Spec 4 Surprise Overhaul and restoration

[slayer44]

Hi all. I recently acquired a spec 4 from the auction site that needs new outputs...I know what I need to buy...my question is, how do I go about ordering these with matching hFe?...the manual states they should be matching. Can you order them this way?...any help is appreciated.

 

[avionic]

Can you order them this way?.

No where that I know of..

What makes you think it needs new outputs ?

 

[slayer44]

Hi, because I bought it that way. The seller pulled them for his other Spec 4 and didnt replace them.

 

[avionic]

Hi, because I bought it that way. The seller pulled them for his other Spec 4 and didnt replace them.

That is a pretty good indication..

 

[slayer44]

I have replaced outputs in other units and nowhere did I find they had to match hFe in the manual. Maybe overthinking it?

 

[slayer44]

Yup!...got it cheap, so I cant complain. Have wanted one for years.

 

[avionic]

Maybe overthinking it?

Perhaps.. Could get pretty spendy if you needed to match yer own.. At $5-$7 dollars apiece.

 

[slayer44]

$8.36...inflation. I looked on AK for a while and never saw any mention of this. I guess I will just get them and mate them the best I can.

 

[slayer44]

So, there is a difference in dynamic distortion between matched/unmatched

 

[markthefixer]

So, there is a difference in dynamic distortion between matched/unmatched

WHERE DID THAT COME FROM????????????

ahhh the manufacturer's datasheet...

each output has the feedback system and the emitter resistors regularizing and swamping them.

look up the benefits of emitter resistors.

The main, most basic advantage is that emitter resistance reduces much of the circuit gain variability that otherwise is highly dependent on actual transistor current gain. The downside to this gain stability is that the emitter resistor causes a reduction in circuit gain.

 

[slayer44]

Ahh, okay...so this is Greek to me..
Do I just buy them as is or try to find matched pairs?

 

[markthefixer]

don't bother trying to match pairs.

They will probably be a far closer match than the originals were back then.

I've had plenty of experience in just that, evaluating how well 4 random output transistors in one batch from a distributor matched and shared loads in the circuit.

 

[slayer44]

Thanks Mark...I have replaced outputs in a few others also and never had an issue...I think I will match up what I get the best I can.

 

[slayer44]

Okay, where to begin?...I got this Spec 4 on the auction site and was told all it needed was new outputs...wrong!. Amp boards were toast. Burnt resistors, missing heatsinks, broken varistor. You name it. And some monstrous relay/base subbed in the chassis. I got it cheap (and it shows), so I am going to begin with a complete restoration. I will pull every component present and test before replacing. I have most, if not all, new transistors and caps, so I will begin with re-sourcing the relay ASR-041 and getting that abomination repaired (See pic). Wish me luck, I'm going to need it.

 

[slayer44]

does anybody have a part number for replacing the two heatsinks for Q10, Q11 on the amp board?...I have two missing. I suppose I could use 1 for each channel for mounting the varistor, and find two close matches for the non-connected side.

 

[slayer44]

I have a couple of questions before I re-populate these amp boards. I have the tools to properly adjust the Bias and Idle, but not the Power Limiter. Is there a workaround for adjusting this without an oscilloscope?...or do I send them out to be adjusted? and does the meter board circuit have anything to do with the amps restoration?...I would like to just concentrate on the amps first, then worry about the meter board after. Any help is appreciated. Also, Mark if you are here, what tool do you use for carving down the stabistor to access enough lead to solder a new wire on? how far can I go?

 

[slayer44]

Quick question here. I have seen a couple of different posts on here where people use 1 or 2 soft start relays in the spec 4. Is it recommended to use two for channel isolation or do I only use 1. In the middle of my mouser order, so wanted to see what the consensus was.

 

[markthefixer]

re: carving - I have posts on that. it's a matter of what you have and can control.it's ticklish and requires good up close vision - something that is fleeing me.....

Spec 4 is not a toroid transformer, so soft start is a custom add on?

 

[slayer44]

Hi Mark, It has a relay right in the center of the chassis in addition to the two on the amp boards. Some posts here show 1 installed, some show 2.

 

[slayer44]

Here is a screenshot from the web