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9090 Driver Board Transistors

G

G Dudds

New Member
Hey all,

I am in the middle of restoring a 9090.
This is my first effort to restore anything so I am learning as I go.
I have done a fair bit of reading earlier on 9090 threads but still have a few questions regarding the driver board, 2436.

What I have done (on this board):
- Replaced electrolytic caps
- Replaced trim pots
- Replaced fuse resistors
- Replaced ZD01/ZD02
- Replaced D05/06
- Added fuses as per service bulletin

When I powered up on a DBT it came out of protection but the bulb was glowing a bit (note that output transistors are not installed yet).
I started checking transistors and found all junctions of TR14 were shorted together.

My questions are:
  1. Since I am replacing TR14, should I also be replacing TR12? Are they sort of a set?
  2. Should I do the other side too (11/13)?
  3. I have already pulled TR06 and TR08 off to check them. I feel like I may as well replace all of TR05 through TR14 at this point (with the exception of TR07/08 because I don't want to bother modifying the heat sink to fit the replacement). Am I getting out of control? Is this a bad idea?
  4. From what I understand only the TR01/TR03 pair and TR02/TR04 pair need to be gain matched. Is this correct? I am not replacing these because of this. Can all the others simply be swapped without considering the gain of the replaced transistor or other transistors in the circuit?

Also, I'd like to double check how I should tune offset and bias after getting the transistors sorted. I read up on this but it was still not 100% clear to me. Procedure:
  1. Set offsets to 0.
  2. Set bias TPs to middle.
  3. Power up on DBT.
  4. Tune offset then bias.
  5. Set offset and bias back to 0 / middle.
  6. Power up directly (No DBT).
  7. Tune offset.
  8. Tune bias.
  9. Wait 30 mins.
  10. Tune again.
Is the purpose of tuning with the DBT just to prove that the circuitry is healthy before hitting with full power? From what I have read the values will not be correct so am trying to understand the purpose of this.

For the record, here is my list of replacement transistors. I derived this from other threads:
TR01-04 : ZTX694B (although I don't plan to replace these)
TR05/06 : KSA1381ESTU
TR07/08 : BD137G (again not planning to replace at this time)
TR09/10 : KSC3503DS
TR11/12 : MJE15030G
TR13/14 : MJE15031G
​
Thanks!
 
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  • The driver amp is built with one channel per side symmetrically mirroring each other. It's advisable to keep the symmetry, so if swapping a stock transistor on the right side (even numbered like TR-12/TR-14 etc...) with a modern replacement, you would typically want to replace the mirror counterpart on the left side (odd numbered TR-11/TR-13 etc...). This is generally true for all components, but it's more important with semi-conductors b/c it's been generations of technological advances with newer replacements. With some semi-conductors, like the differential pairs transistors (TR-01...TR-04), you would also want to match the gain (hfe) of all. With others, like the power transistors (TR-11...TR-14), gain can be just close enough but you do not want to leave the old transistors on one side and swap for new on the other.
  • The protection PCB purpose is to eliminate harmful DC levels from reaching the speakers. It has a relay that will engage when those levels drop to a certain threshold (probably around 300mV - I'm not sure). Without the driver board installed, there's no output going to the speakers, so obviously the protection board does not detect a DC issue and the green LED comes on. The same applies without the output transistors. It's pretty useless other than for debugging the protection board or power supply.
  • Be extremely careful when you turn the bias current adjustment trimpots all the way to any direction b/c that directionality is super counter intuitive and it's a well known recipe for frying a driver amp and output transistors. You're much safer with the bias trimpots smack in the middle than with turning them all the way to either side. Never have a first time go at it without a DBT. For biasing the unit you need to measure current in milliamps in series and you need to know your meter and make sure it's internal fuse is intact. You measure bias current on the fuse holders without the fuse! The service manual has the driver amp board installed backwards facing the back of the unit, thus refers to the opposite fuse holders for channel orientation. It also lists the bias current value as voltage in mV units instead of current in milliamps and it's 50mA (mili-amperes). The driver amp is typically installed with the trimpots facing you at the front of the unit and it's VR04/F08 for right channel and VR03/F07 for the left. DC offset sticky here. The bias current adjustment and DC offset procedure are interlinked. One will affect the other. It is usually being verified more than ones, so proper background read is a must

    Some tools:

    6GSXCz2.jpg


    General Orientation - notice where the probes are and what we measure

    MGJTawo.jpg


    EPwS8o6.jpg


  • a DBT is a current limiter, so if your unit shorts, it cannot draw enough current to catastrophically damage (AKA "fry") it self. It's a safety measure and it protects the relatively expensive output transistors and the driver amp components. However, more than just a one time emergency protection fuse, the behavior of the bulb of a DBT is indicative of general health. When it glows bright (like when the unit is turned on and the filter caps are charged) it means there's lot's of current flowing in. When it stays bright - there's a short (or the unit is just drawing too much current). When the bulb stays dim it shows an idle/normal operation and when it pulsates, it can reveal a problem (possibly a pinout error or a bad semi-conductor etc...). It's a very effective tool, other than a safety measure. The only minor caveats of using a DBT is that depending on the bulb wattage, it may leave the unit a little power hungry. The power transformer unit may, in some cases, exhibit buzzing. Bias current settings are typically 2 - 3mA higher with a DBT than what they need to be with mains voltage, so as a final stage in the bias & DC offset adjustment procedure, it's typically required to re-adjust on mains voltage.
  • F2436 replacement transistors and pinout caveats discussed here.
 
If not familiar with using your meter to measure current. Read and heed your meters owners manual. Meters do not auto range in the (current/amperage) setting. So always start by manually selecting the highest current range ie 20A. Then down range it once you have a good idea what's actually there. The key is not to over range or you will blow a fuse in your meter. Fluke fuses are about $7 a pop.
https://www.mouser.com/ProductDetail/Fluke/943121?qs=sGAEpiMZZMuBVu7Wy0UTywvz8Kf3RXLh
 
Last edited:
Hey guys,

Thanks for your replies. Things are progressing well. I can power up on DBT and tune bias successfully.

TR07/08 also turned out to have failed so I have replaced them with BD137GOS.
After searching other posts regarding these transistors, it looks like the ones I picked have a slightly lower HFE than normal. It is 40 for what I ordered, and 100 is more typical, like the 2N3417.
Is this a problem? I think these are used to compensate for temperature of TR13/14 so the gain may not be critical, but I am not a circuit expert.

thanks,
 
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