Spec 1 & 4 Restoration...First Steps?

Good advice rcs16, thx. I did make note of each component as I replaced. Q6 was definitely shorted. I replaced everything listed in mattsd's recap list as well as EW's suggestions and still no luck on the meters. I'm wondering if anyone has replavced the IC's and if so what was used?
In addition, it appears I have a problem with the left power amp. While I could dial in both DC offset and bias within spec, I noticed some real issues with the power limiter settings. I could only get about 11V at full power and the 1khz signal wave was grossly distorted on my scope.
I had kept the original output transistors and I'm wondering if this could be a symptom of a dying output transistor?
If so, I will likely order a full set of outputs and some replacement IC's tomorrow.
Would love to know what you all think.
Thx very much.
 
Just a quick update.
After the complete recap I am still having the two issues below.

METER BOARD:
I have checked Q5 & Q6 on the meter board and get the +20V and -13V one would expect.
I then moved to the TA7136P2 IC's to check the power there.
Below are the readings I made (I put what the service manual shows as normal in parenthesis):

Pin 1: 0V (19V)
Pin 2: 0V (0V)
Pin 3: -8V (0V)
Pin 4: -13V (-13V)
Pin 5: -11V (11V)
Pin 6: -11V (0V)
Pin 7: 20V (20V)

To try and better understand what is going on, I found the following pinout online for a TA7136P. Can anyone confirm if these are correct for the TA7136P2?

Pin 1: frequency comp - what does 'comp' stand for???? comparator?
Pin 2: non-inverting input (+ input)
Pin 3: inverting input (- input)
Pin 4: V-
Pin 5: voltage comp
Pin 6: output
Pin 7: V+

According to the SM the TA7136P2 operates as part of a 'Logarthmic compression circuit'. Still trying to grasp that concept.
My next plan of attack was to probe from the supply (Q5 & Q6) to the IC pins to try and figure out which components are altering the voltages. Make sense?
Any guidance is appreciated.

LEFT POWER AMP:
The second issue I'm having and I believe it is unrelated the the first is the left power amp. As I mentioned in an earlier post, I can dial in the dc offset and the bias but when I try to do the power limiter adjustments things get weird.
The right power amp goes to full volume at around 28.29V/4ohms and the sine wave is nice and clean. On the left I can only reach about 11V and the sine wave is horribly distorted.
I am still using the original output transistors on both sides and have ordered replacements for all but I'm curious while I wait for parts if this could be a symptom of a failed/failing output transistor?

As always, thanks everyone for your help.
 
Both TA7136's, q14,15 measure the same? i doubt it, so check both
Pin 1: 0V (19V)
Pin 3: -8V (0V)
Pin 6: -11V (0V)
"comp" is for the compensation terminal. used to control the frequency response and phase.
those voltages show a defective chip. pull q16 or 17, to isolate, whatever side you are work on and measure again.

Those are new pots for the limiter controls? Not sure why the one channel distorts at 11V. If you check all the bias voltages between both channels, they are the same? If you think it is the output devices, do they bias up correctly? if you swap them, does it make any difference?
 
I will check both TA7136's again when I can. Hard to reach all IC pins when everything is in place so will have to wait until next week to disassemble and try again.
Thank you for the explanation regarding "comp". Very helpful.

I did replace all the pots on the power amp boards. I adjusted bias and DC offset as per the SM and everything looked ok on both boards. Forgive me 'rcs16' but I'm not sure what you meant by 'If you check all the bias voltages between both channels, they are the same?'. Is it different than the bias setting procedure in the SM? I do have some new output devices on order and will swap those out to see if the fault lies there.

