/diy/ - Do It Yourself

[Anonymous No.1578718]

I've got a QRX9001 quad amp that was left to rot in a garage for an untold number of years. It's been on my todo shelf for a while and I'd like to get to it this summer. When I had first gotten it I, probably foolishly, tried to plug it in and turn it on. It was initially set to 220V, I didn't see that and plugged it into 120V briefly before changing the setting. I don't know if that would have caused horrendous damage but hopefully not. If I recall, once I had it configured correctly the face lights came on and there was some quiet crackling noise through the headphone jack and maybe in the speakers. I've got myself a function generator+oscilloscope, multimeter, soldering iron, lab power supply, and little experience with this sort of work. How should I get started debugging this thing?

[Anonymous No.1578739]

lower voltage you should be fine. good luck. check fuses first, often culprit.

[Anonymous No.1579324]

Get a schematic and check the power supply first, they are the heart of an amplifier. Caps are probably bad, diodes rarely fail in the psu but can, if it's regulated check the VRs. If that checks out, put a signal to an input and follow it till something isn't correct.

[Anonymous No.1579333]

>>1578718
Have you actually tried to get sound out of it? Like tune a channel or play music or something?

[Anonymous No.1579402]

>>1578718
>>1578718
Plug it into fused socket or isolation transformer, check if power supply is fine (transformer + rectifier and caps).
It would be also smart to put light bulb in series (60-100W), it should light up for few seconds and then go dim. If it stays lit up you might have short somewhere or amp is drawing too much power.
If you have voltage on positive and negative rails (i presume it should be somewhere around +-40V, might be more but most 100W amps are around that) go ahead and check for DC voltages on output, try to hear if output relays are switching (if there are any).
Don't plug in speakers before checking DC offset on output, if output stage is damaged and protection doesn't work (or there is no protection) you might burn your speakers.

[Anonymous No.1579671]

Own a QX949 myself.

That may have done damage but likely just fuses.

You need the schematic for it.

Another problem they have are the switching power supplies and the capacitors that are attached to them plus the diodes tend to go bad. If it power's on but you don't get any sound that means that these things haven't turned on power to most of the stereo and you need to do some work on them. Again a schematic will tell you what to do or what pieces you may need to test

[Anonymous No.1579734]

>>1578718

The rotary switches that switch the inputs is likely dirty, you might need to get some contact cleaner and work it back and forth.

A monster receiver like this ought to be re-capped with audio grade electrolytic capacitors (once you have it working). These are really great sounding amps when brought back up to spec.

Pic somewhat related. A small Kenwood receiver I'm working on. I just finished replacing all the electrolytic and tantalum capacitors with audio grade/film capacitors. It sounds awesome driving some JBL speakers of a similar vintage. It still has the failure prone 2sc1213s and 2sc1345s in it, will be replacing those when they get here

[Anonymous No.1580080]

>>1578718
You likely did not damage it over rot. However, the QRX9001 is a twitchy beast, and you won't get far without a service manual. Good luck.

[Anonymous No.1580097]

>>1579734

Very nice work. I'm doing something similar, but I'm having trouble sourcing audio-grade solder.

[Anonymous No.1580124]

OP here, all the fuses I could find seem to be intact. I turned it on and probed the headphone jack, both channels there go to -45V, and left swings up to 40V after a couple seconds of being on, spooky stuff. The front speaker terminals are just some line hum at a little under 0.2V. I do hear a relay click after a moment.
The transformer voltages all seem to check out. There's one line that's a couple volts above what it should be, that works out to 6% which I guess is okay enough.

>>1580080
I had a schematic and users manual, Just found a service manual so I'll be having a read through it.

[Anonymous No.1580241]

>>1580097

>audio grade solder

Just get some quality 60/40 rosin core solder like Kester or Delta

[Anonymous No.1580262]

>>1579734
I picked up a Kenwood KR-4070 at a thrift shop a year or so back just 'cause it looked cool. Was sold "as is" for $9.99 but it was a sale day so I only spent 7-ish on it. Some times it wouldn't turn on fully, other times I'd only get sound out of one channel. Huge amounts contact cleaner on the inside of the rotary switches fixed the issue. I'd hit the switch with a squirt of cleaner, work it through its range several times, wait for it to dry and power it back on to test. It took a good 10-12 attempts before the problem cleared up completely. Outside of replacing the little lamp that illuminates the analog 'signal' and 'tuning' meters it needed no other work. The caps and such are probably well and truly out of spec after 40 years but it sounds good enough to me that I don't give a shit. It's just driving some old Pioneer CS-99 speakers anyway (which I paid $30 for at a different thrift store because the grills had some damaged and they couldn't test them). One thing that is nice about these old system/speaker combos; they get fucking loud even when outputting very little power. A 40-watt receiver and 80-watt max speakers and it is deafening at anywhere near max volume.

Picture from Google.

[Anonymous No.1580322]

>>1580124
Check that headphone jack, i'm not sure if relay click you can hear is disconnecting outputs to the terminals so there is no offset. If you have schematics check where outputs before relay are and see if there is any DC offset. On some amplifiers headphone jack is connected to main amplifier through resistors so if output section of amplifier is blown you might still get rail voltage there like you do.
I presume only output section has some problems (could be only output transistor), or if headphone jack has it's own preamp that might be blown.

[Anonymous No.1580421]

>>1580124
Check input voltages on all audio bypass caps.

[Anonymous No.1580434]

While on the topic of repairing solid state gear, does anyone have any recommendations for new axial lead type electrolytic capacitors? I have an old solid state amp from the 60s that's just filled with them. I know that Nichicon & Illinois Capacitor still make them, but it seems that many manufacturers have discontinued them. Is there anyone else that still has them available?

[Anonymous No.1580506]

When doing repair work run the mains though an incandescent to reduce the current in case if a fault.