1980 Soviet Radio Cassette amplifier problem.

[Richard9025]

Hi!
I'm having some problems after I replaced all the electrolytics in this radio.
The unit in question:


The unit disassembled:



This thread is only about the amplifier, so I won't get into presenting how it works only if necessary (the rotary tv-like tuner, VHF block, cassette-radio switch and tape mechanism)

When I got it, the amplifier worked ok, it wasn't amplifying the sound as loud as my other radios and at high volume (over 70%) it would start to distort the sound.
After I changed all the electrolytics, the radio got 5x louder, so loud that I can't get to 100% because I fear that the speaker might get destroyed. But also something got 5x bigger, the distortion.
If I turn the volume over 25%, it starts distorting very badly, and the speaker transforms into a trampoline, if I put a tic tac or just a piece of paper, it throws it out like a spaceship (remember, this is at 20%, at 100% is 4x worse), not to mention that it moves on the table and it vibrates like hell, it clearly isn't made to work like this.
The unit is a very low hour set, it looks exceptional and hasn't been tampered with (the paint was still on the screws when I opened it), even the original bulbs are working.
Photos with the speaker (not a single hole in the membrane).



Before talking about the electronic circuitry, I want you to hear how bad this unit sounds at a high volume:
at 10%
https://vocaroo.com/i/s028kagZ5hHv
at 25%
https://vocaroo.com/i/s1ZKRCu4U8fB
at 50%
https://vocaroo.com/i/s1fVolpiXJRF

and a perfectly sounding radio receiver from the same time (just to compare the two):
at 50%
https://vocaroo.com/i/s1JYxMIg4Pu6
at 100% (extremely loud, notice how the distortion is very small unlike the unit I'm repairing)
https://vocaroo.com/i/s1PlVjZ5beRo

Now the electronic part, remember that I replaced every electrolytic cap in this radio.
The schematic:



What I did is: I measured the voltages and the transistor junctions:
I hooked the radio to a battery that measured exactly 9V, In-circuit it measures 8,62V that means that 0,4V are lost by the internal resistance of the battery.
The voltages seem fine to me (all are 0,4v lower because 8,62V enter the circuit, not 9V like in the schematic)


THE TRANSISTORS ARE MEASURED IN-CIRCUIT!!!

Also, the transistors look ok, remember, this is a very low hour set.

One thing that I did is I got the amplified audio (at 5-10% volume) and fed it to the phono input of another radio and it sounded very very good, maybe the speaker is shot. I'll hook it to another speaker to see if it does any better, maybe the amp is good.

I had another unit, the exact same model, but I trashed it for parts because it was very low quality and too far gone. I still have the parts, so if I need to replace something, it won't be a problem.

So, the problem may be:
1. an amplifier problem (transistors over-amplify the signal or one transistor distorts the sound then that sound is amplified or some other problem inside the circuitry)
2. a speaker problem (very low impedance leads to an amplified sound to overdrive it).
3. another problem
The problem can't be the detected signal because I hooked it to another amplifier and it was crystal clear.

The amp (it includes the tape preamp and record circuitry on the same PCB)


The old removed caps:

Many thanks!

 

[kellys_eye]

Have you checked the electrolytics to see what their ESR actually was? It would have been a shame to have removed them for no good reason (destroying the provenance of the radio).

Regardless, I'm assuming you got all the polarities correct!

Try giving R38 (preset) a little 'wiggle' as the track/wiper contact may be dirty/corroded and this little control adjust the bias for the output stage.

 

[Kabelsalat]

I suspect that Vbe across transistor labeled V5 is too low, and therefore it takes more time for the output transistors - the period where zeero crossing and none is conducting.

Therefore - a slightly (very tiny) re-adjustment of R38 may be the way to go. But the danger is that adjusting it too much, both output transistors may conduct and you destroy the output stage. So you may want to put something in serie between battery and amp while adjusting this, such as a light bulb.

[edit]
that's pretty much the same point as kellys_eye

 

[Richard9025]

Have you checked the electrolytics to see what their ESR actually was? It would have been a shame to have removed them for no good reason (destroying the provenance of the radio).

