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 Post subject: Re: weird red plate issue
PostPosted: May Tue 12, 2020 11:21 pm 
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I would say that this is not normal. First step would be to measure the B+ for each position of the power plug. Do you have a schematic?

Dave


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 Post subject: Re: weird red plate issue
PostPosted: May Wed 13, 2020 1:44 am 
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I agree, not normal. With no schematic it's just guesswork but check any capacitors that go from the power line to chassis. I have no real reason to think one may be shorted, but a shot in the dark .....

post up a schematic plz.... and watch it working on this.. you may have a hot chassis there.

Also possible some issue with the power transformer perhaps.

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 Post subject: Re: weird red plate issue
PostPosted: May Thu 14, 2020 2:15 am 
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check the bias supply circuit .Especially if it's feeding off the power transformer. Verify a zener diode isn't being used. Also has this unit been recapped?


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 Post subject: Re: weird red plate issue
PostPosted: May Thu 14, 2020 8:14 pm 
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This is very strange, reversing the polarity of the AC cord (power transformer primary) should have no effect on the biasing of the output tubes or other operating conditions. I would first cut out (delete) any cap(s) between the incoming AC line and chassis. Then, it is absolutely imperative (if not already done) to replace ALL the grey paper in oil caps (could be marked Suzuki, Elna, Rubycon,...) which in japanese sets are know to be allways defective (leaky). Then your problem should be solved. Unfortunately the schematic for this unit seems impossible to find but it would be good to check if this amp has a transformer powered heater circuit. Some cheaper japanese units are using AC mains powered series mounted heater circuits (like some old TV's or AC/DC sets) but transformer powered HV (B+), and with these (hazardous) sets everything is possible... (H-K leakage or shorts,etc...) Also, when you're saying "works fine" did you actually checked (measured) the output stage biasing/operating conditions ? You might be on the verge of overloading/red plating where any slight perturbation could start the red plating. Only educated guesses, but some paths to follow...


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 Post subject: Re: weird red plate issue
PostPosted: May Sat 16, 2020 2:51 am 
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It has been recapped and I have a schematic posted on my other post on it. there are no caps between primary and chassis only secondary I agree it is weird that this happens but my latest theory is that since this plug tends to fall out in one position do to alignment of the interlock I think arcing is occurring putting stress on the primary and increasing the secondary voltages by a little. or could be that one of the new tubes just had a internal fault but works great except the A.M. does not work no clue why the 6be6 tube glows and all coils have continuity plus F.M. works fine.
I will re-post the schematic for anyone interested well actually it's the whole manual


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 Post subject: Re: weird red plate issue
PostPosted: May Sat 16, 2020 2:58 am 
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yes the bias is currently at 0 volts dc according to my voltmeter it was at 300 before recapping in fact it was high enough my multimeter had a fit


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 Post subject: Re: weird red plate issue
PostPosted: May Sat 16, 2020 3:01 am 
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also the heaters are powered by transformer except for the 2 phonograph preamp 12ax7's which are wired into the output tube biasing circuit which seems like a stupid design to me cause if the 12ax7's blow down goes with the bias voltage


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 Post subject: Re: weird red plate issue
PostPosted: May Sat 16, 2020 4:20 am 
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swap the tube to see if the problem follows that tube from socket to socket. It might be a cathode and/or grid short.


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sat 16, 2020 1:49 pm 
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Like I said I swapped the tube for one of the old ones and it is working correctly now


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 Post subject: Re: weird red plate issue
PostPosted: May Sat 16, 2020 2:20 pm 
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SpaceFerret42 wrote:
also the heaters are powered by transformer except for the 2 phonograph preamp 12ax7's which are wired into the output tube biasing circuit which seems like a stupid design to me cause if the 12ax7's blow down goes with the bias voltage


Heaters in the cathode circuit of the output tubes is not too uncommon.

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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sat 16, 2020 2:23 pm 
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Oh okay didn't know that


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sat 16, 2020 9:55 pm 
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SpaceFerret42 wrote:
Like I said I swapped the tube for one of the old ones and it is working correctly now


Wait, is it a NEW reissue that's redplating?


