Zenith MC 7050 Receiver - Full Rebuild - this is a neat one!

EastPoint

EastPoint

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So I picked up a Zenith MC 7050 receiver at some point in the past. No idea where it came from, but I got a pile of low-end receivers from a guy a few years ago, so maybe there. It had a note on it indicating it had some distortion, but I fired it up and didn't hear anything wrong. It played fine and sounded great.

The service manual is available at hifiengine.com and it said the power supply should be set to 33v and the amp board biased at 3mv per emitter resistor. I couldn't get any bias on one channel and the other one was low, and the power supply wouldn't get above 28v.

So first I rebuilt the power supply. The big diodes all got swapped for 1N5404-3G and the small diodes got swapped for 1N4004s. The small transistor becomes a KSC2383 and the two heat-sinked transistors are now TIP41Cs. Caps tested ok but had all drifted upwards 20-25%. After the rebuild the power supply adjusted to 33v just fine.

The amp board rebuild was next. Caps were easy. The four diodes were replaced with 1N4004s. This got me bias on both channels, but I had a bit of a drift on one. I replaced the four driver transistors with KSA1220/KSC2690s and also noticed that one emitter resistor was not soldered in completely--it was wiggling through the board. Bias was fine after I fixed that.

As long as I was in there I figured I'd redo the rest of the board. The protection board looks exactly like a Pioneer SX protection board--it has the 470 uf 6.3v caps, a pile of KSC945s, two .22 uf caps, a lot of little glass diodes, and a MY4-02-DC24 relay. I redid the caps, almost all the transistors except for a few that I wasn't sure about subs for, and the relay.

Then I moved on to the preamp board attached to the tone controls. This was ridiculous! It runs almost the entire width of the receiver, and is jammed full of caps and transistors. All the transistors are KSA640 and KSC1222, which I subbed with KSA992 and KSC1845. The original transistors are EBC, not the usual ECB, so be careful! Caps all got replaced with new Nichicons and Wimas, and the controls got cleaned and lubed.

Last was the gigantic phono board. There are 23 capacitors and 14 transistors on here. No clue as to why. Again, the transistors are all KSA640/KSC1222 and sub with the KSA992/KSC1845.

Main caps were 6800 uf 50v Samsung capacitors, which had drifted a bit higher and showed a little leakage but no ESR. I had some Cornell-Dubilier 6800 uf 80v caps with the same diameter, so I used those to replace them.

Total new part count is 54 capacitors, 32 transistors, 13 diodes, and the relay. It sounds amazing! Here's some eye candy:

IMG_3595.JPG IMG_3602.JPG IMG_3566.JPG IMG_3561.JPG IMG_3539.JPG IMG_3560.JPG IMG_3562.JPG IMG_3563.JPG IMG_3567.JPGIMG_3565.JPG
 
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That's really nice.

Question for you and others who do this - do you just buy all these caps, transistors, etc. in bulk? Is there some sort of audio starter pack to buy? Not that I would attempt a full redo like this, or maybe I would on a lesser receiver, but a list of general things to have on hand would be great - not including the main caps, because I assume they vary more from piece to piece than some of the smaller caps(?).
 
That's an interesting line of receivers. I recently sold a 7040 at a local sale. Lots of folks looked at it, then were surprised to see the Zenith badge on it. They were made in Korea and based what I read elsewhere, probably by Samsung. I only sold mine as I have way too many pieces and something needed to go. The guy who bought it seemed happy.
 
HiFi in WYO said:
That's really nice.

Question for you and others who do this - do you just buy all these caps, transistors, etc. in bulk? Is there some sort of audio starter pack to buy? Not that I would attempt a full redo like this, or maybe I would on a lesser receiver, but a list of general things to have on hand would be great - not including the main caps, because I assume they vary more from piece to piece than some of the smaller caps(?).
Click to expand...

I buy parts in bulk from Mouser.com. I go through a lot of them, so I buy 100 or even 1000 at a time and get pretty hefty discounts that way. This ensures that I always have the right parts when I need them. There are parts lists for a number of receivers available on audiokarma that allow you to order exactly what you need from Mouser.

There isn’t really a starter pack but if you’re looking to get into this seriously the best thing you can do is start buying the parts you need and just buy them in quantity when you do buy them.
 
If you ever come across one of these that's dead and primary of power xformer checks open it
most likely is not. Under the fish tape is a small thermal fuse that can be bypassed to
restore normal operation. Mine still works like a champ after 20 years.
 
HiFi in WYO said:
That's really nice.