I didn't check the ESR (I don't have anything to check esr with), but I measured the capacitance, and over two thirds are not in spec, and some of em (2-3) have capacity in nanofarads instead of microfarads.
This radio is a very common one, it isn't an export version/special version/very first units produced. If it was, I would pray-open the old caps and put the new ones inside (thats after I determined they are bad).

Try giving R38 (preset) a little 'wiggle' as the track/wiper contact may be dirty/corroded and this little control adjust the bias for the output stage.

Therefore - a slightly (very tiny) re-adjustment of R38 may be the way to go.

R38, as far as I know, should get the voltage between the shared emitters of T7 and T9 to be half of the voltage that the amp is working on (9V in the schematic, 8.66 in my unit's state)
So I should adjust it so I get half.
Here's the problem, I can't adjust it to get 4.33V (half of 8.66), so the transistors are unbiased every way I tune R38.
More explained:


I can get a maximum of 3,97V at R38's left-most peak.

The other radio's amp had the exact same symptoms and problem with R38. What can I do to get the desired voltage, 4,3V? (more exactly, to get the transistors biased)
Thanks for the replies!

 

[duke37]

R38 is there to set the static current through the output transistors.Do not exceed the specified current.
R18 is used to set the mid point voltage.

 

[Richard9025]

R18 was a little bit off, 4V instead of 4.3V.
I also adjusted R38 to get the best audio.
But it makes little to no change, I still get very distorted audio at high volume (over 50%)
I connected some other speaker and it was exactly the same, no change, so the old speaker is good.
I checked and double-checked the caps polarity and they are all good.
I don't have any ideas... The distortion is progressive, it increases as the volume gets higher.

 

[duke37]

The voltages look all right but perhaps there is a poor output transistor. Set the current to the specification. The higher the current, the less the distortion and the sooner the transistor will melt. The voltage ACROSS R38 should be about 1.2V

A leaking C35 will pull the output one way and cause distortion.

A scope will help to solve the problem.
You could remove V9, V10 and C35 and then listen with headphones connected to the output via a 100Ω resistor.

 

[kellys_eye]

Have you checked the supply voltage at the higher levels? Are there any larger capacitors in the PSU section that you haven't changed yet?

 

[Richard9025]

The voltage ACROSS R38 should be about 1.2V

By voltage across you mean the voltage with the DMM's negative and positive probes across the three terminals of pot? If so, I get 0,1-0,2V across all three terminals.

A leaking C35 will pull the output one way and cause distortion.

Yes, but C35 is replaced with a brand new cap.

You could remove V9, V10 and C35 and then listen with headphones connected to the output via a 100Ω resistor

V9 and V10 are soldered across a radiator and they're kinda hard to remove, I'll better measure and determine where the problem is before desoldering all the components.

Are there any larger capacitors in the PSU section that you haven't changed yet?

All the tests that I do are battery-powered. I removed the power supply and replaced the open capacitors (it had one open capacitor and the PSU wouldn't work) . Now it outputs 9,01 volts but I don't use it, it stays on the shelf until the radio works on batteries.

 

[Richard9025]

Another version of the schematic, with the input/tone switching circuitry:

 

[duke37]

Sorry. I was wrong, I have been wrong before and will doubtless be wrong again. The idea was to get the voltage betweent the bases of VT7 and VT8 to check that the transistors are turned on. The measurements from ground look OK though.

Measure the current running into the amplifier at P7 and see how it changes when you twiddle R38. Too low a current will give a lot of crossover distortion at low output

The bootstrap capacitor C22 should be checked.

Are you sure that the signal is a pure sine wave?

 

[kellys_eye]

The voltages look all right but perhaps there is a poor output transistor.

This.

Many such output stages use matched pairs of transistors. One of them out of spec will cause the issues described.

 

[Richard9025]

Many such output stages use matched pairs of transistors. One of them out of spec will cause the issues described.

