Dynacord hybrid amp repair

[smilan]

Hi,
A friend of mine gave me a Dynacord hybrid amp repair for repair.
I'm not 100% sure about the model number since on the front panel labeled as  Eminent 1 but the Eminent 1 is a 100% tube amp this amp have four 81008 preamp cards with BC147 transistors and a tube powered output section using one ECC 81 and two EL 34 before the output transformer so the electronics looks much more like a  Favorite 2 model, only with four 81008 preamp cards instead of two 81008 and two 83002. Beside this difference all other cards number are identical to the Favorite 2 cards numbers.
While starting checking the voltage of the different boards I've found that the 40V cards (85008) fuse has burnt.
I try to replace it with a 250V/0.8A fuse and the new fuse immediately burnt.
I disconnected the 50V leads coming from the power transformer from the PCB and measured amperage with a load of 100W light bulb and got 0.19A.
So I guess something is wrong with the 85008 board, where should I start looking for the cause that burning  the fuses?

Another thing that is not 100% clear to me is Di1 (ECO 4234 on the schematic and ITT L738B or might be LZ38B, the label is damaged).
This diode is screwed to the chassis and only one wire goes from G terminal on the 85008 card to the diode.
When I've checked this diode with a multi-meter both sides indicated OL.
So I guess the diode has failed?
If I need to replace it , what model can be used?

 

[nashkato]

el-me-se is good site on german 60ies gear

I disconnected the 50V leads coming from the power transformer from the PCB and measured amperage with a load of 100W light bulb and got 0.19A.

What ? , you did what ? sorry , maybe i´m too busy , but i do not understand what and how you measured .
your schematic isn´t very good readable either , .so instead of naming boards with numbers you could call them names , like preamp-card , or in this case 40VDC PS-card ,so people trying to help you don´t need a lot of time to even begin to understand what you´re saying .

since the 40V fuse blows ... you need to disconnect all the voltages that "leave"  that PS-board , in this case A-F and H . Though there´s no need to disconnect GND , but i don´t how how that is realized , maybe it´s just a multipin plug , so
the fault is most probably at any component on any card that gets fed by those 40V

 

[Walrus]

If you suspect the zener diode, disconnect the wire going to it from the G terminal, and use your meter to check if it is short circuit. Googling seems to suggest it is a 39v  15w device. Exact replacements may be hard to come by....

 

[PRR]

I would look at the rectifier and cap right off that fuse.

If that next resistor is (squint) 100r, then it and anything downstream can't be blowing the fuse.

 

[JMFahey]

I would look at the rectifier and cap right off that fuse.

If that next resistor is (squint) 100r, then it and anything downstream can't be blowing the fuse.

1) agree, the only significant suspects are diode bridge and first capacitor.
If second capacitor or downstream load were shorted then first vresisstor would overheat, maybe even smoke but not "instantly blow fuse".

2) it´s very sensible to refer boards by ID number, takes any doubt off about which one are you talking about.
And Board number is clearly showbn on each, so ....
To boot, we are talking about a *different*  amp from what is shown here, yet each Board ID number means same circuit for each type.
Late 60´s Kustom amps (think Creedence Clearwater Revival) used that method, they made 5 or 6 different boards: preamps, reverb, tremolo, power amps, and all amps in that brand, think 25 or 30 of them, were made out of combinations of those boards, it simplified servicing a lot.
And any "orphan/mystery" model could be repaired even if no specific schematic is available.

3) that beefy chassis mounted Zener diode (40V 15W?) looks like the proper part there to regulate ... ummmm .... a 40V rail  

Your "OL" result only shows you used the wrong scale (resistance) to measure it, you need the diode test scale and in that case it "should"  show OL one way, and some 700mV the other way, "same as a regular diode"  , multimeter voltage is not enough to make it "zener" but if not shorted (the results I just mentioned), it should be fine.

 

[smilan]

Thank you all for your replays.

What ? , you did what ? sorry , maybe i´m too busy , but i do not understand what and how you measured .

