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Need help Identifying a diode (pics)

jwxr7

jwxr7

Active Member
I'm working on a Fisher RS-2003 reciever and the amp board needs repair. I'm trying to figure out what type of diode this is in the pictures. Here's a bottom view where you can see a raised diode symbol and the letters "SV"P1000975.jpg Here's the top where it shows a repeating pattern of 8, 0, B. P1000976.JPGThat's a normal sized pencil in the pic.
There are 2 of these per channel and one was physically broken so needs to be replaced. It has a brown plastic body. Any help would be great.

thanks
 
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Multijunction diode. You'll need to refer to the service manual to see 'how many' junctions (3 is most common), but you can get a ballpark idea with the diode test of a digital multimeter (as long as it supplies enough voltage to forward bias the thing). This is kinda half-assed, and I recommend you get the manual.
 
Thanks EchoWars.

Unfortunately I haven't got a service manual for it yet.
Diode tests on one of my meters didn't show a forward bias, but I just tried my meter here at work and it showed a 1.6V drop in one direction for all three of the unbroken diodes. That sounds like 3 junctions in series, maybe. Crazy question; If I can't find the exact replacement, can I use three genaral purpose diodes in series?
 
jwxr7 said:
Thanks EchoWars.

Unfortunately I haven't got a service manual for it yet.
Diode tests on one of my meters didn't show a forward bias, but I just tried my meter here at work and it showed a 1.6V drop in one direction for all three of the unbroken diodes. That sounds like 3 junctions in series, maybe. Crazy question; If I can't find the exact replacement, can I use three genaral purpose diodes in series?
Click to expand...

Best bet would be to replace with same part, but if it cannot be done and not too big current flows in these diodes, a red LED should be able to drop you around 1.7V IIRC. If these diodes are to bias the push-pull outputs, I figure LED's should work. If I'm wrong, correct me someone :)

az
 
These diodes are used for different purposes, but a common one is to vary the Vf of the diode (likely driving the base of a transistor or two) as the surrounding temperature varies. LED's don't do this (much), they are pretty stable regarding Vf vs temp. Thus, the LED may not do what the original circuit is designed to do.

Another major factor is that the max current in nearly all LED's is about 20mA. That diode is likely rated for 50~100mA.
 
Does anyone have a SM for this fisher? Or can anyone help me get a cross or find an original one of these? I'm having trouble finding anything about the original part. If I search NTE for a cross using "80B" I get a return for 903-80B that crosses to NTE 614. That cross is a voltage variable capacitance diode (tuning diode) http://www.nteinc.com/specs/600to699/pdf/nte610.pdf

Does that make any sense?
 
I did a little experiment;
I tested an original diode vf @ 63F (1.620v) and then with a hair dryer on low blowing on it (1.370v). Then I tried the same thing with several types of general purpose diodes in a series of 3. The 1n4001 had close #s, 1.570v @ 63F and 1.370v heated. the 1n4148 had 1.670v @ 63F and 1.400v heated. The 1n4004 was too low on both 1.505v @ 63F and 1.260v heated.
 
The diode in question is specially engineered to maximize the ability of heat from either the ambient air or from the surface of a heat sink to conduct heat into the diode.

Using a transconductance model for a bipolar transistor, to maintain a contact DC current, as the transistor heats up, the forward base-emitter junction bias must be backed off by 2mV/degree Centigrade. The bias diode in your photo is designed to provide three diodes of forward bias, and to back off on that forward bias as the diode array heats up. Technically, the thermal feedback must be unity or greater, or the circuit will go into thermal runaway.

Generally, the diode array must back off on the bias for four transistors, two outputs and two drivers. Since you have three diodes backing off the bias for four base-emitter junctions, the "tracking constant" must be carefully chosen. Too little = thermal runaway. Too much can yield notch distortion.

So . . . is the diode array mounted on the heatsink, or just somewhere on the circuit board?

Fred
 
So . . . is the diode array mounted on the heatsink, or just somewhere on the circuit board?
Click to expand...
They are pretty much mounted in the middle of the board, not on a heatsink or very close to any transistors. See the picture on post #5, the green arrows point to the diodes' locations.
 
here are some better shots of the board with red lines where the diodes were mounted.
 

