Any opinions on this large difference in HFE numbers?

[62vauxhall]

Within the past week I decided to use a (built by someone from a kit) Hafler DH101 preamp that's been more or less idle for some time. I've had it well over ten years and during that time have replaced all the electrolytic capacitors, RCA jacks and the strips the jacks mount to. The times I did use it, other than it's inherent quirks, it seemed to function OK.

My last go at using it, I heard a previously unnoticed background noise which I've determined to be heat caused. The pre-amp was sitting on top of a MOSFET power amp. The noise did not manifest until things warmed up. Fairly sure the noise is transistor related because I blew hot air over the PC board and soon, the noise appeared.

The method was crude and I could not tell which transistor or transistors were to blame. There aren't many transistor types present so I began trying to learn which new transistors could replace what originals are on the board.

Over the past couple of days, I read posts in online forums about Hafler DH101 repairs or restorations. I compiled a list of candidate transistors but am puzzled about some HFE numbers.

Some existing Darlington transistors on the board are MPSA63 (or MPSA65) with an HFE of 20 & 50 respectfully. If I understood correctly, a BC516 can replace it but it's HFE is 30,000.

The companion MPSA13 has HFE of 5. Suggested replacement for that one is BSC517 with HFA of also 30,000.

That is a big disparity between values.

I've already ordered BC516 & BC517 but am now wondering about those substantially higher HFE values.

 

[AngrySailor]

Within the past week I decided to use a (built by someone from a kit) Hafler DH101 preamp that's been more or less idle for some time. I've had it well over ten years and during that time have replaced all the electrolytic capacitors, RCA jacks and the strips the jacks mount to. The times I did use it, other than it's inherent quirks, it seemed to function OK.

My last go at using it, I heard a previously unnoticed background noise which I've determined to be heat caused. The pre-amp was sitting on top of a MOSFET power amp. The noise did not manifest until things warmed up. Fairly sure the noise is transistor related because I blew hot air over the PC board and soon, the noise appeared.

The method was crude and I could not tell which transistor or transistors were to blame. There aren't many transistor types present so I began trying to learn which new transistors could replace what originals are on the board.

Over the past couple of days, I read posts in online forums about Hafler DH101 repairs or restorations. I compiled a list of candidate transistors but am puzzled about some HFE numbers.

Some existing Darlington transistors on the board are MPSA63 (or MPSA65) with an HFE of 20 & 50 respectfully. If I understood correctly, a BC516 can replace it but it's HFE is 30,000.

The companion MPSA13 has HFE of 5. Suggested replacement for that one is BSC517 with HFA of also 30,000.

That is a big disparity between values.

I've already ordered BC516 & BC517 but am now wondering about those substantially higher HFE values.

Interested to hear opinions on this. Ive got one of them little component testers and have noticed variations as well...

 

[Gepetto]

Within the past week I decided to use a (built by someone from a kit) Hafler DH101 preamp that's been more or less idle for some time. I've had it well over ten years and during that time have replaced all the electrolytic capacitors, RCA jacks and the strips the jacks mount to. The times I did use it, other than it's inherent quirks, it seemed to function OK.

My last go at using it, I heard a previously unnoticed background noise which I've determined to be heat caused. The pre-amp was sitting on top of a MOSFET power amp. The noise did not manifest until things warmed up. Fairly sure the noise is transistor related because I blew hot air over the PC board and soon, the noise appeared.

The method was crude and I could not tell which transistor or transistors were to blame. There aren't many transistor types present so I began trying to learn which new transistors could replace what originals are on the board.

Over the past couple of days, I read posts in online forums about Hafler DH101 repairs or restorations. I compiled a list of candidate transistors but am puzzled about some HFE numbers.

Some existing Darlington transistors on the board are MPSA63 (or MPSA65) with an HFE of 20 & 50 respectfully. If I understood correctly, a BC516 can replace it but it's HFE is 30,000.

The companion MPSA13 has HFE of 5. Suggested replacement for that one is BSC517 with HFA of also 30,000.

That is a big disparity between values.

I've already ordered BC516 & BC517 but am now wondering about those substantially higher HFE values.

Are you leaving off HFE zeroes in your text? It is hard to follow if you are using shorthand...

 

[J!m]

Since you identified it was heat-driven, get it hot (and noisy) and blast compnents with an inveterd can of "air" to chill each component until you find your culprit(s).

 

[62vauxhall]

Are you leaving off HFE zeroes in your text? It is hard to follow if you are using shorthand...

Thanks and no, I took that information from a datasheet site I was looking at. I did not look at others but your comment prompted me to do that. According to info on alldatasheet.com, HFE for MPSA63 is either 5,000 or 10,000.



