Transistor: proper continuity / fails hFE test

andrewsrea

andrewsrea

Well-known member
I am battling a client's science project amp repair (1970's Sunn Beta Lead solid state), where I get proper signal right up to the inverter circuit, which is clipping on both half's of the push-pull and phase cancelling the output. Thus the amp output to the speaker is almost nonexistent and has a malformed signal compared to the original 1K sine wave.

The PNP side of the inverter tested well for continuity (diode test) and had an hFE of 27 on my DMM (a little lower than I expected, but my DMM is low voltage vs. the requisite -40VDC).

NPN side is a 2N3440 that passed the diode continuity test, but registered '0' for hFE, which is typically indicative of a short. So I have conflicting results.

I was wondering if any of our solid-state gurus have had similar experiences like this and any recommendations for steps forward. I'd hate to throw money at this amp, by ordering a replacement, especially that replacements are not NOS, but Asian versions (which are typically hit or miss).

Please ring in if you have any ideas.
 
andrewsrea said:
passed the diode continuity test, but registered '0' for hFE, which is typically indicative of a short
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Dunno if this helps, but while ago I cooked couple Ge transistor exceeding their max current limit with trimpot on FF circuit and after that my cheap component tester read cooked transistors as shorted/faulty component. Cannot recall if it was E or C tied to B, but nevertheless that gave a diode reading.
 
JohnnyCreepy said:
Dunno if this helps, but while ago I cooked couple Ge transistor exceeding their max current limit with trimpot on FF circuit and after that my cheap component tester read cooked transistors as shorted/faulty component. Cannot recall if it was E or C tied to B, but nevertheless that gave a diode reading.
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Thanks. I am reading no shorts, proper forward voltage from E to B and C to B, plus proper reverse voltage readings.

The reason I am interested in the gain measurement (hFE) is this and its PNP counterpart are responsible for initiating the push-pull of the larger transistors in the output section. If they are not matched, the output becomes highly asymmetrical to the point that one side of the signal is basically off.


These are the most difficult diagnoses, as the parts are 'kind of working' and 'kind of not.' That dysfunction just becomes magnified downstream.
 
Haven’t done any transistor based PPs, but I’ve messed with tubes. Would your circuit be tweakable with adjustable bias or bias balance as it’s done with tubes? If the old NPN is cooked I’d go replacing PNP and NPN sides with new transistors and adding a bias adjustment addon there.

With tubes bias balance can be adjusted just using your ears by aiming to pot settings giving the lowest hum, so maybe it’s to be done with transistors too? Could make life easier as transistor don’t need to so tightly matched!

If you’re not familiar with this, it can be done with balance pot between each push&pull side or separate bias adjustment pots to each side. Search the web for bias adjustment and bias balance circuits, maybe someone has done this to Beta before?

Edit. https://el34world.com/charts/Schematics/files/sunn/Sunn_Schematics.htm

Looking Beta service manual now and yeah, there’s great info there. Test point voltages, circuit explanation etc. And I re-read your last post. So it’s the unity gain drive/start stage of PA section you have problems. Bias pot for output transistor is there, but is it balanced? Could it affect your too if it’s unbalanced? If test point readings are not ok, would it be easiest just to replace Q7 and it’s counterpart? They are just unity gain buffer stages so does it matter if new transistors have higher gain if other specs are same or greater?
 
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JohnnyCreepy said:
Haven’t done any transistor based PPs, but I’ve messed with tubes. Would your circuit be tweakable with adjustable bias or bias balance as it’s done with tubes? If the old NPN is cooked I’d go replacing PNP and NPN sides with new transistors and adding a bias adjustment addon there.

With tubes bias balance can be adjusted just using your ears by aiming to pot settings giving the lowest hum, so maybe it’s to be done with transistors too? Could make life easier as transistor don’t need to so tightly matched!

If you’re not familiar with this, it can be done with balance pot between each push&pull side or separate bias adjustment pots to each side. Search the web for bias adjustment and bias balance circuits, maybe someone has done this to Beta before?

Edit. https://el34world.com/charts/Schematics/files/sunn/Sunn_Schematics.htm

Looking Beta service manual now and yeah, there’s great info there. Test point voltages, circuit explanation etc. And I re-read your last post. So it’s the unity gain drive/start stage of PA section you have problems. Bias pot for output transistor is there, but is it balanced? Could it affect your too if it’s unbalanced? If test point readings are not ok, would it be easiest just to replace Q7 and it’s counterpart? They are just unity gain buffer stages so does it matter if new transistors have higher gain if other specs are same or greater?
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Correct. The 'trim' has pretty much the function of balancing the symmetry using bias (think of the bias balance in a silverface Fender from the early 70's).

I am trying to avoid driving the cost up for the client by throwing parts at it. Most people get upset when you tell them you had to scrap the 'mojo' vintage original components. What I am trying next is replacing the electrolytic caps, as part of what I am seeing is one side collapsing. If a cap has gone resistive and isn't holding charge, it could exhibit that result.
 
Recalling my amp repair days, I never futzed with wonky components when their test measurements were suspiciously off; I just replaced 'em. Especially where it came to the splitter/output sections. Just my 2¢
 
Cybercow said:
Recalling my amp repair days, I never futzed with wonky components when their test measurements were suspiciously off; I just replaced 'em. Especially where it came to the splitter/output sections. Just my 2¢
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I've been repairing amps since 1977 and when I was doing it to make a little side money, I'd also quickly replace and charge the customer, for any suspicious part - just for time sake. I'd primarily do what you suggest for my own gear, given the plethora of good replacements (with some exceptions, like obsolete MOSFETs).

Things have changed, as I am now retired from my Aerospace career (which unrelated was Program Management and Finance) and do this as a hobby which does not cost me money. Being a musician, I want my work to be lasting quality and tone for the client. Last and mostly due to internet lure, the folks who come to me for my service are in reverence to internet lure about magic old parts. Which means I've become part preservationist.
 
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