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Post by mauro on May 12, 2018 14:01:50 GMT
Saturday morning time to fix things!
I'm rebuilding a Seeburg MRA3-L6 amplifier and am planning to use a 1970's cartridge with adapter to fit tone arm, I've done this many times before and assumed 47K to be the proper input resistor to use in the amp. This time I thought I'd get the group's opinion as I read recently a good argument for using a 22K input resistor in this application. Wondering what others have used and whether a small capacitor in series would help?
I've never used the cap to pad the input but thought it was time to ask.
What do you think?
Mauro
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Post by Ron Rich on May 12, 2018 14:15:50 GMT
Hi Mauro,
Depends on which "70's cart." and how it sounds --I don't like using anything but a "red head" Pickering ! If used in a B,C,G-w model, the tone arm springs must be the long style (R-up), or the supports must be "adjusted" to compensate" for the tracking force difference. Ron Rich
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Post by mauro on May 12, 2018 14:40:00 GMT
Good morning Ron,
I've read enough of your posts to know that the springs should be changed. I always keep a few sets handy, but always good to mention it as one never knows?
I'm curious about matching the output of the cartridge to the amplifier correctly here.
I've always used a 47k resistor in place if the 10k at position R7 but recently read that since both channels are now in parallel the input should be half of that. Not sure how sound that advise is so when unsure I come here.
I'm unsure alot!! lol
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Post by Ron Rich on May 12, 2018 16:30:14 GMT
Hi Mauro,
Guess that there is really "no advise" to that--ALL, "aftermarket carts." differ, and some tend to differ with whom assembled them/when. I start with the 47k--but have been known to change it, at the customer's request--Ron Rich
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Post by mauro on May 12, 2018 21:52:00 GMT
Ok, now I've got a problem!
I finished the amp and triple checked my work.
First test the juke played but very treble dominant so I pulled the amp out did a few more resistor checks and checked contacts, all good.
Put amp back in juke and got a loud high pitch squeal and 2amp fuse blew right away.
I'm going to do some voltage checking now but would love to hear some ideas of what this could be from the group.
SOS!
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Post by mauro on May 13, 2018 2:05:09 GMT
Well, I think I got rid of the squeal, there was a loose connection in the speaker plug, it was the jumper from the field coil to ground.
Now I'm back to a very much treble sound. There is also a very slight hum but I think I'd like to deal with the lack of bass issue.
I've checked everything and all checks out good. If anyone could suggest which components might contribute to this it would really help me out as I'm out of ideas.
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Post by Ron Rich on May 13, 2018 2:49:21 GMT
Mauro,
If all resistors/caps are good--something's "dirty"--
Didja clean ALL the switches-- Deoxit works well here ! Did you clean the tube pins ? Deoxit works well here too-- Didja clean the cap on the 6J6 ---? Ron Rich
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Post by mauro on May 13, 2018 12:52:02 GMT
Yes, to all your questions.
I even verified contacts with an ohm meter, including the relay contacts and the input plug. I even tried 23k as an input resistor but it sounded the same with the 340-xx and the Thorpe cartridge. I also subbed in replacement tubes one at a time.
Seems lacking in bass and not enough volume, I get almost no sound until the volume control is turned 1/3 to 1/2 of the way to full.
I'm going to continue tinkering today but all I can do is reclean and recheck contacts on switches, they were pretty dirty!
Really appreciate the suggestions...
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Post by Ron Rich on May 13, 2018 13:10:53 GMT
Hi Mauro,
Relay ?? What "relay" have you found in that circuit ? Didja check that the "squelch" contact on the mechanism was, in fact, making good contact ?
Ron Rich
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Post by mauro on May 13, 2018 16:32:54 GMT
Sorry Ron, no relay, I was confusing it with the amp from my 1700.
But squelch is making contact. I've checked all the switches, they are making and breaking correctly what I did notice is that the bass control has no effect on the sound whatsoever. Turning to all positions sounds the same. The treble control makes a very audible difference in every position.
Will compare now to schematic but still a hum when music is not playing, can't hear it when music playing.
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Post by mauro on May 13, 2018 17:59:37 GMT
Getting closer, solved the hum issue.
Still had poor connection to ground in speaker plug but the high pitched squeal is back now, started after 30-40 seconds of the record playing.
