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Old 29th March 2016, 07:40 AM   #1
kwape is offline kwape  United States
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Join Date: Nov 2015
Location: Plymouth
Default SAE Mark IIIC - Repair Help

I've purchased this amp as a basket case; it was missing outputs, top cover, and a choke assembly on one channel on the main amp board. Luckily it was not molested too much.

My problems are I am getting 40.1vdc measuring bias across Q13,14 collectors, Bias pots have no effect (should be 2.1vdc here). Relay doesn't click with outputs installed (did click without them) and dc offset measured @ the 0.125 resistors is -750mv channel B, -1.7 to -3.1 volts DC (erratic) channel A. offset pot has no effect on channel A. I've replaced the bias IC on a guess with CA3086 NOS from ebay but has no change. I've tested the bias ICs and I think they're all good. Also I was advised to short collectors of Q9,10 to eliminate the bias IC from the equation and still no effect. So its not the bias IC.

Here's what I've been doing so far:

I've installed all new outputs
MJ11021 for SAE204 (11-0070)
MJ4502 for SAE202 (11-0069)
MJ802 for SAE101 (11-0063)
MJ11022 for SAE103 (11-0064)

according to wardsweb.org these are appropriate equivalents.

hand made new choke assembly on a 2.2ohm wirewound resistor and wrapped with magnet wire. It is an exact replica of the original.

made CAD drawing to have a new cover fabricated.

on the main amp board the SAE301 (11-0075) diodes were replaced with a Motorola part # and installed backwards by someone else. They were also shot. Replaced them with IN5820. Replaced the IN4004 diodes on this board with new al well. Both 1.2ohm resistors to ground on this board were shorted, since been replaced.

On channel A driver board driver transistor NPN 2N6316 was replaced with a sony D291 (still tested good) but I through in a new 2N6316. The 1.2ohm resistor to ground was bad here as well and since been replaced.


I've recapped both driver and main amp board completely and replaced any carbon resistors with metal film.

@ the 51v zener diode on driver board I am getting 49.8v channel A, 50.1v channel B using dbt.

Last edited by kwape; 29th March 2016 at 07:44 AM.
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Old 29th March 2016, 07:50 AM   #2
kwape is offline kwape  United States
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Join Date: Nov 2015
Location: Plymouth
schematic & some pics


not sure where to go from here, if anyone has any suggestions I'd really appreciate it.
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Last edited by kwape; 29th March 2016 at 07:59 AM.
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Old 29th March 2016, 04:35 PM   #3
llwhtt is offline llwhtt  United States
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You should have 50 volts across the collectors of Q13/14, the manual is wrong, the schematic is correct. The approx. 2.1 volts will be across the bases of Q13/14 OR the collectors of Q9/10. The manual should have you measuring the collectors of Q9/10 as they are easier to get to.

Have you tried using the troubleshooting guide in the manual? You'll have to be able to recognize the errors in it though.

Go back and check all of your work. Diodes, transistors, capacitors all installed correctly and the their proper location?

Craig
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Old 30th March 2016, 04:52 AM   #4
llwhtt is offline llwhtt  United States
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Looking at the picture of the output PCB you have the big Schottky diodes installed backwards. The anodes go towards the + rail and cathodes go towards the - rail, in other words forward biased. The flyback diodes, D14/15, are just the opposite, reversed bias. They appear to be installed correctly.

Also note the PCB is NOT laid out in mirror image. Make sure anything polarized is installed correctly, diodes, electrolytics, etc.

Craig
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Old 30th March 2016, 04:58 AM   #5
llwhtt is offline llwhtt  United States
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What's the value of the two resistors next to the connector pins? Should be 62 Ohms, I can see the blue stripe but that's it. I would expect to see blue, red, black, gold or maybe blue, brown, white, gold on those Dales.

Craig

Blue, gray, black, brown? 6.8k Ohms?

Last edited by llwhtt; 30th March 2016 at 05:04 AM.
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Old 30th March 2016, 06:16 AM   #6
kwape is offline kwape  United States
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Join Date: Nov 2015
Location: Plymouth
funny you mention that. I was just removing them, it's the first thing I thought of that could be wrong, except I thought I had Q13, 12 backwards. I was following a pcb printout I have instead of the schematic. Print-out may have some errors, R38 is listed as 68ohm which is what I put in.

Q12,13 (SAE301) are the IN5820 big Schottky diodes. Anode towards + rail. you mean these are backwards?
Q14,15 are the IN4004
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Old 30th March 2016, 06:18 AM   #7
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Location: michigan
Quote:
Originally Posted by kwape View Post
I've purchased this amp as a basket case; it was missing outputs, top cover, and a choke assembly on one channel on the main amp board. Luckily it was not molested too much.

