SHP3 bias adjustment for better sound?

[zelgy]

Hi all!  I have been debating adjusting the bias on the SHP3 amp in my STD4 Mardi Gras to obtain better sound quality.  Please note that I have replaced all the caps in the amp.  It's possible I am being too critical as well as I don't have another juke to compare to and I haven't listened to a Seeburg juke since the late 80's. If I was to describe the sound I would say it is muffled.  I have tried different settings for the base/tremble and cannot seem to find the sweet spot.  I am simply curious how much affect adjusting the bias has on sound quality?  I certainly don't want to attempt to fix something that's not broken.  From what I have read it isn't difficult to adjust.  Your thoughts/opinions are always appreciated!

Thanks much, Jon!

[Ron Rich]

Jon,
The "bias" adjustments are really not for sound quality, and if the amp heat sink, is not getting warm, it's best, if they NOT touched !
If you have quality issues, the first thing is needles--especially if not using gen-U-whine Seeburg/Pickering 253719's. Which SHP-3 Amp do you have installed, and which boards are installed in it ? Have you checked/replaced, the NP's in the cross-over ? Ron Rich

[zelgy]

Hi Rich! I do have new needles and also cleaned the volume adjustment control, which made a huge improvement , but I think there is still additional improvement to be made. I assume the NP's in the crossover you refer to are the 2 electrolytic capacitors in the SN12 Stereo Network & Junction Unit. If so, I think that will be my next component replacement. They are 7 MFD caps. Any suggestions as to a good replacement for them?

[robnyc]

Jon, as Ron points out, bias in a transistor power stage is not for setting sound quality -except in preventing crossover distortion at low levels.  If for any other reason you attempt to adjust the bias pots --be sure to place a 40watt conventional light bulb in series with the amp's power plug. Bias pots in-general can develop dead spot and this with instantly kill the output transistors and the drivers. A bulb will limit that fault and all that will happen that it will flash brightly as you turn the pot. If the amp is not idling hot it is best to leave it alone.

Ad for dull sound, I recall those machines as being rather strident with volume at-near full setting. I did see at least a few of that era with blown tweeters. I suggest hooking up decent externals whose sound you are familiar with (use the "50" watt taps) and disconnect the internals. If the highs are a lot better suspect the faulty tweeters or crossovers.  Similarly, you can attach a receiver or other amp to the internals and compare that as well. 

There are some simple mods to the preamp stages that will boost the highs, but if using that amp for records, I never found it necessary.

BTW: Does the sound fade up at the start of play. It should as the squelch voltage dissipates. Those amps used diodes for level control. If the squelch voltage remains during play the diodes will conduct heavily and this will affect the highs more than bass.   Squelch & active AGC voltage can be measured at this marked pin on the "Mech" female: s1192.photobucket.com/user/Rob-NYC/media/mute_zpskapksovu.jpg.html?sort=2&o=195  In play with no record, the AGC voltage should go to at/near zero. A few millivolts is OK.

RobNYC

[Ron Rich]

Jon,
Rob's post above "jogged" my sleepy memory--- look at the mute/reject relay, be sure the two inner contacts of that relay, when non energized, are making well, or you will get a "muffled" type sound--( Yep--tweeters, {BTW, which are 16 ohm}, do blow on these machines as some "yeahoo" set the balance adjustment too high !) This 24 vdc, relay should have GOLD (flashed) contacts !!
Also--I did not write "new Needles"-- re-read, please----Ron Rich

[zelgy]

As always, appreciate the feedback robnyc and Ron! Regarding needles, I purchased from Needles4Jukeboxes.com (needles found under Pickering 340D). I will stay away from adjusting Bias at this time. I'm not sure about the sound fade at start of play, so I will check that. I'd like to check the squelch and active AGC voltage, but robnyc your link doesn't seem to work. The highs seem to be where the sound is most harsh, so I will try hooking up external speakers to see if it is indeed blown tweeters. Ron, I will check the contacts on the mute/reject relay. I would still like to replace the caps in the SN12 Stereo Network & Junction Unit, if that is the truly part of the cross over circuit and the NP's you referred to in your initial post. Could you please let me know what a good replacement cap would be for these 2 components? I can about imagine what it is like to help us novices, so again thanks! Your patience is impeccable!

[Ron Rich]

Jon,
Sellers of needles (almost)all state something like--good replacement for "all colors", of these type, needles-- This is what the maker(s?) of them "claims", but this is simply not true ! Genuine, "Yellow" (diamond) and "Violet" (sapphire), needles are designed to comply with the Seeburg MAX 2.5 grams of tone arm pressure, which are the systems used in Seeburg 160 selection products, since the SS-160 model. "Green" (d) and "Tan/brown' (S) are designed to comply with 100 selection mechanisms using 4-6 grams of pressure, ( S-100, up, and the aftermarket 345-03x Pickering cartridges) -- RE-pop needles of this type MUST be "compromised" to achieve this ability--and to varying degrees, they all are !

[zelgy]

It appears there is some hesitation on answering my crossover cap replacement questions. ;-) I'd like to do this no matter what, so your feedback on this specific question would be appreciated!

[Ron Rich]

No hesitation-self evident---replace it with the correct, or nearly correct, one, or two, IF, BAD --