M-80 & C-80 newbie stuff

Half Life

New Member
First, a brief introduction. Long time lurker, just joined. I've found lots of good info on this site. I am a former U.S. Navy Electronics Technician. As a hobby I restore/refurbish pinball machines, both electro-mechanical and solid state. I have 7 pinball machines in my collection now.

Second, a big thank you to Avionic, brutal and amr2 for their many posts on the M-80. Good stuff there.

Now on the the rest of the story (nod to Paul Harvey). Got a free M-80 from a friend of my brothers that said he wasn't sure if it worked anymore. Took the cover and bottom plate off to do a visual inspection and didn't find anything out of the ordinary other than the distinct smell of cigarettes. Unit must have been in a bar during its life.

Plugged it in and turned it on. Unit went straight to protection mode. Turned the unit off. I checked the various connectors to ensure they were snug. Turned the unit back on and this time no protection mode. I used my cell phone (playing Pandora) to provide an input although I knew it wasn't going to drive the amp properly. Got very weak sound but sound nonetheless. Adjusted the R/L Level controls, got some scratching and then back to protection mode. Searched the internet and found this place. Learned about the bond glue issue and took stock of the affected components. Also checked all of the transistors on both heat sinks and found them to be ok.

Placed order for the passive components I needed from mouser and while waiting decided to see if I could find a C-80 companion. Was lucky enough to locate a working one. Got it home and opened it up looking for the bonding glue. Yep, inside this unit too but not near as much. Only had to replace two resistors and do some cleaning. Checked the C-80 with my cell phone and some headphones and it works like a champ.

Parts arrived and replaced in the M-80. Connected the unit to its mate and fired it up (hooked to A & B sets of speakers). No protection mode - woo hoo! Noted the display center not lit but I can hear music. Right channel sounds weak, adjusted levels and still get scratching (need to get more deoxit). Then I hear a "pfffttt" and the unit goes straight into protection mode. Turned the unit off and will look at it later this morning.

Sorry for the long post. Just wanted to introduce myself and say thanks to those that help on here. Without your help I wouldn't have gotten as far as I did with my new-to-me gear. Can't wait to get them working correctly.
 
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Ok, did some more checking and found -5.6VDC on the right channel and -7.5VDC on the left channel. Did not visually find any component that I let the magic smoke out of.

I would welcome any suggestions as to where to look next.
 
Ok, did some more checking and found -5.6VDC on the right channel and -7.5VDC on the left channel. Did not visually find any component that I let the magic smoke out of.

I would welcome any suggestions as to where to look next.
Welcome lurker..:D
Check your solder connections on the TO-126 transistors TR111,112,113,114,117,118,123 & 124.Four of these have fusible resistors connected to the base leads. Have a real good look at all the little greenish resistors mounted on end for blackened rings around there centers. These are all fusible resistors.Happy hunting!
 
Thanks for the suggestion. Checked all of the noted transistors and the fusible resistors with meter. All are ok.

Further troubleshooting led me to D159 which was shorted. I also removed R226 (3.9k) as it appeared as though it had been stressed by the additional current. I'll be replacing both the diode and the resistor.

Haven't yet figured out why this diode failed but won't dwell on it too much. Troubleshooting continues.
 
Hello Half Life, welcome

Further troubleshooting led me to D159 which was shorted. I also removed R226 (3.9k) as it appeared as though it had been stressed by the additional current. I'll be replacing both the diode and the resistor.

I would also check R221 - 22Ohm
 
Hello Half Life, welcome



I would also check R221 - 22Ohm

Another possible issue. Look at R103 220Ω CF )its not on the schematic. But is identified on the component location drawing. If it looks stressed or burnt
theres a real good chance that half of TR101 is bad(2SK389).
 
Hello Half Life, welcome



I would also check R221 - 22Ohm

I pulled that one as well to measure it even though it did not looked like it was stressed. No issues with this component.

Thank you for the suggestion though. I genuinely appreciate it.
 
