mc225 PS/BIAS Help

[jertub]

Hi!

I was going to replace the PS and BIAS on a MC225 that is original except the components on the bias board.

Please click below to see photos:.


ie:
SR3 selium rectifier replaced with 1N5408 diode
C30 10 uf was replaced with 22uf 63V.
The two other diodes were replaced with 1N4822

note: The two other caps were not replaced, and there was no additional resistor added to drop the voltage like I have been reading about. However, the bias measured from pin 6 of the power tubes appear correct at -21vdc.

So why am I getting the correct -21VDC but there is not a 33K resistor added to drop the voltage?

Should I just replace the other 2 caps on the bias board and leave it in this configuration?

Or should I start from scratch and rebuild it using the parts from the kit (which uses the voltage dropping resistor , a10uf cap and the other 2 caps and PS diodes)?

Thanks, I sure would appreciate your input.

Best regards,
Jerry

 

[cademan]

I think that if it reads fine I wouldn't mess with it but I am not anywhere near qualified to speak on behalf of Mac gear!

You know the 'ol saying that if it ain't broke........well you know the rest!

 

[jertub]

Im having a little trouble viewing the photos I posted, if there is an issue with them, please let me know.

I tried to attach the rest of the underside PS shots for good measure, let me know if you see something screwball...

 

[cademan]

Pics are working fine on my end.

 

[dcgillespie]

Jerry -- I'm not sure I understand your question. The resistors that produce the correct bias voltage of -21 vdc (R46, R47, R51) in the Mac 225 are all plainly visible in the second pic you posted in your first post.

If you are referring to an additional resistor that is required to be added when the selenium rectifier is changed out for silicon, that is NOT required on the MC225 -- because the rectifier is passing less than 1 mA of current. In other Mac models that use a cathode follower driver stage, a resistor may be required if the original bias voltage is to be maintained. But the Mac 225 has no such driver stage, and so no (or extremely little) current is drawn fthrough the bias rectifier. With current draw levels that low, a selenium rectifier and silicon rectifier will perform virtually identically, so no additional resistor is needed.

This points out an inconvenient fact to those who insist that every selenium rectifier will always explode given time, and therefore must be changed. Selenium rectifiers that pass significant amounts of current -- like in DC heater supplies and the like -- must absolutely be changed out, as the internal heat they develop breaks down the selenium, which causes more heat, causing things to go south pretty quickly. However, in applications like the Mac 225, Dynaco ST-70, MK II MK III, etc., these designs have the selenium rectifier passing just a couple of mA at best, so that in those specific applications, the selenium rectifier will last every bit as long as any silicon piece installed to replace them.

Dave

 

[jertub]

Thanks Dave (again!). I was confusing the information with the 240? I guess where I read in another thread that an additional resistor is needed in series with diode to drop the voltage. It said the value of that resistor is determined by first setting the plate voltage using a variac.

Now the mac schematics show R46 R47 R51 as 30k 33k and 22k. The kit I was going to use uses 2x 33K and 22K.

Why is the kit supplied with different value?

 

[dcgillespie]

Using a 33K for the 30K could be anything from trying to compensate for today's higher line voltage, to (possibly more likely) using a resistor that is a traditional value, and therefore easier to source. If C28 and C30 are good and have been replaced (usually only C30 needs to be replaced unless it's a bumblebee), and your bias and B+ voltage are proportionate to the published Mac values, then you're good to go.

Dave

 

[jertub]

Thanks for clearing that up Dave. I feel better that I don't have to figure out how to add an additional voltage dropping resistor that is required for the MC240, MC30 etc.

 

[mcgjohn]

Hi!

I was going to replace the PS and BIAS on a MC225 that is original except the components on the bias board.

Please click below to see photos:.
View attachment 1039723 View attachment 1039725

ie:
SR3 selium rectifier replaced with 1N5408 diode
C30 10 uf was replaced with 22uf 63V.
The two other diodes were replaced with 1N4822

note: The two other caps were not replaced, and there was no additional resistor added to drop the voltage like I have been reading about. However, the bias measured from pin 6 of the power tubes appear correct at -21vdc.

So why am I getting the correct -21VDC but there is not a 33K resistor added to drop the voltage?

Should I just replace the other 2 caps on the bias board and leave it in this configuration?

Or should I start from scratch and rebuild it using the parts from the kit (which uses the voltage dropping resistor , a10uf cap and the other 2 caps and PS diodes)?

Thanks, I sure would appreciate your input.

Best regards,
Jerry

Measure the AC voltage ahead of the diode and see if that is correct. Usually on the MC225, you'll need to swap out the 33K resistor with a 30K, but that is only going from memory.

Also how does the ratio of bias voltage compare to the B+ plate voltage?

best

John

 

[jertub]

So that last post adds to my confusions then. Your saying I may have to change the value of R46 (30k) or R47(33K)? Something not provided for in the kit I purchased and contrary to how I understand Dave's clear advice above.

Ive seen reference in another thread that suggested changing R46 to a 33K and to check R47 (33K) for drift to get the bias correct.... but as stated before, right now with the old power supply and SR3 selium replaced, the bias is OK.

When I replace the 3 power supply cans and the other two caps on the bias board, can I expect the bias voltage to change?

 

[mcgjohn]

So that last post adds to my confusions then. Your saying I may have to change the value of R46 (30k) or R47(33K)? Something not provided for in the kit I purchased and contrary to how I understand Dave's clear advice above.

Ive seen reference in another thread that suggested changing R46 to a 33K and to check R47 (33K) for drift to get the bias correct.... but as stated before, right now with the old power supply and SR3 selium replaced, the bias is OK.

When I replace the 3 power supply cans and the other two caps on the bias board, can I expect the bias voltage to change?

Sorry about that, did not mean to confuse.

the OEM cellinium rectifier is more lossy than modern silicon diode will be. (Voltage drop on new diode will be less). So you will not need to change R47, but you may need to adjust R46 to make up for the difference. I found I had to change R46 (30k) to a 33K value to bring the bias voltage into spec.

Changing the caps out should not cause much of a voltage difference unless one of the caps is shorted out.

 

[mcgjohn]

Here are a few pics of the bias and rectifier board if that will help.

just remember when you increase the resistor value, it will make the bias less negative, not more....

 

[jertub]

The photos are great. I do not know if changing the caps effected the voltage or not but the bias appears correct by using 33K instead of 30K.


Thanks again,
Jerry

 

[jertub]

I didn't have a terminal strip to add the new bias diode like I saw in other photos online. So I got a little creative. Workmanship is important to me, esp. for something as classic as a mac ....... I was hoping someone could tell me if my method of mounting the diode looks ok?

 

[Pio1980]

I didn't have a terminal strip to add the new bias diode like I saw in other photos online. So I got a little creative. Workmanship is important to me, esp. for something as classic as a mac ....... I was hoping someone could tell me if my method of mounting the diode looks ok?

View attachment 1055710

View attachment 1055713

Looks fine, I did some reshuffling on the ones I refurbed for a similar neat result.

 

[gadget73]

I see no problem there, probably exactly how I would have done it myself if I'd felt the need to change them.

 

[jertub]

probably exactly how I would have done it myself if I'd felt the need to change them.

Gadget your probably wondering why I bothered after seeing the 'before' picture..... up close you would see it looked flakey.

One observation, this is the first time I ever encountered old style turret boards... they are much more robust than I imagined!
.