I am working on this 78 Phonograph

I used the first schematic on this page as a guide and drew up the second. I was experiencing a signal from the plate of the 12SQ7 that was noisy. The plate looked as if another signal was being modulated on it. I was asked to check the filter caps, electrolytic in a can. In this case a 20 and 40mfd. I put a 20mfd 450VDC across the 20 first with no change in the signal. I put two 20 mfd in parallel and attached them. I am certain I observed the correct polarities, negative to the chassis and positive to the circuit. The signal looked much better but then the set died. I somehow destroyed the half-wave rectifier 35Z5. AC in but the output was zero. I removed the three tubes and tested them with a Precision 640 Cathode conductance tube tester. The 35Z5 was bad. All other checked good. I have a spare 35Z5 but am reluctant to install it without some reason for why the first one was destroyed. If you have a moment please look over the schematic and let me know what you think?

I have no information about this phonograph ie model number, name of manufacturer.

Thanks
Rick Churray
Mathews, Virginia

http://phonoold1.tripod.com/my_old_phonograph.htm

I hope the above url gets posted. For some reason it did not in the last post.

You should remove the old electrolytics from the circuit. Then insert the new ones. Paralleling old electrolytics is kind of bad practice. Old electrolytics fail in several ways, and are rather unstable in the failing mode--they can do other things as well. From what I see in your schematic, the first one has a value of 80 MFD. 40 is fine, though. Changing the values certainly shouldn't affect the rectifier. If anything, 80 will create more of a surge when turned off and back on, with the set warm. It is possible that the old ones shorted out when you inserted the new ones. The new ones would have raised the voltage if the old condensers were failing. With the set as-is, check resistance from the rectifier cathode to ground. If a dead short, remove old electrolytics (which should be done anyway). If still a dead short, you have a short elsewhere in the set, possibly even in the new electrolytics. Make absolutely sure that you are inserting them properly. With fresh electrolytics in place, and no other shorts, your meter should swing up to zero, and then slowly fall to infinity. Leakage of other condensers in the set may keep the meter up a bit (thousands of ohms to millions of ohms, in the case of this amplifier).

If you are able to correct the short, check the audio again with fresh electrolytics in place. Be sure to replace the 20 MFD unit across the 50L6 cathode bias resistor, too. This will improve audio quality. If distortion exists, or becomes evident after perhaps 15 or 20 minutes, your audio bypass condensers are leaky. C3 is the most important one. After that, C2 and C4 can cause distortion as well, or at least a little inefficiency. Never parallel the old condensers. Remove them and put in fresh ones. Leakage should be infinite. There shouldn't be a trace of leakage. Leakage that shows on your meter as needle displacement by only the thickness of a hair is enough to cause distortion. The needle should swing up just a very small amount with condensers above .001 MFD, and then should fall back to EXACTLY where it started. If not, then the condenser is faulty, and should be replaced (this doesn't quite hold true for electrolytics...they can have a small amount of leakage in the millions of ohms, and still function fine in their circuits). Many radio restorers simply replace all old paper condensers with new units, such as the poly film units sold at www.tubesandmore.com or www.radiodaze.com.

If strange hums still exist, you could have faulty wiring in the tone arm, or faulty shielding. Other strange distortion can be caused by a weak or gassy tube (the audio tubes, most likely).

Once you get the amplifier working properly, it'd be interesting to see how it sounded with R4 removed. This is an awfully low value resistor, and will cut out a lot of bass from the music.

Don't use this phonograph in moist areas or outside. You'll surely get a shock from the tone arm, even with rubber soled shoes, if your feet happen to be sweaty, or you happen to be standing in moist grass. I've had a few of these sets with the AC-DC amplifiers (motor is of course AC only), and they all give nasty shocks from the tone arm when in moist locations. I have fair luck when standing on concrete, but have received shocks there, too. The best bet is to plug in the set with the set on, and then measure from the chassis to ground with your meter. Find the plug position that creates no voltage on the chassis. Mark the plug, say, with a black mark (or red) on the hot side. You can also replace the cord with one that has a polarized plug, and then put the switch on the 35Z5 side of the line cord. Normally I'm not about this method, but with the exposed tone arm, if you plan on using the phonograph in, say, yard parties, it's a good idea. Then, too, you must be absolutely sure that you're plugging it in properly if you use a non-polarized extension (some extensions can be plugged into very easily with polarized plugs even though the extensions are not polarized). If you wish to retain an attractive cord and plug on the set, simply mark the hot side with a black or red mark. Then always plug the plug in with the red mark facing the small slot on the outlet. Modify the on-off switch if you wish. With the switch as-is, it'll make the chassis hot when off with the plug inserted correctly, and hot when on with the plug inserted incorrectly. Placing the switch on the hot side will keep the chassis from becoming hot whether on or off, with the plug inserted properly. Keep in mind, though, that if the plug is inserted improperly, the chassis will be hot no matter which way the switch is turned. If you modify the switch wiring, be sure to plug in the set properly every time.