Question for you and others who do this - do you just buy all these caps, transistors, etc. in bulk? Is there some sort of audio starter pack to buy? Not that I would attempt a full redo like this, or maybe I would on a lesser receiver, but a list of general things to have on hand would be great - not including the main caps, because I assume they vary more from piece to piece than some of the smaller caps(?).
Click to expand...
Probably more than you asked for, and kind of a thread crap, but:
100 of: KSC1845, KSA992,
25 of: KSC2383, KSA1013, KSC2690, KSA1220, KSC3503, KSA1381, KSC2073, KSA940, TIP31C, TIP32C, MJE15032, MJE15033
100 of: 1N4148, 1N4004, 13V Zener, 14V Zener

100 of:
1, 2.2, 3.3, 4.7uF 50V KL
10uF 25V KL
0.22uF, 1, 2.2, 3.3, 4.7uF 100V
10, 22uF 50V
47uF 25V
47uF 50V
100uF 16V
100uF 50V
220uF 16V
220uF 50V

I keep a ton of bipolar caps, fuses, Bourns trimmers, Omron relays, Vishay resistors, and CDE caps on hand as well.
 
hola tengo problemas con el circuito de proteccion.the transistors ksa 539 and its complementary ksc815 do not work, are the bc546 bc556 ok?
 

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Hi, you can replace 2SA539 with KSA1015 and 2SC815 with KSC1815. You can probably use BC556 to replace the 2SA539 and BC546 to replace the 2SC815, just be careful with the pinouts--I believe they are different. I am not very familiar with those--I generally work with the more common Fairchild/Onsemi transistors.
 
[QUOTE = "EastPoint, publicación: 14724786, miembro: 45975"] Hola, puede reemplazar 2SA539 con KSA1015 y 2SC815 con KSC1815. Probablemente pueda usar BC556 para reemplazar el 2SA539 y BC546 para reemplazar el 2SC815, solo tenga cuidado con los pines, creo que son diferentes. No estoy muy familiarizado con ellos; generalmente trabajo con los transistores Fairchild / Onsemi más comunes. [/ QUOTE]

it worked, the relay hits there is sound !!!
 
EastPoint said:
So I picked up a Zenith MC 7050 receiver at some point in the past. No idea where it came from, but I got a pile of low-end receivers from a guy a few years ago, so maybe there. It had a note on it indicating it had some distortion, but I fired it up and didn't hear anything wrong. It played fine and sounded great.

The service manual is available at hifiengine.com and it said the power supply should be set to 33v and the amp board biased at 3mv per emitter resistor. I couldn't get any bias on one channel and the other one was low, and the power supply wouldn't get above 28v.

So first I rebuilt the power supply. The big diodes all got swapped for 1N5404-3G and the small diodes got swapped for 1N4004s. The small transistor becomes a KSC2383 and the two heat-sinked transistors are now TIP41Cs. Caps tested ok but had all drifted upwards 20-25%. After the rebuild the power supply adjusted to 33v just fine.

The amp board rebuild was next. Caps were easy. The four diodes were replaced with 1N4004s. This got me bias on both channels, but I had a bit of a drift on one. I replaced the four driver transistors with KSA1220/KSC2690s and also noticed that one emitter resistor was not soldered in completely--it was wiggling through the board. Bias was fine after I fixed that.

As long as I was in there I figured I'd redo the rest of the board. The protection board looks exactly like a Pioneer SX protection board--it has the 470 uf 6.3v caps, a pile of KSC945s, two .22 uf caps, a lot of little glass diodes, and a MY4-02-DC24 relay. I redid the caps, almost all the transistors except for a few that I wasn't sure about subs for, and the relay.

Then I moved on to the preamp board attached to the tone controls. This was ridiculous! It runs almost the entire width of the receiver, and is jammed full of caps and transistors. All the transistors are KSA640 and KSC1222, which I subbed with KSA992 and KSC1845. The original transistors are EBC, not the usual ECB, so be careful! Caps all got replaced with new Nichicons and Wimas, and the controls got cleaned and lubed.

Last was the gigantic phono board. There are 23 capacitors and 14 transistors on here. No clue as to why. Again, the transistors are all KSA640/KSC1222 and sub with the KSA992/KSC1845.

Main caps were 6800 uf 50v Samsung capacitors, which had drifted a bit higher and showed a little leakage but no ESR. I had some Cornell-Dubilier 6800 uf 80v caps with the same diameter, so I used those to replace them.

