bendix germanium transistor replacement? HP6823A "audio" amplifier *fixed, trace

[coppercone2]

I fried an audio amplifier doing something complicated, or maybe it broke by itself after light use, hard to tell

The transistor is B1654 in the manual and I found "Bendix 0 - 407" in the sister unit. Its from a HP6823A

I have a feeling I am fucked but I thought I would try the forum. I have a working sister unit so I found that its open on the diode test and the manual seems to point me to the same place.

https://www.keysight.com/us/en/assets/9018-03150/user-manuals/9018-03150.pdf?success=true

I just need something that works I don't care if its authentic or requires mods etc

Can't find any reference about these transistors at all

[floobydust]

schematic download didn't work, it's not at BAMA either. Can you post the sch or portion. Likely need to change the bias circuit for something silicon.

[coppercone2]

ok, i uploaded it

I don't really know about this type of circuit and it is pissing me off because I need the amplifier to continue experiments, unless I want to build something ridiculous by brute force with unknown characteristics, feel like i am getting constantly fucked by old gear 

I think it was on the way out because when I was lookin at the scope trace I had some nasty wiggling going on which I thought was a trigger/noise issue but I think the transistor was just fading out

I don't mind buying old germanium but I have no idea wtf to put in there

[bob91343]

An old popular small signal germanium transistor is the 2N404.  I might have one.  If you need some power dissipation or high voltage, you will have to use something else.

[coppercone2]

no, its a power device, its the to oval package

I don't mind buying an old transistor, this is not a modernization request, unless thats the only way, I just have no idea what parameters I want.


confused myself, not sure how that circuit works, if someone wants to teach me how to rebias that thing for a modern transistor..

[floobydust]

I would wonder about the crossover point, if Q1 and Q2 are ever both on it will cause current-limit I think.
But the circuit looks like it doesn't care if Q2 is Ge or Si. CR18, CR19 seem to both be Si leaving overlap actually    What does CR18 read for Vf?

I'd put in a vanilla PNP like 2N2955; 2N6609, MJ15023. What's in your junk box? Higher speed parts should not be a problem because this is an EF.
Q2 PNP 1850-0407 160V 25A 100W Ge Delco, Bendix, Gpd
Q1 NPN 1854-0225 60V 15A TO-3 like a 2N3055.

[coppercone2]

the datasheet says the CR18 is 1.0 and the cr19 is 1.25 (for the other side)

[coppercone2]

in circuit cr19 reads 0.550 and cr 19 reads 0.135, the working unit is very close at 0.554 and 0.139

cr18 looks like a silver can with a red stripe and cr19 looks like black bell (very weird).

i think the 0.139 = germanium?

[coppercone2]

lifted one leg on cr18, it measures 0.118 when hot so I think the in circuit measurement is accurate

lifted c19 and its 0.496

0.1 for a diode is low, I think thats germanium, I expect between 0.4-0.7 for a silicon? I think their right because the working supply measures the same

measured with 34401A

says its germanium
https://www.datasheets360.com/part/detail/1n91/-7020426734529444701/

[floobydust]

The schematic says:
CR18 Rect. Si 150mA 100PIV 1N91 G.E. but this is really germanium.
CR19 Rect. Si 500mA 200PRV 1N3253 RCA

They're both oddball diodes to give some bias current to Q1 and Q2, by CR18 and CR19 Vf adding up should be a bit less, and at most add up- to the VBE of Q1 and Q2.

If Q2 gets a Si replacement, CR18 as Ge would give a deadzone in the crossover region. The PSU might go unstable at that point around 0V out and squeal I'd guess.
So I would change CR18 to Si but with a bit less Vf than Q2 VBE. I have no idea what idle current goal is, likely very small there and the two diodes aren't thermal tracking Q1, Q2.
Measure your new Q2 VBE and try a 1N4004?

[coppercone2]

Ok I need to buy all the parts, I don't have a power PNP in my junk bin.

If I am buying new parts I might as well buy a germanium one, if you can suggest it, otherwise I should go for a 2N2955; 2N6609, MJ15023.

Not sure which one is the best bet?

What has a higher chance of success? I put alot of work otherwise restoring this amp so I am not trying to get the best dollar value


NTE121  ?

[floobydust]

The original Q2 PNP Ge is hand-selected by HP, I think for high hFE. This is why the part is exclusive and hard to find, as well as going back to 1966, and having a 25A 60V rating.
Maybe look at HEP644, 2N1166, NTE179 https://vetco.net/products/nte179-pnp-transistor-ge-af-amp lists the many parts of the day that are similar.  Vintage smaller Ge transistors suffer from shorts due to tin whiskers.

I'd put in a Si part and go through the pain of setting up the bias current with various Si CR18's, On-Semi MJ2955 or MJ15023, MJ15016G is quite tough. There's no protection diodes across Q1, Q2 so inductive loads could be a problem for this PSU. What are you using this for it seems a bit unusual.

