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 Post subject: To re-cap, or not to re-cap?
PostPosted: Jan Tue 05, 2021 5:31 pm 
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Location: Cedar Rapids, Iowa
I bought a few Russian radios a few months ago and finally decided to work on one of them yesterday...it's a Selga 402. Once I replaced the corroded 9v battery clip it came to life and plays well. I almost always recap old radios but I'm wondering if I should do this one, or leave it alone for originality. I lifted one lead for all of the electrolytics and used my cheap Chinese component tester to test them and came up with these results.
Attachment:
electrolytic_values.png
electrolytic_values.png [ 9.46 KiB | Viewed 2615 times ]
All capacitance values are high, but the ESR values look decent to me, except possibly C20 and C22, and the obvious exception of C26. So I'm wondering if I should

1) Leave it as-is. I haven't done a thorough test, but the testing I did do showed that it plays well.
2) Replace just C26, and possibly C20 and C22
3) Replace all electrolytics

I like refreshing all the electrolytics in radios, but I also like the originality aspect, especially in this one. I've attached the schematic. From what I can tell this radio is from 1972.

Thanks,
Rich


Attachments:
manual_schematic.pdf [327.82 KiB]
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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Tue 05, 2021 6:30 pm 
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Hello Rich. The problem with measuring capacitance and e.s.r. values is they can give false readings with old capacitors, and they don't measure leakage. They're really only accurate with new capacitors. I check values before installing new ones. I almost always replace any electrolytics unless the radio plays so well there's no room for improvement. I do have a few radios that I've kept all original because recapping wouldn't make much difference, or any difference. (they have strong audio) But in 95% of my radios I have replaced them, and it has made a noticeable difference. Sometimes the difference is mild, other times the difference is night to day.


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Tue 05, 2021 6:31 pm 
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Location: Mountains of Mourne. Ireland.
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles
And by opposing end them.


Radios, tube or transistor will work just fine with wide swings in component values, as you have observed.

If it keeps you awake at night - - replace them.
If it keeps you awake at night - - don't replace them.

I have many dozens of transistor radios from the 1960s and 1970s
some have been completely recapped - - many have not.

Yes, some may perk up with new capacitors, but don't get too hung up on keeping old caps for nostalgic reasons.
Nostalgia isn't what it used to be. lol

Not that it matters - - non of these were Low ESR capacitors to begin with.


I hope that doesn't sound too flippant Rich, if it works and sounds acceptable, just leave it alone.

Like an old TV that fades over the years, you may not notice for some time. Then, when you do, pull it off the shelf and recap it.


Happy new year old bean... :)
Greg.

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more→ viewtopic.php?p=3181366#p3181366


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Tue 05, 2021 8:29 pm 
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Location: Cedar Rapids, Iowa
Thanks guys! Good catch on the quote Greg :P

I think I'll leave it as-is for now. I just need to do a little more cleaning and permanently install the 9v battery connector and I'll have finished my first Russian radio :)

Rich

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 1:04 am 
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The one Electrolytic capacitor that would offer the most improvement, assuming there is some to be made, is the one off the wiper of the VC, going to the input of the audio amp circuit.

If you have the set out of it's case, it would be informative seeing a scan of the component side of the chassis.

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 1:51 am 
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Here you go, with the electrolytic locations marked
Attachment:
circuit_board_parts_002_marked_r.jpg
circuit_board_parts_002_marked_r.jpg [ 304.13 KiB | Viewed 2547 times ]
And as a bonus, here is the foil side.
Attachment:
circuit_board_traces_002_r.jpg
circuit_board_traces_002_r.jpg [ 263.73 KiB | Viewed 2551 times ]
Once I got it put together I find that it does not perform quite as well as I think it should so I will try replacing a couple of the caps, at least temporarily.

Rich

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 2:45 am 
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Oops...I made a mistake on my capacitor locations image. I've corrected that in the previous post.

Rich

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 5:05 am 
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One method you could use is to "target" the replacement of the capacitors for specific reasons:

1) Lets say the volume control is noisy & not improved with cleaning, replace the capacitors leading to and from it. Leakage can put DC on the control making it noisy.

2) lets say the audio sounds thin lacking in low range frequencies, then parallel a test capacitor with the suspect coupling capacitor, or emitter resistor bypass caps in the audio stages, does it help, yes, then replace.

3) Is it motorboating; LF oscillations. Yes, then replace power supply bypass electrolytics.

4) Is the capacitor physically leaking, yes, always replace.

It pays to have a sound reason for replacing any component/s . The reason should be based on tests & observations. Otherwise you might find yourself replacing all sorts of components, capacitors included, where you gain no improvement in the performance of the radio and this is simply not logical and diminishes originality of the radio. Plus, in vintage transistor radios, the phenolic pcb's are heat fragile and it also requires controlled temperatures and good technique to avoid lifting tracks. So if there is not a very sound reason to replace a part, it is better left alone.


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 5:31 am 
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The normally sound reason to replace all Electrolytic caps in 60+ year old radios is because they normally make no sound... :wink:

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 1:45 pm 
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Location: dayton oh usa
c26 is a goner.
the rest marginal at best.
if i wanted to keep a radio original i would check esr.
if ok i would measure current .
if in normal range play it often enough to keep them formed.


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Wed 06, 2021 8:46 pm 
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Location: Mountains of Mourne. Ireland.
Transistor radio current draw...
Image
more→ http://antiqueradios.com/forums/viewtop ... 4&t=278833


This Hungarian website shows a similar model...
http://www.szetszedtem.hu/586selga/selga.htm


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Thu 07, 2021 2:49 pm 
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Joined: Mar Thu 02, 2017 1:37 am
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Location: Cedar Rapids, Iowa
Thanks guys!

