Stumped: What Is The Function of These Transistors In The Circuit? - Page 1
Forum section: electronics / projects

author: Frogger
author_rank: Newbie
timestamp: November 12, 2017, 03:58:16 am
content: These two 2N3393 general purpose NPN transistors are connected in series from the output of the full-wave rectifier bridge to ground. It looks to me as if the designer is using the VEB breakdown voltage as a zener diode. So the two emitter-base junctions in series would function roughly as two 8.1 volt zeners for about a breakdown voltage of 16.2 volts.Though I have occasionally seen transistors applied in this way to use the emitter-base junction as a zener, I've never seen zeners connected to the unfiltered output of a simple bridge rectifier power supply circuit. Odd.Any ideas?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 04:43:06 am
content: I've seen C-E used as a snap diode (whether it actually snaps, who knows, but you can get transistors to avalanche with a snap action), like a few H-K scopes did that for input protection.Doesn't seem like a good idea here, especially since the node says 22V normal. That doesn't mesh with the 2N3393 being 5V rated (and from a typical process that will indeed breakdown around 8V each). If it's for transient protection (unlikely anyway, as I don't see the rest of the circuit shown being particularly vulnerable), it's a one-shot deal, as B-E junctions aren't very beefy. Even worse, the bond wire will blow off before the capacitor even gets discharged very far.For replacement, I'd just leave them out...Tim

author: strawberry
author_rank: Super Contributor
timestamp: November 12, 2017, 09:44:56 am
content: @T3sl4co1l : At that power level transistor bond wire wont blow, instead should blow mains fuse when two transistors go short

author: Ian.M
author_rank: Super Contributor
timestamp: November 12, 2017, 10:02:13 am
content: Its a HeathKIT, so its quite possible the transistors were specially selected for their B-E junction to zener at around 12V. I suspect the idea is to provide *some* protection if the input voltage selection jumpers are set incorrectly. A beefy Zener, with a breakdown voltage of about 15% greater than the peak no-load output of the bridge rectifier would be an appropriate replacement.

author: Benta
author_rank: Super Contributor
timestamp: November 12, 2017, 10:19:35 am
content: Note that they are marked as "D", not "Q".

author: nfmax
author_rank: Super Contributor
timestamp: November 12, 2017, 10:49:30 am
content: Possibly the idea is to limit the voltage across the audio output transistors when one of the panel lamps fails?

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 12:36:00 pm
content: That’s a sensible assertion.This is quite common in Heathkit stuff if the era.This probably happened because they had a stupid number of 3n3393 stocked and would have to buy a Zener in for the job that they didn’t have or it was more expensive. Decent Zeners of the era cost a lot of money, more than a lot of transistors. You will see a couple of 2n3393 used as protection with C-E junctions back to back to protect input FETs on their instruments.There is also the possibility that the transistors were less noisy than a Zener diode! Also the output transistors were Ge so a little more voltage sensitive.Lots of reasons for this.A lot of Heathkit designs have very interesting cost reduction techniques applied. Some of them really don’t perform very well. The SW-717 was heavily cost cut and suffers terribly from hum and noise. Earth strapping apparently solves that. I repaired one for someone a few years back and it wasn’t a nice radio. It worked, just, and that’s all you got at the price point.

author: StillTrying
author_rank: Super Contributor
timestamp: November 12, 2017, 12:55:59 pm
content: It wouldn't surprise me if D9 and D10 were supposed to be connected the other side of the nearby R42, there's a reason I call these Heath Robinson Kits.https://people.ohio.edu/postr/bapix/SW-717.htmedity:There's no voltage control and almost no decoupling on the 10V line, so I think that must be it, ...of course this would mean that these things are plagued with mains hum...

