[KCMike]

I finally got back to the main amplifier board.
I removed and tested all the power transistors.
All 4 are giving me a reading btwn. E&C only one way. Example: Positive lead on E neg. on C. the other way around I get no reading I guess I cooked them. All the other tests read like a good transistor should. The old ones test good except for one.
I decided to use two of the old good ones and rebuild one side of the amp.
I replaced C 9,10,-100P C15,16- 47P C17,18,- 0.022, C19,20, 100P.
Tested the transistors and Q6 2sa497 was bad. Replaced it and Retested all the resistors. Replaced R30 ,150 ohm.
Hooked up the amp on the rebuilt side, its the (out 2) side of amp. with jumper wires. connected as follows:
E2,E4, out2, in2, and B+ and a ground jumper wire from the mounting bracket to chassis. All wires on power board hooked up.
Speaker plugged into A socket on back of receiver.
It powered up fine and at 115v it was drawing only .1 amp. Which I believe is good.
Problem is I only get a steady medium hum out of speaker that does not change with Vcontrol. I plan to order another power transistor and finish side 1 of the amp.
The lightes all work and changing dial scale you can see the signal strenght meter respond to where radio stations are.
Not sure what I can check with one side of amp. only hooked up.
Any guidance appreciated. Whew!

[morzh]

So, they were cooked after all.

Now, if you keep touching various tranistors' bases moving back from output to input you should hear AC hum. Like Q4 and then Q2.
While Moving back you could see where it stops and see why. If all touches within the power amp yield hum, start moving back to preamps and do the same, go backwards touching bases.

You could check plenty with one side, it should play and this is what you are after.
Don't worry about hum right now, it may simply be the result of it bing not fully assembled.

[KCMike]

Gotcha. I will start on that in the morning. Thanks

[KCMike]

I went ahead and waited for the 2 new output transistors from mouser.
Installed them and put the main ampl. back in the unit.
It still had the problem. Drawing heavy current. GRRR! I almost just quit on the thing but I'm hard headed.
Took amp back out, took out OP transistors. Rechecked them they are good, and everything on the board again. This time I changed VR3 & VR4 50 ohm pots.
Hooked it up out of unit. I know have AM playing loud and clear and amp. guage is reading .1amp.So thats good but I have no FM, on mono or stereo, the signal meters work . Just barely audible hum from speakers when on FM settings.
I feel I'm getting closer to fixing this and plan to reinstall the main amp. then trouble shoot for the no FM problem.
What should I start with for the no FM? Your help really needed. Thanks

[morzh]

In the post 43 (p4) I asked to short (put in the shorted position) those VR3 and 4.
I guess they did not short after all.
These dictate the quiescent current of the output tranistors.
But even at maximum they will not make it dangerous.
If however they are open or way out of tolerance (on the high side) it is different the output pair gets fully open, especially the upper transistor (Q5 and Q6).

As for FM, I am no expert, maybe others will chime in.


PS. If both those trimpots went bad and the two others, VR1 and VR2 are the same type, maybe you want to change them too.
I would measure them first in circuit, should be the same. They dictate the gain of the amp.
One thing though, both VR1-2 and VR3-4 will need calibration. Before you make the whole thing work I would keep the 3-4 in shorted state.

You will need an audio generator, an ammeter and, if possible, a scope.
VR3-4 should establish such a quiescent current (measured by the ammeter in series with B Power as the difference between the shorted position and a non-shorted one) where the crossover distortion on the scope disappears (or you stop hearing it) and the quiescent current is on the order of 50-200 mA or so.
VR1-2 will establish a desired gain, say full power at 250mV input signal, which can be measured by a scope or a true RMS voltmeter.

[KCMike]

It turned out they were open. Found out when I tested them out of circuit. I ordered two more pots for VR1,VR2. They are very similar but are 100K pots. I will short the 3&4 for know while I check voltages to FM boards.

[Ron Ramirez]

I've been watching this thread with great interest, as I also own an SX-1500TD. Mine was working fine until it lost a channel last year. Eventually, I will try to get it going again.

Mike - big difference between 100K and 50 ohm trimpots? Can you not get the lower value anymore?

[KCMike]

Yes I got them from mouser.Mouser Part #:


652-3352P-1-500LF . 50/ohms. The problem is they are not like pic. The adjustment is on top not at 90 degree so you can't get to it with amp in chassis. I ordered two more along with the 100k pots and will do them all and then install the amp.

[morzh]

Yes I got them from mouser.Mouser Part #:


652-3352P-1-500LF . 50/ohms. The problem is they are not like pic. The adjustment is on top not at 90 degree  so you can't get to it with amp in chassis. I ordered two more along with the 100k pots and will do them all and then install the amp.

Mike


Mouser explicitly writes under pictures:

Images are for reference only

See Product Specifications




You have to look in datasheet. Never use Mouser pictures.

[morzh]

Ron

If this problem with trimpots is common (same type), maybe you also have them starting to quit.

This is a bad design (shame on Pioneer engineers).
The diodes need to be three and the trimpot needs to be in paralel with one of them, then open pot does not then result in fully closed output transistors. There are other ways too.

[KCMike]

While waiting for trim pot order to arrive I was checking around on the chassis looking for anythig I might have missed. I have AM but no FM. With power on I discovered that the control amp board W15-079 caps C3 & C5 and transistor Q1-2sc871 when touched or tapped make a hum and scratching sound out of the speaker. This is with unit in FM mode.
I'm going to recap this board and replace the transistors.
Heres the question. There are 2-2sc871 and 2-2sc870 transistors on the board.
The searches for replacements show use KSC1845 for all four transistors,but 2 of them are pnp.( the 871s) according to schematic and KSC1845s are npn.
[Image: http://i1336.photobucket.com/albums/o654...cd640d.jpg]
Here is schem. What should I use for the pnps? Or am I misreading schem.?
Thanks.

[morzh]

Mike

one thing you should learn is reading datasheets .
If you open the datasheet for KSC1845, in the very beginning it will say

Audio Frequency Low Noise Amplifier
• Complement to KSA992


So there is your compement.


PS. I did not verify the KSC1845 being the NPN equivalent for 2SC870, so I will leave it to you as exercise - look at 1) Unity gain frequency, 2) Vc, 3) Beta (hFE), 4) packaging. Currents are not as important in here.

[KCMike]

Thank you for that info. You are right. I will start using data sheet and get away from trusting just web. searches.
Looked up the KSC1845 data at mouser. It said complement to KSA992.
Unity gain freq.- min. 50mHZ) no max. this said current gain bandwidth the symbol ft Is that same as Unity gain?
Vc--120v)
hFE--200 min. 1200max.)
So this should be a good fit.
I'm slowly learning and appreciate your correction when I need it.

[morzh]

Yes gain-bandwidth product is the same as unity gain frequency.

[KCMike]

Started the recap of control amp. unit W15-079 and C1 & C2 which are connected at( IN-1 & IN-2) according to the schematic and the parts list is 0.47uf. The ones in the board are 47uf. I removed one and used magnfying glass to make sure and it is 47uf. Elna cap. I don't know if that brand was used originally or someone added the 47uf where it should be 0.47.
Which value of capacitor should I use here? Thanks