ACORNVALVE - some nice troubleshooting tips...thanks!

Greg - I should have put this in my first post. One of the first things I did was measure current draw and it was around 8mA - 9mA

I decided to pop out C26 and tack in a 22uF. The difference was quite pronounced...much more volume. I wanted to be sure I wasn't just fooling myself so I put the original back in and the improvement is definite. I found some 1uF and 0uF axial caps at DigiKey so I'll be getting those ordered.

Rich

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Thu 07, 2021 2:58 pm 
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Here's a side question...a learning opportunity for me. What is the device circled in blue? And what is it's purpose?

Thanks,
Rich


Attachments:
schematic_front_end.jpg
schematic_front_end.jpg [ 144.91 KiB | Viewed 2434 times ]

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Thu 07, 2021 6:48 pm 
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That part circled is a diode, but not sure of its purpose in that circuit, or what the "L" means.


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Thu 07, 2021 6:58 pm 
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Joined: Mar Thu 02, 2017 1:37 am
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Yea, the goofy symbol is what I'm wondering about. I found a different schematic for this radio and it shows it as just a plain old diode.

Rich


Attachments:
schematic_front_end_alt.jpg
schematic_front_end_alt.jpg [ 299.43 KiB | Viewed 2413 times ]

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Fri 08, 2021 9:03 am 
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Selga 402 was the first Soviet radio receiver to employ silicon transistors.
Six silicon and two germanium transistors.

The date of manufacture (month and year) is stamped on the thumb-wheel.
https://www.youtube.com/watch?v=CDzvR4tH-LI

Owner and service manuals...
http://rr20.ddns.net/Item.aspx?ItemId=9 ... db&Lang=En

------------

The Zener Diode is for stability - Voltage changes and Temperature.

From the service manual... comrade... 8)
ПРЕОБРАЗОВАТЕЛЬ ЧАСТОТЫ И ГЕТЕРОДИН

Преобразователь частоты собран на транзисторе Т; типа КТЗ15А по схеме
< совмещенным гетеродином. Гетеродин собран по схеме индуктивной трех-
точки Оптимальное условие преобразования частоты обеспечивается при на-
‘пряженни гетеродина на эмиттере транзистора 7, 80—120 мв. Напряжение
сигнала с входных контуров с помощью катушек связи Ё2 или [4 подается на
базу транзистора Ть. В коллекторную непь гранзнстора Г, включен трехкон-
лурный фильтр сосредоточенной селекпни (ФСС), который обеспечивает избн-
фательность приемника по соселиему каналу не менее 26 96. ФСС имеет ши-
рину полосы пропускапия 7,5—8 кгц на уровне —6 06. Связь между контура-
ми ФСС внешнеемкостная, а с преобразователем частоты Т\ и с первым каска-
дом усилителя ПЧ — индуктивная. Для обеспечения стабильности работы пре-
образователя частоты при изменении напряжения питания. окружающей тем-
пературы, а также после замены транзистора в базовую пепь транзистора Г,
включен стабилитрон Д! типа 7ГЕ2А-С. Режимы работы транзистора Т, опре-
деляются величинами сопротивлений резисторов А» и В».

Максимальная чувствительность приемника по промежуточной частоте
с базы транзистора 7, составляет 2—3 мка при выходном напряжении на на-
грузке усилителя НЧ 225 мв.

Text from on-line Russian OCR


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x.jpg
x.jpg [ 440.75 KiB | Viewed 2380 times ]
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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Fri 08, 2021 12:38 pm 
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criageek wrote:
What is the device circled in blue? And what is it's purpose?

According to a review of the radio published in the Russian magazine "Radio", issue no. 12 in 1970, that device is a selenium diode. Its purpose is to supply a stabilized voltage to the base circuit of the converter transistor. The voltage measurements shown in a picture of the PCB indicate a forward voltage of 1.5V for the diode. Here's a quote in Russian from the review:

Quote:
Стабилизированное напряжение, определяющее режим работы каскада, подаётся только базовую цепь преобразователя. С этой целью используется селеновый диод типа 7ГЕ2А-С. Все остальные каскады автостабилизированы в отнощенйи изменйи температуры и напряжения батарей питания.

Translation:
The stabilized voltage, which determines the operation mode of the stage, is supplied only to the base circuit of the converter. For this purpose, a 7GE2A-S selenium diode is used. All other stages are self-stabilized with respect to temperature and battery voltage changes.
Attachment:
Selga-402_Schematic.jpg
Selga-402_Schematic.jpg [ 504.34 KiB | Viewed 2363 times ]

Attachment:
Selga-402_DC_AC_Voltages.jpg
Selga-402_DC_AC_Voltages.jpg [ 821.23 KiB | Viewed 2363 times ]

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Fri 08, 2021 4:34 pm 
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Location: Cedar Rapids, Iowa
Thanks Greg - that's some great info!! I'll say it again...I don't know how you do it, but I'm glad you do it :lol:

Thanks bb.odin - more great info!

I have 2 of these things. The second one has a bad speaker, but even with a good speaker tacked in there is no reception, just static. So all this info will be very helpful!

Rich

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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Jan Fri 08, 2021 6:11 pm 
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Lol...


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 Post subject: Re: To re-cap, or not to re-cap?
PostPosted: Aug Thu 26, 2021 9:51 am 
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Almost all Soviet electrolytic capacitors were of disgusting quality and did not perform well even when new. All must be replaced at once.

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