SAE Mark IIIC Repair

[kwape]

well, I was going to opt to change them. I don't know what else it could be.

found this site, hoping for the best. They seem legit....
http://www.utsource.net/ic-datasheet/CA3086-242106.html

 

[llwhtt]

Using the data sheet from that source test the transistors again, they're either good or not good. They're just single transistors NOT Darlingtons. The voltage you are trying to measure should be steady. If the CA3086 is bad that may well be the reason for your bad voltage. Both channels? Both SKs?

Craig

 

[kwape]

they look identical on both channels but the other one has no labeling on it. the peak meter indicated darlingtons, I'll check them again.

 

[llwhtt]

The hfe spec is too low for them to be Darlingtons. Were there any bad components connected to the CA3086s?

Craig

 

[kwape]

checked again with atlas meter: all darlingtons measuring 107-120hfe except for Q5 measuring 65hfe. I know these meters can be inaccurate so I will dismiss this.

checking with my mm: I do not get a beep indicating that I usually get that indicates the transistor/diode is good, I just get a voltage reading (averaging 770mv).

to be honest, I don't think the SK3543 is matching up with the datasheet of the CA3086, and I cannot find a datasheet on the SK. I'm getting clean connections on pins that I should not be and not getting clean readings on pins that is should be according to the datasheet of the CA3086.

 

[llwhtt]

I can't find my SK cross reference manual but if you cross CA3086 and SK3543 both come up as NTE912 so I would say they're all supposed to be the same. If you are positive on your measurements they must be bad.

Craig

 

[kwape]

yeah that wasn't it. getting the same wacky bias and no relay click. I thought I checked every resistor in this amp. frustration is rising

 

[llwhtt]

Q5 on mine measure weird also, there's continuity between emitter and collector. I'm going to order some LM3046 or CA3046, whatever I can find.

You should be able to short the collectors of Q9 and Q10 together, this will take the bias IC out of the picture and put the bias full cold. Bias and DC offset "shouldn't" be related. What is the DC offset? You need to check it before the relay, either side of the big .125 Ohm resistor would be OK. Does the offset trimmer work as advertised?

Craig

 

[kwape]

ok i'll check the offset @.125 ohm resistors and then proceed to Q9 & Q10 on driver board.

I installed some RCA CA3086 from ebay (NOS) so they say. Still tests weird on my peak (claiming darlington) and multi meter

 

[llwhtt]

On my meter transistors usually measure .6 or so, these measure .7 something, higher than normal transistors. Could be what's confusing your tester.

I drew a schematic of the "thermal block" if you are interested, I can email to you if you want. They managed to use all five of the transistors.

Craig

 

[kwape]

dc offset @ 0.125ohm resistors to ground.

channel A: -1.7 to -3.1 volts DC (erratic) offset pot not having effect here obviously. Also, first I've ever heard this, but the transformer or something in the power supply on this channel is making a strange pulsating sound off and on.

channel B: started @ around -1vdc and adjusted to -750mv then gave up.


something is clearly way off hear.

channel B: @ Q10 base -56vdc. @ Q9 base 59.5vdc

Channel A: Q60 -59.5vdc. Q59 59.1vdc

schematic calls for -72.91v & +72.91v here. Could it be I need a larger bulb on my dbt to get this thing going (using 100w currently)?

 

[llwhtt]

Monitor the voltage across the 51V Zener, until it starts regulating all bets are off.

Craig

 

[kwape]

I'm getting -49.8v channel B\

-50.1 channel A across 51v zener D3

am I supposed to have a negative reading here? measured with negative lead on cathode pos on anode......

 

[llwhtt]

Across the Zener you should have approx. 51 Volts depending on the tolerance of the Zener. With the pos. lead on the cathode and the neg. lead on the anode you should have a + voltage. Another way to measure is to ground, approx. +25.5 on the cathode and -25.5 on the anode. Remember Zeners are in the circuit backwards if you want to look at that way. Your measurements look OK although opposite polarity.

 

[kwape]

yes with opposite polarity I have positive values. I am using dbt so those values seem good. I'll proceed to shorting collectors of Q9 & Q10

 

[kwape]

shorting the collectors of Q9 & Q10 had no effect. So I guess that rules out the bias IC.

man....I'm really not sure where to turn right now.

 

[kwape]

let me update this thread one time:

Just dropped the IIIC off @ High Performance Audio in Hyannis MA. Man seemed knowledgeable in a shop piled with equipment leaving only a pathway through the damn place, even had to hop the counter to get in lol. Anyways he thinks the issue is cold solder joints from his past experience with a IIICM which is the same amp with meters. Considering I've re-solder about 80-90% of all solder joints it's still possible but I thought unlikely, they looked good to me. We'll see how it goes, if he does well or ****s my shit up you'll know about it.

I continued my repair attempts for a few more pages on the diy forum. At this point I really don't have much free time and have become over frustrated with this project. Amp layout is a pita to access and I just want to enjoy this amp. At this point I have 250$ in the amp itself shipped to me, plus another 260$ or so from mouser, couple months of my time, plus whatever the repair bill will be from this shop. oh and plus I need to pay a metal shop to fab that cover for me black anodized aluminum. 900$ all in easy to get this one back to life, **** me.

 

[llwhtt]

That sucks! Your tech will probably find something really really stupid. The MKIVD I was working on is back up and running great. New outputs, new drivers, new bias ICs, new small electrolytics, and a few new resistors got it up and running but the DC offset was out of spec, new input diff. xsistors fixed that. Yes, these amps are a PITA mechanically to work on but I've been doing SAE, GAS, and SUMO for so long I just strip it down to the bare bones, check everything, replace bad/questionable parts and reassemble, and they always seem to work for me.

Craig

 

[kwape]

got the amp back today! clean bill of health and sounds awesome. problem was a few open resistors that was causing the voltage problems. He said he had to pull them to test because the circuit loops and is difficult to track down. Price was 88$ for his labor which I think is very fair.

I would recommend High Performance Audio in Hyannis MA if any akers in this area need a good tech.

We're having the fest in Clinton MA Saturday. I will be bringing this amp to the fest to show off my ba a400s I'm trying to move.
perfect timing!

 

[kwape]

also got some good leads to have the covers fabricated. will likely need to have to sheet metal made at one place, anodized at another. but this will be done, post pics when I get it done.