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Mon 18, 2020 10:33 am 
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It was yes also had a old tube red plate in the same socket but that one got blasted by a bad cap so knew it was bad


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Mon 18, 2020 12:11 pm 
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Reading your other thread about this amp it looks you had this red plating problem (more or less) intermittently right from the start and even after recapping and other parts replacement it's still there. I would not adhere to your theory or "arcing in the AC plug" and at this point I would NOT consider the amp fixed reliably. Apparently it seems your tubes are very (too) close to max. plate dissipation and any small disturbances (or AC mains variations) could start the red plating, or there is an intermittent fault condition. Provided that all the old P.I.O caps have been replaced and you have checked all the solder joints there's no much left. Here's what I would do in this situation:

1) Clean and retight the output tubes socket contacts. Replace the sockets if necessary (excessive wear).
2) Check for poor ground return points around the output stage. (bad solder joints are very common in cheap japanese sets). Re-solder all suspect solder joints.
3) Replace all the four 6BM8 output tubes at the same time with good quality N.O.S tubes. Don't try the single tube swapping procedure to see if the red plating "follows the tube", replace them ALL.
4) We don't have the schematic, but if the output tubes are fixed biased, the (negative) bias circuit must be checked for drifting resistors, bias pot,poor contacts, caps... If the circuit is cathode biased (which it looks it is) check the cathode circuit and the cathode decoupling cap. Since the total cathode current is used to provide heather supply to the input preamp tubes (not as bad idea as you might think, and a commonly used "trick" in old tube amps) measure the output tubes cathode voltage which should be somewhat less than the total (series connected) heater voltages (about 20VDC for a pair of 12AX7's), this will also give you an indication about the biasing of the ouput stage. In this circuit red plating also means too much current through the preamp heaters which could be damaged if left in this condition for too long. Under no circumstances should this voltage ever exceed 25VDC.


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Mon 18, 2020 7:12 pm 
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Tubologic wrote:
Reading your other thread about this amp it looks you had this red plating problem (more or less) intermittently right from the start and even after recapping and other parts replacement it's still there. I would not adhere to your theory or "arcing in the AC plug" and at this point I would NOT consider the amp fixed reliably. Apparently it seems your tubes are very (too) close to max. plate dissipation and any small disturbances (or AC mains variations) could start the red plating, or there is an intermittent fault condition. Provided that all the old P.I.O caps have been replaced and you have checked all the solder joints there's no much left. Here's what I would do in this situation:

1) Clean and retight the output tubes socket contacts. Replace the sockets if necessary (excessive wear).
2) Check for poor ground return points around the output stage. (bad solder joints are very common in cheap japanese sets). Re-solder all suspect solder joints.
3) Replace all the four 6BM8 output tubes at the same time with good quality N.O.S tubes. Don't try the single tube swapping procedure to see if the red plating "follows the tube", replace them ALL.
4) We don't have the schematic, but if the output tubes are fixed biased, the (negative) bias circuit must be checked for drifting resistors, bias pot,poor contacts, caps... If the circuit is cathode biased (which it looks it is) check the cathode circuit and the cathode decoupling cap. Since the total cathode current is used to provide heather supply to the input preamp tubes (not as bad idea as you might think, and a commonly used "trick" in old tube amps) measure the output tubes cathode voltage which should be somewhat less than the total (series connected) heater voltages (about 20VDC for a pair of 12AX7's), this will also give you an indication about the biasing of the ouput stage. In this circuit red plating also means too much current through the preamp heaters which could be damaged if left in this condition for too long. Under no circumstances should this voltage ever exceed 25VDC.


+1


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Mon 18, 2020 9:59 pm 
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Joined: Nov Thu 07, 2019 11:11 pm
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right perhaps I should state that all carbon comp. resistors and caps in the preamp. and power amp sections have been replaced.
the amp is working correctly with the current tube set bias heater voltage and plate voltage are all in accordance on what's listed on the schematic on the back of the manual.
1. What has me concerned now is the capacitor [that is in accordance with the schematic] between the FM antenna and one side of the ac line cord.
2. The fact that the AM tuner is dead in the water.
not sure if the AM turner malfunction is due to the AM exclusive section or a failure in the shared section of the turner that only effects the AM


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sat 23, 2020 5:05 pm 
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Just removed the capacitor from between ac line in and the FM antenna did not effect the function of it but had to reverse the power cord as the chassis became hot.


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sat 23, 2020 11:53 pm 
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Have you verified the B+ is within range? Beginning to wonder if the power transformer has a developing problem


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sun 24, 2020 1:46 am 
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Joined: Nov Thu 07, 2019 11:11 pm
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It does appear to check out with in range according to the schematic. Plus it is also appears to work correctly at the moment so I am just thinking the new tubes a a slight defect in one so it was weak. And since to unit operates the 6bm8s at near overload condition any weak tubes will red plate at the slightest provocation perhaps that is the source of my issues it runs the tubes so hot anyway. Also this might be why this unit has a higher value resistor then the schematic calls for in the cathode circuits of the 6bm8 pairs they had issues at the factory and a revision was made


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 Post subject: Re: Roland/Martel (Rotel) FAX-200C
PostPosted: May Sun 24, 2020 4:04 am 
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Okay just noted something interesting the plug only truly works right one way around the other way it acts like it has a bad connection even when plug into the back of the unit all the way but since it is no polarized just switch it in the outlet and can plug it into the unit the right way up without getting the living daylights shocked out of me


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