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 Post subject: Admiral 20X1 Excessive Width
PostPosted: Aug Tue 24, 2021 7:11 pm 
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Joined: Oct Fri 28, 2011 7:27 pm
Posts: 115
Location: New York, NY
I'm working on a 20X12 right now and having a similar problem as @Fennec. It is is a Run 12, with a date of Nov 14, 1949 stamped on the tuner, so near the end of the model's run.

Complaint: The raster goes well beyond the screen horizontally and no adjustments have been able to reduce it. I'd say 20-25% of the screen width is not visible even with every possible adjustment set to minimum. See attached photos showing crosshatch going beyond CRT face.

The good:

  • Tuner is solid, very clear, no complaints. IF working fine, voltages all within spec.
  • Sync, both vertical and horizontal is rock solid. Horizontal sync holds through full rotation of the control.
  • Contrast/brightness is great, HV measures ~ 9KV
  • Vertical height/linearity controls are responsive and are just about right with controls in the center
  • All paper and electrolytic caps replaced (as is typical they were almost all completely bad)
  • Important and out-of-tolerance resistors checked and replaced

The bad/ugly:

  • Horizontal Output (6BG6) input grid voltage (pin 5) is way too high. With Drive control cranked fully in, is about 55V PP, and opened up is about 75V PP. Docs say it should be between 30-35V PP (depending on if you ask Sams or Riders). Increasing the drive (towards 75V PP) predictably makes the picture even wider. (pic attached)
  • Horizontal Oscillator voltage measures approx 95V PP. Riders says it should be 55V. (pic attached)
  • Horizontal Width and Linearity controls have no perceivable effect. I know they are supposed to be only "fine" adjustment, but full rotation of each literally makes no change whatsoever.

Just the facts:

  • Swapped 6BG6 (Horiz Out) tube - no change
  • Swapped 6SN7 (sync/clip and horiz osc) - no change
  • Swapped 12AU7 (sync amp, vert osc) - no change
  • Have not swapped 6W4 (Damper) - I do not have an extra of these
  • Triple-verified all wiring and replaced component values, and cross-checked Sams, Riders and Admiral schematics.
  • The 470K resistor across the drive control also a 1% and measures 470.7K
  • H.O. Input grid replacement resistor (47 ohms) is a 1% and measures 47.5 ohms
  • H.O. Input grid (pin 5) DC voltage is ~ -6V (Sams says -0.5V, Riders says -9.0V).
  • H.O. K (pin 3) DC voltage is ~7V (Sams says 9.6, Riders says 8 )
  • H.O. Screen (pin 8 ) DC voltage is 258V (Sams says 260V, Riders 270V)
  • Damper plate (pin 5) is ~370V (Sams 350V, Riders 375V)
  • Damper K (pin 3) is 417V (Sams 410V, Riders 450V)
  • Checked continuity on width coil (~1 ohm), and horiz linearity (~35 ohm)
  • The horizontal hold control has somewhat the effect of moving the picture left to right a bit. There is also a faint foldover line (but no visible portion of the picture) that moves left to right when moving the sync control. (see pic)
  • The only tube replaced is the 1X2 HV rect, as the original tested extremely weak
  • Lowering the AC line voltage down to 100V or lower does indeed cause the width to shrink down to finally seeing the edges.
  • All measurements taken with AC line dialed in at 115V. At 117V, the filament supply voltage measures 6.45V, and at raw line (121-ish) is around 6.65V. Never seen this before... bit of a head scratcher.
  • I have not touched the ion trap magnet. Based on dust and debris, it does not appear to have been moved any time recently. Since the horizontal tube voltages are excessively high, I'd think to leave this alone until that is worked out.

Quick links to schematic/service info.

Any tips or ideas greatly appreciated!

