SOLVED: SX-626 Problems

[GlasgowGrip]

Hello All,
I'm checking out an SX-626 for a friend that has a couple of mysteries I could use an assist with. Y'all were a great help with my SA-6500ii problem last month, so thanks again! I'm hoping we can get to the bottom of things again.

To set the stage, this is a clean and totally unmolested SX-626, S/N SB1200468, which translates to a Manufacturing Date of Feb. 1972. It is an "Early" model with a round "FW" sticker and the 5-position voltage selector. The tuner seems to be in decent alignment, all the inputs work, even all the lamps still work. So I haven't done anything to this unit yet except testing.

Some findings:
- Frequency response within +0.5/-2.0 dB
- Clean power before clipping: Right channel 24.1W, Left channel 26.2W (1kHz into AUX, playing into 8 ohm dummy loads)
- THD+N: Right channel 0.15% @ 18.4W, Left channel 0.10% @ 18.7W (1kHz into AUX, playing into 8 ohm dummy loads)

Power Supply seems okay:
- Pin 3 = 60.69v
- Pin 5 = 30.63v
- Pin 7 = 12.56v
- Pin 10 = 9.185v

Power Amp checked with 8-ohm dummy loads connected, volume at 0, no signal into AUX...
- Out1 DC Center Voltage: Q9 C = 60.70v, Q11 C = 26.19v; Bias = 12.3 mV at flying lead
- Out2 DC Center Voltage: Q10 C = 60.70v, Q12 C = 26.73v; Bias = 16.3 mV at flying lead

So I'll cut to the chase, I need to figure out these two mysteries:
1. The Low Filter button makes a loud thump every time it's engaged or disengaged, on both channels, all inputs, and no matter what the volume is set to. I de-ox'd the switch thoroughly, so I'm guessing it's maybe some kind of a Cap problem... But the caps on the switch board are green epoxy films, they don't seem likely culprits, do they? Anyone seen this one before?

2. The owner told me the Phono inputs were "noisy", but I thought things were in good shape the first night I listened to it. But today I went back to check again, hooked the TT back up and sure enough, a loud random CRACKLE in the Right channel. Checked the Mic, FM, FM Mono and AM inputs, and the random crackle is there too! The thing is, it plays perfectly through the AUX input, every time, all the time, so... Selector switch? But why is AUX not affected? Exercised the selector about 200 times, and it makes no difference. I guess I will de-ox it in the morning, but anyone have any other ideas?

Thanks in advance! - Steve

 

[AE Bob]

The Aux and Tape monitor do not go through the W21-004 "Head Amp" board. The designation of Head Amp is a little misleading. It is the phono preamp and it also is used to amplify the signal from the FM and AM tuners. The gain and equalization are changed between the tuner and phono functions. It is VERY likely a noisy transistor on that board. I just change all 4. I use 2SC1815 to replace both types. As far as the pop, I would suspect leaky capacitors on the AWG-008 tone board. If they are blue "robin's egg" Tantalum capacitors REPLACE ALL OF THEM.

 

[GlasgowGrip]

Brilliant! Thank you for the insight Bob, I'll check those out and report back.

 

[GlasgowGrip]

@AE Bob: Many many thanks - you were correct on both counts!

1. The "thump" when the Low Filter switch was engaged or disengaged was solved by re-capping the AWG-008 Control Amp board. The output coupling caps C27/C28 were the Sanyo robin's egg blue tantalums and they tested way out of spec. I replaced C27/C28 with 2.2uF/63V 10% WIMA MKS2 series. I also replaced the small signal BJT's (2SA572/2SC1000) while I was at it due to the 2SC1000 being a known troublemaker, substituting with KSA992/KSC1845. I left the JFET's alone since they seem to be fine.
AWG-008 Before and After pics:

2. The persistent crackling on the Phono/Mic/AM/FM inputs was solved by replacing the small signal BJT's on the W21-004 Head Amp board. I replaced all of the the troublemaker 2SC871 and also the 2SC870 with KSC1845. I then also re-capped the board while I was at it.
W21-004 Before and After pics:

Very happy now, a very nice sounding receiver. Thanks again Bob!