• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Spec 4 Surprise Overhaul and restoration

amp #2? a different spec-4? same transistors? but no excessive idle current?

I'm looking at keeping the R15 220 in circuit,
and paralleling R13 620ohms with 10k or so resistance,
changing 1.24v to 1.16v.

That 1.16v might be too low - but do you have a 10k resistor to try? Tack it across R13.
 
Sorry, other channel, not amp. Yes, I have all resistor values. Will do what you recommend and report back. I have managed to get pretty far today so I dont want to keep you from the family. Lets see how this works.
 
10k ohm across 620 ohm R13, reducing current source current.
new R15 voltage 1.14v / 220 ohms 5.18 mA, showing 123 mV idle.
was 1.194 / 220 = 5.4 mA STV current showed 83mv idle

The other channel is fine?
let's refresh our memories on exactly what transistors are in this duff channel:
(and I would still like to temporarily remove Q6, Q7, Q13, Q14 to conclusively disable any power limiter action)


q1 2sa798
q2 2sc1775a
q3 2sc1775a
Q4 2sc1439
q5 2sa858
q6 2sa733 --pl--
q7 2sc945a --pl--
q8 2sc1904a KSC3503
q9 2sa899a KSA1381
q10 2sd608a (to-220)
q11 2sb628a (to-220)
q12 2sc869 o/c sense
q13 2sc945a --pl--
q14 -2sa733 --pl--

note: --pl-- is power limiter transistor
 
q1 2sa798= 2 ksa992fbu (matched)
q2 2sc1775a= ksc1845fta
q3 2sc1775a= ksa1845fta (matched with Q2)
Q4 2sc1439= ksc3503dstu
q5 2sa858=ksa1381 (Vbe matched with Q4.Not hfe)
q6 2sa733 --pl-- =ksa1013ybu
q7 2sc945a --pl- =ksc2383yta
q8 2sc1904a KSC3503 = yes
q9 2sa899a KSA1381= yes
q10 2sd608a (to-220)= 2sd608a (original tests good)
q11 2sb628a (to-220)= 2sb628 (original tests good and matching q10)
q12 2sc869 o/c sense=ksc2383yta
q13 2sc945a --pl-- =ksc2383yta
q14 -2sa733 --pl-- =ksa1013ybu
 
Ok, took q6,7,13 and 14 out. Idle started out at 124mV and kept climbing at a rate of .1 per second and eventually leveled off at 140mV. Watched for a moment longer and increased very slowly.
 
Thanks for taking out the q6, q7, q13, q14 power limiter transistors.
They should stay out until we have full control of the idle current.

I neglected the outputs in my query. I am rectifying that: what are the output transistors?

This Spec-4 has one working channel, does that channel have the same transistor types as this duff one?

Did / does vr2 have any effect upon the idle current?

Also measure voltage from ( / = the junction of) D2 / R20 (red) to R21 / D3 (black).
The reading should be somewhere in the vicinity of 3.6 volts.

At same vr2 setting, measure Q8 (red) base to Q9 (black) base

At same vr2 setting, measure Q10 (red) base to Q11 (black) base

At same vr2 setting, measure Q10 (red) emitter to Q11 (black) emitter

Measure the voltage across vr2. red probe one side of vr2, black probe the other side of vr2 at the same time.
 
Last edited:
Morning Mark, all of the outputs are the same and new from the restoration lists (MJE...95G, 96G). I will make these checks and report back.
 
markthefixer said:
This Spec-4 has one working channel, does that channel have the same transistor types as this duff one?
Click to expand...
Yes, re-populated them at same time. All new diodes per Matts list..the only difference is the working one has MJE115030/MJE115031 for drivers...I had these left over from another project and they matched Vbe, so I used them.
 
What is funny is the now working board was the one that had burnt everything (transistors and 1/2 watt resistors)...this bad one was untouched since the factory!...I did pull all power resistors to check and left the ceramics/ polyesters alone...that could be my issue. I have seen polyesters fail before.
 
Okay Mark, all bets are off. I arc'ed the Q10 to Q11 bases so now need to find replacements...everybody take note...too much coffee and live board measurements do not mix!...I re-checked all 4 outputs and they didnt perish. Some good news anyway.
So, before my stupidity, I got 3.515V between D2 and D3
3.506V between Q8/Q9 bases.
Now to find new drivers.
 
So I am having some trouble finding a matched pair of drivers (mje15032 and 33's) any other suggestions? I checked my TIP 41 and 42 stock and nada there also. On a chat with Mouser and had to explain to them what hfe and Vbe means!...just how close does the hfe have to be to make this work. This is insane!
 
Another update...so I decided to see what 2 random MJE15032, 33's would do for the drivers that I sparked earlier. Turns out they work just fine. Been looking all day for matched pairs, even sent a service request to Mousers supplier...Mark, changing these to MJE from the originals (608, 628) cured my idle problem. However fully CCW only brought me to 47mV but I think that will be fine but no room for more adjustment. DC bounces around +.005/-.009 mV. That should be okay. I played a source on this amp for 15 minutes at half volume and outputs just luke warm. I think we got it. Once I get my new output for the other channel I can send it out for power limit adjustment.
 
Did you remove the extra resistor across the 620 ohm R13?
You didn't say anything about it in your postings.
Remember we had TOO much idle, so we put that in to tame the idle.

When you do, dial back the idle current first.
 
You must log in or register to reply here.
Back
Top Bottom