60s RCA Stereo/Record Console - Crackling Subwoofer: source partially narrowed down -- need help diagnosing

[bakarrr]

Hello all,

I just picked up an RCA VMT-72W console for free on fb marketplace. It worked nearly perfectly--the only problem was the (stage) right subwoofer cracking intermittently. It would improve some when you fiddled with the balance knob but never fully went away. When i opened the back the thing was riddled with cobwebs. I cleaned the dust and cobwebs and went through all the wiring connections with some Deoxit. Now, the crackling subwoofer is constant and happens regardless of volume level of the system.

My Attempts to Diagnose:

• I first started by swapping the power and ground wires for each of the subs and it swapped my problem (R sub worked and L sub crackled), so the issue was upstream.
• I then messed with all of the connections between the subs and the amp narrowed down to the connector tab where the sub wiring connects to the amp.
• For each side (L + R) there are 2 prongs but only one connected wire. When the crackling sub is disconnected and touched to the free tab on the non-crackling sub side, both sides work and the problem is solved! The only problem is that there is maze of connections unique to each side after these tabs enter the amp, making me feel like there is an electrical reason these 2 sides are separated.

I am clueless when it comes to all the wiring inside, so I am hoping that someone can offer some diagnostic help based on what i've found. I've attached pics of what I (attempted to) describe above.

Any advice is very much appreciated!

 

[raycarr00]

that's a lot of console, nice! Have you searched and found a schematic? Must certainly need servicing by now. Did the dimbulb shine bright?

 

[orthophonic]

That is a 1969 or 1970 model, the first thing to do is replace the electrolytic caps in the amplifier, they are always bad in these models.

This was the last model series to use the unique to RCA integrated circuit cartridge, the preamp is in the cartridge, a chip is under the silver cap on the cartridge headshell.

 

[Namlook]

That's not a subwoofer.

 

[bakarrr]

That's not a subwoofer.

astute observation

that's a lot of console, nice! Have you searched and found a schematic? Must certainly need servicing by now. Did the dimbulb shine bright?

Thanks! I just found a schematic online but it'll take me ages to crack the code given my electronic illiteracy. Also had to google "dimbulb" lol.

That is a 1969 or 1970 model, the first thing to do is replace the electrolytic caps in the amplifier, they are always bad in these models.

This was the last model series to use the unique to RCA integrated circuit cartridge, the preamp is in the cartridge, a chip is under the silver cap on the cartridge headshell.

Very helpful advice--I'll look into that, thanks!

 

[Bradbrews]

Snap crackle pop is not likely to be a capacitor. A switch, connector, or transistor is more likely.
See if any of the transistors in the unit are listed in the Top 10 bad actors list in the pinned post here.

 

[bakarrr]

So I am all but certain it was the output transistors, however in usual DIY fashion I may have created more problems trying to diagnose the original. I noticed 3 of the 4 output transistors had steady current on the multimeter, but the one associated with my popping speaker (Q508 in schematic) was weak voltage and fluctuating like crazy. Then my hand slipped when I was checking the reciprocal transistor again (Q506 in schematic) and it sparked and the power to the receiver went out (the turntable still has power).

All this to say, I need to replace the germanium transistors with silicon (+ rebiasing), replace the diodes and fuses +/- replace some capacitors.

TLDR:
I ordered MJ15025G silicon transistors, but now after reading people's advice I feel these may not have the optimal specs to preserve the original sound. I've tried to find crossover replacements for the originals but have not had any luck. I know I am asking a lot here but does anyone know which silicon replacements may be the best match for the originals?

(I did not expect to fall into this electrical engineering rabbithole when I picked this console up from facebook)


(PDF of schematic attached)

 

[orthophonic]

I did a couple of cross searches and it appears this Germanium crosses over to NTE 127 and ECG 127. There is one original RCA on ebay.

To be sure, I would contact Moyers Electronics, 800-577-6001, they have millions of parts, were RCA distributors for decades and are very helpful with
cross referencing or maybe even having the original RCA.

I hope you get it working soon!