[coppercone2]

will try nte179, looks like high gain, i was fooling around with a motor and dummy loads tryin to do something (I noticed the sine wave looked fuzzy on dummy load, compared to the other one, I think it was a sign it was going to blow)

i had germanium confused with selenium technology, i was thinking I was up shit creek but it looks like those parts are not as terrible

not connected to motor, like a break on a electromagnet dragging on a motor flywheel

[Kleinstein]

I would go for a silicon transistor. They are more tolerant to high heat sink temperature. A germanium transistor may have a slight advantage at saturation, but thus should not be a problem here.

Instead of hunting for diodes with a suitable VBE, one could replace one diode with transistor as a VBE multiplier, so one can adjust the voltage, a bit like in class AB audio amplifiers.

The missing revere diodes to the power transisors could be added for some additional protection.

[coppercone2]

I will try a direct substitution of the high HFE germanium part and if it does not work or blows again following minor abuse I will replace it with a Silicon transistor

I also hope changing that diode does not effect anything on the current limiting circuit?

[floobydust]

The current-limiting is not affected by changing Q2 from Ge to Si.
How did Q2 fail? Sometimes nearby parts get taken out as well so I would check.

If you are using this with inductive or motor loads, add two protection diodes.
Typically across the the C-E of Q1 and Q2 but the current-limiter resistors are in the way, best from output to collector of Q1, Q2 each. Even a 1N4004 will protect the beast.
I notice this PSU is special in that it does not have an output cap and faster than expected 20kHz even with the slow fT 500kHz of the Ge part.

[bob91343]

I have a number of very old transistors, including some germanium.  Some are power transistors.  I suspect the detailed parameters are not critical, so nearly any TO-3 germanium should work.

[coppercone2]

it does say selected for reasons unknown, and its the only part I could find easily that is different when comparing the two PCBs with a diode tester, but its obvious because it says when you measure the voltage in step 2 of the PCB low that does not involve desoldering the transistor I don't have on my revision (which is step 1) (there are 2 revs of the PCB, each of my units are slightly different, I could just not find one of the Q's on my PCB), it says check for Q2 open. and there is a  200mV drop on the working unit but a open circuit / infinity on the non working unit, right at Q2 which is what the manual attributes to that voltage failure. I assume if I fix that the other problems will go away.. I am pretty sure I have my man

[coppercone2]

Hmm, I remeasured the same circuit points yesterday as today. The voltage drop changed. I don't want to take the chassis apart until I get my supposed replacement transistor. It was reading nothing now its reading 0.9V and drifty, other circuit is quick to jump to 0.6V and gives the reading I expect from a proper semiconductor junction

[Kleinstein]

There is some frequency compensation in the stage before the ouput transistors. So I would no consider the power transistor so critical - though some of the Ge power transistors can be awfully slow and this could be a point to check in the selected section.  The old Ge transistors can also vary quite a bit in the initial gain and over time the gain tends to go up.

[coppercone2]

I have a dual HP supply that has which one side which seems to be adjusting to setpoint faster then the other supply, would higher gain in the transistor in a similar configuration from gain drift cause this ? I thought it was capacitance shrinkage but maybe its both.

[coppercone2]

is it possible the matching is related to the silicon NPN transistor driving the positive rail? not related to total gain?

[Kleinstein]

I don't think the transistor gain should be that important in this circuit. It can effect the maximum current, as the base current is limited by the current source around Q3. So a lower gain transistor may not reach the full current.

The speed is mainly set by the capacitors in the compensation (C5,C6, C7, maybe the cap at Q4) - these should be film / keramik types and thus not much aging expected there.

I see only a need to have matching for the sum if the VBE of the power transistors to the 2 diodes. This is to ensure the power transistors are no on at the same time, fighting each other and also keeping the dead zone around zero current small (or keeping a small standing current). There is quite some emitter resistance. So the matching should not be that critical.

[floobydust]

The PSU has a closed-loop gain of 20dB with 20kHz bandwidth, so it doesn't have a lot of gain to spare and a lazy Q2 might be something they wanted to avoid. Is an hFE of 10 going to work there? It's lucky to have fT of 200-500kHz.
But it's strange output current is 0.5A but there are TO-3 transitors rated 10A or more, at about 40V for worst-case SOA.
It might be from a problem with shoot-through current, or the designers mid-1960's just found transistors unreliable or HP being extra conservative. Well, it does weigh in at 16lbs.

[Kleinstein]

The power rating with germanium transistors is a bit tricky, as the maximum temperature for germanium is at some 70-90 C. So if the case temperature is not 25 C as for the nominal power, but more like 50 C for a large heat sink, the permissible power is down 50%. Going higher temperaure it gets worse quite fast.
Germanium power transistors have no real problem with a high current, but are more power dissipation limited. So the > 20 A current rating comes for free when you need a 150 W nominal (maybe 50 W real world) power rating.

For transisents the worst case voltage may reach some 60 V, if the load is reactive, like a capacitor / inductor. I can imagine the SOA could be a problem at higher temperature and voltages of more than some 30 V, as there seems to be very little internal series resistance to ensure even currrent distribution.
The Si transistor may be a bit over-kill, following the rules for the more sensitive germanium transistors.

Selecting the Gemanium transistor for high gain is relative - worst case gain can be really low. There is not that much series resistance at the base - so curent gain of the transistor would not effect the loop gain (mainly voltage based) very much. A low current gain would need a higher current from the constant current source and thus more power loss in the stages in front.