While taking 5 minutes this morning to look at the meter amp board on my bench I noticed something odd. In the dead center of the attached picture (to the left of pin 9) there is what appears to be a 10 ohm resistor. I can't find that anywhere on the foil side view of the board layout. It should appear somewhere between and below Q19 and C27. Anyone shed some light on this component? Short of process-of-elimination going through the schematic I'm not sure how I can figure it out without removing the board and physically tracking. I'm sure I'm not the first one to discover this or perhaps I'm just an idiot and it is clear to everyone else?IMG_6644.JPG
 
'If you check all the bias voltages between both channels, they are the same?'. Is it different than the bias setting procedure in the SM?
All transistors require bias to operate, I should have said measure/compare the voltages on both of the meter amp TA7136 ics. I doubt both can be bad, so one side should match the voltages(bias) as shown on the schematic.
On the o/p stage you have to manually set/adjust the bias current where as the other circuits, the bias voltages are derived using the standard value resistors.
Anyone shed some light on this component? Short of process-of-elimination going through the schematic I'm not sure how I can figure it out without removing the board and physically tracking
Yup that is what you have to do figure out what it attaches to with reference to the schematic. i have not worked on one so I can only go by what i read in the SM.
 
I really appreciate all the help and advice rcs16 (and everyone else who have responded along the way)!
Thoroughly enjoying the process. Funny thing is I don't think I'll enjoy using the amp as much as fixing it.
I did go back and remeasure the TA7136's and I'm getting identical readings on both IC's. I will take your advice and remove either Q16 or Q17, try again and report back. I agree it is strange that they would both IC's would go at the same time. Is it possible/probable that when the -13V supply transistor went (Q6) that it could take both IC's with it?
While turning the meter pots (VR1 & VR2) I do see very brief if sporadic needle movement so I'm hoping they are still functional if supplied the correct supply. I'm thinking I will replace these pots as well as a matter of course. EW, had suggested replacements on another thread. Also, on power up I can see a reading on pin 9 and 17 of about .5V for a second or two before they fall to about 9mv.

While I have the meter amp board out I will trace the mystery resistor and figure out where it goes.

I did replace the output transistors on the left side (right side is next) and everything became far more stable. The DC Offset only floated around 0mv by about 3mv! The bias was spot on 50mv.
The 1KHZ sine wave was stable and I was able to see the full 28.29V/4ohms. At this point I feel like I am just fighting the meter board to get the meters to work.
I may even treat myself to a record or two through it before continuing!
Thanks again for the help.
 
Here's an odd observation. As I said in my last post, I treated myself to a few albums through the Spec 4 as a reward for getting this far (not the odd part). Has anyone experience a 'cat pee' smell coming from the amps modules? I know it sounds crazy but the scent is so clear. I did the sniff test all around my bench but smell nothing. Only when leaning directly over the amps. In the process of repairing and recapping I cleaned everything meticulously with 99.9% pure isopropyl alcohol. I know cat pee is strong but not that strong. Anyway, just thought I would throw it out there. I guess it is possible that the white mesh stuff covering the wires has the last owners cat pee still on it. Weird.
 
Splash/spray/soak some alcohol over the suspect areas where the smell is coming from, obv with the power off/unplugged and let the alcohol thoroughly dry out before powering up.
Is it possible/probable that when the -13V supply transistor went (Q6) that it could take both IC's with it?
Yes, that is a good possibility. How easy is it to find NOS TA7136?, if not, should be able to find a way to replace with a newer opamp. Finding a SIP pkg maybe more difficult as most these days are in a dip pkg. JRC I think still make parts in SIP pkgs, have to look.
 
The Toshiba TA7136P is pretty common. It could be replaced with a modern opamp, but as long as originals are still around, I'd use them. Changing to a new device is a trial you need not visit upon yourself.

http://www.utsource.net/ic-datasheet/TA7136P-1883466.html?gclid=CPSyx9KQ3ckCFZQvgQodqewFnA

I'd not even think about installing a new opamps till you know the +20V and -13V supplies are working properly.