Thanks,
FK


Attachments:
File comment: Grid of Horizontal Output (6BG6), pin 5.
horiz_out_5_1.jpg
horiz_out_5_1.jpg [ 499.74 KiB | Viewed 1108 times ]
File comment: Horizonal Oscillator
horiz_osc_1.jpg
horiz_osc_1.jpg [ 502.66 KiB | Viewed 1108 times ]
File comment: Example of width
width_issue.jpg
width_issue.jpg [ 342.1 KiB | Viewed 1108 times ]
File comment: Foldover? Moves left to right with horizontal hold control.
ghostly_foldover_1.jpg
ghostly_foldover_1.jpg [ 561.81 KiB | Viewed 1108 times ]
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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Tue 24, 2021 9:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2459
Location: Lafayette, CO
Plate resistor of the sawtooth generator may be off. Change the screen resistor to the output tube. Add a 100ohm resistor between horizontal output cathode. Measure B+ power supply as it may be putting out 30 or 40 volts too high. Actually, do the last first or you will be chasing your tail. Craig


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Tue 24, 2021 11:31 pm 
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Joined: Oct Fri 28, 2011 7:27 pm
Posts: 115
Location: New York, NY
analog.tv wrote:
Plate resistor of the sawtooth generator may be off. Change the screen resistor to the output tube. Add a 100ohm resistor between horizontal output cathode. Measure B+ power supply as it may be putting out 30 or 40 volts too high. Actually, do the last first or you will be chasing your tail. Craig


The B+ at the second filter cap (between the field coil and the focus coil) measures 375V with variac set to 115V. The Riders (Sams doesn't call out any B+ measurements really) shows 365V. At the focus coil wiper I get 340V (@115VAC), which I believe Riders says 325V (hard to read), though this would vary a bit based on the position of the focus coil. Lowering it down doesn't really make a pronounced difference though. Now if I dial down the variac to about 90V, I *can* get the horiz output PP voltage down to the 35V it wants, and then the picture is actually about the right width (obviously all other voltages now insufficient).

I assume you want me to install the 100 ohm resistor to be able to measure cathode current on the output tube? Here's the thing... that socket pin (3) on the 6BG6 is the tie point for 3 capacitors and 1 resistor, in the most cramped corner of the chassis and the service info describes that area as "lead dressing is critical", so that might be a bit of a tall order. If I could get a socket adapter and check it with an ammeter that would get me the same thing in the end right?

For plate resistor of the sawtooth generator (the horizontal oscillator, right?), goes right to the horizontal freq/lock coil/slugs. There is an 8.2k resistor in parallel with the coil. Or should I be looking at something else?

Thanks!

FK


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 3:00 am 
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Location: Chicago, IL USA
There's already an 82 ohm cathode resistor on the HOT. You can measure the voltage across that to determine the current. No need to add a 100 ohm resistor.
You stated you measure 7 volts there so that's about 85mA.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 3:48 am 
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Location: New York, NY
bandersen wrote:
There's already an 82 ohm cathode resistor on the HOT. You can measure the voltage across that to determine the current. No need to add a 100 ohm resistor.
You stated you measure 7 volts there so that's about 85mA.


Oh, got it. I thought he meant to insert a 100ohm resistor between cathode and everything else... I've done that before with audio amps which is why I got confused.

@analog.tv, you meant to increase the 82 ohm resistor to a 100 then, or add a 100 making approx 180 ohm?


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 3:14 pm 
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As mentioned previously, the quick and easy way to reduce width when everything else doesn't get you where you want to, is to increase the value of the G2 resistor on the horizontal output tube. Some other TV sets over the years actually had a width control there.

Putting in a slightly weak 5U4 will also reduce B+ and as a consequence, picture size.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 3:54 pm 
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I'm also wondering if something isn't drawing as much current as it should be. This set uses a "stacked" B+ design where the audio output tube circuit is part of the power supply.
If the 6AS5 output tube is weak or not biased properly, it could cause problems.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 4:04 pm 
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Location: Pasadena CA USA
I don't really like changing the circuit or putting in bad tubes to make something work. There is a problem here that needs to be fixed. I am sure the set did not leave the factory like that, and that it worked at one time with the factory circuit. I would want to know if the TV set ever worked properly while in the OP's possession. Is this a recent problem or was the problem here when the set was obtained. If the latter, it is possible that someone has made some modifications that need to be fixed.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 4:14 pm 
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Location: New York, NY
Mr. Detrola wrote:
As mentioned previously, the quick and easy way to reduce width when everything else doesn't get you where you want to, is to increase the value of the G2 resistor on the horizontal output tube. Some other TV sets over the years actually had a width control there.

Putting in a slightly weak 5U4 will also reduce B+ and as a consequence, picture size.