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EchoWars said:
These diodes are used for different purposes, but a common one is to vary the Vf of the diode (likely driving the base of a transistor or two) as the surrounding temperature varies. LED's don't do this (much), they are pretty stable regarding Vf vs temp. Thus, the LED may not do what the original circuit is designed to do.

Another major factor is that the max current in nearly all LED's is about 20mA. That diode is likely rated for 50~100mA.
Click to expand...

Aww man! I always overlook the thermal side of the coin.... Thanks for the info!

az
 
jwxr7 said:
I'm working on a Fisher RS-2003 reciever and the amp board needs repair. I'm trying to figure out what type of diode this is in the pictures. Here's a bottom view where you can see a raised diode symbol and the letters "SV"View attachment 137592 Here's the top where it shows a repeating pattern of 8, 0, B. View attachment 137591That's a normal sized pencil in the pic.
There are 2 of these per channel and one was physically broken so needs to be replaced. It has a brown plastic body. Any help would be great.

thanks
Click to expand...

Hi...

"SV" diodes were often "varactors," also known as "varicaps" or "tuning diodes." These diodes are used as capacitors, by reverse-biasing them. As the reverse DC voltage on the diode changes, its capacitance changes, and frequency tuning can be done. The bad news is that varactors are very specific, and need a precise replacement. The good news is that there aren't that many types, so tracking down a cross-reference should be fairly easy.

What's confusing to me in your situation is that the diodes in question seem to be in the power amp section, while varactors are generally found in the tuner section. However, it's hard to tell from your photo. So, I can think of three possibilities:

1- The diodes are varactors, and that part of the PCB is in the RF or IF stage.

2- The diodes are varactors, and are being used for some atypical purpose.

3- The diodes aren't varactors, and you are back to square one.

I know this isn't a hugely helpful post, but perhaps it is a lead...

-k
 

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Thanks Ken,

I ran accross the "tuning diode" when I was trying to cross it, but didn't know if it made sense. It may actually cross to an NTE614 then?

In the 2nd pg of the pdf link, the case #150 looks like my diode.

I'm pretty sure that whole board is dedicated to AF amp stuff. Mainly I see the 2 channels come in from the eq section at one end and +- power supplies and speakers out at the other end near the stk0040 ICs
 
Their use in audio amps has nothing to do with their variable capacitance, and certainly RF is a rather undesirable thing in a power amp.. They are either used as part of a Baker clamp, or used in the biasing circuitry. You cannot use an NTE tuning diode as a replacement.
 
EchoWars said:
Their use in audio amps has nothing to do with their variable capacitance, and certainly RF is a rather undesirable thing in a power amp.. They are either used as part of a Baker clamp, or used in the biasing circuitry. You cannot use an NTE tuning diode as a replacement.
Click to expand...


Interesting! So they probably used them for their forward bias temp characteristics? Or in lieu of a low voltage Zener?

I learn something new every day! (And forget two...)

-k
 
From the photo, it appears as if these mystery diodes are in series with conventional diodes. So I am going to vote for their use as a voltage regulator, either in a supply or a bias circuit, as others here have already noted.

Based on this, and the other comments in this thread, I'd try a 1.5V reference diode, (ie- fake "Zener" diode). In a pinch, a 1.2V reference like the LM185 would probably work OK.

-k
 
Similar Diode

I'm working on a Sony STR-6055 driver board with diodes that look exactly like yours. Mine are stamped SH-1. The service manual is only a little help listing the diode as SH-1S. Anybody know what this device is or what the modern equivalent is? I need to change one.
 
Interesting ideas.
It sounds like I'm going to do a bit of gambling if I want to fix this unit.

That channel on my Fisher has all these problems; The stk0040 is shorted from outputs to supplies, Q14 is leaky,D14 and D16 are shorted (these have no markings to ID them), D18 was broken (the diode in question), and R26 got over heated. So I have to invest a little money before I can even experiment with the wierd diode we've been discussing :sigh:. The other channel's IC is leaky from one output to supply too, but no other components seem to have been taken out with it.
 
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