RE: Log compression. The meters read logarithmic, not linear, so the meter amp has to output accordingly. The diodes in the opamp feedback loop provide the compression...works well, since the voltage across a silicon diode is proportional to the logarithm of the current through it. With a diode placed in the feedback path of an inverting op-amp, the output voltage will be proportional to the log of the input current.

http://www.circuitstoday.com/log-amplifier
 
Pin 4: -13V (-13V)
Pin 5: -11V (11V)
Pin 6: -11V (0V)
Pin 7: 20V (20V)
I thought that these voltages had been verified already, no reason not to check again. I agree, get new ones from utsource, remove the old devices, butcher a machine contact (prefered) 14 pin dip socket to make two 7-pin sip sockets, solder in, insert the new parts, test.
Utsource is wrong about the part description
"FOR FM IF AMPLIFIER AMPLIFIER AND DETECTOR"
it is an opamp, they copied this description from a part like TA7060P
 
Last edited:
This is great EW and rcs16! I did confirm that both the supplies (+20V and -13V) are correct now that Q6 has been replaced. All pin voltages checked with and without Q16 installed.
I will order up the parts from utsource today. Will report back when I have them in and tested.
Look forward to reading up on 'log amplifiers' as well. Thx for the link EW.
I had bought a couple of adapters from Brown Dog's site but will set those aside in favor of original part.
Thx guys.
 
Waiting for the TA7136P's is killing me! Without paying a fortune in shipping, UTSOURCE will take a month or more! I ordered a couple from a place online that might get here in at best 3 weeks. I fear I've become a member of the 'MTV generation' and I want instant satisfaction! I know, I know, first world problems. I think perhaps I will start on my Spec 1 while I wait.
 
The good news is the TA7136's arrived. The bad news is the meters are still not functioning. I haven't had a chance to check the power on the pins of the replacement IC's yet but did notice something interesting that might help narrow down my issue. With my scope hooked up between ground and the positive pins of the meters (pins 9 & 17) I was getting a lot of noise in the DC when I tapped anywhere on the chassis or front plate. I understand that ground is attached to the chassis but this seemed strange to me. Any ideas? Thx guys.
 
Forgot about this thread. Sorry to steer you to replacing the TA7136 devices when in fact they are not the fault. It was suspicious that both channels behave the same and have blown chips. Have you managed to sort it out?
 
Hi rcs16, Thanks very much for checking in. I still haven't had any luck but honestly I've been swamped with work and family and haven't had a chance to really dig back in. I can tell you that, from a first quick look, the 20V supply seems to now be acting up. Clearly the TA7136 won't function correctly without giving it the correct power. I truly appreciate all your help and please don't hesitate to chime in again! I'm dying to get back to the bench and will post my findings here as soon as I have reliable numbers to share. Thx.
 
Hi All, I found some time this afternoon to get back inside the spec 4. Was originally looking to tackle the still non functioning meters when I noticed that all too familiar 'hot' smell coming from the meter board. I did some poking around and it appears the R40 470ohm 2W resistor I replaced (along with Q7) was getting way too hot. When time permits I will measure the supply from pin#5 again as well as Q7 for possible failure. If anyone has any other suggestions I would sure like to hear them. Thx.
 
It appears that R24 is feeding the collector of Q5. When I take a reading on all 3 of the legs of Q5 I get approximately 4V on all of them. Does this mean Q5 is likely bad? I did replace it but I suppose I could have a faulty one. Thoughts? What concerns me is that there may be a fault elsewhere and this is just the weakest link and no matter how many times I replace Q5 it will keep failing. Any guidance is appreciated. On the upside, I'm sure this has something to do with my meter issue as they aren't receiving the correct 20V supply.
 
I haven't had chance to look at the SPEC-4 schematics for a while but I would work back from Q5 and check that everything behind it and all power supply voltages are good. It could be a problem on the power supplies that's causing things you replace, to fail again...

If you get the same reading on all pins of a transistor, it means either its failed or something behind it is acting up and forcing it to fully conduct.
 
Thanks very much QSilver. I will take some measurements and make sure everything is in order as well as check Q5. Will let you know what I find. Thx.
 
Back
Top Bottom