Portable Colour CRT, no visual output

[Cont3mplation]

Hiya folks,

Got ahold of a 5" colour CRT that on powering up plays audio just fine, but has absolutely no life on the screen end of things. No beam, no hum etc.
I assume the issue is going to be with either the transformer or something else on the power supply end of things, but have had a cursory glance and see no obvious bulging caps or damaged components.

Can anyone here offer some advice on where to start looking? I'm aware the voltage in that area is no joke so I thought I'd see if anyone is aware of common culprits.

Advice would be greatly appreciated! Know these lil guys aren't worth a whole lot, and aren't generally a great quality screen, but would be nice to see some life from it!

[Graham G3ZVT]

It would be useful to know the brand and model number.

[ortek_service]

Yes, knowing the make & model should hopefully assist in locating a schematic etc for it.


And also looking at the CRT's heaters, to see if they are lit ?
- And it is maybe just missing the EHT voltage (fortunately rather lower on these, compared to larger CRT's - Although still a few kV) or there is no drive to the CRT cathodes / Anodes.

However, it could be that the CRT Heater voltage, as well as those on the Anodes / Final EHT Anode, are produced from a single 'Line OuPut Transformer (LOPT), which if faulty / not being driven, will not give any outputs for several supplies.

[Cont3mplation]

Hiya,
My bad for not including the brand, it's a RadioShack branded "portavision", though I believe that they're somewhat generic and just badged RadioShack as I've seen them under a couple other names.

Took a look and I don't see any light from the tube when it's switched on, seems the heaters aren't powered.

[ortek_service]

Thanks for the further info.

But, unfortunately, it seems they used the 'Portavision' model name on several of their products - Inc. multiple 5" Colour TV's.
- Like 16-165 or 16-7062 etc.
So can you find an exact model number on a label on it somewhere?

A picture of it may also help, in order to match it to ones online, like at: https://www.radiomuseum.org/r/radioshack_realistic_portavision_30_cat_nr_16_7062.html
Where there's many Portavision products (inc. some radios), at: https://www.radiomuseum.org/m/radioshack_usa_en_9~model.html

I presume you got it just by itself, without a User Manual or box etc.?
(It seems some have been sold - brand new? - boxed in Auctions for around £20)
As Tandy were often quite-good at giving you the schematic in their manuals.
But I haven't had much luck in locating any service info for these (Possibly linked with Genexxa brand or Memorex).

[Cont3mplation]

Apologies for the delayed response, from looking at the back I believe 16-125A is the model num? The label should be visible in the attached images.

[simpsons]

No suprises that this model is also sold under the RCA label, USA Model RCA 16-3000, Maruman and probably others.

I can't find the service manual for these but Roadstar sold a similar spec set CTV 5501.

https://elektrotanya.com/roadstar_ctv_5501.pdf/download.html#google_vignette

This might give you a start in solving the problem

Chris

[simpsons]

As you will have probably seen the Ebay GENEXXA 16-125A for sale leading you to the equivalent Roadstar CTV505, I'll leave this with you.

Chris

[Welsh Anorak]

ISTR the unusual bipolar line scan coupling capacitor used to fail on these causing no picture.

[ortek_service]

Originally Posted by simpsons

As you will have probably seen the Ebay GENEXXA 16-125A for sale leading you to the equivalent Roadstar CTV505, I'll leave this with you.

Chris

Unfortunately, despite quite a bit of searching, I've not found any sales-listings that also gives an equivalent Roadstar model.

Plus it seems the Roadstar CTV-505 (That's in an online Argos 1993 catalogue archive, at £170)
- was actually a 6" model, from orig. rear-view label (See attached images)

Although rear case moulding / connectors-layouts of these does look virtually identical (Despite Roadstar being made in Hong Kong and Radio-Shack made in Malaysia). So maybe case (and the circuit boards) allowed both size CRT's or there was a slightly-larger similar-looking case ?

I also found a list of Roadstar models, where the CTV-5501 is also a 6" model, but the CTV-5502 MS is a 5" one. With CTV-5512 being 5.5"
https://view.officeapps.live.com/op/view.aspx?src=https%3A%2...HEMITVC_Q_Z.xls&wdOrigin=BROWSELINK


Surprisingly, I've not found any instructions / schematics / service info on the Radio-shack / Genexxa 16-125A (Dating from around 1996).
It seems it mat be an updated version of the Memorex 16-125 (orig. produced around 1994?), but not really any more info on that either.
- These not being on the listed models at: https://www.radiomuseum.org/m/radioshack_usa_en_10~model.html


And I've not found much on Roadstar CTV 505 etc models - Except CTV 5501 service manual, previously found as free download at elektrotanya.
Where it seems there are actually multiple copies / versions of that one on there:

https://elektrotanya.com/roadstar_ctv_5501.pdf/download.html
https://elektrotanya.com/roadstar_ctv-5501_ctv-5501i.djvu/download.html
https://elektrotanya.com/roadstar_ctv-5501-sm.pdf/download.html

As well as at: https://manualmachine.com/roadstar/ctv5501/5865612-service-manual/


There are also some other Roadstar models (But can't find anything for the closest? CTV 5502), like CTV-5512 that may be closer?
https://elektrotanya.com/roadstar_ctv-5512_sch.pdf/download.html
https://elektrotanya.com/roadstar_ctv-5512_sm.pdf/download.html

[ortek_service]

Originally Posted by Welsh Anorak

ISTR the unusual bipolar line scan coupling capacitor used to fail on these causing no picture.

I assume that is the C415 'BP' 10uF 50V (so seems a bit low-voltage?) marked one on the CTV 5501 service manual schematic
(Confusingly shown as a conventional 'Unipolar' Electrolytic).

