Need Substitutes For BF871 and BF872 Transistors - Traynor BLOC 100K amp

[CoopDaKill]

Need Substitutes For BF871 and BF872 Transistors - Traynor BLOC 100K amp. I have some MJE340STU & MJE350STU transistors that seem to be as close as I can find. Would someone please verify that these are ok to use, for me? Thanks a lot. I appreciate your time.Coop

[Jazz P Bass]

Looks good from here.

As long as you use a proper heatsink.

[CoopDaKill]

Thanks Jazz P Bass . The problem I've been having with this amp is, R107 gets too hot and starts smoking. When I got the amp, that resistor was cooked. Output transistors were blown. Most all electrolytic caps were way off spec. So, I replaced all that, All the transistors on the heatsink, and it's still overheating. The amp works and sounds fine, but R107 (220 ohm 1 watt) still gets too hot, and starts smoking after a few minutes. I have a 2 watt in there now, but it still gets so hot, it will unsolder itself. I can't figure it out. Guess, I need to take more voltage readings. Any advise will be greatly appreciated. Thanks
Coop

[g1]

Do you have -15V coming out of Q20? Sounds like something is loading down that line and overheating R107. Have you checked Q20?

[CoopDaKill]

Hi g1 . Good to hear from you too. I changed out Q20 & Q19. Heres some voltage readings for Q20 & Q19:

Q20 (BD140)
____________
Emitter: -15.3 vdv
Collector: -48.3 vdc
Base: -16.0 vdc

Q19 (BD139)
___________
Emitter: +14.9 vdc
Collector: +24.1 vdc
Base: +15.6 vdc

The -48vdc on collector of BD140 is twice what it should be. Possible bad Zener diode?

[J M Fahey]

Killer amp, very good and advanced design.

Dear coopdakill, please when mentioning parts add a little besides plain name, such as "R107, 220r at the -15V supply", "BF871/72 Reverb drivers" and so on.
I wasted 15 minutes looking for it in the reverb area, after all R100/101/102 live there, then near the power amp input, R113 lives there, etc.
And since these are photocopies of photocopies, some numbers are fuzzy or plain unreadable.
Thanks.

[J M Fahey]

Hi g1 . Good to hear from you too. I changed out Q20 & Q19. Heres some voltage readings for Q20 & Q19:

Q20 (BD140)
____________
Emitter: -15.3 vdv
Collector: -48.3 vdc
Base: -16.0 vdc

Q19 (BD139)
___________
Emitter: +14.9 vdc
Collector: +24.1 vdc
Base: +15.6 vdc

The -48vdc on collector of BD140 is twice what it should be. Possible bad Zener diode?

Why would you say that?
Those are 16V Zeners, you do not mention voltage across them which is the first thing to do if you want to pronounce them dead or alive.

That said, BD139/140 bases are held at -16 and + 15.6 VDC so I presume them good, nothing makes me think the opposite.

Emitters are respectively -15.3 and +14.9 VDC , what is expected, and about "1 diode drop" (600 to 700mV) away from Base, so they are working properly.

Only unusual value is that Collector voltage is different ... which might be normal or not.

Now said collector voltage comes from +/-55V supplies, through 220r 1W resistors, negative one is dropping 7V while the positive is dropping 31V .
Which means it´s passing more than 4X the current from the other rail.

Now it might indicate a failure or simply that the +15V rail feeds more "customers".

Repair the Reverb circuit, which so far is the only visibly "bad/wrong" part of the amp (you mention nothing else) and recheck.

[Dave H]

Q20 (BD140)
____________
Emitter: -15.3 vdv
Collector: -48.3 vdc
Base: -16.0 vdc

Q19 (BD139)
___________
Emitter: +14.9 vdc
Collector: +24.1 vdc
Base: +15.6 vdc

I only wasted 10 minutes looking for R107

If those measurements are correct it's R105 that should be smoking not R107
R105 is running at over 4W. R107 is only 0.2W

[g1]

Yes, it should be R105 on the negative side overheating, is that correct? Does it still measure 220R?
If so, please measure across R111 and R112 (22R) in the low voltage supplies there. It will help isolate the fault to the -15A or -15B lines.

[CoopDaKill]

Yes, it is R105 that's smoking. Sorry for my error. This board isn't marked, and I'm no pro at reading schematics. R105 (220R) is still measuring 220R. I measured voltage across R111 & R112 (22R). I get 2.9vdc across R112, and I get 0vdc across R111.

I did notice the reverb effect is very faint, even at max setting. I was checking resistors and found that R86 & R88 (10Mohms) are measuring around 6.7 Mohms each, in circuit. I powered on the amp for 30 seconds, then powered it off, and these two resistors are at 30 Mohms each, and quickly fall back to the 6.7Mohms. Not sure if this is any significance, but thought it was worth mentioning.

[Dave H]

Yes, it is R105 that's smoking. Sorry for my error. This board isn't marked, and I'm no pro at reading schematics. R105 (220R) is still measuring 220R. I measured voltage across R111 & R112 (22R). I get 2.9vdc across R112, and I get 0vdc across R111.

I think you may have R111 and R112 reversed also. R115 (220R) is smoking because it has 30V across it and R111 (22R) has 3V across it so it looks like the excess current is from +15B. Is Q19 (BD139) getting hot? Check C58 for leakage. What are the voltages on +15B and -15B?

[CoopDaKill]

Dave H, I do have R111 & R112 reversed. Sorry again. I'm trying my best. Seems Q19 (BD139) is not getting hot. C58 & C59 (470uf) each have 16vdc. All of these parts are new, as well. As far as checking capacitor leakage, I'm not sure I know how to do that.

[CoopDaKill]

Not sure else what to look for on this amp. Any other advise before I take it out back and shoot it? lol

[Dave H]

Did you measure the +15B and -15B voltages? If the excess current is going where I think it is then +15B should measure low at about +12V. The current is going from +15B to ground as the -15V side looks OK. C58 was the first thing I could see between +15B and ground. Disconnect C58 temporarily to see if +15B goes back up to +15V (assuming it really is low)

[nevetslab]

OK....picking up where everyone left off. Do you yet have +/- 15V thru the zener/pass transistor regulators from the +/- 55VDC supplies? You were having overheating problems on the 220 ohm collector resistors of the Pass Transistors (Q19 & Q20. Since the collector voltage of Q19 has pulled loads of current thru the 220 ohm collector resistor, while the zeners shows they're ok, it suggests you have a high current drain on where the supply connects to off '+15A' . It seems to rule out connections to '+15B' or that would be dragged down.

All of the op amps are running off of +/- 15V, as well as some support circuits, as well as local bypass caps. If there are any Dip Tantalum caps in that role, I'd lift those...though if they shorted, it would normally drag the 15V supply down. If the op amps are NOT socketed, they you have a bit of work to do. But, you may also carefully check with your finger the surface temperature on the op amps. Normally the case temperature is just a bit higher than ambient, though if one or more op amps are bad, they could be pulling all that current, dragging the collector voltage of Q18 down to that 24V level.

It's tedious work to find where the current is being sucked up, but that's the drill.