Troubleshooting Tape Repro: help needed

[glaferri]

Hi,
I just bought a used RS-1520 with a Bottlehead Tape Repro. Both units have been throughly verified and are supposed to be in perfect order. I have an unconditional faith in the seller when he tells me he can’t reproduce the behavior I’m experiencing at his place.
When I play the RS-1520 using the internal preamp everything sounds fine. When I use the direct head connection with the Tape Repro, the output sounds muffled, like you would put a pillow in front of all the microphones.
I have checked and double checked everything and I can’t find why it would sound like that on my sound system.
So far I have tried with different phono cables between the RS-1520 and the Tape Repro and different  XLR and RCA cables between the Tape Repro and my McIntosh C1100 preamp.
I tried adjusting the Tape Repro gain lower and higher.
Heads have been cleaned and demagnetized.
Tubes have been tested and are perfect. Bottlehead have been sent a second time to a tech, verified and play tested and everything is fine.
Could it be an impedance mismatch between the Tape Repro and the C1100?
Another thing I would like to try is making sure the RS-1520 head direct connexion is not faulty. I would like to connect the RS-1520 direct output directly into one of my C1100 phono input and bypass the Bottlehead. I know the sound will not be very good but it could indicate me wether the direct connexion is defective or there’s an incompatibility between the Bottlehead and C1100 line inputs? Isthis something I should try. If yes, should I use an MC or an MM input? What would be the best impedance to use  for the MC input?
Any thoughts on what could cause that?
Thanks for your help.

[Paul Birkeland]

Bottlehead have been sent a second time to a tech, verified and play tested and everything is fine.

When was this done?

Could it be an impedance mismatch between the Tape Repro and the C1100?

No.

I would like to connect the RS-1520 direct output directly into one of my C1100 phono input and bypass the Bottlehead.

Yes, send it to the MM input.

Did you try the unbalanced outputs from the Repro?  Were you careful to insert the 15095 output transformers carefully into the octal socket?  The last Repro I remember getting in for repair had the 15095s jammed in with no regard for the keyway, and once I straightened that out it was working correctly, but this was many years ago.

[Doc B.]

It sounds like you have been very thorough and logical in your tests. Without being able to hear how extreme this muffled sound is it's a little hard to guess where the issue is. If it is mild it could be that trying some different E86 and 6922 tubes could have enough of a different tonality that it would open up the top end. The one thing I didn't see mentioned is whether the playback EQ setting was correct for the tape being played. I imagine it was, but I am just trying to be thorough as well. One possibility would be to try the Repro (from the balanced output and with the trim levels turned down almost all the way to start) running directly into your power amp without the C1100 in the signal path. That might help to identify any incompatibility between the Repro and the C1100. It does seem unlikely as the input impedance of the Mac is 44K balanced, plenty high. If you try this be sure to power up the Repro first and then the amp, power down the amp first and then the Repro.

[glaferri]

When was this done?No.Yes, send it to the MM input.

Did you try the unbalanced outputs from the Repro?  Were you careful to insert the 15095 output transformers carefully into the octal socket?  The last Repro I remember getting in for repair had the 15095s jammed in with no regard for the keyway, and once I straightened that out it was working correctly, but this was many years ago.

Bottlehead arrived from tech 2 days ago. It was sent it back to the previous owner because it was noisy and the sound was muffled. I returned it to the owner and he sent it to his tech which is supposed to be one of the best in Toronto. He changed the installed Telefunken 6922 for a Sovtek 6922. Told the previous owner he didn't like that particular Telefunken tube because there was a danger of burning a resistance. He also change one resistance. He kept the Tape Repro for 3 days on the bench and played music with it make sure everything was fine. Tape Repro came back totally silent but sound is still muffled.

This is a answer of the previous owner this morning after I sent him a video of the tape sound vs the same song on vinyl: "This is quite mysterious and based upon the topography and design of the BH, there is no reason for this to be happening since both in my system and in my Tech's setup, the BH played perfectly. It may be that there is  a mismatch , possibly an impedance issue with your system. My tech has painstakingly checked all the caps, resistors , connections, solders and tested the transformers in the BH to make sure all is as it should be. Everything tests to spec. He then ran it with his own tape deck for a few hours. Similarly , when the deck came back to Tom , I asked him to spend an extra few hours checking the deck completely and he did this as well."

I tried direct output to C1100 MM input. Sound is correct. So no problem with the RS-1520 direct output.
I verified the ouput transformers and they are inserted and seated correctly.
I tried with the unbalanced Tape Repro output and same muffled sound

Actual tubes are : Telefunken EF-86, Sovtek 6922 and original 6CM7 all tested perfect.
New tubes I am waiting for: Siemens E188C/7308 and RFT-EF86/6267

[Doc B.]

Since this is a preamp someone else has owned for a while, a tech I don't know has done some sort of work on it and there has been a change in the circuit it is pretty difficult for me to know what is going on. Beyond the suggestions I have made the best I can offer is to send it to us and we can have a look at it. Siemens tubes will bring up the top end if the sound you are hearing is simply unsuitable to your taste rather than a defect in the equalization or frequency response of the unit. Is the play head stock in the Technics?

[Paul Birkeland]

It's not an impedance issue. 

[glaferri]

Well, I found something interesting reading the owner manual about CCIR vs  NAB equalization. It says if you play a CCIR tape on a NAB eq. it will sound muffled. All my tapes are CCIR and the switch has always been in the up position, which is the right position for CCIR.

I tried to put the switch in the down NAB position and everything sounds fantastic. Sound is crystal clear, airy. Bass is correct, tight and not overwhelming.

For me it looks like the switch is inverted. Is this possible? I attached some pics. No one seems to have ever touched the wires or soldering of the switch. From what you see on the pics, does it seems inverted?

I really don't get this one... I guess it's probably something else but it's kind of strange...

[Paul Birkeland]

Your EQ circuit is indeed wired backwards.  I do recall there being an issue about ten years ago where this popped up.  We could certainly address this for you.

[glaferri]

I'm relieved the problem is finally solved!

Is the switch only inverted or there could be more mismatch in the eq circuit from what you recall 10 years ago?

If it's only inverted, I have no problem leaving the switch in the down position but if something else from those production dates should be verified, I can send it to you.
Your help is much appreciated. I am quite impressed with your customer service!

Thanks!

Gilles

[Paul Birkeland]

The switch wiring is just inverted.

-PB

[Doc B.]

Apologies for the error. I am embarrassed not to have caught it when the unit was built and kind of surprised the original owner never contacted us about the issue. Makes one wonder if that Repro was used much.

We would be happy to correct it for you. However, shipping is so risky these days it might be safer to live with it as is. If you do want to have it fixed and don't have the original shipping box (hopefully you do, they have about $50 worth of a special foam designed for shipping instrumentation inside) let me know and we can make one up to send to you.