Typically, modifying radios so that they're not original, isn't something that radio restorers do. However, if you ever desire a small pilot light gem, it can be done with this set. The first task is to find an attractive gem like those found on the old guitar amplifiers (perhaps red or orange), but not as large as those. This can be mounted below the speaker. Then, rewire the 35Z5 with pin 2 on the line cord side and pin 7 going to the other tubes' filaments. Connect pin 5 to pin 3. Connect a #47 pilot lamp across pins 2 and 3 (sockets and bulbs can be found at Radio Shack, though buying bulbs through a radio supplier may be more affordable...if purchased with other items). Once you make this modification, be sure to replace the pilot lamp whenever it burns out (doesn't burn out very often...many years). Not doing so will put extra strain on the 35Z5 filament, and will burn it out within a matter of months or less.

Thomas

Oh...I see that you said that the schematic on your web page is possible, and not definitely from that set. In that case, I see why you say your set has a 40 MFD on the rectifier, and not an 80....the schematic is for a different set of similar construction.

Thomas

Hello All,
Still at this old phono as time permits. I have pictures and a schematic here
After replacing V3. I installed new filter capacitors in place of C1. I have checked the voltages at V3 pin 8 and V2 pin 4. V2 pin 4 is reading 120vdc with less than 1 vac ripple. This appears to be good! V3 pin 8 on the other hand has 140 vdc with a sawtooth wave riding on it. max =156 min= 136. Is this normal? Any suggestions?

Yep. All is well. What I wrote you before still holds true...that it's okay to tap the speaker directly off of the rectifier, even with this ripple, because the signal won't get amplified anymore past the speaker. However, now that I've looked at your schematic again, I see that there's a field coil in there, and that even the speaker is connected after the field coil, so it will receive the same smooth current that the rest of the amplifier is receiving. Even better! With what you say, your amplifier's power supply is filtering well. It is normal to have a fair amount of ripple at the rectifier. The ripple after the field coil is great--very small.

How does the speaker actually sound? You can test out the amplifier's audio capabilities with a portable CD player. Take a headphone wire (you can buy them at Radio Shack...make sure that you get a stereo one) and connect it to the cartridge wires. The L and R wires should go to the top of R1, and the shield should go to R3. Turn the CD player up to about 7 or 8 (on a scale of 1 to 10). If there's distortion, turn down the player a bit until the distortion goes away. Then use your phono's volume control to do all of the controlling. Sometimes stereo and mono CDs will distort when the channels are mixed directly. If so, disconnect either the L or the R wire. If you're playing a mono CD, you won't miss any of the audio if you disconnect one of the channel wires, so try to play a mono CD (like a CD of Glenn Miller or Harry James recordings).

Thomas

Hello All,
I did as you suggested with the CD player and the amplifier works fine the tone sound good and I am happy with the amps operation at this point in time. I will be sending the cartridge and idler wheel out the be refurbished and then I'll put the whole package together and try a record to see how it sound then.

Thanks all of you for the help on the old un identified 78 Phonograph.

All Blessings
Rick Churray
Mathews, Virginia

:Yep. All is well. What I wrote you before still holds true...that it's okay to tap the speaker directly off of the rectifier, even with this ripple, because the signal won't get amplified anymore past the speaker. However, now that I've looked at your schematic again, I see that there's a field coil in there, and that even the speaker is connected after the field coil, so it will receive the same smooth current that the rest of the amplifier is receiving. Even better! With what you say, your amplifier's power supply is filtering well. It is normal to have a fair amount of ripple at the rectifier. The ripple after the field coil is great--very small.
:
:How does the speaker actually sound? You can test out the amplifier's audio capabilities with a portable CD player. Take a headphone wire (you can buy them at Radio Shack...make sure that you get a stereo one) and connect it to the cartridge wires. The L and R wires should go to the top of R1, and the shield should go to R3. Turn the CD player up to about 7 or 8 (on a scale of 1 to 10). If there's distortion, turn down the player a bit until the distortion goes away. Then use your phono's volume control to do all of the controlling. Sometimes stereo and mono CDs will distort when the channels are mixed directly. If so, disconnect either the L or the R wire. If you're playing a mono CD, you won't miss any of the audio if you disconnect one of the channel wires, so try to play a mono CD (like a CD of Glenn Miller or Harry James recordings).
:
:Thomas