SAE Pre Drive Board (SAE 22-0091)

[jw68317]

Greetings all,

I am in the process of repairing and restoring an SAE 2500 Pre-Drive Board. In taking out the screws to the hear sink, one of the copper rings separated from the pcb (see !MG_1070). It is the bottom screw hole in the photograph and is the bottom hole of the q4 transistor. In looking at the spec sheet (also attached), although there is no direct trace between it, there should be continuity between that heat sink screw and the outside nodes of R31 and R36. There is continuity between those 3 nodes on the twin pre-drive board in the amplifier. IMG_1072 is a photo with my multimeter prongs pointing to the three nodes, and IMG_1073 and 1074 are photos of the twin board for comparison.

As I mentioned before, in looking at the board and the spec sheet, there's no direct trace between these. So my first question is - do I need to re-establish continuity between these nodes despite the fact that they're not connect via trace line? I suspect the answer will be yes, which leads to the second question: how would you go about doing that? My initial thought would be to jumper the bottom screw node with the outside nodes of the two resistors, but let me know if there is other, preferable methods. If using a jumper is the way to go, would you use one wire and solder it to both the resistors on the one side, or two jumper cables, one to each resistor (the latter seems like overkill but thought I'd ask).

Any assistance or answers are welcome. Thanks in advance.

Jack

 

[ivandezande]

I’m confused. You do know the other mounting hole, also connected to the case of the transistor, is the active one, right? They don’t both have to go somewhere.
Should be replacing with MJE15032/3G anyways.

 

[hamrules]

Why did you take the screws out in the first place. They are soldered into place on the circuit board. You just take off the nuts after de-soldering the transistor leads to get the transistor / heatsink off.

 

[jw68317]

I’m confused. You do know the other mounting hole, also connected to the case of the transistor, is the active one, right? They don’t both have to go somewhere.
Should be replacing with MJE15032/3G anyways.

Thanks and yes I understand that and already have those transistors on order. It’s also my understanding that I would only need to connect the mounting holes of Q2 and Q3 (see for example the picture of the same board from hamrules from a while back).

My question is, given that is the case, does that bottom mounting hole of Q4 need continuity to those aforementioned resistors as it is the “inactive” mounting hole? If not, then I wouldn’t go to the trouble of putting in a jumper between that mounting hole to them.

 

[jw68317]

Why did you take the screws out in the first place. They are soldered into place on the circuit board. You just take off the nuts after de-soldering the transistor leads to get the transistor / heatsink off.

Thanks for the response. Yes in hindsight I wouldn’t have messed with them, I was planning replace them simply out of completeness but have realized that’s more trouble than it’s worth. Unfortunately I did remove them and have this potential issue.

As ivandezande mentioned, that particular mounting hole isn’t the active hole for the transistor. So all that being the case, I’m simply wondering if continuity between that hole and the two aforementioned resistors is necessary. If not, I could re-adhere the copper ring within that mounting hole and move forward as there is no trace between those points to repair.

 

[hamrules]

Thanks and yes I understand that and already have those transistors on order. It’s also my understanding that I would only need to connect the mounting holes of Q2 and Q3 (see for example the picture of the same board from hamrules from a while back).

My question is, given that is the case, does that bottom mounting hole of Q4 need continuity to those aforementioned resistors as it is the “inactive” mounting hole? If not, then I wouldn’t go to the trouble of putting in a jumper between that mounting hole to them.

Correct, you don’t need to connect that mounting hole. Just do it like the pic you have of what I did above. On the mounting holes not used by the newer T0220’s I used a hard fiber washer between the heatsink and nut just to be sure there was no continuity.
So it was hard fiber washer first, then steel washer and finally nut. Worked just fine.

 

[jw68317]

A bit of a delay, but I finally finished up the first pre-drive board and it runs through the amp.

Onto the next one (which will hopefully be less of a hassle)…

 

[jw68317]

Second pre-drive board assembled. Onto the drive boards once parts arrive.

 

[hamrules]

I would have insulated the jumpers on the back just because.

 

[jw68317]

Insulated the jumpers on the pre-drive boards. I'm currently working on the drive boards and will send pictures once completed. Question in the meantime - for the relay board, would you guys necessarily replace the relays themselves? If so, what is a modern replacement that would fit in the footprint of this board. All the relays I've found are much too long for the relay board itself.

 

[hamrules]

AZ428-56-201 is what I showed for the one I did. I got some AZ428-56-211's from the auction site and from what I remember I broke off one of the tabs as it was a DPDT instead of a DPST. I do not know for sure if there is a relay from the big electronics stores that would fit, probably is with some research.

 

[jw68317]

Thanks I’ll do some digging on that. For the drive board transistors, I’m assuming I need to put jumpers on some of them? It looks like I might need to do it for at least Q13 and Q14, but let me know if I’m off base.

 

[hamrules]

All of them.

 

[jw68317]

Finished the drive boards and relay board as well. Time to move onto the mainboard. Most of the transistors on the heatsink are shot, so I’ve got to order replacements.

 

[jw68317]

Hi all. Finally got the replacements for the to-3 transistors and mounted them to the heatsink. I’m actually waiting on some shoulder washers to insulate the mounting screws themselves. In the meantime, I had a few questions about setting the bias for this unit, as the service manual isn’t entirely clear to me. First, there appears to be 2 bias pots per side (r61 on the main board and r38 on the predrive and 1 offset pot (r37 on the predrive).

First question is, how do you access r61 to adjust with the unit together and on? I see that it is viewable from the bottom vents of the amplifier itself (see pic), but do you need to unscrew the bottom of the board entirely while the unit is on to adjust the bias for the output or am I over complicating it.

Second, what do I connect to when adjusting the bias on both the main and pre drive boards? I suspect I need to connect to one of the white emitter resistors for the former but don’t know what to connect to for the latter.

Any assistance on this front is greatly appreciated, as I don’t want to wreck my gear immediately upon putting it all back together. Many thanks as always.

Jack

 

[hamrules]

I would leave the unit unscrewed so I could put a piece of wood between the board and case so I could get a long screwdriver in to adjust. I insulated the screwdriver with tape or heat shrink.

 

[hamrules]

The pre-driver has the dc offset and quiescent current adjustments.

 

[hamrules]

You can monitor the QC & BIAS from the pre-drive board.

 

[hamrules]

 

[jw68317]

Thanks for all that information! Once I get the shoulder washers I’ll get to putting this thing back together and make those adjustments.