Pioneer Spec-2 Rebuild

[EchoWars]

QUESTION: have I hamstrung the amp in some way that I am not detecting? Can you explain why our results are so different?

The diodes are most often used to set up the proper bias current...the diodes get it 'in the ballpark' (on the low side), and you fine-tune with the trimpot to get the proper voltage between the bases of the driver transistors, and thus the proper current through the output transistor. The difference between a STV-3H and an STV-4H is about 1/2 a volt or so, and that ought to be enough (if you swapped a 3H for a 4H) to reduce bias current to zero. The amp will work, but not well. You could use a larger trimpot to increase the voltage drop, but it can end up making the amp thermally unstable.

NEXT: do you do a resistance reading for each R when you rebuild,

God no...

or do you read voltages down the line?

In every design there are resistors that take a beating. Learning where they are is part of the game. RE: Voltages. I check all regulated and unregulated voltages at the source. As far as the rest of the voltages in the amp, if the semiconductors and caps and resistors are doing their job as they should, then they'll be about where they should. Bench testing with dummy loads and signal sources can ferret out symptoms that can lead to the discovery of components that are 'working', but perhaps not as well as they should.

Is there any point in checking every transistor on the board for comparative readings in the instrument’s 'diode mode'?

Depends on if there's a problem that you can't find. I got better things to do than pull transistors out of a working amp and test them. The diode test is a crude one, and useful only for finding gross failures.

AND: Should we replace the mica insulators on the output trannies? Recently I just cleaned them up and reused them because I found that the ones I hand on hand were the wrong size (smaller).

Depends on the condition of the old ones. If they are cracked or chipped or too thick (many old mica insulators are too damn thick...you want thin) I replace 'em. Sometimes I just don't feel like cleaning them.

ALSO: In a Pioneer SA-8500 (Jpn model 8900) I found a 2SC1626 with no thermal grease. The 2SA816 on the same board (switch control) had grease. Is this weird?

Pioneer used a clear thermal compound that set kinda like epoxy for some time back in the 70's. Perhaps you had one. The one with white grease may have been removed by someone at sometime.

I'm using the only grease I can get locally in a computer building supply shop. It's from Sunhayato (Tokyo I think) — a silicon grease rated for 200 Centigrade. Sound OK? — LORNE

Sounds good to me.

 

[lorne]

STV-4H 'Forward Reference Diode'

Oh .... :sigh:

The difference between a STV-3H and an STV-4H is about 1/2 a volt or so, and that ought to be enough (if you swapped a 3H for a 4H) to reduce bias current to zero. The amp will work, but not well. You could use a larger trimpot to increase the voltage drop, but it can end up making the amp thermally unstable.

So what do you suggest I do? Is the only option to troll the planet for an STV-4H? The stock bias pot is 10K. AND ... please ... in terms of sonics what fault/s do you think that I should be listening for on the channel with the STV-3H? Imaging, or the whole spectrum. I knew I was going to be ordering the manual to set the bias. This pushes on the button.

 

[restorer-john]

string 4 1N4148 diodes together and set them in epoxy, drill a hole through the epoxy and there you go- STV4H, ready to mount.

 

[lorne]

:thmbsp: :ntwrthy:

string 4 1N4148 diodes together and set them in epoxy, drill a hole through the epoxy and there you go- STV4H, ready to mount.

What would some of us do without AK? In series RIGHT?? Thanks!

EW: I realize my last question was sorda kinna dumb — about the sonic variance with a healthier channel. I was thinking ahead (... er ... panicking) in case I never got the right part in there.

 

[EchoWars]

You can use the STV-3H, but remove it and take off the covering on the long uninsulated leads. Add one 1N4148 diode inline with one of the leads, and cover it back up with the stock covering (or use shrinkwrap to cover it up). Invisible repair...almost. Take care to install the new diode so it will be forward-biased when the amp is turned on...get it wrong and the output will blow up real nicely.

The amp will likely be slightly undercompensated for temperature compared to stock if you do this mod, but most Japanese amps are overcompensated anyway, so it may even out.

I see the 4H diodes on eBay every once in a while. And...you being in Japan, it surprises me that you can't locate one there somewhere.

 

[lorne]

Thanks again EW. This mod would probably be OK. Even in the humid Japanese summer the Kenny monos run very cool.

And...you being in Japan, it surprises me that you can't locate one there somewhere.

It's a long story, my friend. Consider this: perhaps the most expedient way to get a 1N4148 into Sendai (let alone an STV4H) would be to order it from North America. (I might find a Japanese equivalent). And I don't read Japanese past the title line, which makes Japanese web auctions very difficult. Akihabara audio is a fading relic having changed a lot in recent years and $200 train ride away. Nagoya is a $400 train ride and a hotel stay over. Most of the old audio junk is off the resellers’ shelves all over Japan due to stuff you may have heard about. (I wrote about it on AK). Even in the age of bloat machine and technical parts inventories characteristically continue to be limited and or centralised. Ordering parts can take ages. I once ordered a selection of Black Gate NP's from a local supplier. They arrived — 7 months later — from Osaka! Now I use RS (no ... not Rat Shack).

It's an ironic situation best illustrated by the time I had to order fresh axles for the Shimano hubs on my sports bike. It took a long time, and they had to come from the United States because there weren't any in Japan where they had been made in the first place. The only person who seemed surprised was me.

 

[zebulon1]

I wanted to post this before for I forgot:
I used a Littlefuse HE751A2400 for the R1 relay that we "Dead Bug" mount.
Mouser Part number:
934-HE751A2400
It mounts a little better due to the wider back of the IC style relay. I used hot glue to fasten it.

The old part number is obsolete.

 

[rhodus]

Awesome thread