Total new part count is 54 capacitors, 32 transistors, 13 diodes, and the relay. It sounds amazing! Here's some eye candy:

View attachment 1682689 View attachment 1682690 View attachment 1682691 View attachment 1682692 View attachment 1682693 View attachment 1682694 View attachment 1682695 View attachment 1682696 View attachment 1682698View attachment 1682697
Click to expand...
Thank you so much for this post! I recently got my hands on this amp. It had many problems. The protection relay refused to work. For many reasons all solved thanks to your post! I almost rebuilted all sections where I faced the same problems you mention. Power supply was off and I couldn't adjust bias on one channel. Many transistors where dead or leaking. But power transistors are still OK. Thanks to my lack of systematic method, I must say that I solved many problems not being sure of what I really did since I made too many changes at the same time... But anyway, the relay now works. The only thing is that it takes a long time for it to get going. Around 10 secs, and even more if the amp is cold. That doesn't seem normal. Anyone knows what could cause that? I replaced all caps and 2 of the relay board transistors. Maybe I should replace all transistors and diodes and see? Attached is the schematic.
 

Attachments

felixleg said:
Thank you so much for this post! I recently got my hands on this amp. It had many problems. The protection relay refused to work. For many reasons all solved thanks to your post! I almost rebuilted all sections where I faced the same problems you mention. Power supply was off and I couldn't adjust bias on one channel. Many transistors where dead or leaking. But power transistors are still OK. Thanks to my lack of systematic method, I must say that I solved many problems not being sure of what I really did since I made too many changes at the same time... But anyway, the relay now works. The only thing is that it takes a long time for it to get going. Around 10 secs, and even more if the amp is cold. That doesn't seem normal. Anyone knows what could cause that? I replaced all caps and 2 of the relay board transistors. Maybe I should replace all transistors and diodes and see? Attached is the schematic.
Click to expand...
Pretty much all of the Speaker protection board components replaced, but no change.
 
EastPoint said:
So I picked up a Zenith MC 7050 receiver at some point in the past. No idea where it came from, but I got a pile of low-end receivers from a guy a few years ago, so maybe there. It had a note on it indicating it had some distortion, but I fired it up and didn't hear anything wrong. It played fine and sounded great.

The service manual is available at hifiengine.com and it said the power supply should be set to 33v and the amp board biased at 3mv per emitter resistor. I couldn't get any bias on one channel and the other one was low, and the power supply wouldn't get above 28v.

So first I rebuilt the power supply. The big diodes all got swapped for 1N5404-3G and the small diodes got swapped for 1N4004s. The small transistor becomes a KSC2383 and the two heat-sinked transistors are now TIP41Cs. Caps tested ok but had all drifted upwards 20-25%. After the rebuild the power supply adjusted to 33v just fine.

The amp board rebuild was next. Caps were easy. The four diodes were replaced with 1N4004s. This got me bias on both channels, but I had a bit of a drift on one. I replaced the four driver transistors with KSA1220/KSC2690s and also noticed that one emitter resistor was not soldered in completely--it was wiggling through the board. Bias was fine after I fixed that.

As long as I was in there I figured I'd redo the rest of the board. The protection board looks exactly like a Pioneer SX protection board--it has the 470 uf 6.3v caps, a pile of KSC945s, two .22 uf caps, a lot of little glass diodes, and a MY4-02-DC24 relay. I redid the caps, almost all the transistors except for a few that I wasn't sure about subs for, and the relay.

Then I moved on to the preamp board attached to the tone controls. This was ridiculous! It runs almost the entire width of the receiver, and is jammed full of caps and transistors. All the transistors are KSA640 and KSC1222, which I subbed with KSA992 and KSC1845. The original transistors are EBC, not the usual ECB, so be careful! Caps all got replaced with new Nichicons and Wimas, and the controls got cleaned and lubed.

Last was the gigantic phono board. There are 23 capacitors and 14 transistors on here. No clue as to why. Again, the transistors are all KSA640/KSC1222 and sub with the KSA992/KSC1845.

Main caps were 6800 uf 50v Samsung capacitors, which had drifted a bit higher and showed a little leakage but no ESR. I had some Cornell-Dubilier 6800 uf 80v caps with the same diameter, so I used those to replace them.

Total new part count is 54 capacitors, 32 transistors, 13 diodes, and the relay. It sounds amazing! Here's some eye candy:

View attachment 1682689 View attachment 1682690 View attachment 1682691 View attachment 1682692 View attachment 1682693 View attachment 1682694 View attachment 1682695 View attachment 1682696 View attachment 1682698View attachment 1682697
Click to expand...
I just picked up a MC-7050 for cheap.To my surprise it is super clean inside and out. Works like a champ but there is a little humming between songs. That should be the caps, right? I opened it up, and they didn't seem to be swelled and no signs of leaks, but she is an old machine.
 
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