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 Post subject: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 01, 2018 3:34 am 
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I have been working on my Crosley 628 and am a little confused about the troubleshooting procedure using a signal generator. This five tube radio predates the directions I have. I know to start at the output plate, then its screen, followed by the 1st audio plate, screen. Where would you go next?


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 01, 2018 3:49 am 
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Why not save some time and go right to the CT of the volume control?

http://www.nostalgiaair.org/pagesbymode ... 003527.pdf

Phil

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 01, 2018 4:40 am 
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Hi TE,

NEVER inject a signal from your generator at a tube plate.
The high voltage present on that element may damage the sig gen.
This is true even if you have a DC blocking capacitor in the test lead (as you should).

Always inject at the signal grid (grid 1). You still need the blocking capacitor.
When you no longer hear your generator's signal, you're at the grid of the bad stage.

Start at the grid of the output tube and work back toward the antenna.

Obviously, the generator output frequency must be appropriate for the stage being tested.
That falls into three broad categories: audio, IF, and broadcast transmitter frequency.

- Leigh

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 01, 2018 7:00 am 
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Tube_Enthusiast wrote:
I have been working on my Crosley 628 and am a little confused about the troubleshooting procedure using a signal generator.

What's the matter with the radio to begin with, and have you replaced anything?

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 01, 2018 5:38 pm 
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There are two other threads about this radio...

viewtopic.php?f=6&t=333328
viewtopic.php?f=6&t=328513

Based on the other threads, it appears the audio stage is working since you can get a tone through. There's no info on how far you got with fixing the IF cans. Repeating the above post, you can get better help if you can be specific with the problem you're trying to solve.

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Fri 05, 2018 12:19 pm 
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Tube_Enthusiast wrote:
I I know to start at the output plate, then its screen, followed by the 1st audio plate, screen.

That sounds like the instructions for testing DC voltages........not relevant to testing with a signal generator.

As already suggested, a really good approach for any set is to start at the back end---ie the audio circuits.

Assuming that you have already checked all components, my recommended drill is roughly like this:
--check DC voltages (for now, anything rational is OK--eg if a tube is supposed to have 200 volts and you only get 150, save that for later)
--assuming voltages are in the ball park, touch the grid of the 1st audio tube---or the CT of the volume control. You should get some kind of noise. If not, fix the audio stages first---no signal generator required.
--if the audio is responsive, then touch the grid of the IF amplifier. If not noise, then there is an issue with that tube, or the 2nd IF transformer.
at this point, a signal generator will help---connect to the grid of the IF amp tube, and check for a response from the radio.
--continue towards the front end---move the signal generator to the grid of the mixer
--If you get a response through both IF transformers, then do a "trial alignment" of the IFs---basically just verifying that all adjustments are responsive.

NOTE: "grid" normally means the control or signal grid. Many people will refer to other grids as simply "screen" or "suppressor" to avoid confusion

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Sun 28, 2018 12:47 am 
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Joined: Oct Wed 04, 2017 12:27 am
Posts: 37
Thanks for all of the advice. I had not been able to mess with the radio much since the holidays. I finally got back in it and have had a lot of success but also some new puzzles. The radio in question was massacred by someone decades ago and then mice and weather took their toll. I can get some reception on it now after replacing all of the capacitors, except the micas, and most of the resistors.

However, there are some new issues I don't understand which someone on the forum might recognize. Reception stops above 850 KHz. This made me wonder if the oscillator tube was at issue. To test it I set the radio for 600 KHz and injected a 455 IF signal at the grid of the 6A8 oscillator. This had a strong response but when I rotated the signal generator dial to 600 KHz the response was much weaker. So I tried the same procedure at 1000 KHz and the signal was barely audible. Could this be a weak tube? Another strange thing which I noticed is that if I grasp the glass envelope of the tube, reception is a lot cleaner and louder.


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Sun 28, 2018 2:19 am 
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Location: Victoria, Australia
I think I see an acute methodology fail.

Once I have made the appropriate repairs and made it safe to power; I normally fire it up with a meter on B+. That, on the rare occasions it does not make radio noise, or something fails, gives me an idea of what is going on. If the voltage is excessive that says something is not drawing current: Low short perhaps "Kill switch".

It is then a case of checking all voltages & the wiring. When I cannot get radio noise, My quick check; I fire up the signal generator & oscilloscope. If injecting modulated (Tone on) signal (via cap) into the TC of say 6A8 (signal grid) fails (& switches are in their right position) then one moves to grid 1 of the tube feeding the last IF. If that fails, it is either that stage, or the audio, still has an issue & you fix it.

I calibrate with the Oscilloscope as the meter, on fail it becomes the signal tracer. Do not use too much input signal.

I would check for a shorting gang; However, IF calibration is done with minimal signal into the Pentagrid signal grid (50uV). 600KHz via the antenna terminal & I tend to use around 200pf In series with the Generator (5-12 uV). Where there is an RF amp before the Pentagrid Its tune RF coils just be on the same frequency as those of the Pentagrid: If not it attenuates.

Apart from the voltages on plates (which many modern instruments can't handle) coupling to the plate will throw a resonant circuit off & many IF Plates feed / are part of resonant circuits.

Alignment has pitfalls.

Marc


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 29, 2018 7:23 pm 
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Joined: Oct Wed 04, 2017 12:27 am
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I figured it out! All the voltages look about right. What happened was I had only done a rough alignment of the IF transformer which I repaired. The trimmer capacitors in the can were microphonic. I finally had time to really set them with my signal generator and now reception is outstanding across the dial on either band with no distortion. I can't believe that after decades of -20 in the winter and 105 in the summer all of the tubes are functional. The dial cord was missing, the first electrolytic had ruptured, the 1st IF cap was shattered, the field speaker lines cut and spliced straight together, and a dead short hardwired in for good measure. I would love to work on a radio that hasn't been "worked" on before . :wink:


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Jan Mon 29, 2018 11:21 pm 
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Location: Victoria, Australia
Working on hacked sets is educational, you learn a lot trying to work out "why in the hell they did that". You need to know why its wrong to reach that conclusion.

Bad IF alignment is one of the major causes of "deaf" sets.

Lots learned with that set.

Marc


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Feb Wed 07, 2018 1:35 am 
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Joined: Oct Wed 04, 2017 12:27 am
Posts: 37
I have another question about my Crosley 628. This question is purely mechanical. As I noted earlier, this radio came in pieces and not all of the pieces are necessarily present. I can't figure out how the radio frame is supposed to fasten to the case. No photographs online seemed to show the arrangement. Does anyone on the forum have one of the radios and be willing to explain or post a photo?


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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Feb Wed 07, 2018 3:07 am 
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Ahhh I can answer this one(I have a 628)...

There are supposed to be long brackets with 90* bends on each end, these are secured with screws to front and rear face of chassis... Brackets have threaded openings for screws to hold chassis in cabinet(and maybe feet as well)... I'll guess you're missing the brackets??

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 Post subject: Re: Troubleshooting Five Tube Radio
PostPosted: Feb Wed 07, 2018 3:41 am 
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Joined: Oct Wed 04, 2017 12:27 am
Posts: 37
Okay, I do have the brackets thankfully. Each one has two large holes and 4 small threaded holes (one on each side of the large holes). However, the case only has the four large holes. If you would, 35Z5, could you take a closer look at yours and see how it is assembled exactly. Thanks for the help in advance. It's a really outstanding five tube radio.


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