SHP3 amp heat sink/transistor temperature

[e093116]

Hello,

Now that I've once again got a functioning mech in my current STD160 project, I was able to play a few records for the first time. Much to my surprise, the sound was good/better than I expected.  I did notice that the heat sink got really hot. I think this has to be a problem, I'm surprised it hasn't burned up!

 I measured (with an IR thermo-meter) temps in the 180-220F range on the heat sink and 225-250F on the transistors. I also noticed that the heat sink appears to be a light purple color, I think this is supposed to be a black anodize color. I mention this because it's possible evidence that the amp has been doing this for a while (long enough to fade the color from black to light purple) and might be normal?

I'm also sort of surprised that the back of the cabinet isn't "baked" looking, if the amp was always running this hot, it would have had to age the paint on the inside of the cabinet.  I guess this just tells me that this amp isn't original to this particular jukebox (which isn't a huge surprise).  On my other machines the heat sinks have never been hot to the touch.

I did notice that the transistors heat up immediately without playing a record, if this is some sort of clue as to where the problem could be?

Any suggestions on if this is "normal" and if not, places to investigate?  I just got the Tony Miller book on the SHP amps on Saturday and the bias pot information on page 24 seems like a place to start.

Thanks.

Bill

[Ron Rich]

Hi Bill,

Yep--bias problem (s--if hot on both sides)-- RTFM ! (Read Tony's Friendly Manual ) Should idle with zero heat--Run warm, but not uncomfortable to the touch when playing 15, or so selections at full volume-- NEVER anywhere near these temps ! BTW--I think some heat sinks were poorly anodized--turned white !

[e093116]

Thanks Ron.

Tony's Friendly Manual (TFM) came in handy. It took me a while to source replacement pots, but with the replacements installed the amp runs cool. I'm surprised something didn't burn up along the way. I guess the amp has passed it's 40 year ESS check!

I have most of the bugs out of this machine. The last problem I have is getting the mechanism to pick up records across the complete record rack. It does fine from 100-165 (and 200-265), but doesn't want to pick up records higher than 165. I've done the battery test with no problems, and also cleaned the DES (it was actually much cleaner than I thought it would be).  The interesting thing is that if I start a scan from the "home" position of the mechanism, it will pick up 275-265 in order as the mech scans from right to left. But if I let it scan and come back left to right, when it reverses again the selections are missed. It also stops and picks up on the "100 side" when I have selected on the "200 side"  (example get 177 when I pick 276).  Other times if I pick 178( for example) it will scan twice and not pick up, but if I pick some other selections, every once and a while it will stop and pickup on the subsequent scan. I'm thinking I have an intermittent connection at the gray box connectors or need to study the DES more carefully.  I have found an intermittent connection on the black box connector for the first digit light, which was a bit of a surprise since all of the connector pins looked good on that connector (none were obviously squashed or mangled), so I'm going to start with the gray box connectors.  Any other guidance/suggestions would be highly appreciated.

Bill

[Ron Rich]

Hi Bill,
Where did you "source those pots" ?? (I use a fixed resistor, as per TFM).
From what you are writing, I have to assume the TMU has been slightly bent--Try loosening the two attaching screws on the 179 end of the mech, and pushing the TMU "down"-- After that examine the "read-out contact block adjustments"--especially #3. If it will not select using a known good D cell, the DES, and Gray box will not do so either.

[e093116]

Thanks Ron.

I got the pots from Newark.  I wanted the capability to easily make future adjustments and I didn't want to run the amp anymore in order to dial in the resistance value.  I also couldn't get a stable resistance reading on the old pots, which to me confirmed the high resistance/oxidation failure mode scenario.

I found the outermost contact on the contact block was bent and making contact with the TMU when it wasn't supposed to!  I looked at that a couple times, I didn't realize the contacts would toggle on/off of the TMU with each pass of the mech.  I'm assuming the design of the contact block was changed to minimize wear/maximize life compared to the design used on the LS1/LS2.

The next job is to hook up a consolette to this phonograph, which will be installed in the dining room.  My wife and I are tired of the kids running out to the garage to play songs while we're eating dinner.  The instructions for connection from Tony's website are straightforward and I'm looking forward to hooking it up.

Thanks again for your help.

[Ron Rich]

Hi Bill,
Thanks ! If by chance you still have Newark's part number, could you post it-- What value did you use ?
As for installing a DEC--Easiest way for me is to skip the "10 wire contact block" If you do not care about the audio info, you can use 4 less wires. I just use a Molex connector plugged into the consolette jack on the accessory junction. BE SURE that logic, and power supply, grounds are common! If you are just installing 1 - 4 DEC's you can use the DCC transformer, DBV plug for power--I suggest a smaller fuse for home use (3, or less SB). Thanks, Ron Rich

[e093116]

Ron,

I used Newark # 05N1625.  100 ohm, 2W trim pot.  I drilled holes in the front "face" of the amp chassis to mount and adjusted for 1.65V as noted in TFM.

These were the only thing I could find that weren't $20+ dollars each, actually available, and easy to solder to.

On the DEC, I'm going to try for the full hook up...part of the fun is depositing the coins and my kids like to play operator and collect "the money".

Thanks for the comment on common grounding.

Bill

[robnyc]

The bias circuits in these amplifier and many consumer grade receivers is classic Asinine Engineering.

There is NO good reason for the adjustment to range from zero to destruction.

There is also the problem of intermittents on these pots. Adding a simple resistor across the pot with value selected to limit the possible bias to some high, but safe level would have prevented tons of failures that I and most other technicians have had to deal with.

Early-on the idea of wide bias range was (sort of) explained by the wide range of parameters within a transistor type. That, along with the fact the in the case of germanium transistors, the slow increase of internal leakage as they were used would move the external bias point downward. With the introduction of silicon that reason disappeared.

Seeburg did use better pots beginning with the SHP series, but I still preferred to eliminate them with fixed resistors on the units I serviced for others. The fewer adjustments to tempt Gepettos -the better.

At the very least one should --never-- power one of these up after service or long dormancy without a current limiter of some sort. A 40 watt bulb in-series with the AC plug will save a lot of frustration and wasted components.

Rob-NYC

[Ron Rich]

Hi Rob,
I have had the same experience with the SHP amps-- but---
I beg to dis-agree with you, a little--IMHO, Seeburg used CRAPPY bias pots, starting with the SHP series amps ! They also used different value "bias pots"--but I think ALL types/brands, were subject to intermittently, becoming open. Ron Rich