Another possible issue. Look at R103 220Ω CF )its not on the schematic. But is identified on the component location drawing. If it looks stressed or burnt
theres a real good chance that half of TR101 is bad(2SK389).

Thanks for the pointer. Would have looked right past that one using the schematic. My first visual inspection seems to have captured all of the visibly stressed components. However, I'll take a closer look.

Bought the parts to make a DBT today and will assemble it tomorrow. Never knew about that gadget until I read about it here.

If the dual FET is bad is there an acceptable replacement? Definitely worried about getting fakes.

Considering using led's for the center display but the purist in me (from my pinball restorations) tells me to replace with like. Any suggestions here? Would Mouser carry the incandescent's?
 
If the dual FET is bad is there an acceptable replacement?
2SK389 GR or BL... LinearSystems supposedly has a direct replacement LSK389 I believe.

99.99% of the ones on ebay are no good fakes.
 
Ok, new parts in and replaced (D159 & R226). Also replaced incandescent bulbs in center display. Re-soldered all connections on both upper boards. De-oxed all controls. Plugged unit in and turned it on. YES!!! No protection lamp lit.

Let unit sit and warm up to adjust idle circuit. Right channel adjusted just fine and is stable. The left channel, although adjustable, reads about 10mv low (5.6mv). Back to troubleshooting. So close.
 
Did you erraticate all the dried out glue and corroded component leads around C162/163 1000µf/100vdc and...
C111/112 220µf/6.3vdc ?

reads about 10mv low (5.6mv)

Its suppose to be 10mv not 15mv unless...

There is also two distinct idle current adjustments for over/under serial # 11051. 10mV for those under, and 15.5mV for those 11051 and over.
 
Did you erraticate all the dried out glue and corroded component leads around C162/163 1000µf/100vdc and...
C111/112 220µf/6.3vdc ?

I did but will definitely take a closer look under some magnification to be sure.


Its suppose to be 10mv not 15mv unless...

There is also two distinct idle current adjustments for over/under serial # 11051. 10mV for those under, and 15.5mV for those 11051 and over.

My serial# is greater than 11051 and I used the adjustment instructions related to that.
 
Does the trimmer potentiometer ohm out correctly. I would suggest replacing them if you haven't already. Might want to check TR115 2SC1953 it is a small TO-126 fastened to the main heatsink on the edge of the main board.
 
what are you using to check your transistors ?

Sorry for the delay in responding. I was using my Fluke 87 (in diode mode) to test the active components. But decided to take a break from that and build myself an octopus. Finished that and have tested the transistors using it. So far, all of the ones checked exhibit the same, in-circuit, characteristics. I know this method is not the most accurate but it can reveal variations across similar circuitry.

I've swapped VR103 and VR104 to see if the problem moves, it does not. Therefore I can rule out the trimmer pots as the cause. I did order a couple of replacements (MJE344G) for the 2SC1953. I tested the originals and although they look good on the octopus, I'll replace them anyway.

Back to the shop and will follow up with my findings. Still have not listened to music through this yet but am anxious to do so. I keep reminding myself to just go slow and make this right first.
 
:dammit:
Stupid, rookie mistake. MJE's are TO-225 case which has the collector also on the back of the case. These are not exact replacements for 2SC1953's (TO-126 case). I succeeded in letting the magic smoke out of TR111 (base-collector short). Checked other components downstream and cannot find any damaged. Unbelievable!! I am such a dope. :bash:

Mouser is getting good business from me lately. Back to waiting on parts.
 
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:dammit:
Stupid, rookie mistake. MJE's are TO-225 case which has the collector also on the back of the case. These are not exact replacements for 2SC1935's (TO-126 case). I succeeded in letting the magic smoke out of TR111 (base-collector short). Checked other components downstream and cannot find any damaged. Unbelievable!! I am such a dope. :bash:

Mouser is getting good business from me lately. Back to waiting on parts.

The C1935's are available from B + D Enterprise.:thmbsp:
 
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