Phonoland

Hi all:
Having a problem with a 100j. I recapped the wsr and the hfma1-l6 amp. The amp had sound before the recap, now it doesn't. Following the trouble shooting part of the manual. I pulled the 5879 tube for a pop(none) and pulled the muting plug. no go. All tubes check good. I did voltage checks on all the tubes and the only one way off is pin 8 on 6sk7. rather than the 280 volts it reads somewhere around 40-60. Power supply voltages on all the filter caps also are good. This juke has been modified in that it has the stereo redhead cart, and has an extra circuit near the 5879 tube that includes a 6c4 tube. Not sure where to go next. I do not have the juke at my house to check it. I have to go on site to do anything. I have the amp home now and am waiting for any suggestions. Thanks Mark

Pin 8's the plate - you've got something drawing down the plate voltage, or an off-value B+ feed resistor there.

Mark, in this sort of situation you start from the last stages. So try pulling out the 12AX7 (splitter-driver) near the 6L6s. You should hear a pop. If not, check the speaker connections and wattage selector switch. Alternately, if a signal generator is available, inject a signal of around 400hz @ approx 400mv (anything near that) to pin 2 of the 12AX7.

If a good signal gets through work your way back checking injecting to the grids of earlier stages and checking that the volume control dummy plug and AVC switch are making contact.

As Matt mentioned, the odd voltage on the 6Sk7 may be due to the plate resistor or excessive conduction. However, that tube is simply a shunt attenuator in this circuit so remove it and the 6SN7 while shooting the no sound problem.

The likely scenario is simply that a mistake was made when installing a new component. If the amp had sound before rebuilding, this should be a straightforward issue.

The little 6C4 was probably added to boost the low output of the later stereo cart so that proper AGC action would take place. I initially did something similar using an op-amp, but later went to replacing the 5879 with a 12AX7 with a gain pot between the sections. How was the 6C4 mounted?

Rob/NYC

thanks for the reply. the feed resistor is r27 i will check it out. you mentiond draw down the other components are c24 and c18 both new. what else should i look for related to those or other items. Thanks Mark

Rob: just saw your post. i do not have a signal injector. but will start with pulling the 12ax7. I will also look at double checking the caps and the connections. Thanks Mark

mb9513 wrote:thanks for the reply. the feed resistor is r27 i will check it out. you mentiond draw down the other components are c24 and c18 both new. what else should i look for related to those or other items. Thanks Mark

These days, never assume a "new" capacitor is not the cause of an issue.
There have been times that brand-new caps can be leaky, causing issues.
Been there, done that.

Guy's,
I "shoulda learnt" this long ago , but alas, I'm a slow learner !
I attempt NOT to assume anything is "good" just because it's new.
I have seen all type "electronics", mis-marked, open and shorted--brand new--
In the "old daze", we purchased tubes ( valves ) directly from RCA, and other "major brands".
This was always done with the provisions that 10 % would be "bad". The companies would not "take back defectives", they just said to toss them out, as that was figured in the price we paid. They claimed that to test each one, would far exceed the cost of the 10% allowance they gave us for the defective ones. RCA also claimed, that the 10% figure was high--they claimed that less then 8% actually were bad.
I recall one incident, where a "flock" of newly produced, sub-assemblies, tested bad. We had been making this unit for a long time, and never had any major problems. Once in awhile, we ran into a bad solder joint, or a solder "splash-short", but they were rare. After a lot of "hair pulling" we discovered that the diodes we were using were mis-marked ! They had apparently, somehow, "flipped over", prior to being painted---
Ron Rich