I replaced the 100 ohm G2 (pin 8 on 6BG6) with a 150, then 200 then 250 and made no difference in either width or the oscillator or G1 AC voltage (still 55V PP).

I'm still getting the high voltage from the oscillator at the test point (C on horiz lock), 95V PP where Riders shows 55V. I traced it upstream a bit further and seeing some odd results.

This is at the plate of the horiz oscillator, if you ignore the pulse (this seems to be a result of the retrace suppression mod) and count based on the 50V divisions you get somewhere in the area of 150V, which is high in the same proportion as the other readings.

Attachment:
File comment: Plate of Horiz Oscillator (Riders TP15)
tp15_1.jpg
tp15_1.jpg [ 653.74 KiB | Viewed 1015 times ]


Then at the grid, also seeing it too, this time the voltage is off-scale for the scope to read. Same here but pulses are too high.

Attachment:
File comment: Grid of Horiz Oscillator (Riders TP14)
tp14_1.jpg
tp14_1.jpg [ 681.54 KiB | Viewed 1015 times ]


For comparison here's the Riders images:

Attachment:
tp14_15_riders1.JPG
tp14_15_riders1.JPG [ 99.88 KiB | Viewed 1015 times ]


I'll keep tracing, but this seems like a promising clue...

EDIT: Wait no... those pulses might actually be a result of the "@bandersentv retrace suppression" mod that I did. :) To be clear, the width problem existed exactly the same before that, so had no effect on the issue. However if I take that out of the measurement, the overall PP voltages are still too high. Going to keep looking...

FK


Last edited by FrankieKat on Aug Wed 25, 2021 5:46 pm, edited 4 times in total.

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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 4:22 pm 
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Posts: 115
Location: New York, NY
J. Hill wrote:
I don't really like changing the circuit or putting in bad tubes to make something work. There is a problem here that needs to be fixed. I am sure the set did not leave the factory like that, and that it worked at one time with the factory circuit. I would want to know if the TV set ever worked properly while in the OP's possession. Is this a recent problem or was the problem here when the set was obtained. If the latter, it is possible that someone has made some modifications that need to be fixed.


Yes, I put it back to the 100 ohm after changing the values made no difference. I picked up this set at an antique mall, so I don't know the full history but it was in overall excellent condition -- virtually no rust or corrosion and only minor repairs ever done, the majority of caps original with only a few ancient replacement caps. Based on settled dust, my assessment is that this has sat for a long time (in a climate controlled environment) and hasn't been touched or fixed in decades. The slugs and ion trap position had undisturbed dust, so clearly hadn't been touched any time recently. In short, I have no reason to believe that anyone has monkeyed with it or even tried to fix it since it was last used.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 4:32 pm 
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Joined: Oct Fri 28, 2011 7:27 pm
Posts: 115
Location: New York, NY
bandersen wrote:
I'm also wondering if something isn't drawing as much current as it should be. This set uses a "stacked" B+ design where the audio output tube circuit is part of the power supply.
If the 6AS5 output tube is weak or not biased properly, it could cause problems.


I wondered about this because the B+ supplies are not straightforward. I did do some quick DC voltage checks on the the audio tube, and all seemed to be fine. I didn't spend a lot of time up there because I've got plenty of volume and is clean (other than the characteristic Admiral inter-carrier sound noise). I don't have a spare one of those, but it tested "good" on my cheap-o emissions tester, so I moved on. I'll go through it all again though!


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Wed 25, 2021 6:01 pm 
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The retrace suppression circuit is only for the vertical. It picks a pulse off the vertical output, shapes it and feeds it to the CRT to cut it off during the vertical retrace interval. Got it from the Riders series of vintage TV tips and tricks.

Looks like the circuit is resonating at some harmonic. I wonder if tweaking the horizontal oscillator coil a bit might help ? Or try a difference 6SN7. You can swap it with the other one in the set. Also check that 100pF feedback cap.


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 4:42 pm 
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Location: New York, NY
bandersen wrote:
The retrace suppression circuit is only for the vertical. It picks a pulse off the vertical output, shapes it and feeds it to the CRT to cut it off during the vertical retrace interval. Got it from the Riders series of vintage TV tips and tricks.

