Heathkit AP-1800 Preamp PS Transistor Substitution Question

[RealtoReel]

Greetings all –

I’m working on restoring a Heathkit AP-1800 preamp, and am stuck trying to find replacements for two transistors that support the +/- 35 VDC power supply. The original Q703 / Q707 transistors (tested shorted, with resulting PS output voltages too high) are:

Heathkit # Cross Reference #

Q707 417-283 SMO7275 NPN

Q703 417-284 SM62186 PNP

I have tried using 2N3906 (tested HFE =190 @ 5mA) and 2N3905 (tested HFE =163 @ 5mA) transistors for the positive rail, but apparently their gain is wrong, as the resulting output voltage was too low.

All of the other transistors and electrolytic caps have been replaced, and measured voltages reasonably track spec until the collector of Q703 (and emitter of Q704). I have attached the PS schematic and text from Heathkit’s Theory of Operation section.

Does anyone have suggestions for a replacement set, or perhaps what specifications (e.g., gain) I should be searching for?

 

[Binkman]

Hope this helps.. no subs listed
From a heathkit cross ref source: https://www.qsl.net/w6ovp/heathkitparts.htm
417-284 SM62186 PNP 15V 200mA 850MHz
417-283 SMO7275 NPN 40V 800mA 300MHz

 

[fred soop]

I think that Q704 needs to be something heavier than a 2N3904. Is the original a TO-92 case? It should probably be a tab mount of some sort. Almost any NPN tab mount should work as this is not a really critical circuit.

 

[RealtoReel]

I think that Q704 needs to be something heavier than a 2N3904. Is the original a TO-92 case? It should probably be a tab mount of some sort. Almost any NPN tab mount should work as this is not a really critical circuit.

Good morning Fred - Yes, both Q704 (MJE181) and Q708 (MJE171) are in a TO-225-3 package, and affixed to a good sized heat sink. I've attached a picture of the power supply. From what I can tell, those two transistors are OK, it's the two upstream (Q703/Q707) replacements that I'm guessing are not right.

 

[RealtoReel]

Hope this helps.. no subs listed
From a heathkit cross ref source: https://www.qsl.net/w6ovp/heathkitparts.htm
417-284 SM62186 PNP 15V 200mA 850MHz
417-283 SMO7275 NPN 40V 800mA 300MHz

Good morning - I Googled that info, and found for example that SM07275 has a rough equivalent of NTE123AP. I haven't tried one of those yet given the circuit's poor response to a 2N3903 or 2N3904. Since it's a DC circuit (providing a regulated +/- 35vdc), I assume the bandwidth parameters aren't important, correct? So does that leave gain (hFE) as the spec I need to somehow match with the original transistors?

 

[Binkman]

morn'in back at ya.. I'm on my 1st cup of java and brain ain't kick'n yet. with the lil bit of info in that link kinda hard to find an equivalent. Maybe you can get a clue from measuring from and Q702 & Q706 or rational clue. Nothing printed on the transistors?
I blew up the schema maybe it'll help.

 

[Binkman]

I need to somehow match with the original transistors?

Are you sure the transistors you indicated match the V's input? got the right ones?
Well who says all data sheets are correct?
Lets try this. pretty good listing but digging required. might get it off the limited specs.
https://rtellason.com/trans-2n.html

 

[fred soop]

Transistor beta for this circuit should not be critical at all, especially with the CFP connection. Probably something else wrong.