• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

1967 Pioneer SX-34, 11W, WORTH FIXING???

I got this receiver in pretty poor condition.
Have to say, I'm an amateur and my knowledge is not great at all.
However, plugged this baby in and hooked up some speakers. IT WORKED - AND THE SOUND WAS AMAZING.
I decided to clean the face plate, knobs, dial glass and dust off the inside.
When I turned it back on, the sound didn't seem to be as deep and warm anymore. Don't know what happened.

The bass and treble pots are toast (they were before).
The stereo light comes on during warm up, than goes off and never comes back on again.

I understand and I read, it should be re-capped, but I don't know if I can do this. May be, if someone could lead me and guide me through.
I do have the schematics.

Any advice is appreciated!!!
Scrap it, or fix it.
The big issue with working on tube amps is SAFETY. You will be working around potentially lethal voltages and currents.

How much experience do you have with electronics? Can you read a schematic and replace components with confidence, so that the issues are differences between SS and Tube technology?

The amp probably has several problems as you have already seen. (1) Caps, (2) base and treble pots (clean first with Deoxit or similar), (3) possible contaminated tube sockets (clean and retension pin sockets), (4) other age related issues.

Post the schematic please.

Read the Stickies on safety as a good introduction.
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Thanks Gimp

I wasn't aware of the high voltages, but know now that you say it...., it depends on the current too. The current is the lethal one. Capacitors hold voltages long after an amplifier is unplugged.

It is 35 years ago that I did an apprenticeship in electronics, back in Germany. Not all is forgotten yet:), but I didn't practice very much. :(Yes, I can read a schematic..., a simple one. And yes, I can replace components.
In the SS technology it is the transistor that amplifies the signal. I know this. Tubes do the same thing, different technology though.

Unfortunately, the schematic is 860 KB and the spec sheet is 340 KB, they exceed the forums upload limit and I don't know how to reduce them.
I'll send them to you in an email.

It appears that you probably have the background bur are rusty.

Looking at what I could find on the web, it looks like that amp should be worth fixing unless something major is bad, like Output Transformers and power transformer.

Hone up your skills and dive in.

Fixing it will take a bit of time as you are on a learning curve.
Hi Beaver 59,
Came across your schematic a few minutes ago while searching for a Pioneer ER-420 / LX420A. I really like the ECL82 valves the sx-34 uses and coupled with Pioneer output transformers it would be worth the effort to restore.

Or you could strip out the parts and just rebuild the amp section If that's all you need :devilr:
Hi Beaver 59,
Came across your schematic a few minutes ago while searching for a Pioneer ER-420 / LX420A. I really like the ECL82 valves the sx-34 uses and coupled with Pioneer output transformers it would be worth the effort to restore.

Or you could strip out the parts and just rebuild the amp section If that's all you need :devilr:

Thanks Soonerorlater

Is it from the AK data base?
That is where I got it from, just couldn't upload it, because the file was to large.
This little receiver will be a dandy, when it's fixed
The stereo lamp is probably indicating FM stereo signal at acceptable level. It mingt not light unless you are in the FM stereo mode and tuned to a suffiently strong station.

Yes, I believe so too.
Usually I get 2 local stations in stereo, but not with this receiver.

It gives you the choice of 2 FM settings. 1.) "FM", 2.) "FM mpx"
The reception on "FM mpx" is not as good as the other "FM" setting.
Isn't it, that for stereo reception, one would have to choose the "FM mpx" setting, the straight "FM" setting is for mono reception only.

However, I haven't seen the stereo light working during any operation, except during warm-up, when turning on receiver.
Changing Pots

Just looking at 2 variable resistors, VR 501, 250K (B) and VR 502, 250K (B).
In the schematic, they are about 1 3/4 inches from the top, about 2 inches and 4 inches from the left.
These seem the Bass and Treble pots?
Definitely have to change them.

