cap in the RF circuit? Your opinions please.

Secondly the radio's AVC curcuit seems to work poorly or not at all. Could be related to the first problem? Your opinions please.

Thanks

- BDM - 06-21-2008

Bill, antenna connection are you using? The post on top of the chassis, or the screws on the back of the chassis?

- wonderwrench - 06-21-2008

BDM Wrote:Bill, antenna connection are you using? The post on top of the chassis, or the screws on the back of the chassis?

I'm using the post on top. I just found an intermittent problem. On rare occasions the audio will drop down. I did some poking around but the problem went away before i could find the cause. At one point I thought I found a intermittent at the 75 tube. The darn thing refuses to act up.

- BDM - 06-21-2008

If I remember right, the 75 has had issues like that. Hopefully someone else will comment on that tube further. Are you sure you have the local oscillator adjusted high, and now low of the carrier freq? Do the antenna trimmers peak?

- rghines1 - 06-21-2008

I have been hit a couple of times with a bad 75 tube but don't recall it being intermittent. The tube soon loses gain after a few minutes of warm up. Someone suggested resoldering the heater pins but keep forgetting to give that a try.

Richard

- BDM - 06-21-2008

Bill, has the set been fully re-capped? Did you check the values against the schematic. I see cap #23 should be an .05/mf. Is it? I could see that cap being off or bad, effecting all the RF trimmers. It sounds like the capacitance is too high, and you're not able to trim low enough. Someone please correct if I'm wrong.

- wonderwrench - 06-23-2008

BDM Wrote:If I remember right, the 75 has had issues like that. Hopefully someone else will comment on that tube further. Are you sure you have the local oscillator adjusted high, and now low of the carrier freq? Do the antenna trimmers peak?

I have not had time to work on the 620 but hope to later in the week.
"Are you sure you have the local oscillator adjusted high." I think so but I'm not sure of anything yet. "and now low of the carrier freq?" ???.
" Do the antenna trimmers peak? Yes, all trimmers act normal except the 3 RF trimmers.

- wonderwrench - 06-23-2008

BDM Wrote:Bill, has the set been fully re-capped? Did you check the values against the schematic. I see cap #23 should be an .05/mf. Is it? I could see that cap being off or bad, effecting all the RF trimmers. It sounds like the capacitance is too high, and you're not able to trim low enough. Someone please correct if I'm wrong.

With the limited time I have had to investigate I'm not 100% sure. I have not pulled the Bakelite blocks to see if they have been re stuffed but from the top side it looks like they were. The only caps I see that have not been replaced are a few mica caps. All other caps including the electrolytic's have been replaced. I need to set a day aside to verify all component values and confirm their correct installation. This radio is such a basket case its blinding me. Cap #23 sure does look suspect after you pointed it out, thanks BTW. I will check it out for sure. Could the resistors that are in parallel with #23 cause a similar affect if they were the wrong values?

The last time I was looking it over I determined its a early model 620 because the B.C wirewound (#70 on the early diagram) resistor only has 2 sections not 3 as the later models have. I also found that the volume control was replaced with the wrong part as it has no tap for the tone control. I have the correct part so no big deal.

rghines1 thanks for your comments also. My father was a old radio guy and he claims the grid cap solder joints are problematic on 75's. I take this with a grain of salt as my father is not the god he once was. It's H**l getting old. I wish I could turn back time.

- etech - 06-23-2008

I don't have first hand experience with the 75 tube problem. But, I have read elsewhere that it was in fact common to have a bad solder joint on the 75 tube grid connection causing intermittant or fluctuating audio levels. Your father may be right! I would re-solder that grid cap.

I think Brian mentioned this earlier, but check to make sure that you have the local oscillator running on the high side of the received signal (high side injection). On many radios it is possible to mis-adjust the oscillator to the low side of the signal (low side injection). Low side injection will cause the problems you are having with alignment.

Regards,

Ed