And HFE for MPSA65 either 50,000 or 20,000



I can't recall from which site I obtained those low numbers. I did not make note and it was obviously wrong.

HFE for the enroute replacements are still higher but a lot closer.

 

[mlucitt]

Datasheets can be tricky. Some (such as the ones shown above) give minimum values, others give maximum values. The manufacturers can pick what they want to show for data.
I would replace the all the Darlington transistors with the exact part from the top manufacturer. That is what David Hafler did.
I would also put my preamplifier somewhere besides on top of the MOSFET power amplifier.

 

[Gepetto]

Thanks and no, I took that information from a datasheet site I was looking at. I did not look at others but your comment prompted me to do that. According to info on alldatasheet.com, HFE for MPSA63 is either 5,000 or 10,000.

View attachment 74250

And HFE for MPSA65 either 50,000 or 20,000

View attachment 74251

I can't recall from which site I obtained those low numbers. I did not make note and it was obviously wrong.

HFE for the enroute replacements are still higher but a lot closer.

You can go right to ON Semi's website and get the authentic datasheets for the devices you mention. These are all Motorola devices that ON Semi acquired when they took over the Motorola library of discrete parts.

 

[Gepetto]

You can still buy MPSA13 and MPSA63 devices so I do not know why you would substitute any other device type for them...

 

[NavLinear]

Mouser carries them.

https://www.mouser.com/ProductDetai...r/MPSA63-PBFREE?qs=u16ybLDytRYfz7Z%2BhBMR0w==

https://www.mouser.com/ProductDetai...nGdrLjumseOZPRVL0y4E7eCiRw/3qzRP5qU%2Bc%2BQ==

 

[62vauxhall]

Interesting.

The last time I ordered from Mouser, shipping took a couple of weeks so I have taken to using Digikey. Digikey's shipments arrive in a couple of days.
What popped up first on Digikey's site was the description for On Semi's MPSA63 which said "obsolete" and likewise for MPSA13. That's what prompted my looking for alternatives. That and an AK thread by someone re-building a DH101 who used BC516 & BC516 as substitutes.

I did not check further but now see that "obsolete" referred only to On Semi's offering and that there are others brands.

Oh well - live and learn.

I could leave those two types alone for now and replace the other BJT's in the meantime with what I have on hand and see what happens. If noise persists I could install those BC516 & BC517 when they get here. Or, order some non-On Semi versions of MPSA13 & MPSA63.

 

[nakdoc]

Noise is generally NOT limited to transistors, and heat usually makes noise worse, not better. Resistors and capacitors (rarely) can cause noise. Make a nozzle on your heat source and carefully narrow the heated air to identify a smaller number of parts which seem noisy. The DH101 suffers badly from intermittent connections between top side and bottom side of the circuit board. These connections are called vias or feedthrus. Vias are heat sensitive! I believe there are 30 vias and I recommend you solder 1/4" wires through each and solder top and bottom. Then test again for noise.

 

[62vauxhall]

Replacing transistors in this DH101 fast became a frustrating experience. The diagram I was following (contained in the service manual) used triangle symbols to indicate transistor positions and designated position numbers - Q1, Q2, etc. I have not encountered transistors depicted with triangle symbols before.

As it happened, Q1 & Q2 are the phono preamp and I had no intention to replace those.



At first glance, I thought it was logical that the triangle's base represented the flat face of a TO92 transistor. But after scrutinizing the transistors in positions Q1 & Q2, it appeared that the reverse was true - that the triangle's apex represented the transistors flat face. So I merrily proceeded to replace every TO92 transistor (excepting Q1 & Q2) but the DH101 did not work afterwards.

Obviously I made a mistake.

Over the past several days, I made enlarged drawings by hand so as to confirm I oriented the transistors correctly. My first go at confirmation made me think I got a few of them wrong so out they came and new ones installed. Since the transistors are in pairs, I replaced not just the suspected mistakenly installed transistor but it's compliment as well yet still no joy.

After a couple of sessions of removal and replacement, I went back to what prompted my interpretation of the triangle symbol vs transistor orientation.

I discovered that transistors Q1 & Q2 were not what was specified in the parts list. What was intended to be there was MPSA13 and MPSA65 with pinouts of EBC. What was actually on the board were BC413 and BC415 and those have pinouts of CBE. So my interpretation of the triangle symbols I was using as a guide to orient the bulk of transistors on the board was ass backwards.

I now have a small pile of TO92 transistors that may or may not be OK since they had power applied while backwardly installed.

With a new batch of transistors installed the right was round, the DH101 does work. Whether it works as well as it should with new but different transistors, I don't know. Has the background noise that prompted this chore been eliminated? Time will tell.