I increased the input resistor to 470K and decreased the value of R46 and it helped a little but volume still has to be at 1/2 to get to a listening level.
Treble still very dominant and bass control still has no effect.
Looking for suggestions here.
Thanks, Mauro
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Post by robnyc on May 13, 2018 20:52:59 GMT
Mauro, I have a few suggestions/tests.
1) Understand that the later cart has much lower output than what the circuit was designed for. The mono red w/spikes was approx 30mv vs 8-10mv for the 345 and even that number was based on original Pickering 345 with beryllium cantilever. Generic's will have even less output -often as low as 2-3mv. However, the bass control should be effective till about 3/4th of maximum volume rotation even with low level.
2) it is possible that the bass choke is open. That choke is variably bypassed with resistors connected to the bass switch. Set the bass to max and jump the bass choke. You should hear a big change with a lot of mids and highs and these will vary a bit in level w/bass setting. If you see this result I'd check the bass choke for continuity (if you have not already). If still no change with bass setting, look at the pots and jumper.
Even with original styli in the 345 when used in those old mono circuits, I always had to either bypass the existing pre with a newer type (K-201) or build a new pre with higher gain using a 12AX7 or 6SN7 in the case of octal sockets. When the other issues are settled, if you are interested in going this route, here is the typical schematic I've used for 12AX7 and similar values can be used when connected to the correct pins on 6SN& or 6Sl7. I may have a preamp schematic specifically for those tubes in an old tube manual.
RobNYC
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Post by mauro on May 14, 2018 0:15:11 GMT
Rob,
I jumped the bass choke and did get more mid and highs but obviously less bass.
I did some voltage checks and found about 32v across C20 the AVC capacitor, it's my understanding that the voltage here should be 17 to about 23.
I've got a couple of other off voltages but they dont make sense, pin 3 of the 6sl7 shows there should be 0.5v and I get 30v. Pin 3 of 6sn7 again shows 0.5 on schematic and I get 45v.
Still can't figure out why I dont have bass?
You're cartridge output explanation makes alot of sense for overall volume but would it affect the bass?
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Post by robnyc on May 14, 2018 7:42:43 GMT
I'll see if i can help here:
1)" I did some voltage checks and found about 32v across C20 the AVC capacitor". That is just over the typical cathode voltage and it represents either a very loud record with the mono spikes (not used here) or some other fault. It may be that the squelch voltage is not being removed during play. Select slot w/no record. Measure C20 in quick hits to avoid loading that very high impedance point and distorting the readings. The voltage should drift down to near-zero after about something like 15 seconds, the time isn't important. If it stays anywhere near the initial reading there is a fault. Check that Pin 2 on 6SL7 goes to ground during play. This is necessary to change that tube section into an AGC rectifier for the incoming signal from the record.
2)"pin 3 of the 6sl7 shows there should be 0.5v and I get 30v" That will happen in conjunction with either squelch or a fault in the circuit. The near-zero is what you should see only in play but with no record as in the test I suggested. Possible causes of a errant voltage are a leaky 0.01 (IRRC) that goes from Pin 8 to 4 on 6SK7. Again, this can be caused by the squelch remaining present when it should not.
3) " Pin 3 of 6sn7 again shows 0.5 on schematic and I get 45v" I think you or the schematic may be mistaken here. Pin 3 on Sn7 is the cathode follower and should have either 45V (MRA3) or 90v according to the MRA5 print.
4) "You're cartridge output explanation makes alot of sense for overall volume but would it affect the bass?" Absolutely, because it affects the position of the volume control. As you set it higher, the bass boost goes down. However, I think there is more going on here -particularly in the AGC/squelch section.
I didn't mention removing the AGC tubes (Sl7 & SK7) because in a normally functioning circuit and using that newer cart, the AGC will often have too little input to have any effect.
RobNYC
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Post by mauro on May 15, 2018 23:41:52 GMT
Guys, I will be off this amp for a few weeks as I'm going in for shoulder surgery again tomorrow.
Once I'm back up and around I'd like to continue this as I feel I'm close now.
Rob, I'd love to see a schematic/plans of the preamp you build to use in this application as I have a few of these 345-xxx cartridges and was planning on converting my Seeburgs.
Thanks, Mauro
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