The volume/balance control is weird too.
The balance changes as volume is increased. But this problem may disapeer once everything else is fixed. Do you know? Could this happen?
The control is one of those split controls..., like the left channel volume is separate to the right channel volume..., in one unit though..., a lower knob and an upper knob on one spindle...., or rather a sleeve and a longer spindle inside....
You know what I mean.
Anyway, I can not find it/them in the schematic.
I received this advise

Before changing the pots I'd try cleaning them with a specific electronic cleaner like Deoxit. It works wonders.

If that does not work I'd plan on changing the pots, but be advised that with equipment like this you may have to spend some time finding the right part.

You will need to determine if these are linear pots (equal resistance both sides of the wiper when in the center) or Log tape pots (resistance will be un-equal). 250K is a pretty common value.
forgot to mention

the Bass and Treble control rotate right through, all the way around. I can feel only a tiny little click, where they suppose to start/stop.
They both work only in some positions and completely cut out in other positions.

Also, I should mention, the receiver gets very, very hot in the right hand back corner , near one of the transformers.
Thanks Gimp

I wasn't aware of the high voltages, but know now that you say it...., it depends on the current too. The current is the lethal one.

There is no current until you short voltage source by your body parts. According to Ohm's Law, the current will depend on your body resistance and voltage applied to you. "Lethal voltage" means voltage that can provide lethal current through your body.
I hope, these are not the kind of people I run into every day

Actually you should be glad to run into Wavebourne, a most helpful and knowledgeable fellow. And w.r.t his comment, he was only expressing concern over your safety, and FWIW he's right. (Even though you know what you are doing.)

Good receiver btw, worth fixing up.

Moving along it is not unusual for the volume pots in vintage tube gear to have very poor tracking from section to section, hence the need for means to balance the two channels. Sources and cartridges of the day also often had some level mismatch between the channels. Passive components were not what they are today - this is one area where improvements have been massive. The tubes not so much, mostly the reverse in fact.

Now you need to decide if you are going to keep it original in so far as it is possible or "improve" it with better pots. You'll find electrically equivalent but likely mechanically incompatible parts most easily.

I'll have a look at the schematic and advise on volume pot.

Not exactly sure why you couldn't find the volume pot, it is VR3 and appears right before the power output section. It has a loudness tap and switch and follows the recorder/norm switch. (today known as a tape monitor switch) The control is clutched and you just adjust it at a given volume level so that the two channels match - note that until you verify everything else is working (channel to channel gain match) I would do nothing more than clean the control with a little deoxit.

You should replace all of the electrolytics in the power supply immediately C61, C62, and C63 - multi-section, rectifiers D4 and D5 too (UF4007 should be fine). You should also replace C27 - C32, and C35 in the power amplifier - this will prevent self immolation. Many others should also be replaced as Japanese coupling caps of this era fail leaky or short. (Paper dielectric deteriorates.)
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Many others should also be replaced...

Thank You kevinkr

Wow, you are putting on a lot of work on me.
Hey, but this is fine, I look forward to have this piece of history fixed.

You say, MANY others should be replaced... : )
I'm sorry, could you please give me a better idea/frame, on which ones you mean?
Oh, you mean the coupling caps...
well, I'm not so familiar with this stuff..., would you mind pointing them out for me, please?

What do you think about the Bass/Treble pots, I mentioned above?
To me, they seem to be beyond Deoxit. Please read 4 or 5 posts above.

It is getting late in Nova Scotia and I'm heading to Halifax tomorrow ( 3 hour drive).
I wish there was something, that I could get in the City while I'm there. I should be be back tomorrow night or Friday afternoon.

Can't wait to order some capacitors or other components to get this baby fixed.

I'll be around another half hour, but than I have to sleep.
Good Night and Thank You everyone!!!
Did a review of the schematic to post recommended replacements BUT Kevinkr has covered it expertly. He mentioned all that I would do as a first cut.
Many of these amps used Suzuki Brand coupling capacitors with "OIL" prominently written on the side of the capacitor body. These things go leaky easily and have been responsible for many an output stage melt down. These are the C27 to C32 that Kevin mentions. Do replace them with modern polypropylene capacitors - particularly important if they happen to be those Suzuki "Oil" caps.