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 01:42:46 pm
content: That's a good point. That's a more reasonable place for a 10v zener!BTW if you ever do this, don't ever use the transistors for anything else. hFe is degraded completely after a few minutes of being a zener.

author: Frogger
author_rank: Newbie
timestamp: November 12, 2017, 04:32:35 pm
content: Please hold on, everyone. I have the SW-717 on the bench and was reading the assembly instructions in the manual. Assembly instructions indicate, along with the two transistors, that there is a zener on the same terminal strip. No zener is shown in that area of the schematic. The schematic may be corrupt. My fault forassumingthere couldn't be this big of a screw-up on a simple schematic.I'll open the radio and trace out what is going on in the power supply area and get to the bottom of this.

author: Frogger
author_rank: Newbie
timestamp: November 12, 2017, 04:46:33 pm
content: If anyone is curious about this Heathkit radio model, here is a complete review.

author: Frogger
author_rank: Newbie
timestamp: November 12, 2017, 06:53:41 pm
content: Thanks for the comments and suggestions.For the area in the circuit of interest,schematic=assembly instructions=physical circuit.Voltage at the emitter of D9 is 24.54 VDC.Voltage at the connection between D9 and D10 is a few millivolts less than 24.54 VDC.Voltage across D9 is a few milivolts.Voltage across D10 is a few millivolts less than 24.54 VDC.D9 is conducting, D10 is not.This question arose because I'm starting a restoration of the radio, full electrolytic re-capping, and an alignment. I think I'll just replace D9 and D10 and that's it as far as those two components. Not going to obsess on their function. The goal is to enjoy restoring the radio and then use it daily for shortwave listening and casual ham CW contacts.Notes:1.) A ham commented from experience with the Heathkit ham radio products that, "Heathkit had a tradition for using 2N3393's as avalanche diodes, an example is the voltage regulator in the HW-100 VFO." Below is a schematic of the Heathkit HW-100 VFO circuit showing just that use of a 2N3393 transistor.2.) The SW-717 manual lists in the BOM a replacement price of a 2N3393 as $0.40, whereas the price of a 9.1V 25mA zener diode is $1.50 (1971 prices.)

author: StillTrying
author_rank: Super Contributor
timestamp: November 12, 2017, 07:27:43 pm
content: "Voltage at the emitter of D9 is 24.54 VDC.Voltage at the connection between D9 and D10 is a few millivolts less than 24.54 VDC.Voltage across D9 is a few milivolts.Voltage across D10 is a few millivolts less than 24.54 VDC.D9 is conducting, D10 is not."That sounds like D9 and D10 are connected exactly as in the schematic, andworkas expected. In the few first seconds of the very first power up one of D9 or D10 blows O/C, and from then on they do absolutely nothing!Have you checked the 10V line, perhaps that's where the unknown zener is connected.

author: bd139
author_rank: Super Contributor
timestamp: November 12, 2017, 07:42:52 pm
content: Kamikaze diode?

author: Frogger
author_rank: Newbie
timestamp: November 12, 2017, 07:43:01 pm
content: >Have you checked the 10V line, perhaps that's where the unknown zener is connected.The unknown zener has been resolved. It is not part of the power supply circuit.>In the few first seconds of the very first power up one of D9 or D10 blows O/C, and from then on they do absolutely nothing!It appears that would be the case.I'll order a few 2N3393 "fuses" and see what they do in circuit.

author: bson
author_rank: Unknown
timestamp: November 14, 2017, 12:00:36 am
content: I'd just leave them out.

author: Fred M
author_rank: Newbie
timestamp: August 28, 2021, 04:36:31 pm
content: So this was exactly the thread I was looking for this morning.Hope you guys are still out there.My "D9" and "D10" are not open-circuited but appear to be functioning as reversed biased Zener diodes. But the voltage is a little high-- 25 volts vice 22-- and the voltage changes, goes up a few tenths, as the thing "warms up". So I'm interested in better voltage regulation of the power supply.I also remember from Electronics that leaving the collector floating like this is a good way to make an unwanted RF noise generator.I have a couple 11V .5W Zeners in my junk box. Anyone see a problem with replacing D9 and D10 with them?

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2021, 05:38:02 pm
content: Welcome to the forumsI think a 22V zener combo would be best- compared to E-B junctions. As a shunt regulator, the excess voltage gets dropped as heat in the transformer's winding+rectifier diodes resistance - so if this is an issue due to high mains voltage, there are limits to what Heathkit's regulation scheme can deal with.1N4748A 22V 1W at 11.5mA but 1N5358B at 50mA. 1N5241B 11V 0.5Wat 22mA. I'd check the current to see if the zeners are OK.

author: Fred M
author_rank: Newbie
timestamp: August 28, 2021, 06:51:44 pm
content: Oh wow thanks for the instant reply and the recommendations. I will check the current draw. I also plan to mitigate any transformer heat impact by replacing the bulbs with LEDs too.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 28, 2021, 08:49:29 pm
content: Are you absolutely sure the transistors are connected to the bridge? It just makes zero sense. I have to wonder if they just drew the line down the schematic and didn't notice it passed through the dot where the bridge connects.

