Bottlehead Forum
General Category => Technical topics => Topic started by: porcupunctis on August 24, 2013, 08:38:28 AM
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A young lady in my Trig class brought in an old record player a few days ago and asked if I could fix it. I told her that I would take a crack at it. When I opened up the case I was amazed by two things. First, it is a very simple design and second, they schematic is plastered to the bottom of the case.
When I checked it out, I found that the whole thing was basically functional. Motor runs, speed control, power and volume all to their thing but the overall volume barely overcomes the sound of just the needle in the groove. It's not bad sound, just not much sound.
The tube tested bad in my TC-3 so I replaced it along with the filter caps (I had a few electrolytics laying around the 50/150 was replaced with a 47/450 and the 30/150 with a 33/450). I then gave the potentiometer and socket a good dose of Deoxit and figured I was good to go. When I tested it again there was only the slightest improvement in volume. A little louder but not the way it should be.
My question now is this: Could it be the Selenium Rectifier? I could pull it out and test it but I'm not sure exactly what to look for. Should it test exactly like a diode? or not?
So here is the player and the schematic:
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All good ideas to start with.
Selenium is bad stuff, so you could get rid of it on general principles. If the rectifier is bad you will have no or low voltage on the capacitors.
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I pulled the Selenium Rectifier out of the circuit and checked it with the ohmmeter. I got 250K in one direction and 500k in the other. That didn't seem like much of a diode, so I replaced it with a 1N4007.
With a new tube, new filter caps and new diode, the sound is stronger but it still has a long way to go. The schematic shows a connection to pins "2-5" on the tube but pin 2 is clean like it's never had anything connected.
I'm now starting to suspect the output transformer as a possibility. Not much else to check at this point.
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Occam's Razor....check the stylus.
Cheers,
Geary
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Geary, good point. Soooo, how do I actually "check" the stylus? Is there an expected voltage I can check with the oscilloscope? Do I need a microscope?
I think I've been staying in familiar territory while ignoring the obvious.
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The stylus is replaceable. There should be a conical nub at the end of the cantilever. A magnified look will be helpful,
Although the cartridge suspension has likely hardened and effecting playback.
Since they were designed as a "system", output voltage was all over the place on old phonographs. But you should see somewhere around 50-500mv. Wide range, I know, but don't know the spec's or whether it is a ceramic or crystal cartridge.
Cheers,
Geary
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OK, looks like we're on to something here. My scope is only reporting 26-38mV. That would be on the order of how much the volume is below what I think it should be.
This model has a cartridge with a stylus on each side. You can flip it around by way of a handle that protrudes through the front of the tonearm. One side is for 33/45 and the other side is for 78.
I guess it is time to go to the needle finder.
Thanks for the info, Geary.
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OK, looks like we're on to something here. My scope is only reporting 26-38mV. That would be on the order of how much the volume is below what I think it should be.
Remember that is still a "healthy" output, (but in a totally unreferenced way!), it really depends on the gain of the amplifier. Does the stylus(li) look OK?
Cheers,
Geary
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Can you post a picture of the schematic, and check the tube pin voltages? Armed with those bits of information, the Forum denizens can identify anything grossly wrong with the electronics part.
Those flip-over cartridges were ceramic all right; that means no EQ and less gain needed, making the circuit simple. The wikipedia article "Phonograph" has a section on these cartridges. Since it's old, the needle is probably worn - replacements are available and should be inexpensive.
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One other thing to note. Selenium diodes had a bit of internal resistance inherent to the design. Simply substituting with a modern silicon diode may rectify properly but pass too much voltage. A series resistor may be required.
More here...
http://w3hwj.com/index_files/RBSelenium2.pdf (http://w3hwj.com/index_files/RBSelenium2.pdf)
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Here are the voltages on the 25C5 tube pins:
1. 0.5 VDC
2. Not used
3. 28.5 AC
4. -0.2 VDC
5. -0.2 VDC
6. 21.8 VDC
7. 23.0 VDC
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I am obliged to point out that this thing is not isolated from the power line and is thus dangerous to use and more dangerous to work on.
Excellent help with the (theoretical) problem of why it is not working, though!
You should have around 135 volts on pin 7. The much lower measured voltage indicates that the power supply is not working. Since the 220 and 5600 ohm resistor are the only components in the power supply you have not already replaced, you should probably check them, as well as the orientation fo the parts that have been installed (a backwards cap will not do good things, even before it explodes).
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Paul, I appreciate the safety warning. I've been very careful with the probes when testing and always check that the power plug is unplugged (twice) before doing anything inside.
I checked the 220 ohm resister while I had it out of circuit during the rectifier replacement. I will check the 5600 when I pick up the fight tomorrow.
I do appreciate everyone's help. It will be fun to get this thing working again.
As always, I'm learning a lot thanks to the awesome Bottlehead forum.