First thing I wanted to check the voltage and current coming from the power transformer.
I've used the light bulb as alternative load for this measurement.
This is what I've done:

[quote author= JMFahey ]
1) agree, the only significant suspects are diode bridge and first capacitor.
[/quote]
I've checked the diode bridge with a multi meter  (disconnected from the PCB) and it seems to function well.
I'll install new filter caps after the weekend and see if it helps.

[quote author= JMFahey ]
3) that beefy chassis mounted Zener diode (40V 15W?) looks like the proper part there to regulate ... ummmm .... a 40V rail  

Your "OL" result only shows you used the wrong scale (resistance) to measure it, you need the diode test scale and in that case it "should"  show OL one way, and some 700mV the other way, "same as a regular diode"  , multimeter voltage is not enough to make it "zener" but if not shorted (the results I just mentioned), it should be fine.
[/quote]

OK so if I understood you right, as long as I have no short circuit on the zener diode I can't remove the suspicious from it ?

Anyhow I'll keep you posted and report after replacing the filter caps.

 

[PRR]

> 0.8A fuse and the new fuse immediately burnt.

Let's reemphasize this.

IF that is a 100 Ohm resistor, and IF the AC is less than 80V, than anything downstream (here, to the left) of that resistor can NOT blow a 0.8A fise.

You have rectifier and first filter cap. Why have you not already replaced them?

 

[smilan]

> 0.8A fuse and the new fuse immediately burnt.

Let's reemphasize this.

IF that is a 100 Ohm resistor, and IF the AC is less than 80V, than anything downstream (here, to the left) of that resistor can NOT blow a 0.8A fise.

You have rectifier and first filter cap. Why have you not already replaced them?

I received this amp on Thursday afternoon and the weekend vacation here in Israel is from Friday to Saturday so the stores was closed all this time...
I'll get new caps tomorrow morning.
I've checked the diode bridge earlier today and it works fine.
I disconnected the first filtering cap and the 100 Oham resistor (both connected to the + leg of the rectifire) and measured 46Vdc between the + and - of the rectifire.

 

[smilan]

Hi, here's some updates about this repairing project:
1. I've re-cap the filter caps on the 40Vdc PSU and now it works good!
2. I've replaced one faulty BC147 transistor on the master amp card (82005) and now the amp is working!
3. The only issue occurs is when I'm turning up the bass pot on the master amp there's a very strong AC ripple comig from the master amp output.
When I've checked the 40Vdc card disconnected from the master amp I can't see this ripple, only when the master amp card is connected to the 40Vdc supply I can see this ripple.
I guess there must be a capacitor that should filtering this AC coming from the tube output amplifier or I'm going on the wrong direction?

 

[abbey road d enfer]

3. The only issue occurs is when I'm turning up the bass pot on the master amp there's a very strong AC ripple comig from the master amp output.
When I've checked the 40Vdc card disconnected from the master amp I can't see this ripple, only when the master amp card is connected to the 40Vdc supply I can see this ripple.
I guess there must be a capacitor that should filtering this AC coming from the tube output amplifier or I'm going on the wrong direction?

The voltage is first shunt-regulated by the power Zener D1, then it is distributed via the 3 transistors T500/501/502.
They are connected in a way that is called a capacitance mutiplier. You may want to check capacitors C501/503/505 and indeed these transistors.
Have you checked the correct voltage (40V) at points A, C, E and G?

 

[smilan]

The voltage is first shunt-regulated by the power Zener D1, then it is distributed via the 3 transistors T500/501/502.
They are connected in a way that is called a capacitance mutiplier. You may want to check capacitors C501/503/505 and indeed these transistors.
Have you checked the correct voltage (40V) at points A, C, E and G?

I replaced C501/503/505 and now the amp works fine with no AC ripple

Thanks a lot for the guidance

 

[smilan]

One more question, is it normal that the EL34 tubes hitting up to 150-155 Celsius (300-310 Fahrenheit ) ?
This amp have a plastic case so I'm wondering if this temperatures are normal?

 

[PRR]

> is it normal that the EL34 tubes hitting up to 150-155 Celsius (300-310 Fahrenheit ) ?

The tube is made for 250 deg C (482F).
http://www.mif.pg.gda.pl/homepages/frank/sheets/164/e/EL34.pdf

How hot does your plastic get?

 

[smilan]

Thanks