Author Topic: Loss of reception at top of the AM band Magnavox Regency  (Read 853 times)

bastardbus

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Loss of reception at top of the AM band Magnavox Regency
« on: August 11, 2016, 11:44:51 AM »
   
Was going over a `48 Magnavox Regency Symphony model 155B. This console consists of a stand alone amplifier, a control unit with AM and SW as well as an optional plug in FM tuner and separate phono turntable.


-The amp is re-capped and all resistors-tubes checked and is working great.
-FM re-capped and resistors gone over, alignment done and sounds great as well.
-A turntable plugged in sounds GREAT too.
-The main control unit with AM-SW was also re-capped and all resistors checked as well as tubes. Pots cleaned etc. It also was working GREAT! then while listening to AM it started to fade on me. I have almost completely lost any station above 1000khz. Stations below will still come in and sound decent but once above 900-1000khz it quickly fades to almost silent. You can hear one or two stations if you crank it to full volume and listen closely in the background. If let sit idle you will occasionally it will give a loud crackle and pop in the upper band if left sitting idle with the volume up.

So the phono and FM both perform perfectly. The issue isolated to the upper AM band and it WAS working beautifully so I know it is not an alignment issue. It has to be a tube or component guessing. The unit is a monster and upstairs in a back room while my work area is in the basement. I will need to yank the amp-control unit and two 12 inch speakers (field coil) to drag it downstairs to trouble shoot it on the bench. Before I do that I was hoping someone might have tip or two for places to look before I have to resort to all that work. I am getting a bit of a SMD vibe. I have yet to check voltages and DC on sec of IF trans that is what I planned next. Tips, comments appreciated.

Here is link to the Riders Schematic http://www.nostalgiaair.org/Resources/322/M0010322.htm


T

Motorola Minion

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #1 on: August 11, 2016, 12:46:16 PM »
Ill take a SWAG guess its the 6SA7 tube if you replaced the paper caps (part 19 on Riders) in the osc-converter stage
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Dave

electra225

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #2 on: August 11, 2016, 01:37:12 PM »
My SWAG would be alignment, particularly oscillator alignment on the tuning cap.  It may not be tracking properly on the upper end of the dial.  I agree with the previous comment, I'm just offering another prospective.  Good luck.
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bastardbus

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #3 on: August 11, 2016, 02:28:27 PM »
Thanks for the ideas guys...here is replies.

All caps replaced and I check em before install.  All tubes are fine, tested and swapped with known GOOD working tubes from another magnavox.

Alignment...I would say NO as if you read the unit played perfectly for a couple hours before developing the problem.  Thus alignment was dead on and it just does not change without someone fooling about or changing components...again after all the work done it played unbelievably well so alignment is-should be fine.

Here is a post I made on the ARF about an idea I had.  Maybe someone else with a Maggie of this era might have had the same issue as well?

Quote
I have another Magnavox here, same era and similar design though a lower model. When I purchased it AM worked great but no SW. Upon a recap-service I noticed the SW had been completely disconnected from the elaborate wafer switch mechanism this unit had. Curious and wanting SW to work I hooked it back up. Neither worked and I noticed DC leaking between the AM and SW band apparently through someplace on that wafer switch. Disconnect one and the other worked.

I am wondering if that is the same problem with this unit. It has the same wafer switch set though more complex.

I checked voltages and they seemed fine, resistors all ok.

I have it sitting here on AM right now.  It is near silent...station very faint in background volume is up VERY high.  I will occasionally hear a POP or crackle every minute or so.  Like DC leaking through someplace.  Reminds me of SMD or bad cap.  Though it could be what I mentioned above, I recall it was similar symptom.

Hmmmm

electra225

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #4 on: August 11, 2016, 02:54:12 PM »
Like the response from ARF, I have had experience with a similar issue.  Not on a Magnavox, per se, but on another radio.  If that instrument had not been used in several years, the alignment might drift.  It is almost 60 years old.  Stuff sets around, collects moisture, ozone corrodes terminals.  And those mica sheets in the tuner may get compromised.  When you warm up the chassis and run it awhile, component values change, thus changing the alignment.  You may not have allowed the instrument or the test instrument to warm up before you did the alignment.  Several things like this could take place.  It seems to me that alignment is an easier and less intrusive condition to check as a first step before you go into SMD repair.  The guys on ARF really like to talk about SMD, so I understand that.  I have never had a case of SMD in a Magnavox, for what that is worth.  Not saying it can't happen, I've just never experienced it.  Converter tubes are a finicky lot.  Just because a tube will work in one receiver is no guarantee it will work in that one.  Try several different 6SA7's and see what happens, if you have them on hand.  Good luck.
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bastardbus