Makes sense, we know that the mod won't create this issue for sure and I still had the width problem before. I was just noticing though that the excessive voltage measurements in scope traces started even in the vertical circuits right at the point where the blanking pulse is injected (input of vertical amp), however this may just be coincidence. I suppose I could undo the mod just to see if it makes those beats/peaks go away.

bandersen wrote:
Looks like the circuit is resonating at some harmonic. I wonder if tweaking the horizontal oscillator coil a bit might help ? Or try a difference 6SN7. You can swap it with the other one in the set. Also check that 100pF feedback cap.

Very interesting thought. I have touched up the horizontal oscillator, but didn't make any difference in width or drive voltage. Have actually tried 4 different 6SN7's and all the same. Do you mean the 180pF cap between oscillator coil and grid of oscillator? I did sub that out and no change, as well as the 180pF between sync clipper and AFC input. I suppose I could try to sub out that 7.5 pF cap in the horizontal feedback (I'd have to parallel a 4pF and a 3pF and hope the extra lead length makes up the difference... not sure how critical that value is down to the 1/10 of a pF, but I'd assume the tubular ceramic that's in there now is not 1%...).

The root problem seems to be with excessive horizontal drive: 1.5-2x the AC PP voltages called out in the Riders waveforms because I'm seeing high voltages as early as before the Horiz AFC/oscillator (test point M6 on the Sams, TP13 on Riders). Riders shows 17V PP and I'm getting 35V with hold control extreme left and 45V extreme right. After that all scope voltages are about proportionately high. The DC voltages on the damper, sync clipper and AFC/osc are largely within reason.

Width control and flyback windings both match DC resistance measurements on Sams. Have quadruple checked all resistor, cap values and wiring for the entire section and all checks out. B+ all seems within reason, though it's hard to tell sometimes because neither schematic has any DC voltage measurements other than tube pins, and Sams and Riders have a lot of discrepancies (for example plate of horiz AFC, Sams says 100V and Riders says 165V).

Have subbed out the horiz osc and output tubes with several ones and no change. Only tube I haven't changed is damper, which I had to buy one but is a few days away. Am starting to sub out small value caps...

FK


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 5:56 pm 
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It is the mica caps in the horizontal circuits that are failing. Ceramic caps are not known to fail. But anything is possible. Mica caps should be replaced with mica caps.

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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 7:33 pm 
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Tom Schulz wrote:
It is the mica caps in the horizontal circuits that are failing. Ceramic caps are not known to fail. But anything is possible. Mica caps should be replaced with mica caps.

There's only 3 domino caps (not certain if they are mica or paper) in the whole set - 2 in the sync/integrator/vertical and 1 horizontal sweep coupling.
All of the other low value caps are old-style tubular, dog bone or disc ceramic. I pulled the sweep one out and tested it for ESR, value and leakage and it passed all. I tried a ceramic of the same value and didn't change anything, so just put back the original one.

Now I did notice one thing... the vertical output transformer is a replacement. Clearly a very old replacement (the brand and model date back to the 50's), since it does not have the original Admiral letter stamp on it. The primary is part of the horizontal output, width and flyback circuit, so if the values of this transformer aren't quite the same, could that create some of the excessive voltages I'm seeing? The service info indicates a DC resistance of 730ohm on primary and 12ohm on secondary. I measured the one in there now and the primary is about 1200ohm and secondary about 11ohm. The plate voltage of the vertical amp, coming on the top of the primary of the VOT is right on... Sams says 380V, Riders says 390V, I've got 385V.

Actually this make and exact model number is listed in Sams as a good replacement. Found another thread where the OP replaced the VOT on a similar set with this exact model and reported no excessive width issues.

Back to the drawing board...


Attachments:
vot_1.jpg
vot_1.jpg [ 931.1 KiB | Viewed 936 times ]
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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 9:54 pm 
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FrankieKat wrote:
Now I did notice one thing... the vertical output transformer is a replacement. Clearly a very old replacement (the brand and model date back to the 50's), since it does not have the original Admiral letter stamp on it. The primary is part of the horizontal output, width and flyback circuit, so if the values of this transformer aren't quite the same, could that create some of the excessive voltages I'm seeing? The service info indicates a DC resistance of 730ohm on primary and 12ohm on secondary. I measured the one in there now and the primary is about 1200ohm and secondary about 11ohm. The plate voltage of the vertical amp, coming on the top of the primary of the VOT is right on... Sams says 380V, Riders says 390V, I've got 385V.