My problems are I am getting 40.1vdc measuring bias across Q13,14 collectors, Bias pots have no effect (should be 2.1vdc here). Relay doesn't click with outputs installed (did click without them) and dc offset measured @ the 0.125 resistors is -750mv channel B, -1.7 to -3.1 volts DC (erratic) channel A. offset pot has no effect on channel A. I've replaced the bias IC on a guess with CA3086 NOS from ebay but has no change. I've tested the bias ICs and I think they're all good. Also I was advised to short collectors of Q9,10 to eliminate the bias IC from the equation and still no effect. So its not the bias IC.

Here's what I've been doing so far:

I've installed all new outputs
MJ11021 for SAE204 (11-0070)
MJ4502 for SAE202 (11-0069)
MJ802 for SAE101 (11-0063)
MJ11022 for SAE103 (11-0064)

according to wardsweb.org these are appropriate equivalents.

hand made new choke assembly on a 2.2ohm wirewound resistor and wrapped with magnet wire. It is an exact replica of the original.

made CAD drawing to have a new cover fabricated.

on the main amp board the SAE301 (11-0075) diodes were replaced with a Motorola part # and installed backwards by someone else. They were also shot. Replaced them with IN5820. Replaced the IN4004 diodes on this board with new al well. Both 1.2ohm resistors to ground on this board were shorted, since been replaced.

On channel A driver board driver transistor NPN 2N6316 was replaced with a sony D291 (still tested good) but I through in a new 2N6316. The 1.2ohm resistor to ground was bad here as well and since been replaced.


I've recapped both driver and main amp board completely and replaced any carbon resistors with metal film.

@ the 51v zener diode on driver board I am getting 49.8v channel A, 50.1v channel B using dbt.
I have to concur with Craig as i look at that 3 section driver board. It seems odd the diodes would go the same way and I realize that not having screening on the board is a PITA but you should be able to figure it out with your ohmmeter where these components connect on both sides.
It is not uncommon for bias pots to open in SS amps.I typically set the bias for a change in current like McIntosh does in their setups after setting the bias at minimum..You can't do it this way on all amps but I have done this on Ampzillas and other GAS amps with great success..
Have you measured the base to emitter on each of those transistors so its at least .5 to .7 vdc. This can tell you in which area the problem may exist.Keep the outputs out when you do this but you may just find which area is the culprit.
One more thing and this is for later..Putting in all metal film resistors is going to compress the sonic nature this amp is so famous for..Put in 3% Carbon films especially on the base where the signal comes in and maybe use non inductive wirewounds on the collectors..I use CC on the base that are measured within tolerance because you want that wide bandwidth and CC resistors are non inductive.
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Old 30th March 2016, 04:34 PM   #8
llwhtt is offline llwhtt  United States
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The schematic says 62 Ohms, I didn't think anything of it as James used this value in this position in many of his designs. If 68 Ohms was in it that's fine, that's what is in the parts list and locator diagram.

Not having silk screening makes things a bit more difficult especially when the service manual is wrong as the arrows on the Schottkys are backwards on the component location diagram.

Ds NOT Qs. The 1N4004s appear to be installed correctly, the 1N5820s are backwards. You can leave them out for now as the amp will run fine without them.

Craig
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Old 3rd April 2016, 06:54 AM   #9
kwape is offline kwape  United States
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Join Date: Nov 2015
Location: Plymouth
unfortunately that was not the only problem. still same wacky bias and offset.

I'm getting -20.78 @ Q18 collector
+19.95 @ Q17 collector
both channels comparable

Last edited by kwape; 3rd April 2016 at 07:16 AM.
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Old 3rd April 2016, 10:32 PM   #10
llwhtt is offline llwhtt  United States
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Without a reference that voltage is meaningless. That voltage should be 1/3 of the total B+/-. If your B+/- rail is 60VDC then what you are measuring is correct. Check the voltage on the collectors of Q16/19, should be about 40VDC referenced to gnd. Your actual B+/- should be +/-75VDC without the DBT and 120VAC input. At full line voltage you should have +-75VDC, 50VDC and 25VDC on the appropriate collectors.

Provided all of your parts are correct and in the correct location I doubt your bias and DC problems are in the output stage. Also if you think that voltage will change with the bias setting it will not, you need to look further upstream. Concentrate on the collector voltage of Q9/10, that's the voltage the bias will change.

Craig
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