author: Fred M
author_rank: Newbie
timestamp: August 28, 2021, 11:20:13 pm
content: Yep

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 28, 2021, 11:58:20 pm
content: Quote from: Conrad Hoffman on August 28, 2021, 08:49:29 pmThat line feeds power to the audio amplifier chain, so that part is probably right. I think they're supposed to be connected on the other side of R42, because transistor era Heathkit liked to use transistors as zeners and brag about zener regulating everything, even if it didn't really work very well.

author: Fred M
author_rank: Newbie
timestamp: August 29, 2021, 12:30:10 am
content: I'll try that. Does make more sense. But the other side will require a 10V Zener, not the 22-25V that these two "diodes" amount to. So must be more to it than a printing or wiring error. The designers were up to no good. ;-)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 29, 2021, 03:03:59 am
content: Don't know about larger transistors, but E-B breakdown on smaller signal transistors I've tested is more like 5-7 volts, so it makes sense if they're on the other side of the resistor. Heathkit designs may not be the greatest, but they usually make sense.

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2021, 08:56:43 am
content: Quote from: Fred M on August 28, 2021, 04:36:31 pmYes we're still alive."Show new replies to your posts" wakes us up.It looks like even after 4 years we still think those transdiodes are meant to be connected to the other side of R42.

author: bd139
author_rank: Super Contributor
timestamp: August 29, 2021, 09:07:26 am
content: Not so sure about that. So my last post for this was a naive one in 2017 and I might have been right. I tripped over something else which appears to have the same function the other day in a Farnell TOPS1 power supply. The purpose of the Farnell one is to clamp the DC voltage so that if you get a high mains voltage then the secondary voltage doesn’t get too high and exceed the SOA of stuff connected to it. In the Heathkit cases above the transistor is being “zenered” to serve as a voltage reference for the tube circuit or a clamp for the power supply.Clearly heathkit must have got a massive crate of these transistors and it worked out cheaper to use them than a proper zener which wouldn’t surprise me at all.

author: StillTrying
author_rank: Super Contributor
timestamp: August 29, 2021, 09:33:46 am
content: The voltage across the 2 transdiodes is 22V on the diagram and 24V measured, I think it's too high, and dissipation in them would be much too high. Connected as a zener the other side of R42 390R makes much more sense.https://cr4.globalspec.com/thread/72501/Emitter-Base-Breakdown-Voltage-of-NPN-Used-as-Zener

author: bd139
author_rank: Super Contributor
timestamp: August 29, 2021, 09:36:17 am
content: Peak power perhaps. Average power not so much. Don’t forget it will just be clipping the AC peaks. The higher impedance of the tiny little transformer probably helps it not to blow them up.I agree it should be the other side of R42 but it probably isn’t and it was intentional.

author: Fred M
author_rank: Newbie
timestamp: August 29, 2021, 08:02:21 pm
content: Just disconnected D9 and D10. Didn't change the power to the audio section much-- raised slightly from 25.4 to 25.8V. And the "10V" side of R42 went from 10.6 to 11.8 V. Slapped a 1W 10V Zener on that side and it's come down to 9.9V. All seems well. Many thanks for the discussion!I'm surprised this mod hasn't been documented anywhere else besides this group.Now to ferret out the electrolytics that have changed value. So far they have all moved up in capacitance, even though this set had very little use. I remember it didn't do much better for me than the 3-transistor regenerative Radio Shack Globe Patrol receiver I had been using. The 717 was full of images and the sensitivity wasn't any better. I also remember writing a letter to Heathkit about the ringing and they sent me a ferrite bead for the gate of Q1. As I recall that didn't help.I'm going to keep trying to make it better. I see that because I lacked a 60W soldering iron back then that all the braids going to ground really aren't soldered well. Will resolder them shortly.I replaced the lamps with AC LED bayonet bulbs from Pinball Life. Brighter and should increase the life of the transformer. And I'm contemplating adding a broadband RF amp between the antenna jack and the tuned RF coils. It'll probably overload the whole thing but I'll give it a try externally first. Big plans here if the wife will stop assigning me honeydos.Thank you all for the advice and I hope the discussion continues.