The primary of the vertical output transformer is not really part of the horizontal output, width and flyback circuit, but rather it gets power for vertical circuit from the boosted B+. The boosted B+ is created by the horizontal output, width and flyback circuit. The exact characteristics of the vertical output transformer should have no effect on the width as long it is connected to the correct point in the horizontal output, width and flyback circuit. Sams 100-1 shows an alternate connection where the vertical output transformer is connected to the standard B+ instead of the boosted B+. If that alternate connection was made then the lack of load on the horizontal circuit could cause excessive width.
Double check how the vertical output transformer is connected.

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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 11:11 pm 
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On some models, the horizontal width is simply too large no matter how you adjust it. The manufacturer wanted to make sure the customer would never see a black bar on the left or right, regardless of line voltage, etc.

An easy method to reduce width, while keeping the HV unchanged (better for brightness, focus) is to wrap a little aluminum foil around the neck of the CRT where the yoke sits. Experiment a bit to figure out how much foil to add and you can fine tune it pretty well.

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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 11:34 pm 
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Tom Schulz wrote:
The primary of the vertical output transformer is not really part of the horizontal output, width and flyback circuit, but rather it gets power for vertical circuit from the boosted B+. The boosted B+ is created by the horizontal output, width and flyback circuit. The exact characteristics of the vertical output transformer should have no effect on the width as long it is connected to the correct point in the horizontal output, width and flyback circuit. Sams 100-1 shows an alternate connection where the vertical output transformer is connected to the standard B+ instead of the boosted B+. If that alternate connection was made then the lack of load on the horizontal circuit could cause excessive width.
Double check how the vertical output transformer is connected.

My set is the later revision (Run 12) and is wired according to the dotted line version. According to the Admiral Supplement, that change was made in Run 7:

Vertical Oscillator Plate Voltage Changed in Late Production 20X1 chassis to improve control of height and linearity

"Boot Strap" voltage supplies the vertical oscillator with plate voltages in early production 20X1 chassis and all 20Y1 chassis. In these sets, R416 is 2.2 megohms. In late production 20X1 chassis, the vertical oscillator plate voltage is supplied directly through the focus coil L404 (approximately 340 volts) and R415 is changed to a 2.7 megohm resistor."


Note: I assume "R415" is a typo because in the previous "Run 6" change, R415 was removed. Rider goes on to say "if vertical linearity or height is poor in these 20X1 chassis, try replacing R416 with the 2.2 megohm resistor". Mine has the factory 2.2M 2 watt carbon (tests as 2.4M), and I swapped it out but didn't make any difference (because I'm willing to try anything at this point).

For the sake of science I swapped it to the earlier revision, but alas made no difference...


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 11:47 pm 
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Location: New York, NY
Tom Albrecht wrote:
On some models, the horizontal width is simply too large no matter how you adjust it. The manufacturer wanted to make sure the customer would never see a black bar on the left or right, regardless of line voltage, etc.

An easy method to reduce width, while keeping the HV unchanged (better for brightness, focus) is to wrap a little aluminum foil around the neck of the CRT where the yoke sits. Experiment a bit to figure out how much foil to add and you can fine tune it pretty well.

That's an interesting experiment - I'll give that a try.

Personally, the width doesn't bother my watching... having it zoomed out a bit might even benefit watching something current that's in a 16:9 format anyway! It's more that fact that something isn't working right though -- the width and linearity controls have zero effect (I'm sure the manufacturer didn't send it out that way) and the drive voltages being almost 2x from what the service info says.

I re-read the other post about replacing the VOT with a Merit A-3036, and OP stated "the best linearity comes when the horizontal drive trimmer is screwed down tight" and "best focus adjustment being at the end of its range"... both of which are problems I have. He did not mention width as an issue though, but perhaps this replacement might have these issues and some might just not be concerned about it? I do have a hard time believing that you could drop in a transformer with nearly double the primary windings but almost the same secondary and not have any effect though... ?

Thanks for the suggestions!

FK


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 Post subject: Re: Admiral 20X1 Excessive Width
PostPosted: Aug Thu 26, 2021 11:54 pm 
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I noticed an old sand coated power resistor above the VOT. These are often bad. Be sure to double check or replace it.


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