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #5 on: August 11, 2016, 05:44:33 PM »
That was actually my post I copied from the ARF site.  I actually got no replies over there  ::)

This not being my first Rodeo, I am quite anal about when I do alignments I actually am the only guy around here that does FM alignments as well and do a couple a week for folks.  FM is quite a bit more entailed then AM so I am good there when I do one.  On the alignment point, I think I mentioned I have NOT done one to the AM (FM yes I did but not AM or SW).  Reason being she was performing so well off the bench there was not a need to.  And I must politely disagree about the alignment just changing so radically-suddenly after sitting.  If it does, it is because of a component issue.  Since the unit performed so well for hours and suddenly when south, I am not going to start messing with just adjusting the alignment right off the bat.  I need to eliminate any and all other factors first, which I have been doing just now. 

Here is update....

Voltage checks and Resistance checks via Sams folder.  Voltages are good and all resistance measurements in specs except Tube 1, Pin #4 is OL and it says it should read 1.5 Meg according to Sams. Looks like the tuning capacitor and secondary of the antenna coil.  Going to look into that now after my break here.  Now I always take Sams folders with a grain of salt as 3 out of 5 of them have mistakes I have found.

No SMD which is good.  I had the same feeling about SMD, actually thought it a bunch of hog wash until I came across 2 radios in one week with it....!  Both were RCA AA5s.  I actually did the "fix" and got them back up and working perfectly.  It is not an old wives tale...it does exist and SUCKS to deal with.  You check for DC on the secondaries of the IF transformers.  Should not be any there if there is the SM caps are leaking.

Going to look into this tuning cap situation next...

T

electra225

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #6 on: August 11, 2016, 07:08:12 PM »
I'm familiar with SMD.  My point was that someone "discovered" SMD and everyone else feels left out unless they have the condition themselves.  My procedure to fix SMD is to replace the offending IF transformer.  Good luck.
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bastardbus

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #7 on: August 11, 2016, 08:44:48 PM »
Yeah...finding a correct fitting IF trans ain't that easy.

Figured out the missing 1.5 meg measurement mentioned above.  I took the measurement without the antenna connected and that was where the 1.5 meg went. Was dead on with antenna installed.

Anyhow...for shits and giggles ran through the alignment.  The IF was spot on.  Oscillator was a kinda funky can.  This is a later chassis not covered entirely by the Sams or Riders.  This one has slightly different osc tran can.  Three padders-trimmers down the side.  Seemed to find which was which.  The low one peaks at quite a bit below 600 and is at the end of the adjustment...hmmmm not right.  The top 1400 was peaked dead on at 1400 but the padder seems to not do much adjustment.

Was wondering if the osc was working at all and verified it is with another radio next to it but I think the issue is in lurking in there someplace.

I also went through and tested every single mica cap for value and leakage.  All were good. WHEW...I think I have checked every single component in this thing twice now. LOL  Was wondering about that 10K whistle filter it is about the only component in this bastard I have not checked.

The pisser is this thing was working beautifully and now...not.  I am getting in 760 strong as can be and a few stations up across the band until about 1000 and then she is pretty much silent.  You can hear the radio is working.  The tuning cap is not dead shorted or anything.  I can get 1370 (strong Toledo station) very very faintly with the thing cranked.  It is like something is loading down the top end of the band.


TC Chris

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #8 on: August 11, 2016, 08:54:48 PM »
How about the tuning cap grounding?  Most use a spring brass device as you know.  But the contact can get dirty and go high-resistance.  Try cleaning up the contact area between the gangs.

This problem usually leads to intermittent performance across the dial, but it's easy to check.

Chris Campbell

bastardbus

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Re: Loss of reception at top of the AM band Magnavox Regency
« Reply #9 on: August 12, 2016, 01:26:01 AM »
Thanks for the out of the box idea Chris.  I will try that. 

This thing has the crazy push button tuning assembly that gets sticky.  I lubed that all up pretty good but will check into that.  I pondered something with the tuning cap assembly but just did not see anything out of the ordinary at a glance.  The darn thing is I am getting in stations in certain areas of the dial real nice and strong even on SW.

This has been a real head scratcher.