Phonoland

Hi

I was playing our A for about 2 hours and everything was operating fine. I shut it off and changed out an album. When I turned it back on no sound If I hold my ear right by the stylus I can hear a faint sound. I have pulled the mute plug and no sound. I have checked the tubes and find the 12AX7 tube and 5879 tube to be questionable.

Would these tubes cause the sudden unexpected loss of sound like I had?

Any other ideas where to trouble shoot the A?

Best
Darrell

Amp is MRA 4 L6 so per.trouble shooting guide with record playing I pulled the 5879 tube and quickly reinserted, no noise at all from amp, indicating it could be the tubes I mentioned.

Darrell, some things to check:

-5U 4 Lit?
-Jiggle RVC plug or jumper.
- " Speaker plug.
-Speaker wattage switch.
If this amp has a AVC switch -check that.
Try removing the 12AX7 para-driver in play mode. You should get loud noise.

Tubes can fail suddenly. Signal flow: 5879-6Sn7-12Ax7-6L6s. Only those first three can kill all sound.

Use your meter on low-AC and check for audio at the volume control and (w-control up) at Pin-2 of the 12AX7.

I'm a bit confused as to model, Is this an M100A-? w/MRA-4 amp?

Rob

Rob

Yes, that is correct it is not the original amp but has worked fine for a couple of years now. I suspect it is an amp out of a Seeburg G?

I will try the things you have suggested. I strongly feel that if I replace the 12AX7 and 5879 tubes that the problem will go away. When I test tha 12AX 7 tube it starts out good and then the needle moves into the bad zone if you hold the test button down for more than 2 or 3 seconds. The 5879 checks right at the line of good/weak, and of course I am not real sure how accurate my tester is

Best
Darrell

Rob

-5U 4 Lit? (Yes tube is lit and checks good on tube tester)
-Jiggle RVC plug or jumper. (No reaction)
- " Speaker plug. (No reation)
-Speaker wattage switch. (No reaction)
If this amp has a AVC switch -check that. (No reaction)
Try removing the 12AX7 para-driver in play mode. You should get loud noise. (Removed 12AX7 tube while record playing and no noise)

Will replace two tubes that are suspect and report back

Thanks for ideas

Best
Darrell

Hi Darrell,
Are all of the tubes still in the amp, or is one or two "missing? Has this amp been rebuilt in the last ten or so years?
Has the "AVC kit"Switch and wiring, been added to the mechanism, or is the squelch, internally grounded in the amp ?
Ron Rich

Morning Ron

Yes, the amp was rebuilt by the person (well respected jukebox person that I first met at the Chicagoland Jukebox Show) that built this unit for my wife about 2/3 years ago. Yes, all tubes are in amp and none are missing.

"Has the "AVC kit"Switch and wiring, been added to the mechanism, or is the squelch, internally grounded in the amp ? " Not sure how I would determine that other than I do not see anything added to the mech????? so I assume squelch is internally grounded??

Best
Darrell

Hi Darrell,
Did the re-builder know you were using it in an A ? If so, he should have had you check that, or he may have switched the AVC off. That switch MUST be off, if the kit was not added--trick to determine this--look at the "mute plug" going to the amp--if it has an "outboard wire", going to a switch in the area of the mute switch on the mech, it's been added. Ron Rich

Ron

Yes, builder put that amp in the unit. No kit installed. Grounded pin 3 to ground if that make sense to you.

What can/would happen if switch was not in off position?

Best
Darrell

Darrell, if an MRA-up amp is used it has AGC to level the volume between records.

Seeburg also made a kit to ADD AGC to the older amps. In both cases a set of two contact blades are added to teh top of the vertical stack on the mech. The open during scan and this allows a squelch voltage to build up on the AGC time constant. When the mech goes into play, those contacts close and the squelch voltages now slowly dissipates causing the gain of the circuit to slowly increase until the voltage generated by a record establishes a level. The idea is to prevent blasting at the start of a "loud" record

if Pin-3 is not grounded (ever) the squelch voltage will always be present and amp gain will be low.
If Pin-3 is always grounded the amp may blast (called: overshoot) at the start of a loud record since no squelch voltage will ever build up.

If you have a cart & styli that has lower output than original the level may never be enough to cause overshoot anyway.

None of this will kill all volume though, this is something unrelated.

Rob

FYI, Darrel's Seeburg A is on 33 1/3. The tonearm has been modified to use a later cartridge so as to get the tracking down to 4 grams and to use a decent set of .7 needles. There is no problem of blast at start of record, probably largely due to the lower output of new cartridge and needles. I have built several Album boxes and I don't like to use the original blackhead cartridge on 33's. You have to run to much needle pressure to get it to work reliably without some modification, so if you have to do that much, you may as well update the cartridge while you're at it.

Rob,
On the model A, the squelch switch is mounted to, or in the vicinity of the "safety plunger", not on the stack switch,as on the B's. I done one, once, about 47 years ago--It is not a "fun job" to add/adjust correctly !! Ron Rich

Rob

You said: "Use your meter on low-AC and check for audio at the volume control and (w-control up) at Pin-2 of the 12AX7."

I did not get any reading at the volume control or at pin 2 of the 12AX7 tube socket. I used a digital meter and an analog meter.

Best
Darrell

Darrell, if you are seeing no audio at the volume control (with record playing) check for audio coming off pin 3 of the 6Sn7 before and after C16.
Moreover, look for proper DC voltages too. It is possible that one of the large dropping resistors in the power supply section has opened. Again, is the 5U4 lit, if not, check the fuse.

Rob

H Darrell,
If the 5U4 is NOT lit, but the fuse is good, check the interconnect plug between the WSA-amp--
These often fail to make good contact on the 5.8 AC line--(Deoxit !)
Ron Rich