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Looked up the data sheet:
http://www.shinjo.info/frank/sheets/093/5/50C5.pdf
I see spec operation is with 7.5v bias at 50mA, which would be a 150 ohm cathode resistor. This circuit has dropped the screen voltage quite a bit (with the 5600 ohm resistor). I speculate that this is in order to get away with a lower grid voltage, and hence a lower required cartridge voltage - very clever!
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I will reinforce that warning for two reasons -
One, is it's not our product and we can assume no liability for any well intended but inaccurate safety info you might receive here.
Secondly, I worked on an ACDC chassis myself a few months ago. The cautions routinely given should be heeded, as these things are very tricky. On the radio I restored the design was such that the chassis is hot when the radio is switched OFF. Not only did I have to use care in working on it, I had to instruct the owner to never remove the plastic knobs and touch control shafts. So make sure you thoroughly understand the setup, either by getting the service manual or by doing some careful measurements of the chassis potential with the unit both on and off.
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Are my eyes getting tired, or are the markings on the diode in the schematic the opposite of what you would expect? Isn't the cathode on the bar side?
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Welcome to the wonderful world of electronics... the cathode goes to the positive capacitor terminal, which is correct. It's not usual to use + and - on diodes these days. This is probably why!
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Is this another one of those things we can blame on Ben Franklin?
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I think Ben was a DC guy like Edison. But maybe!
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That thing looks older than the hills. I'm enjoying the heck out of this - don't stop until victory isin your grasp! 8)
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I fully intend to keep up the fight until this thing sings once again. I just can't get anything done during the week while I'm teaching. I'm sure I will be back at it this weekend after grading papers and coaching a cross-country meet.
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OK, got some new parts in and spent part of my weekend basically rebuilding the whole system. The only items that I did not replace at this point is the stylus/cartrigde, the output transformer and the switch/potentiometer.
My voltages are much stronger, but still not up to spec and we're still not actually amplifying. It does seem to pass whatever input it gets to the speaker, though.
Here is the schematic with voltages labeled. All voltages are DC unless noted with VAC. I have to think that the only remaining culprit would be the output transformer.
Here it is:
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Per your posted voltages, it looks like it is working fine. Keep in mind that the tube has to drive the speaker as well as provide gain. I am too lazy to look up the tube. Try driving it with a high output line stage, like a Foreplay 2 and see how that works...John
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Unfortunately, it isn't really working. Paul Joppa mentioned that I should have closer to 135v on pin 7 and I'm a bit shy on that. I even hooked up my signal generator to the input and go no amplification whatsoever. It basically sounded the same if I hooked the generator up to the inputs or just straight to the speaker. It is passing but not amplifying the signal.
I will probably swap out the output xformer with a speco 7010 (old style) when I get a chance. May be a few days.
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I wouldn't bother replacing the output transformer, you can generate 60Hz and measure the input vs. output voltage (AC) to see if the ratio still looks good.
The 25C5 is drawing a pretty healthy amount of current, and it looks like you have ~254 Ohms of resistance on the primary of the output transformer, which is generally a good sign.
Looking at the 25C4 datasheet, there's an operating point that is 110V plate, 110V screen, 7.5V bias, 4ma screen current, 50ma plate current.
This isn't horribly off from your datapoint, the decreased screen voltage will raise the gain slightly. With the book operating point, you get 1.9 watts output, which would be 3.9V into 8 Ohms (2.75V into 4 Ohms), but with more voltage needed at the grid of the 25C5 to get you there (the output transformer step-down isn't helping).
You could have a bit of a dead 25C5, did you try another? If you dropped the screen voltage even lower, I would expect gain to go up bit. Also, a ceramic phono cartridge can have in excess of 1V of output, which would not require much voltage gain from the 25C5. (Frankly, if the ceramic cartridge was removed and replaced by a moving magnet cartridge, you'll really spin your wheels trying to get the player to work properly)
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You did replace the 30u & 50u caps?
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Yes, every cap and resistor was replaced. I have three tubes. The original plus two NOS tubes recently purchased. Could be the ones I bought were not all that good. I will do some tube swapping Friday night.
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The Selenium rectifier as well?
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Rectifier got replaced with the caps. The only remaining original parts are the socket, switch/pot, and the cartridge/stylus.
Last night I connected the cartridge directly to a rat shack "Archer" 9v amp and got a little gain but not a lot and it was pretty noisy since it was not properly grounded. This at least tells me the cartridge is putting out some signal. Just do not know if it is up to spec or not.
I have had one online vendor tell me I would not be able to find a new replacement cartridge. Buggers!
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If it is set up for a ceramic cartridge you can feed it one leg of your CD player's output to test it.
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You could try this circuit with the TL-431. You won't need a transformer, but you will need the volume control. I'd set this up to drive the 25C5, gain will no longer be an issue.