Yup, to test them, I decided to insert audio from a tablet in some audio points that I mapped on the schematic.
For protection, I used a 0,1ufd capacitor in series with the tablet's audio jack.
More exactly:


So, I removed one lead of C14 and I injected Boney M- Rasputin into points A1 to A7, results:

A4 - Crystal clear audio (no need to record)
A3 - https://vocaroo.com/i/s1wLjP2we2ZS
A2 - https://vocaroo.com/i/s1x83LJ4nxOz
A1 - https://vocaroo.com/i/s134crK18jgR

A4 and A3 outputs are crystal clear, so V10 V9 V8 and V7 are perfectly fine.
A2 has some background hiss and A1 has a lot of hisses.
One thing to note is that the amplifier produces a hiss without any audio injected, so there is a bad component in the amp.
I do have a lot of KT3107D and KT315B transistors, so I can replace them easily. I also have their new European equivalent, more exactly the BC566B for the KT3107D.
Awaiting your replies on what to do next.

 

[duke37]

The amplifier runs as a feedback loop. Putting a signal in part way is not going to tell you much.

The signal is compared with an attenuated version by VT2 and VT3. The attenuation is R31/R32 so 15k/58 = 260
This gain is very high and a little bit of hiss in an input transistor will be amplified a lot.

A complex signal such as Rasputin will not be ideal to find clipping level.

 

[Richard9025]

A complex signal such as Rasputin will not be ideal to find clipping level.

With a 1khz sine wave tone injected, it is exactly the same, the hiss increases as I go from A3 to A1.

The amplifier runs as a feedback loop. Putting a signal in part way is not going to tell you much.

Then what can I do next? All electrolytics are changed. Do I change the transistors one by one?

 

[kellys_eye]

Check the values of R31/R32. If they have changed value (R31 high or R32 low) they can increase the gain and introduce distortion.

 

[Richard9025]

R32 - 56.66 Ω
R31 - 15.39 kΩ

 

[Richard9025]

I think there are/is some noisy transistor that also distorts the sound besides the hissing.
I could use some freeze spray and see what transistor is acting up but I don't have some.
I went the "hard" way and pulled one (or two) transistors at a time and put some new replacements to see if there's a difference, if not, I just put the old ones back.

T5 - KT3107D with BC556B - no difference, I put the KT3107D back
T4 - KT315B with another KT315B - no difference, I put the KT315B back
T3 and T2 - 2 KT3107Ds with 2 BC556B- no difference, I put the 2 KT3017Ds back
T7 and T8 - 1 KT315B and 1 KT3107D with another KT315B(from another radio) and BC556B - no difference, I put the old transistors back.

I don't have any replacements for T9 and T10 to test them. I'll get some and see what it does. I think that those are the bad ones.

Is Transition Frequency important for replacing these transistors? For the KT814A/815A, I found the BD137/138 or BD139/BD140 (they have a 50mhz transition frequency and the Russian ones have only 3mhz but I don't think that's a problem) and BD237/238, but I don't like those two because they have a lower min. gain (25 instead of 40).

 

[Richard9025]

You could remove V9, V10 and C35 and then listen with headphones connected to the output via a 100Ω resistor.

I removed V9 and V10 to test them.
I measured the gain on V9 and it said 345, I think that's too much (the datasheets says minimum 40) but the multimeter is a cheap one so I don't expect much.
The gain on V10 is 001, so that's a bummer. (maybe the multimeter doesn't like it) (also the voltage across base-collecter is 598mV)
I'll go Monday and get some replacements for them.
Then I did what you said, connected some headphones via a 100ohm resistor and a 22nf ceramic cap.
At 50% it sounds very good but anything over 50% sounds terribly distorted. At 100% volume, it is so distorted and clipped that I couldn't understand anything...
Awaiting advice.

 

[duke37]

When you get the new transistors, measure them to check the meter.
Do you realise that V9 is npn and V10 is pnp and may well need a different connection to the meter?
A gain of 345 is possible.

I believe that you set R18 to get half the supply voltage at the emitters of V9 and V10 but I think you needlessly twiddled it afterwards so messed up the setting. It is not too critical, within 0.1V is plenty good enough.
R38 needs to be set to give a reasonable standing current, measured from the battery. I think you twiddled this to modify V8 and V9 voltages - wrong.. The standing current should be recommended in the insructions but could be 10mA for a battery power supply and up to 100mA for mains. If you cannot find a value, then set to 10mA to keep the heat down.
Any small transistor should do for V9 (npn) and V10 (pnp).

Measure the supply current and th supply voltage when distortion commences. Do not strain the amplifier by running into extreme distortion. Treat it gently, give it some TLC (tender loving care).