SHP3-B with Transistor Bias - no bias voltage

[paradisecs]

Thanks again for the info. All four of my amps have the 2amp fuse but only one of them has the 180 Ohm resistor and diode. That's what through me.

Stephen

[Ron Rich]

Stephen,
Then "someone" removed the resistor, and the diode ! I don't know how the relay can operate with-out them ?? Ron Rich

[robnyc]

Oct 20, 2018 2:39:13 GMT Ron Rich said:

Rob,
Iffin you look at the schematic-- the -32, nor the + 32 is fused in so far as amp operation goes--the main 3.2 A., fuse "covers" them (lol).
The 2 amp fuse was an added fuse that covers ONLY the RVC and mic power supply. Once again--the dropping resistor for the relay, and the diode have been in EVERY one of the thousand or so (probably more ?) of them I have seen-- ?? Ron Rich

I thoroughly get the "lol" -in that :-) As for the dropping resistor, I assumed that was standard, maybe Stephen is not seeing it due to it being buried under wires(?).

BTW: I use 1 1/2 amp slow as main fuse.

I set bias @ 1.5 to 2 ma. That should be fine for any but the coldest locations.

RobNYC

[paradisecs]

I checked again, even pulled the boards out to look and my STD160 doesn't have either the 180 Ohm or the diode, and it's the unit in my living room that works. The diode I see as Seeburg 309384 which should be a 1N4817 or 1N4818. How close of rating should a replacement be in this case. Will at 1N4003 work or is the current to high? Of all the parts I've sourced out of the cross reference in the AMP book this is one part# nobody has a replacement for.

Stephen

[Ron Rich]

Stephen,
Get thee to a Wallgreens near you and purchase a pair of "reading glasses"-- HONEST, the diode IS there--somewhere !! Which relay socket is installed in that "working" one ? Follow the coil wires to the terminals, and you will find that diode ( Any GP Signal diode, will be fine there-- I ran down to Radio Shack, once, when the factory ran outta them !) If you can't find a source, let me know--I have thousands--well, at least hundreds of them ! Ron Rich

[paradisecs]

A few updates. I changed the bias pots back to 3.3k and the left side started giving me resistance readings more in line with what I was reading in Tony's Amp book. The right side still was way off. I took the bias circuit out of the heat sink to test the parts and the transistor didn't pass my standard transistor test for NPN. Any advice on a replacement? My standard electronic parts suppliers don't seem to have anything close to a 2n3405. I even cross referenced against the H.A.B cross-reference manual I found and still can't find a replacement.

So I thought, okay, I'll just convert it. Well I can't find 1n251 stabistors either. I'll check more places after the kids go to bed.

Stephen

[robnyc]

Stephen, the bias pots are 75 ohm as original.

I looked at my ancient note on these amps and found RCA Sk9142 (that's how old these notes are) and ECG 192 or NTE 192.

Note that there is nothing special about "stabistors" that two or three regular diodes in-series won't replace. There are minor differences between the diode vs transistor bias circuits. I only have schematics for the diode type.

When I was faced with repeated meltdowns of these amps due to a-holes, I didn't bother replacing the transistor types when I ran out of them, I just went to a straight fixed bias with whatever slight compensation three diodes offered.

I found that 1.5 mv was fine and the amps worked well...except when I put one out on the terrace in February then brought it back and tried it. Bad crossover distortion till it warmed. That was never a problem on location.

RobNYC

[paradisecs]

The schematic I have shows the bias pots with the transistor bias as 3.3k, for early SHP1 builds, but I didn't know that until I had replaced with 100 Ohm. That was the odd thing for a newbie like me, I just assumed and SHP3 had the SHP3 bias.

Either way, I figured it was best to update to the stabistor style, then I couldn't find those either. The conversion seemed pretty straight forward, swapping Q11, R61, R61 for CR17, R84, R85.

What you suggest would be to use multiple diodes in place of CR17 all strung ( I know, hi-tech terminology) together then? Any particular diodes to use? How do diodes in series sense temperature or is it more just making sure the current can never spike up to cause the issue in the first place? I'm not sure I understand this principle but I trust your knowledge of it.

NTE192 is a 1A but NTE192A is a .5A so that might be a good one but I'd rather upgrade to the newer style or use the diodes as you suggest.

Edit -  Just found the 192A at one of the two local Fry's ready for pickup cheaper than online so maybe I'll try that for now and see if it works.


Ideally I'd like to get the bias set then pull the bias pots out and replace with a resistor as you previously suggested though.

Stephen

[paradisecs]

It works. Thanks for all the help on this. I've learned so much from both of you doing this. I tested on the dim bulb and no shorts. Got the bias settled at 1.7 before it stopped creeping up. I put it in my STD160 and it sounds great (once I figured out that the gain on the right was off).

Stephen

[Ron Rich]

Stephen,
If using driver base to base readings to set bias, 1.7 is too hi for the transistor bias circuits--set it at less then 1.5, if on the dim bulb tester--then remove the dim bulb and reset it to below 1.6. Let it idle--no nothing plugged in, or attached, for an hour or so --heat sink must remain at the most, warm to the touch. Ron Rich

[paradisecs]

I didn't check base to base on the driver this time (too scared to mess it up). Instead I re-flowed all the solder joints on the bias circuit and all TB points and got the test points working. My 1.7 readings were on the test points to case of 17 & 27. But I will check it again this week. I still have work to do on the machine before the amp gets installed anyways.

Stephen

[Ron Rich]

Hi Stephen,
I have never used the test points--I just measure (1.5 dc)voltage on the drivers--base to base, using mini-clip meter leads (careful not to short one out !! ) Ron Rich

[paradisecs]

And yes, I found the missing diode and resistor. They weren't on the relay directly but further up the wiring attached to a terminal strip. Makes more sense to me to do it that way than directly on the relay socket. Why are some done one way vs the other?

Stephen

[Ron Rich]

Hi Stephen,
Did you get a pair of spect's. at Walgreens ( I want my commission !) ?? I knew you would find them--some day --LOL--
That amp (or a version of it) was produced for over ten years, by several different persons,and as a madder of fact--several "Seeburg" companies. They also used a different source for the relay sockets-- which used a different assembly method--soldered or crimped--  Ron Rich

[paradisecs]

I'm cheap, I got mine at the Dollar Store.

Stephen