However, if this went open-circuit, then I'd presume you'd just get no horizontal-scan and just get a vertical line on the CRT?
- With the Line-output transformer still being driven, producing a voltage for the heaters etc. ?
Whereas it seems CRT heaters aren't lit (but might be worth measuring voltage there, to check), which would indicate a different problem.
- Unless failure of C415 causes line-output stage to shut-down?

I think it's probably worth checking that the 12V supply is getting through L403 (15uH) choke in supply to the line output stage, as maybe that's prone to burning-out?
And, if present, maybe worth checking HT supply (Probably > 100V, but < 160V rating of smoothing capacitor) to the RGB driver-transistors.
The EHT is surprisingly-high (12 to 13kV) for a 5"/6" CRT. And Focus / screen voltages are likely to be > 1kV, so best to ensure contact with these is avoided!


Unfortunately, it seems the CTV-5512 schematics (inc. those in fuller service manual copy) on Elektrotanya have been scanned rather low-res - So basically unreadable as to component values / makes it very-difficult to follow these.
But may be able to tell from PCB layouts, whether this one or the CTV-5501 is actually closest to the Radio-Shack / Genexxa 16-125A

[Lloyd 1985]

I have one of these sets, I remember mine had a focus problem which was caused by some of that horrible ‘Sony glue’ you know, the stuff that starts out beige then goes brown and crispy and also goes conducive at the same time!! The CRT base connector had been stuck on with the stuff, and you could see it arcing in a dark room between the pins. Took ages to get it off, and I’m not sure it’s all gone…

[ortek_service]

Did you have the original 'Tandy' Radio-Shack instructions for yours
- Assuming you have that branded version ?
- If so, do they give you a schematic etc. in this, that they often did, in order to check whatt version of the Roadstar might be being used ?

[simpsons]

Hi Owen

I do hope that Cont3mplation is able to use the additional extremely well-resourced information you have provided and in particular, the importance of the ic complement to confirm the relevance of the schematic.

The Roadstar CTV 505 was identified by Google lens as an equivalent model and which I then visually confirmed. In addition, the Goodmans CT510 Compact TV for sale on eBay # 226970366609 was visually identified as the equivalent Roadstar CT505 and has the same schematic.

I have a JVC CX-60 which is working fine but should it go wrong, I know where to ask for help!!

Chris

[Lloyd 1985]

Originally Posted by ortek_service

Did you have the original 'Tandy' Radio-Shack instructions for yours
- Assuming you have that branded version ?

Sadly mine didn’t come with the instructions, I got it second hand many years ago. It is the RadioShack one.

[af024]

Mine fizzed on one of the CRT pins. Focus varied with it. I had to obtain a replacement CRT (from one with a broken case) and it went the same way about a year later. All very strange.

[Cont3mplation]

Thank you folks endlessly for the advice thus far, have an idea now where to begin!
This may seem a stupid question, but when working on tvs of this sort is it alright to just power it on and measure voltages? That's how I've done every other repair I've worked on, but the joy of cassette machines is no kVs in sight...
Just want to make sure that testing points on the main board shouldn't fry me or the equipment!

Besides that Ill try as suggested and check whether power is reaching the heaters. Will let you know how I fair.

[ortek_service]

The safest way is really to use clip-on probes and make the necessary connections, before switching-on - So no-chance of accidentally touching anything.

But few do this in-practice, as can rather slow down taking measurements and can be a bit over-cautious for those used to working on 'live' equipment.

Fortunately, as your TV is mainly powered off a fully-isolated 12V supply, from its PSU, then not much of it is actually at a high-voltage. With only really the CRT final anode (Which should be well-insulated with rubber cap etc) and the 'Focus' & 'Screen' pins on the CRT at the highest voltages in the kV's (But at low < 1mA max. currents so not likely to be fatal).

Although there is an 'HT' supply, of around 100-160V produced to power the RGB drive transistors.
However, if the Line Output Transformer (LOPT) stage isn't actually running, then there won't actually be any of these voltages present until the fault is fixed.
- But still need to be fairly-careful, in case the LOPT is actually being-driven / there is an intermittent fault, and it suddenly sprigs to life!

So really need to establish what is happing around the LOPT stage, and easiest way to check is by measuring the lower-voltages that it produces (Trying to directly-measure voltages >1kV on a DMM etc., without a special divider-probe is likely to damage it / you !).
And if it doesn't seem to be producing any of these, then need to check the supply and drive to it / the transistors etc. that drive it.
Ideally, an oscilloscope may be needed here, to check drive signal from the main IC through the transistors. But just measuring supply-voltages to the LOPT stage, to start with, may identify what is happening.

And hopefully, the schematics / service-manuals found so-far will closely-match what you TV actually is
- Maybe take some internal photos of the PCB's, and we can verify they match what's in the manuals.

[simpsons]

Hi Cont3mplation

You might find the YouTube videos on repairing the RadioShack/Realistic Portavision helpful in giving you confidence in your project.

Once you have searched for "Radio Shack Portavision repair," you will have a few videos to choose from. I would start with Discharging and venting (what ever that means) and then the video "Small Poratavision CRT." The last will show you how simple it is to assemble the TV after repair. Reverse engineering is perhaps required, but you get the message.

Then search "Radio Shack Portavision Repair" and whilst you will see again some videos, others are new.

The Samsung 5" color CRT repair" might also be helpful, although a scope is used.

Chris

[ortek_service]

Yes, if there are actually some Youtube video on this (that I hadn't seen) then
worth a watch.

If it hasn't worked for months / you're not removing the CRT, then you shouldn't have to discharge anything like the CRT.
And any HT supplies capacitors would normally lose their charge after a few minutes (or at least a few-hours if no on-board bleed resistors or discharge via circuitry powered from it if that isn't functioning.